Rain Shelter Experiment Research Articles

Water use efficiency and canopy temperature response of soybean subjected to deficit irrigation

Deficit irrigation is a key strategy for improving water use efficiency (WUE) under irrigated conditions. However, there is a lack of information regarding the optimal water replacement that have minimal negative effects on soybean productivity. The objective of this study was to determine the water replacement levels associated with insignificant grain yield (GY) losses in soybean crops. A rain shelter experiment was conducted using a randomized complete block design with six replicates. Eight irrigation replacement levels, L120, L100, L90, L80, L70, L60, L50, and L40, were applied, where L100 was the reference treatment that kept soil moisture content along the soil profile under field capacity conditions and all other replacement levels were a fraction of this reference level. Grain yield ranged from 2.2 Mg ha-1 in L40 to 4.4 Mg ha-1 in L100, with a significant GY reduction in irrigation levels below 70%. The average crop water stress index (CWSI) ranged between 0.26 at L120 and 0.66 at L40 irrigation levels. WUE varied significantly only for the extreme irrigation levels studied, with the greatest value at the L40 irrigation level (1.2 kg m-3) and the lowest value at the L120 irrigation level (0.65 kg m-3), whereas for the intermediate irrigation levels from 50 to 100%, the WUE was equal to approximately 1.1 kg m-3. The relationship between CWSI and GY (R2 = 0.85) suggested that the maximum GY occurred at a CWSI of 0.34. In addition, the relationship between CWSI and WUE (R2 = 0.73) showed that as evapotranspiration decreased, crop temperature increased. In conclusion, the implementation of a continuous water deficit in soybeans is feasible for farmers in water-scarce areas, but the minimum value of area productivity must be considered, even though WUE increases under more intensive values of water deficit.

Water productivity and canopy thermal response of pearl millet subjected to different irrigation levels

Pearl millet is a valuable alternative crop to biomass and forage production. However, water productivity (WP) in this crop under limited water conditions remains unclear in literature. In addition, rapid and reliable techniques to detect water stress spatial variability under field conditions are necessary. The objective of this study was to verify the yield and canopy thermal responses of pearl millet subjected to different water replacement levels (WRL). A rain shelter experiment was conducted using a randomized block design with four replications at University of São Paulo in Piracicaba, São Paulo, Brazil. Pearl millet cultivar BRS-1501 was subjected to four WRL treatments: WRL40, WRL70, WRL100 and WRL130. The reference WRL treatment, WRL100, was irrigated to maintain soil moisture at field capacity level, whereas the other treatments were a fraction of the irrigation depth applied in WRL100 treatment. The highest dry biomass yield obtained was 12.1 Mg ha−1 under WRL130 treatment, and this value diminished as WRL decreased. On the contrary, WP increased as the WRL decreased, recording the highest WP in WRL40 (9.0 kg m−3). Pear millet canopy showed different thermal responses as a function of WRL treatments imposed, being an alternative tool to detect water stress zones in the field by remote sensing technologies.

CROP WATER STRESS INDEX FOR PREDICTING YIELD LOSS IN COMMON BEAN

The crop water stress index (CWSI), an index derived from canopy temperature, has been widely studied as a physiological indicator of plant water status to optimize irrigation in common beans. However, it is not clear how this index could contribute to yield prediction as a decision support tool in irrigation management. This paper aimed to use the CWSI for predicting yield loss in common bean (Phaseolus vulgaris L.) subjected to water stress under drip irrigation. A rain shelter experiment was conducted using a completely randomized design with five replications. The indeterminate growth cultivar TAA Dama was subjected to three irrigation treatments: 100% of the field capacity (FC), 75 and 50% FC from 20 days after sowing (DAS) until the end of the crop cycle. Grain yield was reduced by 42% under 50% FC treatment. Furthermore, stomatal conductance was reduced under this treatment, whereas the CWSI and canopy temperature increased as irrigation levels decreased. The relationship between grain yield and CWSI (R2=0.76, RSME=2.35g) suggests that canopy temperature data could be used to forecast grain yield losses. In conclusion, farmers can have a low-cost, effective technique for making water management decisions in common bean.

Predicting Equivalent Water Thickness in Wheat Using UAV Mounted Multispectral Sensor through Deep Learning Techniques

The equivalent water thickness (EWT) is an important biophysical indicator of water status in crops. The effective monitoring of EWT in wheat under different nitrogen and water treatments is important for irrigation management in precision agriculture. This study aimed to investigate the performances of machine learning (ML) algorithms in retrieving wheat EWT. For this purpose, a rain shelter experiment (Exp. 1) with four irrigation quantities (0, 120, 240, 360 mm) and two nitrogen levels (75 and 255 kg N/ha), and field experiments (Exps. 2–3) with the same irrigation and rainfall water levels (360 mm) but different nitrogen levels (varying from 75 to 255 kg N/ha) were conducted in the North China Plain. The canopy reflectance was measured for all plots at 30 m using an unmanned aerial vehicle (UAV)-mounted multispectral camera. Destructive sampling was conducted immediately after the UAV flights to measure total fresh and dry weight. Deep Neural Network (DNN) is a special type of neural network, which has shown performance in regression analysis is compared with other machine learning (ML) models. A feature selection (FS) algorithm named the decision tree (DT) was used as the automatic relevance determination method to obtain the relative relevance of 5 out of 67 vegetation indices (Vis), which were used for estimating EWT. The selected VIs were used to estimate EWT using multiple linear regression (MLR), deep neural network multilayer perceptron (DNN-MLP), artificial neural networks multilayer perceptron (ANN-MLP), boosted tree regression (BRT), and support vector machines (SVMs). The results show that the DNN-MLP with R2 = 0.934, NSE = 0.933, RMSE = 0.028 g/cm2, and MAE of 0.017 g/cm2 outperformed other ML algorithms (ANN-MPL, BRT, and SVM- Polynomial) owing to its high capacity for estimating EWT as compared to other ML methods. Our findings support the conclusion that ML can potentially be applied in combination with VIs for retrieving EWT. Despite the complexity of the ML models, the EWT map should help farmers by improving the real-time irrigation efficiency of wheat by quantifying field water content and addressing variability.

Water and nitrogen stress effects on canopy development and biomass allocation in fodder beet (Beta vulgaris L.)

ABSTRACT The effects of water stress on fodder beet (Beta vulgaris L.) production in the field are confounded by the influences of variable inter- and intra-season rainfall. In this study, a rain shelter experiment investigated the effect of water and nitrogen (N) stress on dry matter (DM) production, DM allocation and N partitioning in fodder beet (‘Rivage’). Two water-management treatments, irrigated (replacement of the profile water lost weekly) and unirrigated (limited water applied only during fertigation), and three fertiliser N treatments (0, 50 and 300 kg N ha−1 (hereafter, 0N, 50N, 300N)) were evaluated during the 2016–2017 season. Under irrigation, the yield was greater (P = .02) for the 50N than 0N treatments (29.4 and 24.9 t DM ha−1, respectively). Yield did not differ (P = .42) between irrigated 50N and 300N (30.6 t DM ha−1) treatments, indicating surplus N application to the latter. Water and N stress restricted leaf growth, lowered N uptake and reduced yield by 48%–51%. Surplus N in irrigated 300N treatments resulted in increased N uptake and leaf proportion but reduced the storage root proportion. These results demonstrate the plasticity of fodder beet biomass and N allocation in adapting to the availability of water and N.

Topsoil Hardening: Effects on Soybean Root Architecture and Water Extraction Patterns

Topsoil hardening is one of the major causes of poor root growth although its effects on subsoil roots are still not well-known. Our aim was to examine the effects of topsoil hardening on the growth and functioning of shallow and deep roots of soybean plants. Two rain shelter experiments were conducted in two consecutive years. Plants were grown in topsoil monoliths (0–20 cm) with low (LR) or high mechanical resistance (HR), extracted from adjacent no-tillage cropping fields, and placed above 180 cm-high containers filled with a sandy loam soil. The effects of topsoil hardening were largely regulated by the level of water stress. In stressed plants, HR conditions reduced total aboveground biomass (up to 13%), total root biomass, root length density, and root surface area (up to 23, 38, and 37% respectively). Mechanical impedances reduced root biomass and length in both shallow (0–20 cm) and very deep layers (+ 160 cm). No changes were observed in specific root length or specific surface area. Plants growing in HR topsoils showed lower total water extraction but greater specific water uptake rates (29–47% higher in year 1 and 2 respectively). No clear architectural (i.e., root density) or morphological (i.e., specific root length/area) responses of enhanced root foraging capacity were observed in subsoil roots. However, soybean root system responded through functional mechanisms (i.e., specific water uptake) which partially attenuated the negative effects of mechanical impedances.

Root Contact between Maize and Alfalfa Facilitates Nitrogen Transfer and Uptake Using Techniques of Foliar 15N-Labeling

Belowground nitrogen (N) transfer from legumes to non-legumes provides an important N source for crop yield and N utilization. However, whether root contact facilitates N transfer and the extent to which N transfer contributes to crop productivity and N utilization have not been clarified. In our study, two-year rain shelter experiments were conducted to quantify the effect of root contact on N transfer in a maize/alfalfa intercropping system. N transfer occurred mainly one direction from alfalfa to maize during the growth period. Following the N0 treatment, the amount of N transfer from alfalfa to maize was 204.56 mg pot−1 with no root barrier and 165.13 mg pot−1 with a nylon net barrier, accounting for 4.72% and 4.48% of the total N accumulated in maize, respectively. Following the N1 treatment, the amount of N transfer from alfalfa to maize was 197.70 mg pot−1 with no root barrier and 139.04 mg pot−1 with a nylon net barrier, accounting for 3.64% and 2.36% of the total N accumulated in the maize, respectively. Furthermore, the amount of N transfer without no root barrier was 1.24–1.42 times higher than that with a nylon net barrier regardless of the level of N addition. Our results highlight the importance and the relevance of root contact for the enhancement of N transfer in a maize/alfalfa intercropping system.

Effects of water stress on photosynthetic characteristics, dry matter translocation and WUE in two winter wheat genotypes

Determining the effects of a water deficit during periods of vegetative growth on photosynthetic traits and grain yield will provide a reasonable strategy for water-saving management of winter wheat (Triticum aestivum L.) and exploited photosynthetic traits for the selection of drought tolerant winter wheat genotypes. A mobile rain shelter experiment was conducted using winter wheat cultivar to assess the effects of different levels of water stress on photosynthetic characteristics, dry matter translocation and water use efficiency (WUE) in the Shijiazhuang 8 (drought resistant) and Yanmai 20 (drought sensitive) cultivars at different growth stages. Three winter wheat growing stages were selected for assessment at follows: recovering-jointing, jointing-flowering and grain-filling, and the effects of four levels of soil water which were selected based on field capacity, on plants from seeding to mature stage were examined by controlling the irrigation as follows: 40–45% (severe stress), 55–60% (moderate stress), 65–70% (mild stress) and 75–80% (full irrigation) The results indicated that mild stress during the recovering-jointing stage improved the canopy structure prior to anthesis and maintained high canopy photosynthesis after anthesis, thus increasing winter wheat yields. Mild stress during all of the growth stages improved the distribution of assimilate to the grain prior to anthesis and increased the yield. Although moderate stress during all growth stages could improve dry matter translocation, the resulting yield was not high, as the accumulation of dry matter decreased after anthesis. Therefore, mild soil water stress can improve grain yields and WUE. Shijiazhuang 8 displayed a higher grain yield and WUE than Yanmai 20 under drought stress, and throughout the different stages of growth, the leaves exhibited a lower net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration (E) under drought stress. Furthermore, Shijiazhuang 8 which maintained relatively higher, Pn, Gs and E under water stress, maintained higher values of WUE than Yanmai 20. Under the severe stress treatments, some varieties showed an increase in the concentration of intercellular CO2 (Ci), indicating an inhibition of photosynthetic activity due to non-stomatal effects. Overall, we conclude that the genotypic differences in the photosynthetic response could be useful for mapping the photosynthetic traits that promote tolerance to drought. We recommend that mild water stress (65–70% water field capacity) be considered for irrigation scheduling in winter wheat under conditions of water limitation.

Intraspecific differences in responses to rainshelter-induced drought and competition of Fagus sylvatica L. across Germany

Future climate projections for Central Europe indicate a decrease in summer precipitation which might range between 15% and 50%, and equally important, changes in the climate variability, resulting in consecutive years with drought periods. With respect to Central European forests, we asked to which degree realistic drought conditions are tolerated by the recruits of the dominant tree species Fagus sylvatica L., and how the effects depend on biotic interactions. To test the combined effects of drought, competition and provenance of F. sylvatica recruits we set up a rain shelter experiment at three sites in different regions of Germany. Transposable roof panels allowed a flexible precipitation reduction between 10% and 70% corresponding with a return period of 40years. We planted F. sylvatica saplings of three provenances, exposed them to drought and competition. We tested if understorey herbaceous competitors have a negative impact on F. sylvatica saplings, and thus, exacerbate drought effects and that F. sylvatica provenances from drier regions are adapted to drought conditions and cope better with drought conditions. Six months after the drought treatment started, we encountered significant drought effects, seen in a reduced leaf stomatal conductance, although there was not yet a response in growth rates. Overall, the site had the greatest impact on phytometer performance, while we found no indication of adaptation to drought of the different provenances. Furthermore, drought effects increased in interaction with site effects, being highest at the driest site. At the driest site, leaf stomatal conductance decreased in the presence of competition but increased in the control subplots, while the site of intermediate moisture conditions showed the opposite pattern and the wettest site displayed no differences. Our results highlight the fact that biotic interactions can mitigate or exacerbate drought effects, depending on regional site conditions.

Parameterisation and evaluation of the FAO-AquaCrop model for a South African taro (Colocasia esculenta L. Schott) landrace

Promotion of taro, a neglected underutilised crop, as a possible future crop under water-limited conditions hinges on availability of information describing its yield responses to water. Therefore, AquaCrop was calibrated and validated for the first time for an eddoe type taro landrace from South Africa, using data from pot, field and rain shelter experiments conducted over two seasons (2010/11 and 2011/12) at two locations (Pretoria and Pietermaritzburg) representative of semi-arid climates. Observed weather and soil physical parameters for specific sites together with measured crop parameters from optimum experiments conducted during 2010/11, were used to develop climate, soil and crop files in AquaCrop and to calibrate the model. Observations from the 2011/12 growing season and independent data were used to validate the model. Model calibration showed a good fit (R2=0.789; d-index=0.920; RMSE=2.380%) for canopy cover (CC) as well as good prediction for final biomass (RMSE=1.350tha−1) and yield (RMSE=1.205tha−1). Model validation showed good simulation for CC under irrigated conditions (R2=0.844; d-index=0.998; RMSE=1.852%). However, the model underestimated CC under rainfed (R2=0.018; d-index=0.645; RMSE=20.170%) conditions. The model predicted biomass (R2=0.898; d-index=0.875; RMSE=5.741tha−1) and yield (R2=0.964; d-index=0.987; RMSE=1.425tha−1) reasonably well for pooled data [field (RF and FI) and rain shelter (100, 60 and 30% ETa)]. The model also predicted biomass (R2=0.996; d-index=0.986; RMSE=1.745tha−1) and yield (R2=0.980; d-index=0.991; RMSE=1.266tha−1) well for the independent data set.

Adaptation of Trifolium repens × T. uniflorum hybrid clovers to drought stress

Abstract White clover (Trifolium repens L.) is valued for its contribution to pasture quality and utilisation by animals, compatibility with grass, and fixation of nitrogen. However, it is limited by poor adaptation to drought. Hybridisation with Trifolium uniflorum L. may have potential to improve the drought resistance of white clover. An experiment in a rain shelter facility with contrasting moisture treatments, and a field evaluation under dryland conditions, were used to compare the agronomic potential of these interspecific hybrids (ISH) with white clover in moisture limited conditions. In the rain shelter experiment, there were smaller effects of water stress on shoot dry weight (DW), leaf area, internode length and senescence of first backcross generation hybrids compared with white clover and second backcross generation hybrids. Differences in photosynthetic responses were possibly influenced by the effect of root DW allocation on water uptake. In the field evaluation, growth scores of a wider range of hybrid families during summer moisture stress concurred with the results under water stress in the rain shelter. Growth of some ISH families outperformed the best white clover cultivars, particularly in the third and most stressful summer and this result was a key performance indicator of the value of the hybrids for drought prone areas. These findings using early, unselected, hybrid populations indicate the potential for further selection of elite, adapted cultivars from ISH breeding strategies. Keywords: Interspecific clover hybrid, drought resistance, rain shelter, field evaluation

Parameterization and Testing of AquaCrop for a South African Bambara Groundnut Landrace

The aim of this study was to parameterize and test the generic crop model AquaCrop for a local bambara groundnut [Vigna subterranea (L.) Verdc] landrace. Such a model should be water driven and assist in the promotion of neglected and underutilized species as possible future crops under water‐limited conditions. AquaCrop was parameterized for a South African bambara groundnut landrace using data from controlled field and rain shelter experiments conducted during two seasons (2010/2011 and 2011/2012) at Pretoria, South Africa. Observed weather, soil physical, and measured crop parameters from optimum experiments conducted during 2010/2011 were used to develop respective climate, soil, and crop files in AquaCrop and to parameterize the model. Model parameterization for bambara groundnut showed a very good fit for canopy cover (R2 = 0.94, Willmott’s d index of agreement = 0.99, RMSE = 3.37%) and biomass (R2 = 0.96, d index = 0.99, RMSE = 1.29 Mg ha–1). The model also predicted final biomass (RMSE = 1.70 Mg ha–1) and yield (RMSE = 0.29 Mg ha–1) reasonably well. Model testing showed good fit for canopy cover under irrigated (R2 = 0.86, d index = 0.96, RMSE = 9.72%) and rainfed field conditions (R2 = 0.95, d index = 0.97, RMSE = 6.18%) compared with simulation of results from rain shelter experiments. The model simulated final biomass and yield of bambara groundnut very well under field conditions. The model’s performance under rainfed conditions make it particularly suited for extrapolation to marginal areas of agricultural production in South Africa and the region.

Moderate drought alters biomass and depth distribution of fine roots in Norway spruce

SummaryA rain shelter experiment was conducted in a 90‐year‐old Norway spruce stand, in the Kysucké Beskydy Mts (Slovakia). Three rain shelters were constructed in the stand to prevent the rainfall from reaching the soil and to reduce water availability in the rhizosphere. Fine root biomass and necromass were repeatedly measured throughout a growing season by soil coring. We established the quantities of fine root biomass (live) and necromass (dead) at soil depths of 0–5, 5–15, 15–25 and 25–35 cm. Significant differences in soil moisture contents between control and drought plots were found in the top 15 cm of soil after 20 weeks of rainfall manipulation (lasting from early June to late October). Our observations show that even relatively light drought decreased total fine root biomass from 272.0 to 242.8 g m−2 and increased the amount of necromass from 79.2 to 101.2 g m−2 in the top 35 cm of soil. Very fine roots (VFR), that is, those with diameter up to 1 mm, were more affected than total fine roots defined as 0–2 mm. The effect of reduced water availability was depth‐specific; as a result, we observed a modification of vertical distribution of fine roots. More roots in drought treatment were produced in the wetter soil horizons at 25–35 cm depth than at the surface. We conclude that fine and VFR systems of Norway spruce have the capacity to re‐allocate resources to roots at different depths in response to environmental signals, resulting in changes in necromass to biomass ratio.

Using a Scanner-based Minirhizotron System to Characterize Sweetpotato Adventitious Root Development during the Initial Storage Root Bulking Stage

A scanner-based minirhizotron (MR) system detected initial adventitious root (AR) development associated with transplant establishment. The system also documented the transition of ARs into pencil roots (PRs) and, in some cases, storage roots (SRs). In general, the MR system underestimated destructive sampling-based (DS) estimates of newly initiated AR (NAR), PR, and SR counts. Angled or vertical single sampling tubes underestimated NAR count by 85% and 79%, respectively. Regardless of installation position, single tube-based measurements underestimated PR and SR count by 83% and 95%, respectively. However, it was found that two vertically installed tubes underestimated NAR count by only 48%. The potential ability of paired sampling tubes to discriminate NAR count differences in response to experimental treatments was confirmed in a simple rain shelter experiment. The paired MR and DS systems detected 83% and 56% reduction in NAR count among plots with rain shelters, respectively. However, it appeared that the presence of tubes interfered with SR formation of monitored AR segments. Despite this limitation, the results show the potential for incorporating MR systems in ongoing and future studies that aim to qualitatively and quantitatively document sweetpotato AR system response to agroclimatic variables and management interventions during the initial SR bulking stage.

Simulated wheat growth affected by rising temperature, increased water deficit and elevated atmospheric CO 2

The cropping systems simulation model APSIM-Nwheat was tested against detailed field measurements representing possible growing conditions under future climate change scenarios. Increasing average temperatures by 1.7 °C observed over several seasons at Obregon, Mexico reduced the time to flowering by 11 days and resulted in a decline of total biomass and grain yield. These effects were reproduced by the model, except when the observed total biomass inexplicably rose again in the fourth and fifth year, despite higher temperature and a much shorter growing time. In a water stress experiment, the effects of different timing and duration of water deficit on crop growth and yield were reproduced with the model for a rain-shelter experiment at Lincoln, New Zealand where observed grain yields were reduced from 10 to 4 t ha −1 due to increased water deficit. In experiments from Western Australia, reduced growth and yields due to extreme terminal water deficit were also reproduced with the model where measured yields fall below 0.5 t ha −1. In the Maricopa Free Air Carbon-Dioxide Enrichment (FACE) experiment in Arizona, USA, the largest yield increase occurred with elevated CO 2 in the dry and high N treatments, whereas little or no response was observed in the wet and low N supply treatments, as simulated with the model. Combining elevated CO 2 with increased temperature in a sensitivity analysis, two levels of water supply and a range of N applications indicated a positive effect of elevated CO 2 on yield as long as N was not limiting growth. Increased temperature and reduced water supply reduced yields and the yield response to N supply under ambient and elevated CO 2. Grain protein concentrations were reduced under elevated CO 2, but the difference was minor with ample N fertiliser. Evapotranspiration was reduced under elevated CO 2. Higher temperatures increased evapotranspiration with low N input, but reduced it with ample N fertiliser, resulting in a reduction and an increase, respectively, in drainage below the root zone. In the Mediterranean environment of Western Australia the impact of elevated CO 2 and increased temperature on grain yield was in average positive, but varied with seasonal rainfall distribution. Based on the range of model testing experiments and the sensitivity analysis, APSIM-Nwheat was found suitable for studies on directional impacts of future climate change on wheat production. Due to some large discrepancies between simulated and observed data, field experiments representing only a limited range of possible climate change scenarios and the large possible range of factorial interactions not tested, simulated quantitative effects with the model should be interpreted cautiously.

Losses in Wheat Due to Waterlogging

Waterlogging stress is one of the limiting factors influencing wheat (Triticum aestivum L.) production, especially in the lower Mississippi valley. A rain-shelter experiment was designed to evaluate the trend response of nine wheat genotypes to four levels of waterlogging treatment: 0, 10, 20, and 30 d of flooding. Genotypes planted in polyvinilchloride (PVC) containers 25 cm long by 10 cm in diameter were waterlogged in plastic tanks under controlled rain conditions. Results indicated significant linear responses for kernels per head and tillers per plant, significant linear and quadratic responses for yield and chlorophyll content, and significant linear and cubic responses for plant height. The linear trend was the most important component, explaining from 92 to 99% of the variability due to waterlogging. Linear prediction equations were obtained to describe the relationship between different traits and waterlogging stress. Losses in yield and yield components were evaluated in a field experiment with 15 genotypes under control and waterlogging treatment. Average yield losses of 44% were mainly caused by a decrease in tiller number and kernels per head. Under waterlogging treatment, tiller number and kernels per head were reduced by 41 and 20%, respectively. Screening of wheat genotypes revealed the potential for waterlogging tolerance in breeding material and identified tolerant cultivars useful for waterlogged environments. ‘Terral LA 422’, ‘Shelby’, and ‘Pioneer 2691’ were the most adapted genotypes for waterlogging treatments. Because of a significant interaction with waterlogging treatment, some of the high-yielding genotypes under non-flooded conditions such as ‘Coker 9663’ and ‘FFR 502W’ showed low tolerance to waterlogging. The results provided information on the methods quantifying losses from waterlogging and identified selection criteria for waterlogging tolerance in wheat.

Modelling radiation interception and radiation use efficiency for sugar beet under variable climatic stress

European sugar beet crops suffer drought stress, and climate change is likely to increase the frequency and severity of drought. Changes in rainfall pattern may also affect the radiation environment. Modelling is an appropriate tool to assess the scale of these problems. In a sugar beet model we included the effects of water stress on foliage dynamics (radiation interception) and of variable radiation use efficiency (RUE). Foliage expansion and senescence were related to the relative water content in the root zone and drought response factors were calibrated from rain-shelter experiments for early and late drought in England. Simulating variable drought response in different growth phases explained 70–96% of the foliage variation for rain-fed growth. The dependence of RUE on the fraction of radiation that is diffuse was described theoretically; measured variation of RUE (0.9 g DM MJ −1) matched well the empirical representation of this theoretical relationship. We evaluated these modifications using experimental total dry matter and sugar yields in the UK and in Germany (1965–1995). Including foliage dynamics and variable RUE improved the simulation of observed yields in the UK in some years. For continental sites mean sugar yields were well described when differences in the antecedent soil water balance were accounted for. The agreement between estimated and observed yields was close to 1:1, explaining between 30 and 45% of observed variation, though not appreciably different from when the original model was used. This was because the test data included only a few seasons with either severe water stress or abnormal fractions of diffuse sunlight. Ideally the model should be tested against crop growth data from more extreme environments. Nevertheless, we conclude that the expanded model is now more suitable for use across much of Europe and future climatic change.

Yield and quality of malting barley (Hordeum vulgare L. ‘Valetta') in response to irrigation and nitrogen fertilisation

Interactions of soil water and nitrogen (N) fertiliser application on the quality of barley (Hordeum vulgare L. ‘Valetta') for malting were studied in a rain shelter experiment at Lincoln, New Zealand. Treatments were arranged in a factorial design, consisting of five levels of soil water (fully irrigated, rain‐fed, early drought, late drought, and full drought) and four levels of N fertiliser (nil, 1 × 50, 2 × 50, and 3 × 50 kg N/ha). N treatments were applied at emergence, appearance of the second node, and at flowering, respectively. Variables indicative of grain quality (N concentration, grain size, and screenings) and micro‐malting characters (water uptake, malting loss, N index, wort‐N, β‐glucan, fine extract, coarse extract, and fine‐coarse difference) were evaluated for responses to the water and N treatments. Drought influenced the distribution pattern of kernel mass along the main stem and tiller spikes and also influenced the relative proportions of grain in the standard size categories. The level of N fertiliser had little effect on the kernel size properties. Screenings originated from both the proximal and distal ends of tiller spikes and from the proximal end of the main stem spike. The less severely water stressed treatments had fewer small kernels and mean weight of kernels was greater over all kernel positions. Grain N concentration responded to N fertiliser application but was unresponsive to water treatment. The reverse was true for kernel size and the malting characters of water uptake, malting loss, extract levels, and fine/coarse difference. Drought increased the levels of wort β‐glucan and lowered wort N and N index whereas N fertiliser caused an increase in wort β‐glucan and wort N but lowered the N index. Interactions between water and N effects were only significant for the latter. Both the timing of drought and fertiliser treatment had strong effects on the biochemical and physical characteristics of grain.

Effects of soil moisture deficits on yield and quality of ‘Russet Burbank’ potatoes

Abstract A ‘Russet Burbank’ potato crop was subjected to 14 irrigation treatments in a rainshelter experiment at Lincoln, New Zealand in the 1987–88 growing season. In 12 treatments, plots were trickle irrigated and covered automatically by the shelter whenever rain fell, so that precise responses of yield and quality to water deficit timing and severity could be obtained. These treatments ranged from overwatering to withholding water to develop maximum potential deficits of up to 138 mm. The water stress treatments were applied either ac tuber initiation, early tuber bulking, or late tuber bulking (assessed by sampling spare plants). Except when fit was raining, these plots were exposed to prevailing climatic conditions. The two remaining treatments, which were outside the rainshelter, checked for direct effects of the shelter on the crop. One treatment was irrigated to achieve the same deficits as one of the treatments inside the rainshelter, and the other was not irrigated. Highest yields of process grade tubers (63 t/ha} were obtained from the overwatered plots. Among the drought-stressed treatments, the highest yields were 58 t/ha of process grade tubers, obtained from plots subjected to drought during tuber initiation; these had fewer bit larger tubers. Drought during late tuber bulking reduced process grade yields by 20%, with both tuber size and tuber number being reduced by up to 20%. Non-irrigated rainfed plots outside the shelter had low yields and small tubers because the season had half the average rainfall. The levels of water deficit imposed had no effect on tuber processing quality.

DETECTION AND CORRECTION OF MIDSEASON P DEFICIENCY IN IRRIGATED POTATOES

Three experiments were performed in an automatic rainshelter and two in the field to determine the role of soil moisture management and phosphorus (P) fertilization in controlling P nutrition of potatoes (Solanum tuberosum L. ’Russet Burbank’). The rainshelter experiments indicated that permitting the upper layer (25 cm) of soil to remain dry during the early part of the growing season depressed the total P concentrations in the leaf blades at the 10% bloom stage to well below the sufficiency range of 0.45 – 0.50%, in spite of high P application rates at planting. Relieving stress at 10% bloom and maintaining soil water potential between −60 and −20 kPa until harvest significantly increased P concentrations. Tuber yields were only slightly less than on those soils without water stress throughout the growing period, provided ample P had been applied. By delaying stress relief in the upper soil layer for 3 wk, 6 wk, or until maturity, tuber yields were reduced 28, 47 and 49% respectively. Without P fertilization of this P-deficient soil at planting, leaf-P levels at 10% bloom were very low (0.26%), but application of P at this stage (banded, broadcast, or in solution) increased leaf-P concentrations and yields were similar to treatments receiving P at planting. Trimetaphosphate was particularly effective in increasing P concentrations in the leaves. In the two field experiments, tuber yields were high on all plots and treatment differences were small, even though leaf-P concentrations were relatively low. However, in the highest-yielding treatment (banded at planting) leaf-P levels averaged from 0.40 to 0.49% which is close to that considered as a sufficiency range (0.45 – 0.50), reported previously. From the practical standpoint, leaf analysis at the early bloom stage can be used to detect P deficiency, which may be caused by inadequate P fertilization or early season soil water stress. If soil and fertilizer P are insufficient, immediate application of fertilizer P will correct deficiencies and enhance yields if adequate soil water is also provided. Soil moisture stress from early bloom to near maturity should be avoided with this crop to obtain efficient use of applied P and maximum yields of tubers.Key words: Leaf analysis, P fertilizers, trimetaphosphate, soil water stress, automated rainshelter.