Ultrahigh CO2 levels enhances cuphea growth and morphogenesis

Applications of ultrahigh CO₂ treatments accelerated cuphea (Cuphea viscosissima × C. lanceolata ‘PSR23’) growth and development and aided in seedling establishment. The growth (fresh weight) and morphogenesis (number of leaves and roots and seedling length) were determined in cuphea seedlings exposed to 350, 1500, 3000, 10,000, or 30,000 μmol mol⁻¹ CO₂ for 30 days under greenhouse conditions. Greater CO₂ levels, especially the ultrahigh levels (i.e. ≥3000 μmol mol⁻¹ CO₂) resulted in significantly higher (P ≤ 0.05) fresh weights, leaf numbers, root numbers, and seedling lengths compared to seedlings grown under ambient air (350 μmol mol⁻¹ CO₂). For example, cuphea ‘PSR23’ Morris heavy seedlings showed the greatest seedling fresh weight, leaf number, root number, and seedling length when supplemented with 10,000 μmol mol⁻¹ CO₂ increasing 607%, 184%, 784%, and 175%, respectively, when compared to seedlings grown without CO₂ enrichment.     

Effects of water vapor pressure difference on leaf gas exchange in potato and sorghum at ambient and elevated carbon dioxide under field conditions

High leaf to air water vapor pressure differences often substantially reduce rates of assimilation of carbon dioxide, especially in C₃ species. Rising concentrations of carbon dioxide [CO₂] in the atmosphere could reduce the sensitivity of assimilation rate to partial stomatal closure caused by high vapor pressure difference by a variety of mechanisms. However, field data addressing this question are scarce. In this study, we examined day-to-day variation in midday gas exchange rates of upper canopy leaves of potato and sorghum grown at the current ambient [CO₂] and ambient+350 μmol mol⁻¹ [CO₂] in field plots. Stomatal conductance and assimilation rate were negatively correlated with vapor pressure difference across days. Assimilation rate was not less sensitive to vapor pressure difference at elevated than at ambient [CO₂] in either species. For both potato and sorghum short-term increases in vapor pressure difference for individual leaves produced significantly smaller responses of leaf gas exchange than did the day-to-day variation in vapor pressure difference, again with no reduced sensitivity at elevated [CO₂]. The smaller response of gas exchange to short-term manipulations of vapor pressure difference than to day-to-day variation may indicate that much of the response to high vapor pressure difference apparent in the day-to-day variation resulted from leaf water deficits caused by exposure of the whole canopy to high vapor pressure difference, rather than from direct effects of high water vapor pressure difference. The lack of a [CO₂] effect on the sensitivity of assimilation rate to vapor pressure difference, and the substantial sensitivity of assimilation rate to vapor pressure difference in the C₄ species both resulted from reductions in assimilation at a given internal [CO₂] at high vapor pressure difference. An implication of these results is that that high leaf to air water vapor pressure difference may continue to be a major limitation to assimilation rates in C₃ and C₄ crop species even at twice the current concentration of carbon dioxide in the atmosphere.     

An improved water-use efficiency for winter wheat grown under reduced irrigation

Single irrigation, compared to the conventional four or five irrigations, has been practised in northern China on winter wheat on a relatively large scale since 1991. In a field study, irrigation was reduced from normally four times (I₄, 4×75 mm) to one (I₁, 75 mm at the end of the second internode elongation) in an area with an annual rainfall of about 600 mm. A control without irrigation (I₀) was also included. Late sowing and early soil drying at seedling stage resulted in a relatively deep root system. Leaf area index, the size of upper leaves and the length of base internodes were also significantly reduced under I₁, but kernel number per panicle was not reduced, suggesting that the development of inflorescence was not disrupted. During the active grain-filling stage, it was found that leaf water potential under I₁ was maintained similar to that of I₄, while daytime stomatal conductance was substantially reduced. Leaf temperature was increased, indicating an inhibited leaf transpiration. Early senescence was induced in I₁ and I₀ crops and resulted in a substantially lower kernel weight. Although the grain yield of I₁ was reduced by about 15% from I₄, the water-use efficiency (WUE) for total water consumption was increased by 24–30%. Single irrigation can potentially make wheat cropping sustainable in this area in terms of water usage and prevent further depletion of the underground water resource. Explanations for the small or zero reduction in yield are: (1) the encouraging development of a deep root system that enabled the plants to use more water at depth (below 1 m), which is recharged annually by the relatively high summer rainfall. (2) A large portion of root system in the drying soil and its induced shoot physiological changes, that is, reduced leaf expansion and stomatal conductance, which helped the plants to establish a better canopy structure with a much reduced water consumption. (3) An improved harvest index.     

Safflower yield,chlorophyll content,photosynthesis, and water use efficiency response to nitrogen fertilization under rainfed conditions

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 very important for the growth and yield of safflower, however, the effect of N level on chlorophyll content, assimilation rate, transpiration rate, stomatal conductance, substomatal CO₂ concentration, and water use efficiency (WUE) have not been determined. A 2-year field study was conducted with the objective to determine the effect of N fertilization on yield, yield components, chlorophyll content, photosynthetic characteristics, and WUE of safflower grown under rainfed conditions. Three rates of N were used (0, 100, and 200 kg N ha⁻¹) and two hybrids (CW9048 and CW9050). N fertilization increased seed yield by an average of 19%, the seed weight per plant by 60%, the seed weight per head by 18%, the number of heads per plant by 32%, and the number of seeds per plant by 41% compared with the control. N level also affected chlorophyll content, N concentration at anthesis, protein, and oil yield. N application increased assimilation rate by an average of 51%, stomatal conductance of water vapour by an average of 27%, and WUE by an average of 60% over the 2 years of the study when compared to the control. The present study indicates that N fertilization can affect yield, yield components, photosynthetic efficiency, and physiology of safflower under rainfed conditions.     

Some characteristics of reduced leaf photosynthesis at midday in maize growing in the field

Maize was grown in the high-radiation arid summer environment of Davis, California, and its leaf photosynthetic rate was measured over diurnal courses on cloudless days with the leaf held perpendicular to the sunlight. On days of high atmospheric vapor pressure deficit (VPD), leaf photosynthesis reached a maximum in the late morning and then decreased gradually as the day progressed, though the soil was well irrigated. When CO₂ concentration in the measurement chamber was raised to about 1000 μmol mol⁻¹, photosynthesis was enhanced, but more in the afternoon than in the morning. As a result, rates measured at high CO₂ in the morning and afternoon were essentially the same. There was also no difference in the curves of photosynthetic rate (A) versus intercellular CO₂ concentration (C_i) for the morning and afternoon. Hence, photosynthetic capacity was similar for the two periods and there was no evidence of photoinhibition by the high photosynthetic photon flux density at noon. Further, C_i and photosynthetic rates A measured over a range of photon flux density were lower in the afternoon than in the morning. These results indicate that A at noon and early afternoon was more limited than in the morning by epidermal conductance (mostly stomatal). On a day of low VPD, however, midday depression in A and epidermal conductance were not evident for the well-irrigated plants. Without irrigation and with leaves at a lower midday water potential, midday reduction in conductance and A was much more marked, beginning late in the morning. Epidermal conductance of maize grown in the field in Davis is are not sensitive to VPD. Therefore, the midday reduction in conductance and A was more likely the result of low leaf water potential caused by high transpiration rates.     

Seasonal changes in the effects of free-air CO2 enrichment (FACE) on dry matter production and distribution of rice (Oryza sativa L.)

It is reported that stimulating effect of elevated atmospheric [CO₂] on photosynthesis of rice (Oryza sativa L.) is likely to be reduced during the plant growth period. However, there is little information on seasonal changes in dry matter (DM) production and distribution of rice under elevated atmospheric [CO₂]. A free-air CO₂ enrichment (FACE) experiment was conducted at Wuxi, Jiangsu, China, in 2001–2003, using Wuxiangging 14, a japonica cultivar. The rice was grown at ambient or elevated (ca. 200 μmol mol⁻¹ above ambient) [CO₂] and supplied with 25 g N m², which is the normal N application rate for local farmers. DM accumulation of rice in FACE plots was significantly increased by 40, 30, 22, 26 and 16% on average at tillering, panicle initiation (PI), heading, mid-ripening and grain maturity, respectively. Rice DM production under FACE was significantly enhanced by 41, 27, 15 and 38% on average during the growth periods from transplanting to tillering (Period 1), tillering to PI (Period 2), PI to heading (Period 3) and heading to mid-ripening (Period 4), respectively, but significantly decreased by 25% in the period from mid-ripening to grain maturity (Period 5). In general, seasonal changes in crop response to FACE in both green leaf area index (GLAI) and net assimilation rate (NAR) followed a similar pattern to that of the DM production. Under FACE the leaves decreased significantly in proportion to the total above-ground DM over the season, the stems showed an opposite trend, while the spikes depended on crop development stage: showing no change at heading, significant increase (+4%) at mid-ripening and significant decrease (−3%) at grain maturity. Grain yield was stimulated by an average of 13% by FACE, due to increased total DM production rather than any changes in partitioning to the grain. We conclude that the gradual acclimation of rice growth to elevated [CO₂] do not occur inevitably, and it could also be altered by environmental conditions (e.g., cultivation technique).     

Leaf photosynthesis is enhanced in normal oil maize pollinated by high oil maize hybrids

The grain yield of normal oil maize (Zea mays L.) might increase when pollinated by high oil maize (HOM) hybrids because of heterosis. To testify that the grain yield increase might be a result of improved photosynthetic rate and related traits, the normal oil maize (NOM) hybrid, Nongda108, was cross-pollinated by three HOM hybrids, HOM202, HOM115 and HOM4515 (for short as ND108pHOM202, ND108pHOM115 and ND108pHOM4515). We found that the ND108pHOM202 and ND108pHOM115 exhibited higher net photosynthetic rate (P_n), accompanied by larger stomatal conductance (g_s) and transpiration rate (E). Moreover, delayed leaf senescence was observed in their leaves, including larger leaf area index (LAI) and higher Chl content and Chl a/b ratio. Apart from higher phosphoenolpyruvate carboxylase (PEPCase) activity, the soluble proteins were also higher in the two cross-pollinations. The higher leaf photosynthesis could explain the grain increase in ND108pHOM202 and ND108pHOM115. However, ND108pHOM4515 exhibited a decreased photosynthetic characteristic and yield performance. Significantly positive relation between grain yield and biomass (r² = 0.96, P < 0.05), P_n and biomass (r² = 0.74, P < 0.05) also suggested that the yield increase in the two cross-pollination treatments was generally owing to the higher photosynthetic rate and related photosynthetic traits.     

Effect of SWD irrigation on photosynthesis and grain yield of rice (Oryza sativa L.)

A study was conducted with the objective to determine the influence of (shallow water depth with wetting and drying) SWD on leaf photosynthesis of rice plants under field conditions. Experiments using SWD and traditional irrigations (TRI) were carried out at three transplanting densities, namely D1 (7.5 plants/m²), D2 (13.5 plants/m²) and D3 (19.5 plants/m²) with or without the addition of organic manure (0 and 15 t/ha). A significant increase in leaf net photosynthetic rate by SWD was observed with portable photosynthesis systems in two independent experiments. At both flowering and 20 DAF stages, photosynthetic rate was increased by 14.8% and 33.2% with D2 compared to control. SWD significantly increased specific leaf weight by 17.0% and 11.8% over the control at flowering and 20 DAF stages, respectively. LAI of D2 under SWD was significantly increased by 57.4% at 20 DAF. In addition, SWD with D2 significantly increased the leaf dry weight (DW) at both growing stages. At all the three densities, SWD increased the leaf N content and the increase was 18.9% at D2 density compared with the conventional control. In SWD irrigation, the leaf net photosynthetic rate was positively correlated with the leaf N content (R² = 0.9413), and the stomatal conductance was also positively correlated with leaf N content (R² = 0.7359). SWD enhanced sink size by increasing both panicle number and spikelet number per panicle. The increase in spikelet number per panicle was more pronounced in the 15 t ha⁻¹ manure treatment than in the zero-manure treatment. Grain yield was also significantly increased by SWD, with an average increase of 10% across all treatments. SWD with D2 had the highest grain yield under the both cultivars with or without 15 t ha⁻¹ manure treatment, which was 14.7% or 13.9% increase for Liangyoupeijiu and 11.3% or 11.2% for Zhongyou 6 over the control, respectively.     

The effect of irrigation regime on biomass and resin production in Grindelia chiloensis

Grindelia chiloensis (Corn.) Cabr. is a shrub native to Patagonia, Argentina and can accumulate as much as 25% resin in its leaves, with net primary productivity between 90 and 170 g per year per plant when growing in native stands. Under cultivation, 67.4 g of resin per plant have been produced (about 2.24 Mg ha⁻¹). The objective of this study was to assess the effect of irrigation regime on biomass and resin production on G. chiloensis. In order to achieve this objective, four irrigation treatments were performed during 1996–1997 and 1997–1998: (i) weekly irrigation (7d), (ii) irrigation at 20-day intervals (20d), (iii) irrigation at 40-day intervals (40d), (iv) non-irrigated (N-I). It was found that the intermittent water supply at 40d was sufficient to promote canopy development, and increase water use efficiency (WUE) and resin production per plant (RP) with highest resin production (approximately 5.12 Mg ha⁻¹ in 1997). In order to achieve high levels of RP, above ground biomass was maximized at the expense of a reduction in WUE. A concomitant increase in WUE (at the leaf level; WUE_L) and leaf resin content with water stress and time was found. This result supports the hypothesis that epicuticular resin could influence water transpiration (E), as it represents an additional barrier to gas diffusion from the epidermis and through the stomatal pores.     

Crop responses to compacted soil: capture and efficiency in the use of water and radiation

We combined field and modelling experiments to investigate crop-level responses to soil compaction. Our working hypotheses are that the effect of soil compaction on crop growth is (i) primarily mediated by reduction in capture of water and photosynthetically active radiation (PAR), and (ii) secondarily affected by reduced transpiration efficiency (biomass per unit transpiration) and radiation-use efficiency (biomass per unit intercepted PAR).Three field experiments were carried out in the Mediterranean-type Mallee region of south Australia where the landscape alternates sand dunes (hills) and swales (flats) of sandy loam soil. All three experiments compared wheat crops grown in compacted (control) soils, and soils in which compaction was alleviated with deep tillage (ripped); additional sources of variation include season and soil type as related to topography.All soil and crop responses to ripping were more marked in sand hills than in sandy loam flats. Penetration resistance of undisturbed soil had a peak ∼2 MPa at 0.1–0.2 m depth in sandy loam flats and ∼3 MPa at 0.2–0.3 m depth in sand hills. Ripping dramatically reduced soil penetration resistance between 0.10 and 0.3–0.4 m. Control crops yielded between 1.2 and 2.9 t ha⁻¹ and yield improvement attributable to alleviation of soil compaction ranged from nil to 43%; yield response to ripping remained for at least two cropping seasons.Increased transpiration and PAR interception fully accounted for the increase in crop growth associated with alleviation of soil compaction; ripping did not affect transpiration efficiency or radiation-use efficiency. The proportion of evapotranspiration accounted for by soil evaporation (E:ET) declined from 0.58 in controls to 0.36–0.45 in ripped sand hills.A limited modelling study showed that water availability, as characterised with the lower limit of plant available water, could partially account for the effect of soil compaction and deep tillage on crop growth and evapotranspiration. Long-term simulations indicated important changes in the fate of water in response to ripping in sandy soils, including a moderate increase in evapotranspiration, a substantial reduction in E:ET, and important reductions in the frequency and rate of drainage beyond the crop root zone.     

Salinity reduces radiation absorption and use efficiency in soybean

The potential rate of plant development and biomass accumulation under conditions free of environmental stress depends on the amount of radiation absorption and the efficiency of utilizing the absorbed solar energy to drive photosynthetic processes that produce biomass materials. Salinity, as a form of soil and water stress, generally has a detrimental effect on plant growth, and crops such as soybean are usually sensitive to salinity. Field and greenhouse experiments were conducted to determine soybean growth characteristics and the relative impact of salinity on radiation absorption and radiation-use efficiency (RUE) at a whole plant level. Cumulative absorption of photosynthetically active radiation (∑APAR) was estimated using hourly inputs of predicted canopy extinction coefficients and measured leaf area indices (LAI) and global solar radiation. On 110 days after planting, soybean plants grown under non-saline conditions in the field accumulated 583 MJ ∑APAR m⁻². A 20% reduction in ∑APAR resulted from growing the plants in soil with a solution electrical conductivity (EC) of about 10 dS m⁻¹. Soybeans grown under non-saline conditions in the field achieved a RUE of 1.89 g MJ⁻¹ ∑APAR for above-ground biomass dry materials. The RUE reached only 1.08 g MJ⁻¹ ∑APAR in the saline soil, about a 40% reduction from the non-saline control. Salinity also significantly reduced ∑APAR and RUE for soybeans in the greenhouse. The observed smaller plant and leaf sizes and darker green leaves under salinity stress were attributed to reductions in LAI and increases in unit leaf chlorophyll, respectively. Reductions in LAI exceeded small gains in leaf chlorophyll, which resulted in less total canopy chlorophyll per unit ground area. Analyzing salinity effect on plant growth and biomass production using the relative importance of ∑APAR and RUE is potentially useful because APAR and total canopy chlorophyll can be estimated with remote sensing techniques.     

Effect of N fertilizer top-dressing at various reproductive stages on growth,N2 fixation and yield of three soybean (Glycine max (L.) Merr.) genotypes

Soybean (Glycine max (L.) Merr.) is one of the most important food and cash crops in China and a key protein source for the farmers in northern China. Previous experiments in both the field and greenhouse have shown that N₂ fixation alone cannot meet the N requirement for maximizing soybean yield, and that N top-dressing at the flowering stage was more efficient than N top-dressing at the vegetative stages. However, the effect of N fertilizer application at other reproductive stages of soybean is unknown. Thus, a field experiment was conducted to study the effects of N applications at various reproductive stages on growth, N₂ fixation and yield of three soybean genotypes. The results showed that starter N at 25 kg ha⁻¹ resulted in minimum yield, total N accumulation and total amount of N₂ fixed in all three genotypes. N top-dressing at 50 kg ha⁻¹ at either the V2 or R1 stages, significantly increased N accumulation, yield and total amount of N₂ fixed in all three genotypes. However, N top-dressing at the same rate at either the R3 or R5 stage did not show this positive effect in any of the three genotypes. Thus, the best timing for N top-dressing during reproduction is at the flowering stage, which increased seed yield by 21% for Wuyin 9, 27% for You 91-19, and 26% for Jufeng, respectively, compared to the treatment without N top-dressing.     

Emissions of ammonia,nitrous oxide and carbon dioxide from urban gardens in Niamey,Niger

Urban and peri-urban agriculture (UPA) contributes significantly to meet increasing food demands of the rapidly growing urban population in West Africa. The intensive vegetable cultivation in UPA gardens with its high nutrient inputs is often reported to operate at large surpluses of nutrients and presumably high turnover rates of organic matter (OM) and nitrogen (N) losses via emanation and leaching. Many of these claims are lacking solid data which would allow suggesting mitigation strategies. Therefore, this study aimed at quantifying gaseous emissions of ammonia (NH₃), nitrous oxide (N₂O), and carbon dioxide (CO₂) in three representative urban gardens of Niamey, Niger using a closed chamber gas monitoring system. Mean annual N emissions (NH₃-N and N₂O-N) in two gardens using river water for irrigation reached 53 and 48 kg N ha^?1 yr^?1, respectively, while 25 and 20 Mg C ha^?1 yr^?1 was lost as CO₂-C. In the garden irrigated with sewage water from the city's main wadi, N₂O was the main contributor to N losses (68%) which together with NH₃ reached 92 kg N ha^?1 yr^?1, while CO₂-C emissions amounted to 26 Mg ha^?1 yr^?1. Our data indicate that 28% of the total gaseous C emissions and 30–40% of the N emissions occur during the hot dry season from March to May and another 20–25% and 10–20% during the early rainy season from June to July. Especially during these periods more effective nutrient management strategies in UPA vegetable gardens should be applied to increase the nutrient use efficiency in UPA vegetable gardens.     

Long-term effect of tillage systems on soil microbiological,chemical and physical parameters and the incidence of charcoal rot by Macrophomina phaseolina (Tassi) Goid in soybean

A 20-year field experiment was employed with the aim of evaluating the effect of tillage systems on biological, chemical and physical aspects of the soil, and to establish whether there was a correlation of these parameters with the incidence of charcoal rot (Macrophomina phaseolina) of soybean and crop yield. The tillage systems evaluated were direct seeding (DS), DS + scarifier (DS + S), minimum tillage (MT) and conventional tillage (CT). DS presented higher values than CT in culturable total fungi (26.33 × 10⁵ vs. 2.33 × 10⁵ CFU g⁻¹ dry soil), total bacteria (182 × 10⁷ vs. 64 × 10⁷ CFU g⁻¹ dry soil), microbial respiration (0.77 mg CO₂ g⁻¹ week⁻¹ vs. 0.45 mg CO₂ g⁻¹ week⁻¹) and fluorescein diacetate (FDA) hydrolysis (4.17 ug fluorescein g⁻¹ h⁻¹ vs. 1.70 ug fluorescein g⁻¹ h⁻¹ in CT. Fungal and bacterial community fingerprints, by terminal restriction fragment length polymorphism (T-RFLP) analysis, of Intergenic spacer regions of rRNA and 16S rRNA genes, respectively, were influenced by the tillage system. Also FAME (fatty acid methyl ester) profiles showed that microbial community structure in DS and CT was clearly different. DS samples contained significantly higher total microbial biomass than the other tillage treatments, but there were no significant differences in fungal biomass or any consistent trend with respect to stress index. Our results showed that microbial communities were more abundant and active in DS than in CT in response to high nutrient content in soil. Indeed, DS systems presented higher soil OM, total N, K and Ca than CT. Electrical conductivity and aggregate stability (AS) were also improved by DS. Soybean grown in high-quality soil was not affected by charcoal rot, however, under CT, disease incidence in soybean was 54%. These differences were correlated to the higher microbial abundance and activity under DS, the biological component being a key factor determining soil capacity to suppress the soilborne pathogen.     

Water balance and rice growth responses to direct seeding,deep tillage,and landscape placement: Findings from a valley terrace in Nepal

For maximizing water retention and attaining high yields, transplanting into puddled soil (TPR) is often considered the optimal method of rice (Orzya sativa L.) establishment. Alternative management techniques like direct seeding (DSR) and deep tillage have been proposed as mechanisms to improve soil physical properties for subsequent dry-season crops, but the risks to rice are uncertain. In this full factorial study on a valley terrace in Nepal, the influence of tillage (shallow—T₁, deep chisel—T₂, deep chisel + moldboard plough—T₃) and establishment practice (TPR, DSR) on the field water balance and rice performance were evaluated in two adjacent landscape settings (terrace edge “upland”, central terrace “lowland”). Although deep tillage had only modest influences on seepage and percolation (SP) rates in both years (Y₁, Y₂), landscape placement and establishment practice had significant implications for the water balance (e.g. Y₂ SP cm day⁻¹: TPR-lowland = 1.6, DSR-lowland = 2.3, TPR-upland = 4.1, DSR-upland = 6.1). During low rainfall periods, however, soil water potential and drought vulnerability were governed solely by landscape placement. Despite water balance differences, there was little evidence that rice rooting behavior was substantially modified by landscape or establishment method. Weed biomass was higher in DSR, but was uncorrelated with water balance and productivity trends. In Y₁, lower SP rates and more days with continuous flooding were positively associated with rice productivity. DSR yields were significantly lower than TPR in both landscape positions, with the lowland outperforming the upland (Y₁ mt ha⁻¹: TPR-lowland = 6.4, DSR-lowland = 5.2, TPR-upland = 5.7, DSR-upland = 4.7). To determine if N dynamics were contributing to productivity differences, fertilizer nitrogen was increased from 120 to 150 kg N ha⁻¹ in Y₂. Results suggest that DSR performance is comparable – and landscape less important – if nitrogen is non-limiting (Y₂ mt ha⁻¹: TPR-lowland = 6.9, DSR-lowland = 6.5, TPR-upland = 7.0, DSR-upland = 6.5); no aspect of the field water balance was associated with yield variability in Y₂. For direct seeding in N-deficient farming systems, landscape criteria may prove useful for minimizing production risks by identifying field areas with lower SP rates.     

Responses of potatoes (Solanum tuberosum L.) to irrigation and nitrogen in a hot,dry climate: I. Water use

The effects of differential irrigation and fertiliser treatments on the water use of potatoes (Solanum tuberosum L. cv. Desirée) were studied over 2 years in the hot dry climate of northeast Portugal. Total actual evapotranspiration (ET_c) ranged from 150 to 320 mm in 1988, and from 190 to 550 mm in 1989 depending mainly on irrigation treatment, potential evaporation rates (ET_p) and duration of the growing season. By comparison, the effects of nitrogen fertiliser on total water use were relatively small. Although nitrogen increased transpiration (larger leaf canopy), it reduced evaporation from the soil surface, in frequently irrigated plots, by similar amounts. As a result, in well-irrigated crops, the ET_c/ET_p ratio averaged 0.85 over the season, regardless of nitrogen level. Evaporation from the soil surface represented 15–25% of total water use by well-fertilised plants, but as much as 30–50% from the sparse stands of unfertilised crops. The proportion of water extracted from each depth increment of the silt-loam soil declined logarithmically, from the surface to 1.1 m depth, the maximum measured, for irrigated crops, and linearly when rain-fed. The ET_c/ET_p ratio fell below unity when 25–30% of the available water in the top metre had been depleted, equivalent to soil water deficits (SWDs) of 45–50 mm. By comparison, ET_c declined to zero when 75–90% of the available water had been extracted, corresponding to actual deficits of 135–150 mm. Peak ET_c rates reached 12–13 mm per day on days immediately following irrigation, nearly twice ET_p (possibly due to the influence of advection) but then declined logarithmically with time to about 3 mm per day within 5 days. Using the same data, a companion paper reports the influence of climatic conditions on the yield responses to water of potato crops grown in the region.     

The effect of increasing NaCl in irrigation water on growth,gas exchange and yield of tobacco Burley type

Despite the economic importance of tobacco, there is limited field study on the quantitative response of growth and yield to increasing soil salinity. The effects of irrigation with saline water on yield components of field-grown tobacco (Nicotiana tabacum L.) “Burley” type plants were studied over two growing seasons. Growth, dry matter partitioning and gas exchange were measured either in rainfed or fully irrigated plants growing in a clayey–sandy–loam soil. The four fully irrigated treatments received amounts of saline waters at 0.54, 2.5, 5.0 or 10 dS m⁻¹ electrical conductivity (EC_w) equal to crop evapotranspiration. In both years, the electrical conductivity of the saturation phase (EC_e) across the 0.6 m topsoil profile increased with increasing salinity of the irrigation water. Soil moisture was markedly lower in the rainfed treatment than in fully irrigated treatments. Different saline concentrations of irrigation water had virtually no effect on soil moisture. Carbon assimilation rate, stomatal conductance and water use efficiency of the saline treatments were lower than the fully irrigated plants at 0.54 dS m⁻¹ (NW treatment) in 1996, but not in 1997. Transpiration rates were unaffected by salinity in both years. The highest yield was produced by plants irrigated with good quality water. The number of leaves per unit land area was greater for the NW plants, whereas there were no differences between the other four treatments. Salinity decreased plant dry matter and height at harvest, increased dry matter partitioning into leaves and decreased that into stems in both years. Dry matter partitioning to leaves was also greater for the rainfed plants than for the NW plants. Tobacco plants grown under field conditions showed a maximum reduction of relative yield at the highest salinity level of only 31%. The threshold values (0.56 and 0.96 dS m⁻¹) and the EC_e at which a 10% yield reduction was obtained (3.12 and 2.55 dS m⁻¹) calculated from the linear model of response of relative yield to increasing EC_e were typical of moderately sensitive crops. The EC_e values at which 50% yield was reduced (13.34 and 8.91 dS m⁻¹) were indicative of moderate tolerance to salinity.     

Radiation interception and the accumulation of biomass and nitrogen by soybean and three tropical annual forage legumes

Field experiments were conducted at Gatton and Dalby in southeastern Queensland to determine parameters associated with radiation interception and biomass and nitrogen (N) accumulation for the ley legume species, phasey bean (Macroptilum lathyroides (L.) Urban) and vigna, (Vigna trilobata (L.) Verdc.). Sesbania (Sesbania cannabina Retz.), a native legume species, and soybean (Glycine max (L.) Merrill)) were included in the study for comparison. The most important differences between species related to differences in radiation interception, radiation-use efficiency (RUE), N-accumulation efficiency and the partitioning of N to plant parts. During early growth, soybean intercepted more radiation than the other species, primarily because of its greater leaf area index (LAI). Sesbania had the highest RUE (1.08 g MJ⁻¹) followed by phasey bean (0.94 g MJ⁻¹), soybean (0.89 g MJ⁻¹) and vigna (0.77 g MJ⁻¹). The efficiency of N-accumulation was greater in soybean (0.028 g N g⁻¹) and phasey bean (0.030 g N g⁻¹) than in vigna (0.022 g N g⁻¹) and sesbania (0.021 g N g⁻¹). In all species, the proportion of N allocated to leaves declined throughout the experimental period, being more rapid in soybean than in sesbania and phasey bean. Despite this decline in total N partitioned to the leaves, both soybean and phasey bean maintained a relatively stable specific leaf nitrogen (SPLN) throughout the experimental periods although sesbania and vigna displayed rapid decreases in SPLN. The large variation between species in RUE and N-accumulation efficiency indicates that the development of ley legume cultivars with a combination of traits for more efficient legume production, water use and soil N-accumulation in the water-limited environments of the grain belt of eastern Australia may be possible. The sensitivity of forage production, water use and soil N-accumulation to variation in RUE and N-accumulation efficiency needs to be quantified using modeling techniques prior to embarking on screening programs to select appropriate germplasm for evaluation studies.     

Soybean–chickpea rotation on Vertic Inceptisols: I. Effect of soil depth and landform on light interception,water balance and crop yields

Vertic Inceptisols are prone to land degradation because of excessive run-off and soil erosion during the rainy season. Productivity of soybean-based systems on these soils needs to be improved and sustained by better management of natural resources, particularly soil and water. During 1995–1997 a field study was conducted in Peninsular India on a Vertic Inceptisol watershed to study the effect of two soil depths, namely shallow (<50 cm soil depth) and medium-deep (≥50 cm soil depth) and two landform treatments, namely flat and broadbed-and-furrow (BBF) systems, on productivity and resource-use efficiency of soybean–chickpea rotation (soybean in rainy season followed by chickpea in post-rainy season). Soybean grown on flat landform on medium-deep soil had a higher leaf area index and more light interception compared to the soybean grown on the BBF landform. This resulted in an increase in mean seed yield for the flat landform (2120 kg ha⁻¹) compared to the BBF landform (1870 kg ha⁻¹). However, the landform treatments on shallow soil did not affect soybean yields. The soybean yield was higher on the medium-deep soil (1760 kg ha⁻¹) than on the shallow soil (1550 kg ha⁻¹) during 1995–1996, but were not different during 1996–1997. In both years chickpea yields and total system productivity (soybean + chickpea yields) were greater on medium-deep soil than on the shallow soil. Total run-off was higher on the flat landform (25% of seasonal rainfall) than on the BBF landform (20% of seasonal rainfall). This concomitantly increased profile water content (10–30 mm) of both soils in BBF compared to the flat landform treatment during 1995–1996, but not during 1996–1997. Deep drainage was higher in the BBF landform than in flat, especially for the shallow soil. Across landforms and soil depths, water use (evapotranspiration) by soybean–chickpea rotation during 1996–1997 ranged from 496 to 563 mm, which accounted for 54–61% of the rainfall. These results indicate that while the BBF system is useful in decreasing run-off and increasing infiltration of rainfall on Vertic Inceptisols, there is a need to increase light use by soybean on BBF during the rainy season to increase its productivity. A watershed-based farming system needs to be adopted to capture significant amount of rain water lost as run-off and deep drainage. The stored water can be used for supplemental irrigation to increase productivity of soybean-based systems leading to overall increases in resource-use efficiency, crop productivity, and sustainability.     

Nutrient and assimilate partitioning in two tropical maize cultivars in relation to their tolerance to soil acidity

The use of Al-tolerant and P-efficient maize cultivars is an important component of a successful production system on tropical acid soils with limited lime and P inputs. Grain yield and secondary plant traits, including root and aboveground biomass, nutrient content and leaf development, were evaluated from 1996 to 2002 in field experiments on an Oxisol in order to identify maize characteristics useful in genetic improvement. Here we present the results of the 2002 trial and compare them with previous results. The aim of this experiment was to assess the effect of assimilate and nutrient partitioning on the growth and grain yield of two tropical cultivars having different Al tolerance (CMS36, tolerant, Spectral, moderately tolerant). The soil had an Al saturation of 36% in topsoil (pH 4.5) and >45% below 0.3 m depth (pH 4.2). Measurements made from emergence to grain filling included: root, stem and leaf biomass, P and N content, leaf area index (LAI), radiation use efficiency (RUE), soil available N and root profiles at anthesis. The experiments consisted of two P treatments, zero applied or 45 kg P ha⁻¹ (−P and +P). All the treatments received N and K fertilizers. In −P, root biomass and LAI at anthesis were twice as great in CMS36 as in Spectral. In +P the differences between cultivars were negligible. Roots were deeper in CMS36 due to its higher Al tolerance. Total biomass and grain yield were not strongly related to root biomass and LAI. Other factors such as the leaf biomass and the amount of nutrients per unit leaf area were highly correlated with RUE and biomass. In −P, Spectral had the same total biomass but a higher grain yield than CMS36 (2.1 Mg ha⁻¹ versus 1.5 Mg ha⁻¹). This was due to a higher leaf P content (+40%), a greater RUE (+74%), and a lower number of sterile plants. In +P, CMS36 had higher total biomass and grain yield (4.1 Mg ha⁻¹ versus 3.1 Mg ha⁻¹). This was due to its higher leaf P (+25%) and leaf N (+43%) contents, and an increased RUE (+130%) that were associated with higher P and N uptake. Our results indicated that although root tolerance to Al toxicity is necessary for good crop performance on acid soils, assimilate and nutrient partitioning in the aboveground organs play a major role in plant adaptation and may partially compensate for a lower root tolerance.