The effects of regulated and continuous deficit irrigation on the water use, growth and yield of olive trees

Important savings in the levels of irrigation without an associated penalty in yield have been reported for olive under deficit irrigation strategies. Full irrigation (C), continuous deficit irrigation (CDI) and regulated deficit irrigation (RDI), were compared from 2004 to 2006 in Cordoba, southern Spain, in terms of seasonal evapotranspiration (ET), growth and yield in mature olive trees (Olea europaea L. cv. ‘Arbequina’). In deficit treatments, the total amount of irrigation was around 25% of that of the Control while ET was 65–70% of that of C. Deficit treatments strongly reduced vegetative growth, but only slightly reduced the final fruit volume. Water stress caused a higher reduction in fresh fruit yield than oil yield due to a higher oil concentration in deficit irrigated trees, without differences between CDI and RDI. Therefore, both irrigation strategies may be used in olive to save a significant amount of irrigation with moderate reductions (about 15%) in oil yield. The amount of oil produced per unit intercepted PAR was almost the same for all the treatments, which suggests that olive oil yield may be calculated from intercepted radiation even under moderate water stress.     

Biomass yield and energy balance of giant reed (Arundo donax L.) cropped in central Italy as related to different management practices

In order to evaluate the possibility of reducing energy input in giant reed (Arundo donax L.) as a perennial biomass crop, a field experiment was carried out from 1996 to 2001 in central Italy. Crop yield response to fertilisation (200–80–200 kg ha⁻¹ N–P–K), harvest time (autumn and winter) and plant density (20,000 and 40,000 plants per ha) was evaluated. The energy balance was assessed considering the energy costs of production inputs and the energy output obtained by the transformation of the final product. The crop yield increased by +50% from the establishment period to the 2nd year of growth when it achieved the highest dry matter yield. The mature crop displayed on average annual production rates of 3 kg dry matter m⁻², with maximum values obtained in fertilised plot and during winter harvest time.

Fertilisation mainly enhanced dry matter yield in the initial period (+0.7 kg dry matter m⁻² as years 1–6 mean value). The biomass water content was affected by harvest time, decreasing by about 10% from autumn to winter. With regard to plant density, higher dry matter yields were achieved with 20,000 plants per ha (+0.3 kg dry matter m⁻² as years 1–6 mean value).

The total energy input decreased from fertilised (18 GJ ha⁻¹) to not fertilised crops (4 GJ ha⁻¹). The higher energetic input was represented by fertilisation which involved 14 GJ ha⁻¹ (fertilisers plus their distribution) of total energy costs. This value represents 78% of total energy inputs for fertilised crops.

Giant reed biomass calorific mean value (i.e., the calorific value obtained from combustion of biomass sample in an adiabatic system) was about 17 MJ kg⁻¹ dry matter and it was not affected by fertilisation, or by plant density or harvest time. Fertilisation enhanced crop biomass yield from 23 to 27 dry tonnes per ha (years 1–6 mean value). This 15% increase was possible with an energy consumption of 70% of the overall energy cost. Maximum energy yield output was 496 GJ ha⁻¹, obtained with 20,000 plants per ha and fertilisation. From the establishment period to 2nd–6th year of growth the energy production efficiency (as ratio between energy output and energy input per ha) and the net energy yield (as difference between energy output and energy input per ha) increased due to the low crop dry biomass yield and the high energy costs for crop planting. The energy production efficiency and net energy yield were also affected by fertilisation and plant density. In the mature crop the energy efficiency was highest without fertilisation both with 20,000 (131 GJ ha⁻¹) and 40,000 plants per ha (119 GJ ha⁻¹).     

Growth and development of bambara groundnut (Vigna subterranea) in response to soil moisture: 2. Resource capture and conversion

A study was conducted in controlled environment glasshouses to investigate the effects of soil moisture on resource capture and conversion of three landraces (DipC, S19-3 and UN from Botswana, Namibia and Swaziland, respectively) of bambara groundnut (Vigna subterranea (L.) Verdc.). The study was conducted under two soil moisture treatments: an irrigated control and a drought treatment where irrigation was withheld from approximately flowering to final harvest. Drought reduced the mean fractional intercepted radiation (f) from 0.8 to less than 0.7 across landraces. The mean light extinction coefficient (=0.46) was not affected either by landrace or watering regime, while cumulative intercepted radiation (S_ci) reduced under drought because of the reduction in f. Drought reduced total transpiration (E_c) only in DipC while it had no effect on the other two landraces. Crops under irrigation extracted most of the water from the top 50 cm of the profile while those under drought extracted water down to 90 cm. The conversion coefficient for intercepted radiation (_s; g MJ⁻¹) was reduced by 32%, from 1.51 to 1.02 g MJ⁻¹. Similarly, drought reduced the dry matter/transpired water ratio (ε_w; g kg⁻¹) by 20% from 2.05 to 1.65 g kg⁻¹.     

Productivity and water use efficiency of sweet sorghum as affected by soil water deficit occurring at different vegetative growth stages

The growth and production of sweet sorghum [Sorghum bicolor (L.) Moench] crops under semi-arid conditions in the Mediterranean environment of southern Italy are constrained by water stress. The effects of temporary water stress on growth and productivity of sweet sorghum were studied during three seasons at Rutigliano (Bari, Italy). The aim of this research was to evaluate the sensitivity of phenological stages subjected to the same water deficit. In a preliminary study it was observed that stomata closed when pre-dawn leaf water potential (Ψ_b) became lower than −0.4 MPa. This criterion was used in monitoring plant water status in three different plots: one never stressed and two stressed at different phenological stages (‘leaf’ and ‘stem’) when mainly leaves or stems were growing, respectively. An evaluation of the sensitivity of phenological stages subjected to identical water stress was obtained by comparing the above-ground biomass and WUE of drought crops with those of the well-irrigated crop (up to 32.5 t ha⁻¹ of dry matter and 5.7 g kg⁻¹). The sensitivity was greatest at the early stage (‘leaf’), when a temporary soil water stress reduced the biomass production by up to 30% with respect to the control and WUE was 4.8 g kg⁻¹ (average of three seasons). These results help quantify the effects of water constraints on sweet sorghum productivity. An irrigation strategy based on phenological stage sensitivity is suggested.     

Climatic conditions and concentrations of carbon dioxide and air pollutants during ‘ESPACE—wheat’ experiments

A major objective of the ESPACE—wheat programme was to perform by means of open-top chambers (OTCs) ‘standardised’ experimental investigations of spring wheat responses to increased atmospheric CO₂ and O₃ concentrations and to other environmental stresses at different locations in Europe, representing a broad range of different climatic conditions. From 1994 to 1996 a total number of 25 OTC experiments were carried out. In addition, four growth chamber experiments focusing on key physiological processes of wheat growth in CO₂-enriched air were performed. According to the specific needs for subsequent modelling purposes, environmental data were collected during experiments, i.e. air temperature, global radiation, humidity and trace gas concentrations. In the present paper results of these measurements are summarised. It was shown, that the OTC-experiments covered a considerable range of growing season mean-air-temperatures (13.0–23.4°C) and global irradiances (10.8–18.1 MJ m⁻² d⁻¹), the most important driving variables for crop growth simulation models. Mean concentrations of CO₂ and O₃ in ambient air and in different treatments illustrated the observed variability of trace gas exposures between different experiments. Implications for subsequent analyses of biological response data are discussed.     

Seed yield, oil and phytate concentration in the seeds of two oilseed rape cultivars as affected by different phosphorus supply

A greenhouse pot experiment was conducted for studying seed and oil yield, P uptake and phytate concentration in the seed of two oilseed rape cultivars (Brassica napus L. var. Oleifera, cv. Bristol and cv. Lirajet), grown on a soil substrate at different levels of plant available phosphorus (6, 19, 31 and 106 mg P-CAL kg⁻¹ soil, resp.). All other nutrients were maintained at a high level. At maturity, seed yield and seed quality were investigated. An increase in the phosphorus soil supply resulted in a significant (P<0.05) increase in seed and oil yield, oil and P concentration of the seeds, and P transported to the seeds. The phytate-phosphorus concentration ranged from 0.5 to 6.9 g kg⁻¹ in the seeds and from 0.9 to 12.8 g kg⁻¹ in rapeseed meal. Insufficient P supply resulted in a reduced concentration of phosphorus and phytate in the seeds. Significant interactions between the factors cultivar and P supply were found for the traits seed yield, oil yield, and P-harvest index.     

Effect of fruit maturity on efficiency of 1-methylcyclopropene to delay the ripening of bananas

Three bunches of unripe ‘Williams’ banana fruit of different maturity, 173, 156 and 71 days from bunch emergence, were harvested. Fruit from the top, bottom and middle hands from each bunch were fumigated for 24 h with 1-methylcyclopropene (1-MCP) at 0, 5, 50 or 500 nl l⁻¹ at 20^oC. All fruit were then stored at 20^oC in air containing 0.1 μl l⁻¹ ethylene and the time taken for each fruit to ripen (green life) was noted. The green life of fruit treated with 500 nl l⁻¹ 1-MCP varied with fruit maturity. In the two most mature bunches it was 27.9±2.3 days, 4-fold longer than fruit fumigated with 0 nl l⁻¹ 1-MCP (6.7±0.6 days). In the least mature bunch, green life was 39.7±3.0 days, 1.5-fold longer than fruit fumigated with 0 nl l⁻¹ 1-MCP (25.7±2.5 days). Most fruit treated with 500 nl l⁻¹ 1-MCP showed an unacceptable uneven skin colouration when ripe. There was no significant effect on green life of 1-MCP at 50 nl l⁻¹ and 5 nl l⁻¹. Other fruit from these bunches were not exposed to 1-MCP and were held in ethylene-free air until ripe. In the two most mature bunches, these fruit had a significantly shorter green life (11.2±5.6 days in hand 1; 18.9±4.1 days in hands 4 and 6) than fruit that were fumigated with 500 nl l⁻¹ 1-MCP. In the least mature bunch, however, these fruit had a significantly longer green life (56.0±5.9 days) than 1-MCP treated fruit. Since the effectiveness of 1-MCP varied with fruit maturity and in any commercial consignment there is a mixture of fruit maturity, it is concluded that 1-MCP has limited commercial potential for the storage of unripe ‘Williams’ bananas.     

Fertilisation of irrigated maize with pig slurry combined with mineral nitrogen

Maize (Zea mays L.) is a very important crop in many of the irrigated areas of the Ebro Valley (NE Spain). Intensive pig (Sus scrofa domesticus) production is also an important economic activity in these areas, and the use of pig slurry (PS) as a fertiliser for maize is a common practise. From 2002 to 2005, we conducted a field trial with maize in which we compared the application of 0, 30 and 60 m³ ha⁻¹ of PS combined with 0, 100 and 200 kg ha⁻¹ of mineral N at sidedress. Yield, biomass and other related yield parameters differed from year to year and all of them were greatly influenced by soil NO₃⁻-N content before planting and by N (organic and/or mineral) fertilisation. All years average grain yield and biomass at maturity ranged from 9.3 and 18.9 Mg ha⁻¹ (0 PS, 0 mineral N) to 14.4 and 29.6 Mg ha⁻¹ (60 m³ ha⁻¹ of PS, 200 kg ha⁻¹of mineral N), respectively. Grain and total N biomass uptake average of the studied period ranged from 101 and 155 kg ha⁻¹ (0 PS, 0 mineral N) to 180 and 308 kg ha⁻¹ (60 m³ ha⁻¹ of PS, 200 kg ha⁻¹of mineral N), respectively. All years average soil NO₃⁻-N content before planting and after harvest were very high, and ranged from 138 and 75 kg ha⁻¹ (0 PS, 0 mineral N) to 367 and 457 kg ha⁻¹ (60 m³ ha⁻¹ of PS, 200 kg ha⁻¹of mineral N), respectively. The optimal N (organic and/or mineral) rate varied depending on the year and was influenced by the soil NO₃⁻-N content before planting. For this reason, soil NO₃⁻-N content before planting should be taken into account in order to improve N fertilisation recommendations. Moreover, the annual optimal N rates also gave the lowest soil NO₃⁻-N contents after harvest and the lowest N losses, as a consequence they also could be considered as the most environmentally friendly N rates.     

The dynamics of lucerne (Medicago sativa L.) yield components in response to defoliation frequency

The productive life of lucerne (Medicago sativa L.) stands depends on the rate of mortality of individual plants. However, self-thinning of plant populations may be compensated for by increases in other yield components, namely shoots/plant and individual shoot mass. Frequent defoliation reduces lucerne yield but it is unclear whether this is caused by an acceleration of plant mortality or changes in these other yield components. To investigate this, crops with contrasting shoot yields were created using constant 28 or 42-day regrowth cycles applied to a ‘Kaituna’ lucerne crop in Canterbury, New Zealand during the 2002/2003 and 2003/2004 growth seasons. Two further treatments switched from 28 to 42 or 42 to 28 days grazing frequency in mid-summer (4th February) of each year. The annual yield of shoot dry matter (DM) ranged from 12 to 23 t/ha for the treatments defoliated consistently each 28 or 42 days, respectively. Plant population was unaffected by treatments and declined exponentially from 130 plants/m² in June 2002 to 60 plants/m² in September 2004. The dynamics of plant and shoot population were associated with the light environment at the base of the canopy. The slope of the size/density compensation (SDC) of plants was −1.67 for the treatment defoliated each 42 days, near the expected self-thinning slope of −1.5 for stands at constant leaf area index (LAI). Self-thinning of shoots resumed after each defoliation when the LAI reached 2.1 and the transmission of photosynthetically active radiation (PAR_t) was 0.20. At this point the proportion of aerial DM in the tallest (dominant) shoots increased non-linearly from 30 to >80%, due to the mortality of intermediary and suppressed shoots. The average maximum shoot population in each rotation was 780 shoots/m² and unaffected by the decline in plant population due to a compensatory increase from 6 to 13 shoots/plant as the stand thinned. A lower asymptote of 43 plants/m² was estimated as the minimum plant population at which yield component compensation would maintain the productive potential of these ‘Kaituna’ stands. Differences in shoot yield were explained (R² = 0.97) by changes in the individual shoot mass (ISM) that were consistently lowered by frequent defoliation treatments. Frequent defoliations reduced crop productivity by limiting the assimilation of biomass into each individual shoot with negligible impact on shoot appearance rate, the number of shoots per plant at an LAI of 2.1 or the rate of plant population decay. Inter-specific competition for light was proposed as the main factor controlling self-thinning of plants and shoots regardless of their individual C:N status.     

Long-term effects of N fertilization on cropping and growth of olive trees and on N accumulation in soil profile

Two experiments were conducted for 13 years in two olive groves of southern Spain to study the long-term effect of nitrogen (N) fertilization on trees and soil. In the first experiment, 12-year-old ‘Picual’ olive trees were arranged in a split plot design with method of N application (soil versus a 50% soil:50% foliar combination) as the whole plot factor, and amount of N applied annually (0, 0.12, 0.25, 0.5 or 1.0 kg N tree⁻¹) as the subplot factor. In the second experiment, N application to 50-year-old ‘Picual’ trees was based on the previous season's leaf N concentration. Urea was the source of N in both experiments. During the last 4 years, soil samples were taken at 0–20, 20–40, 40–60, 60–80, and 80–100 cm depth to evaluate the effect of N application on soil eutrophication. Fertilization with N had no significant effects on yield, fruit characteristics, and growth of olive trees for the 13 years of study, even when leaf N concentration increased with the amount of fertilizer N applied. Combining soil and foliar application may reduce the amount of fertilizer N necessary to correct a possible N deficiency because our experiments showed this practice to be more effective in increasing leaf N that applying N only to the soil. Our results question the established deficiency threshold of 1.4% of N in dried leaf because no reduction in yield or growth was observed for lower concentrations. However, leaf N concentration did not drop below 1.2% after 13 years with no N application, probably because of N inputs from rainfall and the mineralization of organic N. Whereas under natural conditions of the non-fertilized treatments NH₄⁺–N represented the dominant fraction of mineral N in soil, accumulation of high amounts of NO₃⁻–N in the soil profile occurred in the fertilized plots, which represents a high risk of N leaching from soil. All these results suggest that annual applications of fertilizer N are unnecessary to maintain high productivity and growth in olive. Applying N only when the previous season's leaf analysis indicates that leaf N concentration is below the deficiency threshold, is thus a recommended practice to optimize N fertilization in olive orchards and to reduce N losses by leaching.     

Light interception and radiation use efficiency in temperate quinoa (Chenopodium quinoa Willd.) cultivars

Sea level quinoas are grown at low altitudes in Central and Southern Chile. Both sensitivity to photoperiod and response to temperature largely determine quinoa adaptation, but crop biomass production must be quantified to evaluate agronomic performance. The objectives of this work are: (i) to characterize development effects on leaf area evolution for genotypes of sea level quinoa differing in cycle length, (ii) to quantify the extinction coefficient (k) for photosynthetically active radiation (PAR) and radiation use efficiency (RUE) from emergence up to the beginning of grain filling and (iii) to identify which crop attributes related to canopy architecture should be considered to improve biomass production. Four cultivars (NL-6, RU-5, CO-407 and Faro) were cropped in Pergamino (33°56′S, 60°35′W, 65 m a.s.l.), Argentina, at three densities (from 22 to 66 plants m⁻²) in two consecutive years under field conditions with adequate water and nutrient supply. Thermal time to first anthesis and maximum leaf number on the main stem were linearly correlated (r² = 0.87; p < 0.0001). Leaf area continued to increase during the flowering phase, notably in NL-6, the earliest genotype. There were significant differences in maximum plant leaf area between cultivars. Increasing density reduced plant leaf area but effects were comparatively small. Estimated k was 0.59 ± 0.02 across genotypes and was higher (p < 0.05) for 66 plants m⁻². Values for RUE changed as cumulative intercepted PAR (IPAR) increased; at initial stages of development RUE was 1.25 ± 0.09 g MJ IPAR⁻¹, but if cumulative IPAR was higher than 107.5 ± 10.4 MJ IPAR m⁻², RUE was 2.68 ± 0.15 g MJ IPAR⁻¹. That change occurred when leaf area index (LAI) and fraction of PAR intercepted were still low and ranged from 0.61 to 1.38 and from 0.33 to 0.51, respectively. No significant association was found with any developmental stage. Our results agreed to the notion that RUE variation during pre-anthesis phases is largely determined by LAI through its effect on radiation distribution within the canopy. Biomass production could be improved if periods of interception below 50% of incoming PAR were reduced to ensure high RUE. This seems to be possible in temperate areas both by the use of late genotypes with a higher number of leaves on the main stem and by early genotypes provided adequate plant density is chosen. Early increment in LAI and overlapping of the leaf area increase period with the flowering phase are desirable strategies for earliest genotypes to maximize yield.     

Tillage, cover crops, and nitrogen fertilization effects on soil nitrogen and cotton and sorghum yields

Sustainable soil and crop management practices that reduce soil erosion and nitrogen (N) leaching, conserve soil organic matter, and optimize cotton and sorghum yields still remain a challenge. We examined the influence of three tillage practices (no-till, strip till and chisel till), four cover crops {legume [hairy vetch (Vicia villosa Roth)], nonlegume [rye (Secaele cereale L.)], vetch/rye biculture and winter weeds or no cover crop}, and three N fertilization rates (0, 60–65 and 120–130 kg N ha⁻¹) on soil inorganic N content at the 0–30 cm depth and yields and N uptake of cotton (Gossypium hirsutum L.) and sorghum [Sorghum bicolor (L.) Moench]. A field experiment was conducted on Dothan sandy loam (fine-loamy, siliceous, thermic, Plinthic Paleudults) from 1999 to 2002 in Georgia, USA. Nitrogen supplied by cover crops was greater with vetch and vetch/rye biculture than with rye and weeds. Soil inorganic N at the 0–10 and 10–30 cm depths increased with increasing N rate and were greater with vetch than with rye and weeds in April 2000 and 2002. Inorganic N at 0–10 cm was also greater with vetch than with rye in no-till, greater with vetch/rye than with rye and weeds in strip till, and greater with vetch than with rye and weeds in chisel till. In 2000, cotton lint yield and N uptake were greater in no-till with rye or 60 kg N ha⁻¹ than in other treatments, but biomass (stems + leaves) yield and N uptake were greater with vetch and vetch/rye than with rye or weeds, and greater with 60 and 120 than with 0 kg N ha⁻¹. In 2001, sorghum grain yield, biomass yield, and N uptake were greater in strip till and chisel till than in no-till, and greater in vetch and vetch/rye with or without N than in rye and weeds with 0 or 65 kg N ha⁻¹. In 2002, cotton lint yield and N uptake were greater in chisel till, rye and weeds with 0 or 60 kg N ha⁻¹ than in other treatments, but biomass N uptake was greater in vetch/rye with 60 kg N ha⁻¹ than in rye and weeds with 0 or 60 kg N ha⁻¹. Increased N supplied by hairy vetch or 120–130 kg N ha⁻¹ increased soil N availability, sorghum grain yield, cotton and sorghum biomass yields, and N uptake but decreased cotton lint yield and lint N uptake compared with rye, weeds or 0 kg N ha⁻¹. Cotton and sorghum yields and N uptake can be optimized and potentials for soil erosion and N leaching can be reduced by using conservation tillage, such as no-till or strip till, with vetch/rye biculture cover crop and 60–65 kg N ha⁻¹. The results can be applied in regions where cover crops can be grown in the winter to reduce soil erosion and N leaching and where tillage intensity and N fertilization rates can be minimized to reduce the costs of energy requirement for tillage and N fertilization while optimizing crop production.     

Residual nitrogen effect of clover-ryegrass swards on yield and N uptake of a subsequent winter wheat crop as studied by use of N methodology and mathematical modelling

The residual effect of 2-year-old swards of clover-ryegrass mixture and ryegrass in monoculture on yield and N uptake in a subsequent winter wheat crop was investigated by use of the ¹⁵N dilution method and by mathematical modelling. The amount of N in the wheat crop, derived from clover-ryegrass residues was 25–43% greater than that derived from residues of ryegrass which had been growing in monoculture. Expressed in absolute values, the N uptake in the subsequent winter wheat crop was 23–28 kg N ha ⁻¹ greater after clover-ryegrass mixture than after ryegrass in monoculture. Up to about 54 kg N ha⁻¹ of the N mineralised from the clover-ryegrass crop was calculated to be leached, whereas only 11 kg N ha⁻¹ was leached following ryegrass in monoculture.     

Manganese requirement of sunflower (Helianthus annuus L.), tobacco (Nicotiana tabacum L.) and triticale (x Triticosecale W.) at early stage of growth

Manganese deficiency symptoms are more often observed in crops at early stages of growth since Mn²⁺ can be easily mobilized from the surface soil. The objectives of this study were to evaluate some of the popular rotation crops grown in Hungary for tolerance to low external Mn²⁺ levels and to determine the critical tissue concentration for Mn²⁺ deficiency during early stages of growth. Indicator plants of sunflower (Helianthus annuus L.) were grown with NPKCaMg-fertilization induced of 0.0425–0.0700 g kg⁻¹; of tobacco (Nicotiana tabacum L.) 0.0237–0.0337 g kg⁻¹; of triticale (x Triticosecale W.) 0.0103–0.0327 g NH₄-acetate + EDTA extractable soil Mn²⁺ kg⁻¹; and were grown for 73, 50, and 191 days. The minimum Mn²⁺ concentration required in soil nutrient contents was 0.0425 g kg⁻¹ for sunflower, 0.0243 g kg⁻¹ for tobacco, and 0.0103 g kg⁻¹ for triticale. Sunflower, tobacco and triticale achieved optimum growth from 0.048 to 0.065 g Mn²⁺ kg⁻¹, from 0.0249 to 0.0321 g Mn²⁺ kg⁻¹, and from 0.0287 to 0.0296 g Mn²⁺ kg⁻¹, respectively. Critical ABP's dry weight Mn²⁺ concentration at early stages of growth was 0.0536 g kg⁻¹ in sunflower, 0.458 g kg⁻¹ in tobacco, and 0.1938 g kg⁻¹ in triticale. Our results demonstrate that the tolerance to low external Mn²⁺ (triticale <0.0302 g kg⁻¹; sunflower <0.0562 g kg⁻¹; tobacco <0.0693 g kg⁻¹) and the critical tissue Mn²⁺ levels for deficiency varied significantly among crop species tested.     

Spear yield and quality of white asparagus as affected by soil temperature

In three consecutive years two asparagus cultivars (Gijnlim, Grolim) were cultivated at three different temperatures in the ridge surface ranging from 12 to 26 °C to assess (1) the onset of spear yield, (2) the mean daily yield increase per plant, and (3) the establishment of spear quality defects. The objective was to determine the temperature dependence of the spear growth and quality of white asparagus with respect to different cultivars and the harvest year.

The mean time interval Δt from the beginning of the temperature treatment to harvest start was 16.5 d at the reference temperature of 20 °C. This time interval corresponded to a thermal time of 255 °C d at a base temperature of 4.4 °C. The mean yield increase per plant at the reference temperature was 17.4 g d⁻¹. The temperature dependence of Δt and was expressed by the relative change of these quantities per 1 °C alteration, which were on average 0.08 and 0.14 °C⁻¹, respectively. The frequency of quality defects (rusted spears, split spears, hollow/club-shaped spears, spears with open heads) and grade I quality depended significantly (P < 0.05) on the temperature, cultivar, and harvest year. Generally, ‘Gijnlim’ showed fewer quality defects than ‘Grolim’, whereas spears without defects were most frequent for those grown between 18 and 22 °C. With rising temperatures, the frequency of split or hollow/club-shaped spears and spears with open heads increased, while the frequency of rusted spears decreased. The derived functions of yield and quality parameters should improve the controlling of the yield and quality of white asparagus spears at varying temperatures in the ridge.     

Wheat productivity in the Mediterranean Ebro Valley: Analyzing the gap between attainable and potential yield with a simulation model

Water deficit is an important constraint for wheat yield generation under Mediterranean environments. However, nitrogen (N) availability could limit yield in a more important way than poor water conditions. The aim of the work was to analyze, using the Ceres-Wheat crop simulation model, to what degree N fertilization constitutes a tool for reducing the gap between attainable and potential yield. Firstly, the model was calibrated and validated under a wide range of N and water conditions for the region of the Ebro Valley (NE Spain). Anthesis and maturity date were adequately predicted by the model. Predictions of yield tended to be quite accurate in general, though under severe water deficits precision was lower. We then assessed the gap between attainable and potential yield considering different N availabilities at sowing taking into account a weather database of 17 years for the location of Agramunt (NE Spain), representative of cereal growing conditions of the Mediterranean Catalonia. Potential yield ranged between 3.5 and 8.1 Mg ha⁻¹. Variations in potential yield were explained by the duration of the period from sowing to anthesis and by the level of incident radiation during the period immediately previous to anthesis. Average attainable yield was 1.8 Mg ha⁻¹ for N availability of 50 kg_N ha⁻¹; but increased to 2.8 Mg ha⁻¹ for higher N availabilities (100–250 kg_N ha⁻¹). In the 25% of the worst years there was no effect of N availability on attainable yield. Increasing N availability beyond 100 kg_N ha⁻¹ generated a gain in yield only in 6% of the years. Variations between years in attainable yields were mainly explained by rainfall during the period from sowing to anthesis, whereas differences in attainable yield between N treatments increased with increases in rainfall. The gap between potential yield and attainable yield was higher in years with higher potential yield. On the other hand, the higher the attainable yield, the lower the gap. Thus, the proportion of the yield gap ascribed to N availability varied depending on the conditions of the growing season. In the high-yielding potential years, the main restriction for growth was water shortage, and fertilizing only slightly reduced the gap. Conversely, in rainy years characterized by low potential yields and mild water stresses, N management may constitute a simple tool for effectively reducing yield gap under rain-fed conditions.     

Delaying senescence of wheat with fungicides has interacting effects with cultivar on grain sulphur concentration but not with sulphur yield or nitrogen:sulphur ratios

Winter wheat was grown in three field experiments, each repeated over two or three seasons, to investigate effects of extending flag leaf life by fungicide application on the concentration, kg ha⁻¹ and mg grain⁻¹ of nitrogen (N) and sulphur (S) as well as N:S ratio and sodium dodecyl sulphate (SDS) sedimentation volume. The experiments involved up to six cultivars and different application rates, timings and frequencies of azoxystrobin and epoxiconazole. For every day the duration to 37% green flag leaf area (m) was extended, N yield was increased by 2.58 kg ha⁻¹, N per grain by 0.00957 mg, S yield by 0.186 kg ha⁻¹ and S per grain by 0.000718 mg. The N:S ratio decreased by 0.0135 per day. There was no evidence that these responses varied with cultivar. In contrast, the relationship between flag leaf life and N or S concentration interacted with cultivar. The N and S concentrations of Shamrock, the cultivar that suffered most from brown rust (Puccinia recondita), increased with the extension of flag leaf life whereas the concentrations of N and S in Malacca, a cultivar more susceptible to Septoria tritici, decreased as flag leaf senescence was delayed. This was because the relationships between m and N and S yields were much better conserved over cultivars than those between m and thousand grain weight (TGW) and grain yield ha⁻¹.     

Potential for nitrogen fertilization and Hessian fly-resistance to improve Morocco's dryland wheat yields

Hessian fly (Mayetiola destructor, Say) is a perennial scourge of cereal production in the Mediterranean region, particularly in North Africa. In Morocco, it accounts for considerable yield losses of wheat (Triticum spp.), especially in the semi-arid southwestern coastal provinces. Breeding for resistance is the only feasible approach to abate its effects. Nine major Hessian fly-resistance genes have been identified in bread wheat. Two bread wheat varieties have these characteristics; the first variety with complete resistance was ‘Saada’, released to farmers in 1989, whereas a tolerant variety ‘Massira’ was released in 1994. Another widespread limiting factor for all cereals in Morocco is lack of adequate nitrogen (N). With favorable farmer acceptance, Saada became the focus of on-farm N fertilizer trials throughout the low rainfall (250-450 mm year⁻¹) zone, where it consistently out-yielded the susceptible common bread wheat, Nesma, except when no Hessian fly infestation occurred. In most cases, 40 kg N ha⁻¹ was adequate for maximum yield. Substituting resistant cultivars for Hessian fly-susceptible cultivars and increased N use could have an immediate and positive effect on wheat production in Morocco, especially in areas where the insect is endemic. The future impact will be greater when Hessian fly resistance is also transferred to other bread wheat cultivars and to durum (T. durum) wheat, the major staple food in the Mediterranean region.     

Stomatal conductance and ozone exposure in relation to potato tuber yield—results from the European CHIP programme

Measurements of stomatal conductance on field-grown potato (Solanum tuberosum L.) cv. Bintje from the CHIP programme were combined to study the response to environmental factors. 3274 data points were used. Data were obtained from five sites: Jokioinen in Finland, Östad in Sweden, Giessen in Germany, Tervuren in Belgium and Sutton Bonnington in UK. Measurements were made in open-top chamber treatments with ozone and carbon dioxide exposure and in the ambient air. A typical light response curve was obtained with light saturation at approximately 400 μmol m⁻² s⁻¹ photosynthetically active radiation (PAR). The leaf temperature optimum for stomatal conductance was 29 °C. The stomatal conductance declined strongly at leaf-to-air vapour pressure differences >20 hPa. An elevated carbon dioxide concentration (680 μl l⁻¹) reduced the stomatal conductance by up to approximately 20%. Elevated ozone reduced the stomatal conductance towards the end of the growth period, in addition to the negative effect by ordinary senescence on stomatal conductance. A multiplicative model, based on the boundary line technique, was used to estimate the relationship between stomatal conductance and the environmental variables. Test with the data sets from two sites providing sufficient data, Östad and Giessen, showed that the multiplicative model had R²-values of 0.60 and 0.42, respectively, for the relationship between calculated and observed conductance. Test of the model with an independent data set from an open-top chamber experiment with the potato cultivar Kardal showed an R² of 0.59 between calculated and observed conductance. The conductance model was used to estimate the accumulated ozone uptake (CUO₃) by potato leaves from emergence to harvest. The relationship between CUO₃ and relative yield loss, using a threshold for the ozone uptake rate of 7 nmol m⁻² s⁻¹, provided a higher R²-value (0.45) than CUO₃ without any threshold and relationships based on the accumulated exposure over 40 nmol mol⁻¹ (AOT40) or the sum of all hourly average ozone concentrations exceeding 60 nmol mol⁻¹ (SUM06). All four relationships were however statistically significant.     

Effect of ultraviolet-C light on quality and microbial population of fresh-cut watermelon

The influence of ultraviolet (UV-C) light (1.4–13.7 kJ m⁻² at 254 nm) on the quality and microbial populations of fresh-cut watermelon [Citrulus lanatus (Thunb.) Matsum. and Nakai] was investigated and compared to that of common sanitizing solutions used for fresh-cut produce. Dipping cubes in chlorine (40 μL L⁻¹) and ozone (0.4 μL L⁻¹) was not effective in reducing microbial populations and quality was lower in cubes receiving these aqueous treatments compared to UV-irradiated cubes or control. In commercial trials, exposing packaged watermelons cubes to UV-C light at 4.1 kJ m⁻² produced >1 log reduction in microbial populations by the end of the product's shelf life without affecting juice leakage, color and, overall visual quality. In further experimentation, lower UV-C dose (1.4 kJ m⁻²) reduced microbial populations to a lower degree and only when complete surface exposure was ensured. Higher UV-C doses did not show any difference in microbial populations (6.3 kJ m⁻²) or result in quality deterioration (13.7 kJ m⁻²). Spray applications of hydrogen peroxide (2%) and chlorine (40 μL L⁻¹), without subsequent removal of excess water, failed to further decrease microbial load of cubes exposed to UV-C light at 4.1 kJ m⁻². When properly utilized, UV-C light is the only method tested in this study that could potentially be used for sanitizing fresh-cut watermelon.