Higher crop performance of rice hybrids than of elite inbreds in the tropics: 2. Does sink regulation,rather than sink size,play a major role?

Higher harvest index, widely reported as a main trait supporting major improvements in rice yield potential, also supports the higher performance of hybrids over inbreds. Although higher sink size is generally claimed for being the driver of higher grain filling in hybrid rice, it is relevant to question whether efficient sink regulation, that is timing and magnitude of the successive sink activities over crop growth from early stage, plays a role in supporting higher harvest index and is a key feature ensuring higher performance of hybrid rice. The partitioning coefficient (PC), as the increase in dry matter of one single organ over that of shoot, was calculated over short and successive periods to quantify the dynamics of dry matter partitioning. Four hybrid and four inbred genotypes of similar crop duration were grown under the same cropping management on the IRRI farm during a wet and a dry seasons. While PC to blade and sheath was similar for both plant types in both seasons at the early stage, PC to culm of hybrids was higher than that of inbreds, and PC to blade was lower, in the late vegetative and early reproductive phases. During the late reproductive phase, PC to panicle of hybrids was higher than that of inbreds, and PC to culm was lower whereas culm elongation was similar for both plant types. During grain filling, PC to culm was significantly more negative with hybrids which indicated stronger remobilization with hybrids. Specific culm length before anthesis, as low as 40 cm g⁻¹, accounted for storage ability, and at maturity, as high as 130 cm g⁻¹, for remobilization ability. Sink strength index, as a better indicator of dry matter partitioning efficiency than harvest index, and unfilled grain size, as an indicator of assimilate wastage, were suggested as key indices to account for the better sink regulation in hybrid rice and to improve screening protocols for increasing yield potential and tolerance to lodging.     

Interactions of sunflower (Helianthus annuus) and sunflower broomrape (Orobanche cumana) as affected by sowing date,resource supply and infestation level

The holoparasitic weed Orobanche cumana (sunflower broomrape) constrains sunflower (Helianthus annuus) production in many countries. The development of efficient control strategies requires an understanding of the processes underlying the complex environment–host–parasite interrelations. Growth and development of O. cumana and sunflower were quantified under field conditions in southeastern Romania. Sunflower hybrid Florom 350 was sown at two dates, in plots infested with 0, 50, 200 and 1600 viable O. cumana seeds kg⁻¹ dry soil, under low-input (rainfed, low nitrogen supply) and high-input (irrigated, high nitrogen supply) conditions. Sunflower shoot biomass reached peak values of 760–1287 g m⁻² between the end of anthesis and physiological maturity. Seed yield varied from 221 to 446 g m⁻². Sunflower biomass and yield were affected by all experimental factors. Seed yield responded positively to delaying sowing from early April to late May as well as to irrigation and fertilisation, and negatively to O. cumana infestation. Yield reductions, which were a product of reduced seed number and size, amounted to 13%, 25% and 37% at parasite seed densities of 50, 200 and 1600 viable seeds kg⁻¹ soil, respectively. Maximum O. cumana attachment numbers, recorded in late-sown high-input crops in 2004, ranged from 11 m⁻² in plots with 50 parasite seeds kg⁻¹ soil to 188 m⁻² with 1600 seeds kg⁻¹ soil. Parasite attachment number was a function of crop sowing date, water and nutrient supply, seedbank density, and sunflower biomass and root length density, via mechanisms of parasite seed stimulation, host carrying capacity and intraspecific competition. Delayed sowing and improved water and nitrogen supply were associated with increases in parasite number that neutralised yield-boosting effects of irrigation and fertilisation at the highest infestation level. Sunflower shoot biomass was significantly reduced by O. cumana infection, with reductions affecting organs in the order head > stem > leaves. Most of the discrepancy between infected and non-infected plants was accounted for by O. cumana biomass. Parasites mainly acted as an extra sink for assimilates during sunflower generative growth and impaired host photosynthesis to a much lesser degree. Results suggest that similar mechanisms govern infection level and host–parasite biomass partitioning across different Orobanche–host systems.     

Intra-specific competition in maize: Early established hierarchies differ in plant growth and biomass partitioning to the ear around silking

Early interferences among plants within a maize stand determine the establishment of extreme plant types (i.e. dominant and dominated individuals). The development of these hierarchies takes place well before [from the seventh leaf stage (V₇) onwards] the start of the critical period for kernel set (i.e. a 30-day period centered in silking). Kernel number per plant (KNP) is significantly related to plant growth rate around silking (PGR_S) and biomass partitioning to the ear during this period. Previous evidence has demonstrated that at high stand densities, extreme plant types may exhibit similar PGR_S values but set different KNP. We tested the hypothesis that early established plant hierarchies differ in biomass allocation to the ears during the period around silking. Two hybrids of contrasting tolerance to crowding (DK752 and DK765 as the tolerant and the intolerant hybrid, respectively) were cropped at different interplant competition intensities (6, 12, 12 pl m⁻² thinned to 6 pl m⁻² at V₉ and 6 pl m⁻² shading from V₉ onwards) during 2003/2004 and 2004/2005 in Argentina. For all treatments, the coefficient of variation (CV) of plant biomass increased from V₃ (ca. 1.2%) to V_9-10 (ca. 22%). From V₇ onwards, plant growth rate of dominant individuals was higher (P < 0.05) than that of the dominated plants. Hence, dominant plants exhibited higher (P < 0.05) PGR_S (ca. 4.5 g pl day⁻¹) than dominated individuals (ca. 3.7 g pl day⁻¹). As PGR_S declined in response to increased plant population density (ca. 5.1 and 2.8 for 6 and 12 pl m⁻², respectively), biomass partitioning to the ear was reduced (ca. 0.44 and 0.33 for 6 and 12 pl m⁻², respectively). For all treatments, however, dominant plants exhibited a greater biomass partitioning to the ear (ca. 0.41) than the dominated individuals (ca. 0.36). Consequently, the former were the individuals with the highest ear growth rate (ca. 1.9 and 1.4 g per ear per day for the dominant and dominated plant, respectively) and KNP (ca. 623 and 490 kernels per plant for the dominant and dominated plant, respectively) of the stand. We identified three traits on DK765 related to the low tolerance to high-density stress of this genotype: (i) a higher plant-to-plant variability (CV ca. 26% and 19%, for DK765 and DK752, respectively), (ii) a lower biomass partitioning to the ear around silking (ca. 0.26 and 0.39 for DK765 and DK752, respectively), and (iii) a higher response rate of KNP to ear growth rate around silking (ca. 370–738 and 360–414 kernels per g, for DK765 and DK752, respectively). Hence, as stand density was increased, KNP of DK765 was sharply reduced, especially in the dominated individuals of the stand.     

Relations of rice seeding rates to crop and weed growth in aerobic rice

Aerobic rice describes a management adaptation to reduced irrigation water supplies but, due to reduced intervals of flooding in this system, this requires revised weed management approaches to reduce costs and provide effective weed control. One approach is to make the crop more competitive and reduce the effects of weeds on the crop by using higher rice seeding rates. A study was conducted in the Philippines and India in 2008 and 2009 to assess the relations of seeding rates (15-125 kg ha⁻¹) of hybrid and inbred varieties to crop and weed growth in aerobic rice. Plant densities, tillers, and biomass of rice increased linearly with increased in seeding rates under both weedy and weed free environments. Weed biomass decreased linearly with increasing seeding rates from 15 to 125 kg ha⁻¹. Panicles and grain yields of rice in competition with weeds increased in a quadratic relation with increased seeding rates at both locations; however, the response was flat in the weed free plots. A quadratic model predicted that seeding rates of 48-80 kg ha⁻¹ for the inbred varieties and 47-67 kg ha⁻¹ for the hybrid varieties were needed to achieve maximum grain yield when grown in the absence of weeds, while rates of 95-125 kg seed ha⁻¹ for the inbred varieties and 83-92 kg seed ha⁻¹ for the hybrid varieties were needed to achieve maximum yields in competition with weeds. On the basis of these results, seeding rates greater than 80 kg ha⁻¹ are advisable where there are risks of severe weed competition. Such high seeding rates may be prohibitive when using expensive seed, and maximum yields are not the only consideration for developing recommendations for optimizing economic returns for farmers. Results of the present study do suggest however that increasing seeding rates of aerobic rice does suppress weed growth and reduce grain yield losses from weed competition. This information could be incorporated in integrated crop management packages to manage weeds more effectively.     

Dry matter and nitrogen accumulation,partitioning, and retranslocation in safflower (Carthamus tinctorius L.) as affected by nitrogen fertilization

Safflower (Carthamus tinctorius L.) is a deep-rooted crop which can tolerate water stress and can be grown in rotation with other crop species. Nitrogen is one of the most important nutrients for the growth and development of safflower; however, the effect of N level on dry matter, accumulation, partitioning, and retranslocation has not been extensively studied. A 2-year field study was therefore conducted with the objective to determine the effect of N fertilization on crop phenology, dry matter, N accumulation, partitioning and retranslocation of safflower grown under rain-fed conditions. Three rates of N were used (0, 100, and 200 kg N ha⁻¹) and two hybrids (CW9048 and CW9050) of safflower were selected. The experiment was conducted during the 2003–2004 (2004) and 2004–2005 (2005) growing seasons on a calcareous sandy loam (Entisols, Orthents, Typic Xerorthent) at the experimental farm of the Aristotle University of Thessaloniki, in Northern Greece. During 2004 spring was quite mild with significant rainfall whereas during 2005 spring was hotter with lower rainfall. Our study found that N fertilization increased biomass at anthesis by an average of 24% and at maturity by an average of 25% compared with the control. Total above ground biomass increased after anthesis in both years, in both hybrids and for all fertilizer treatments. N fertilization increased the dry matter partitioning in leaves + stems and heads at anthesis and also in leaves + stems, seeds, and head vegetative components at maturity. Dry matter translocation was not affected by N fertilization but lower values were found during the second year. N content was affected by the fertilization treatments and increased in those plants treated with fertilizer compared with the controls. In addition, N fertilization increased N retranslocation from the vegetative parts of the plant to the seed, but it did not affect N gain. During the second year, which was drier, there were significant N losses but also greater N translocation efficiency and higher contribution of pre-anthesis N to seed. Seed yield was correlated with the dry matter and N translocation indices, and was higher for the fertilized plants, compared with the control. The present study indicates that N fertilization promoted the growth of safflower and increased the dry matter yield, N accumulation, translocation and seed yield under rain-fed conditions.     

Maize kernel growth and kernel water relations as affected by nitrogen supply

Improving maize (Zea mays L.) growing conditions near flowering by applying nitrogen (N) could affect both kernel number per unit area (KN) and potential kernel weight (KW). Potential kernel weight can be estimated with maximum kernel water content (MKWC), as final kernel weight and kernel water relations are strongly associated in maize. At the crop level, the product of KN per unit area and MKWC could provide an appropriate index of potential sink capacity. The main objective of this study was to determine if the decrease in potential sink capacity (i.e. the product of MKWC and KN), under N deficit and with a late planting date, is due to MKWC or KN reductions. Additionally, we evaluated sink growth rate per unit area (i.e. the product of KN and kernel growth rate) during grain fill period as related to potential sink capacity. Three N rates under optimal and late planting dates and two hybrids were evaluated in experiments carried out at Paraná, Argentina (31°50′S; 60°31′W) during 2002–2003 and 2003–2004 growing seasons (seasons 1 and 2, respectively). Except for the late planting date during season 1, there was a significant positive response on grain yield, KN, KW, and MKWC to N supply. Experiments explored a broad range of KN, from 1645 to 5066 kernel m⁻². MKWC and KN were positively correlated for DK682. Nitrogen increased the potential sink capacity and sink growth rate only as KN increases from a particular threshold in both hybrids. The sink growth rate was largely related to potential sink capacity, as MKWC was highly correlated with KGR across all treatments. Our analytical approach, considering potential sink capacity as a product of KN and MKWC, is integrative of sink demand both for individual kernel and crop levels. This work highlights the role of early establishment of potential sink capacity on yield determination under a wide range of N conditions.     

Reproductive partitioning in sea level quinoa (Chenopodium quinoa Willd.) cultivars

The proportion of growth allocated to reproductive organs can be an important determinant of yield variation between cultivars and environments. The main aim of this paper was to evaluate the adequacy of a model assuming constancy in partitioning coefficients (PC, the slope of organ weight to total weight relationship) within periods whose limits are associated with phenological phases to describe variation in reproductive growth (including seeds when present) in the Andean seed crop quinoa. A second objective was to analyze the dynamics of panicle and stem growth to advance our understanding of factors determining yield in this species. To do this, we used data from two experiments conducted in 2 years under field conditions in the Argentinean pampas, using four cultivars belonging to the Sea Level Type and adapted to temperate environments, under three densities. Reproductive partitioning followed a bi-phasic pattern; panicle biomass increased gradually until reaching a total biomass value, and then there was an increase in the slope of panicle vs. total aerial biomass relationship. Partitioning coefficients for the initial stage varied between some cultivars and densities in the first year, but not in the second. No significant differences were detected when PCs for the second stage were considered. The start of panicle growth was associated with thermal time to first anthesis (R² = 0.62) while thermal time to change in partitioning from low to high PC and that to end of flowering were strongly related (R² = 0.93). Combining data across cultivars, years and densities gave a PC of 0.15 for the initial stage and 0.90 for the second stage. Using these relationships and parameters dynamics of panicle biomass accumulation was predicted satisfactorily in an independent data set for a different environment, confirming the usefulness of a single model approach to describe partitioning across cvs. and environments in this crop. Besides, crop yield estimations improved when compared to those obtained by a seed number estimation model, predictions were only 7.25% lower than observed values compared to −24.5% using a seed number approach. There is a trade-off between final partitioning to reproductive structures (higher in short-cycle cvs.) and total crop biomass, one of the factors contributing to this trade-off being a negative association between the panicle–stem relationship at harvest and duration in thermal time units of stem growth; so, selection for high partitioning rate should be targeted at long duration cvs. within this germplasm.     

The high yield of irrigated rice in Yunnan,China: ‘A cross-location analysis’

A number of field trials on rice productivity have demonstrated very high yield, but reported limited information on environmental factors. The objective of this study was to reveal the environmental factors associated with high rice productivity in the subtropical environment of Yunnan, China. We conducted cross-locational field experiments using widely different rice varieties in Yunnan and in temperate environments of Kyoto, Japan in 2002 and 2003. The average daily radiation throughout the growing season was greater at Yunnan (17.1 MJ m⁻² day⁻¹ average over 2 years) relative to Kyoto (13.2 MJ m⁻² day⁻¹). The average daily temperature throughout the growing season was 24.7 °C at Yunnan, and 23.8 °C at Kyoto. The highest yield (16.5 tonnes ha⁻¹) was achieved by the F1 variety Liangyoupeijiu at Yunnan in 2003, and average yield of all varieties was 33% and 39% higher at Yunnan relative to Kyoto in 2002 and 2003, respectively. There was a close correlation between grain yield and aboveground biomass at maturity, while there was little variation in the harvest index among environments. Large biomass accumulation was mainly caused by intense incident radiation at Yunnan, as there was little difference in crop radiation use efficiency (RUE) between locations. Large leaf area index (LAI) was also suggested to be an important factor. Average nitrogen (N) accumulation over 2 years was 49% higher at Yunnan than at Kyoto, and also contributed to the large biomass accumulation at Yunnan. The treatments of varied N application for Takanari revealed that the ratio of N accumulated at maturity to the amount of fertilized N was significantly higher at Yunnan than at Kyoto, even though there was no great difference in soil fertility. The Takanari plot with high N application showed a N saturation in plant growth at Kyoto, which might be related to low radiation and relatively high temperatures during the mid-growth stage. These results indicate that the high potential yield of irrigated rice in Yunnan is achieved mainly by intense incident solar radiation, which caused the large biomass and the N accumulation. The low nighttime temperature during the mid-growth stage was also suggested to be an important factor for large biomass accumulation and high grain yield at Yunnan.     

Ecophysiological determinants of biomass and grain yield of wheat under P deficiency

Grain yield of crops can be expressed as a function of the intercepted radiation, the radiation use efficiency and the partitioning of above-ground biomass to grain yield (harvest index). When a wheat crop is grown under P deficiency the grain yield is reduced but it is not clear how these three components are affected. Our aim was (i) to identify which of these components were affected in spring bread wheat under P deficiency at field conditions and (ii) to relate the grain yield responses to processes of grain yield formation during the spike growth period. Three field experiments were conducted in the potentially high wheat yielding environment of southern Chile. All experiments had two levels of P availability: with (155 kg P ha⁻¹) or without P fertilization (average soil P-Olsen concentration of 10 ppm, a medium level of P availability). High wheat grain yields were obtained varying between 815 and 1222 g m⁻² with P applications. Experiments showed a grain yield reduction caused by P deficiencies of 35, 16 and 18% in experiments 1, 2 and 3, respectively. This was related (R² = 0.99, P < 0.01) to a reduction in the total above-ground biomass at harvest and not to the harvest index. Reductions in above-ground biomass were due to a reduction in radiation intercepted under P deficiency without effecting radiation use efficiency. Grain number per square meter was the main yield component (R² = 0.99, P < 0.01) that explained the grain yield reduction caused by the P deficiency which was due to low spike biomass at anthesis (R² = 0.96, P < 0.05). The reduction in spike biomass at anthesis was related (R² = 0.86, P < 0.01) to reductions in crop growth rate during the spike growth period as a consequence of a lower radiation intercepted during this period. This study showed that under high wheat yield conditions the main effect of a P deficiency on grain yield reduction was a negative impact on the total above-ground biomass due to the negative impact on intercepted radiation, particularly during the spike growth period, affecting negatively spike biomass at anthesis and consequently grain number and yield.     

Arthropod communities related to different mixtures of oil (Glycine max L. Merr.) and essential oil (Artemisia annua L.) crops

Plants can host many herbivores and their natural enemies during their growth cycles. For this reason, changes in the relative abundance of crop and weed plants in a monocropping system as well as different crop plants in an intercropping system may produce great bottom up impacts in the specific and functional structure of spontaneous communities of arthropods. The hypothesis of this study was that the combination of two contrasting species, soybean (Glycine max, Fabaceae, N₂ fixing plant) and annual wormwood (Artemisia annua, Asteraceae, VOCs plant), would be related to different spontaneous communities of arthropods depending on the proportion of each species, and this would favor crop biodiversity without compromising crop production. The objectives of the study were: (a) to analyze the differences of spontaneous communities of arthropods related to different soybean (S)-annual wormwood (W) mixtures, using standard crop management for S production in Argentina, (b) to determine S and W total biomass and W essential oil content and yield and, (c) to analyze the relationship between arthropod communities and crop productivity. Factorial field experiments with 3 replications were done during 2006 and 2007. S density was kept constant (40 plants m⁻²) and different W densities (plants m⁻²) were added. Treatments were pure S, S + 2W, S + 4W, S + 8W and pure W (8 plants m⁻²). Arthropods were sampled at soybean full flowering and were classified in functional groups as herbivores and non-herbivores. S and W total and relative biomass and W essential oil content and yield from leaves and inflorescences were estimated in reproductive stage. Arthropod morphospecies abundance and richness were determined for each treatment. Data were analyzed using uni (ANOVA) and multivariate (CCA) techniques. Arthropods belonging to 7 orders presented a total richness of 48 morphospecies in 2006 and 36 in 2007, while total abundance was 379 in 2006 and 318 in 2007. The proportion of non-herbivores was higher than the proportion of herbivores. Different arthropod communities were observed according to each treatment. No differences were found among treatments in S + W and S total biomass production, while W total biomass and essential oil yield were both different among treatments. Relative biomass production of S and W was the main explanatory variable related to the contrast of arthropod communities between pure annual wormwood (W) and the rest of the treatments. Annual wormwood could be used as an accompanying essential oil crop or left as a weed in the densities tested in this work, favoring biodiversity and, eventually, pest management without compromising soybean crop yield.     

A quantitative approach to characterize sink–source relationships during grain filling in contrasting wheat genotypes

We present a simple generic framework to quantify source–sink relationships during grain filling, by using a determinate growth function which has a unique property, namely being able of explicitly describing the time for the end of a growth process. This model framework was applied to analyze these relationships in plants of six wheat (Triticum aestivum L.) genotypes grown in pots in climate-controlled greenhouses under two temperature regimes (day/night: 20/15 and 25/20 °C). The function accurately described the sigmoid pattern of grain growth (sink activity), as its modified form did for the reversed sigmoid shape of flag-leaf area (source capacity), during grain filling. The six genotypes differed significantly in grain number as well as in grain yield, ranging from 54 to 81 grains and from 2.67 to 4.52 g DM per culm, respectively, when grown at 20/15 °C. Biomass and grain yield were significantly reduced by a rise of 5 °C. Grain nitrogen contents raised from 2.1 to 2.6% as a consequence of less carbon accumulation resulting in lower grain weights at the high temperature. On average, a rise of 5 °C in temperature reduced the duration of grain growth by 12 days (>30%), and increased the growth rate from 1.32 to 1.67 mg grain⁻¹ d⁻¹ (20%). Genotypic differences in grain-filling duration were also larger than in rate of grain growth. The genetic variation in the flag-leaf area duration (a proxy for the capacity for intercepting radiation and photosynthesis) was positively associated with sink size. Model analysis showed that whether or not the timing for the cessation of grain filling and for the end of post-anthesis source activity was synchronized depended on temperature. The quantitative approach yielded parameters that characterize genotypic differences of post-anthesis source and sink capacity in responding to environmental variables.     

Sweet sorghum productivity for biofuels under increased soil salinity and reduced irrigation

Sweet sorghum (Sorghum bicolor (L.) Moench.) is a drought-tolerant crop with high resistance to saline-alkaline soils, and sweet sorghum may serve as an alternative summer crop for biofuel production in areas where irrigation water is limited. A two-year study was conducted in Northern Greece to assess the productivity (biomass, juice, total sugar and theoretical ethanol yields) of four sweet sorghum cultivars (Sugar graze, M-81E, Urja and Topper-76-6), one grain sorghum cultivar (KN-300) and one grass sorghum cultivar (Susu) grown in intermediate (3.2 dS m⁻¹) or in high (6.9 dS m⁻¹) soil salinity with either low (120 mm) or intermediate (210 mm) irrigation water supply (supplemented with 142–261 mm of rainfall during growth). The soil salinity and irrigation water supply effects on the sorghum chlorophyll content index, photosystem II quantum yield, stomatal conductance and leaf K/Na ratio were also determined. The sorghum emergence averaged 75,083 plants ha⁻¹ and 59,917 plants ha⁻¹ in a soil salinity of 3.2 dS m⁻¹ and 6.9 dS m⁻¹, respectively. The most affected cultivar, as averaged across the two soil salinity levels, was the Susu grass sorghum emerging at 53,250 plants ha⁻¹, followed by the Topper-76-6 sweet sorghum emerging at 61,250 plants ha⁻¹. The leaf K/Na ratio decreased with decreasing irrigation water supply, in most cases, but it was not significantly affected by soil salinity. The dry biomass, juice and total sugar yields of sorghum that received 210 mm of irrigation water was 49–88% greater than the yields of sorghum that received the 120 mm of irrigation water. Sorghum plants grown in a soil salinity of 3.2 dS m⁻¹ produced 42–58% greater dry biomass, juice and total sugar yields than the yields of sorghum plants grown in a soil salinity of 6.9 dS m⁻¹. The greatest theoretical ethanol yield was produced by sweet sorghum plants grown in a soil salinity of 3.2 dS m⁻¹ with 210 mm of irrigation water (6130 L ha⁻¹, as averaged across cultivar), and the Urja and Sugar graze cultivars produced the most ethanol (7620 L ha⁻¹ and 6528 L ha⁻¹, respectively). Conclusively, sweet sorghum provided sufficient juice, total sugar and ethanol yields in fields with a soil salinity of 3.2 dS m⁻¹, even if the plants received 50–75% of the irrigation water typically applied to sorghum.     

A simple model for chickpea development, growth and yield

Chickpea (Cicer arietinum L.) yield is unstable and low in major producer countries. A robust crop model can assist in evaluation of possible genetic improvements and cultural management practices to improve yield. The objectives of this study were to develop and test a chickpea simulation model that could be used across a wide range of environments. This model simulates phenological development, leaf development and senescence, mass partitioning, plant nitrogen balance, yield formation and soil water balance. Responses of crop processes to environmental factors of solar radiation, photoperiod, temperature, nitrogen and water availability, and genotype differences were included in the model. The model uses a daily time step and readily available weather and soil information. The model was tested using independent data from a wide range of growth and environmental conditions. In most cases, simulated grain yield were similar to observed yield (ranging from 20 to 379 g m⁻²) with a root mean square root of 26 g m⁻² (15% of average measured yield). It was concluded that the model generality, i.e., constant parameters for genotypes across locations, and applicability to a wide range of environmental conditions factors made this model especially useful.     

Inter-plant competition for resources in maize crops grown under contrasting nitrogen supply and density: Variability in plant and ear growth

Increased plant population density in irrigated and fertilized maize crops enhances plant-to-plant variability since early vegetative stages, because the most suppressed individuals of the stand intercept less radiation per unit leaf area than the dominant ones (i.e. a size-asymmetric competition for light). Contrarily, a size-symmetric competition has been proposed for the acquisition of soil resources in a plant community (e.g. N capture per unit root length is similar among plants of different size). Hence, N fertilization effect on the variability of maize plants would depend on the initial plant-to-plant variability or on that promoted by a high plant population density. Two maize hybrids with contrasting tolerance to crowding (tolerant AX820 and intolerant AX877) were cultivated under different combinations of stand densities (6, 9 and 12 plants m⁻²) and N supplies (0 and 200 kg N ha⁻¹) without water restrictions. Variability in plant growth rate among plants was computed along the cycle, especially after fertilizer was applied (i.e. the early reproductive period; PGR_ER) and during the critical period around silking (PGR_CP). Plant-to-plant variability in biomass partitioning to the ear (partition index; PI), ear growth rate during the critical period (EGR_CP) and kernel number per plant (KNP) was also established. Reduced N supply increased the coefficient of variation (CV) of PGR_ER, PGR_CP, EGR_CP and KNP (0.05 < P < 0.10). The CVs of PGR_CP, PI, EGR_CP and KNP augmented (0.001 < P < 0.10) at the highest stand density. The CVs of PGR_ER, PGR_CP, PI and KNP were larger for hybrid AX877 than for hybrid AX820 (0.001 < P < 0.10). N fertilization smoothed the initial plant-to-plant variability, but the extent of this benefit in a maize crop is genotype dependent; it was much larger in the hybrid tolerant to crowding stress than in the intolerant one. For the latter, the variability held during the critical period around silking and produced a high CV of KNP.     

Indigenous legume fallows (indifallows) as an alternative soil fertility resource in smallholder maize cropping systems

Widening the range of organic nutrient resources, especially N sources, is a major challenge for improving crop productivity of smallholder farms in southern Africa. A study was conducted over three seasons to evaluate different species of indigenous legumes for their biomass productivity, N₂-fixation and residual effects on subsequent maize crops on nutrient-depleted fields belonging to smallholder farmers under contrasting rainfall zones in Zimbabwe. Under high rainfall (>800 mm yr⁻¹), 1-year indigenous legume fallows (indifallows), comprising mostly species of the genera Crotalaria, Indigofera and Tephrosia, yielded 8.6 t ha⁻¹ of biomass within 6 months, out-performing sunnhemp (Crotalaria juncea L.) green manure and grass (natural) fallows by 41% and 74%, respectively. A similar trend was observed under medium (650–750 mm yr⁻¹) rainfall in Chinyika, where the indifallow attained a biomass yield of 6.6 t ha⁻¹ compared with 2.2 t ha⁻¹ for natural fallows. Cumulatively, over two growing seasons, the indifallow treatment under high rainfall at Domboshawa produced biomass as high as 28 t ha⁻¹ compared with ∼7 t ha⁻¹ under natural fallow. The mean total N₂ fixed under indifallows ranged from 125 kg ha⁻¹ under soils exhibiting severe nutrient depletion in Chikwaka, to 205 kg ha⁻¹ at Domboshawa. Indifallow biomass accumulated up to 210 kg N ha⁻¹, eleven-fold higher than the N contained in corresponding natural fallow biomass at time of incorporation. Application of P to indifallows significantly increased both biomass productivity and N₂-fixation, translating into positive yield responses by subsequent maize. Differences in maize biomass productivity between indifallow and natural fallow treatments were already apparent at 2 weeks after maize emergence, with the former yielding significantly (P < 0.05) more maize biomass than the latter. The first maize crop following termination of 1-year indifallows yielded grain averaging 2.3 t ha⁻¹, significantly out-yielding 1-year natural fallows by >1 t ha⁻¹. In the second season, maize yields were consistently better under indifallows compared with natural fallows in terms of both grain and total biomass. The first maize crop following 2-year indifallows yielded ∼3 t ha⁻¹ of grain, significantly higher than the second maize crop after 1-year indifallows and natural fallows. The study demonstrated that indigenous legumes can generate N-rich biomass in sufficient quantities to make a significant influence on maize productivity for more than a single season. Maize yield gains under indifallow systems on low fertility sandy soils exceeded the yields attained with either mineral fertilizer alone or traditional green manure crop of sunnhemp.     

The effects of cover cropping on yield and weed control of potato in a transitional system

Cover cropping can have various beneficial effects to the cropping system such us the increase of soil nutrient content and weed suppression. In this respect, the species used for covering is of great importance. This paper reports results on the yield and weed control effects in potato crops preceded by different cover crops over a 2-year period (2003 and 2004) in Central Italy (Viterbo). Results were obtained in the frame of a more complex study set up in 2002 where in a 3-year chick-pea/potato/tomato rotation, each crop was preceded by 7 different soil managements: 5 cover crops (rapeseed, Italian ryegrass, hairy vetch, snail medick and subclover) + 1 unfertilised weedy fallow (cover crop absent) + 1 control (weedy fallow fertilised with mineral N at a rate of 170 kg ha⁻¹ for potato). Two different weed control regimes in potato were also applied [weed-free crop (1 inter-row hoeing + 1 hilling up + manual weeding on the row); mechanical control (1 inter-row hoeing + 1 hilling up)]. Cover crops were sown in September and cut and ploughed just before potato planting in March. The potato crops following the cover crops were only fertilised with green manure. Averaged over years, all the cover crops produced more above-ground dry biomass than the weedy fallow (4.79 t ha⁻¹ on average vs 2.36 t ha⁻¹). Hairy vetch and subclover accumulated the highest N in the incorporated biomass (169 and 147 kg ha⁻¹), followed by snail medick (108), rapeseed (99), ryegrass (88) and weedy fallow (47). Rapeseed and ryegrass were the most efficient weed suppressors and had the least proportion of weed biomass (<1%) of the total produced by the cover, while they also reduced weed emergence in the following potato crops (8.8 plants m⁻²vs 25.5 plants m⁻² with all other cover crops). Following subclover and hairy vetch the potato crop yield was similar to that obtained by mineral N-P-K fertilisation (48.5 t ha⁻¹ of fresh marketable tubers). Mechanical weed control compared to weed free crop always reduced potato yield and the reduction, averaged over years, was greater in N-P-K mineral fertilised control (−23.6%) and smaller in ryegrass (−7.9%).     

A comprehensive study of plant density consequences on nitrogen uptake dynamics of maize plants from vegetative to reproductive stages

Nitrogen (N) use efficiency (NUE), defined as grain produced per unit of fertilizer N applied, is difficult to predict for specific maize (Zea mays L.) genotypes and environments because of possible significant interactions between different management practices (e.g., plant density and N fertilization rate or timing). The main research objective of this study was to utilize a quantitative framework to better understand the physiological mechanisms that govern N dynamics in maize plants at varying plant densities and N rates. Paired near-isogenic hybrids [i.e., with/without transgenic corn rootworm (Diabrotica sp.) resistance] were grown at two locations to investigate the individual and interacting effects of plant density (low—54,000; medium—79,000; and high—104,000 pl ha⁻¹) and sidedress N fertilization rate (low—0; medium—165; and high—330 kg N ha⁻¹) on maize NUE and associated physiological responses. Total aboveground biomass (per unit area basis) was fractionated and both dry matter and N uptake were measured at four developmental stages (V14, R1, R3 and R6). Both plant density and N rate affected growth parameters and grain yield in this study, but hybrid effects were negligible. As expected, total aboveground biomass and N content were highly correlated at the V14 stage. However, biomass gain was not the only factor driving vegetative N uptake, for although N-fertilized maize exhibited higher shoot N concentrations than N-unfertilized maize, the former and latter had similar total aboveground biomass at V14. At the R1 stage, both plant density and N rate strongly impacted the ratio of total aboveground N content to green leaf area index (LAI), with the ratio declining with increases in plant density and decreases in N rate. Higher plant densities substantially increased pre-silking N uptake, but had relatively minor impact on post-silking N uptake for hybrids at both locations. Treatment differences for grain yield were more strongly associated with differences in R6 total biomass than in harvest index (HI) (for which values never exceeded 0.54). Total aboveground biomass accumulated between R1 and R6 rose with increasing plant density and N rate, a phenomenon that was positively associated with greater crop growth rate (CGR) and nitrogen uptake rate (NUR) during the critical period bracketing silking. Average NUE was similar at both locations. Higher plant densities increased NUE for both medium and high N rates, but only when plant density positively influenced both the N recovery efficiency (NRE) and N internal efficiency (NIE) of maize plants. Thus plant density-driven increases in N uptake by shoot and/or ear components were not enough, by themselves, to increase NUE.     

The effects of nitrogen and iron fertilization on growth,yield and fertilizer use efficiency of soybean in a Mediterranean-type soil

Poor seed yield of soybean in Mediterranean-type environments may result from insufficient iron (Fe) uptake and poor biological nitrogen (N) fixation due to high bicarbonate and pH in soils. This study was conducted to evaluate the effects of N and Fe fertilization on growth and yield of double cropped soybean (cv. SA 88, MG III) in a Mediterranean-type environment in Turkey during 2003 and 2004. The soil of the experimental plots was a Vertisol with 176 g CaCO₃ kg⁻¹ and pH 7.7 and 17 g organic matter kg⁻¹ soil. Soybean seeds were inoculated prior to planting with commercial peat inoculants. N fertilizer rates were 0, 40, 80, and 120 kg N ha⁻¹ of which half was applied before planting and the other half at full blooming stage (R2). Fe fertilizer rates were 0, 200 and 400 g Fe EDTA (5.5% Fe and 2% EDTA) ha⁻¹. It was sprayed as two equal portions at two trifoliate (V2) and at five trifoliate stages (V5). Plants were sampled at flower initiation (R1), at full pod (R4) and at full seed (R6) stages. Application of starter N increased biomass and leaf area index at R1 stage whereas Fe fertilization did not affect early growth parameters. N application continued to have a positive effect on growth parameters at later stages and on seed yield. Fe fertilization increased growth parameters at R4 and R6 stages, and final seed yield in both years. This study demonstrated an interactive effect of N and Fe fertilization on growth and yield of soybean in the soil having high bicarbonate and pH. There was a positive interaction between N and Fe at the N rates up to 80 kg N ha⁻¹. However, further increase in N rate produced a negative interaction. Fertilization of soybean with 80 kg N ha⁻¹ and 400 g Fe ha⁻¹ resulted in the highest seed yield in both years. We concluded that application of starter and top dressed N in combination with two split FeEDTA fertilization can be beneficial to improve early growth and final yield of inoculated soybean in Mediterranean-type soils.     

Comparative analysis of physiological characteristics and yield components in sugarcane cultivars

Sugarcane in an important crop due to the economic value of its products. Physiological characteristics and yield components of sugarcane were studied in three field-grown sugarcane cultivars B 63118, POJ 2878 and Ja 60-5. Three growth stages were identified: formative phase (until 140 DAP), grand growth (140–300 DAP) and maturity (after 300 DAP). Results indicated that cultivars showed contrasting yield mainly after 300 DAP. At ripening, the most productive cultivar (Ja 60-5) achieved higher leaf area, an optimum leaf area index for light interception, a high and stable net assimilation rate and an elevated leaf area and biomass duration. In addition, this cultivar showed the higher density and lower area of leaf sieve elements as compared with other, which could influence the high translocation rate (1.85 cm min⁻¹) at 8 MAP. The higher efficiency of this process in Ja 60-5 might also be supported by a higher (15–25%) apparent free space of stem parenchyma as compared with POJ 2878 and B 63118. Our results suggest that Ja 60-5 reduced carbon partitioned to foliar respiration which led to a higher partitioning of sucrose to stems evidenced by a higher Pol%.     

Estimating maize nutrient uptake requirements

Generic, robust models are needed for estimating crop nutrient uptake requirements. We quantified and modeled grain yield–nutrient uptake relations in maize grown without significant biotic and abiotic stresses. Grain yield and plant nutrient accumulation in above-ground plant dry matter (DM) of commercial maize hybrids were measured at physiological maturity in on-station and on-farm experiments in Nebraska (USA), Indonesia, and Vietnam during 1997–2006. These data were used to model the nutrient requirements for yields up to 20 Mg ha⁻¹ using the QUEFTS (QUantitative Evaluation of the Fertility of Tropical Soils) approach. The model required estimation of two boundary lines describing the minimum and maximum internal nutrient efficiencies of N, P and K (IE, kg grain per kg nutrient in plant DM), which were estimated at 40 and 83 kg grain kg⁻¹ N, 225 and 726 kg grain kg⁻¹ P and 29 and 125 kg grain kg⁻¹ K, respectively. The model predicted a linear increase in grain yield if nutrients are taken up in balanced amounts of 16.4 kg N, 2.3 kg P and 15.9 kg K per 1000 kg of grain until yield reached about 60–70% of the yield potential. The corresponding IEs were 61 kg grain kg⁻¹ N, 427 kg grain kg⁻¹ P and 63 kg grain kg⁻¹ K. The model predicted a decrease in IEs when yield targets approached the yield potential limit. A spherical model was derived from QUEFTS model outputs and found to be particularly suitable for practical applications such as estimating fertilizer needs. The proposed spherical model offers generality across environments and management practices, allowing users to estimate the optimal N, P and K uptake requirements based on two inputs: estimated yield potential and yield target. Further improvements in modeling the relationship between N uptake and grain yield can be made by taking into account differences in harvest index. Accuracy in the simulation of N uptake using the spherical model was improved from an RMSE of 35 kg N ha⁻¹ to 25 kg N ha⁻¹ when harvest index was accounted for, suggesting that the relationship between N uptake and actual yield is affected by both yield potential and efficiency in biomass partitioning.