Drought is probably the most important factor limiting crop yields worldwide, therefore it is not surprising that there has been continuing interest in the ways in which drought affects crop yield. Efforts have been concentrated in this area in the hope that it would prove possible to use a knowledge of drought physiology to provide a rational basis for the development of rapid methods of breeding drought tolerant cultivars, and also to help in the improvement of crop management for dry conditions. The last five years have seen some important reassessments of the underlying principles and concepts involved in plant response to drought and these will be outlined in this brief review. Some of these important shifts in emphasis have been highlighted by Kramer (1988), Passioura (1988), Schulze el al. (1988) and Boyer (1989), particularly in relation to the question of what measure of water stress is most relevant to plant function. As it is not possible to cover all aspects of drought physiology in a brief review of this nature, we highlight four topics where recent findings may have particular relevance to the improvement of drought tolerance in agricultural crops.

Simulation models are heuristic tools for integrating diverse processes and help to increase our understanding of complex processes and systems. Models that predict crop development can serve as decision-support tools in crop management. This paper describes a phenology simulation model for the winter wheat shoot apex and reports validation and sensitivity analysis results.

The complete developmental sequence of the winter wheat shoot apex is quantitatively outlined and correlated with commonly recognised phenological growth stages. The phyllochron is used to measure the thermal time between most phenological growth stages, thereby increasing the flexibility over the growing degree-day (GDD) and photothermal approaches. Nineteen site-years covering a range of climatic conditions, cultural practices and cultivars across the Central Great Plains, USA, are used to validate the model.

Validation results show that the predicted phyllochron (108 GDD) agrees well with the observed phyllochron (107 GDD) for ten cultivars. Mean seedling emergence is predicted to within 2 days in almost all of the 19 site-years. The ability of the model to predict growth stages accurately increased successively from jointing to heading to maturity. Maturity is generally predicted to within 5 days of the observed day.

After validation, recalibration of the phyllochron estimates between growth stages are provided, and corrections for mesic and xeric conditions are suggested. Further validation of the entire developmental sequence of the shoot apex is recommended.

Body composition in wild animals is regulated within a band that encompasses variations due to seasonal food supply and physiological state (e.g. that of the reproducing, lactating female and the growing juvenile). This body composition is characteristic of the genotype and is notable for the avoidance of obesity (Pitts & Bullard 1968). Urgent behavioural responses are also invoked to minimize weight loss. Figure 1 illustrates this homeostasis diagrammatically for the non-lactating, mature animal.

In this integrated complex, both food intake (quantity) and diet choice (quality) play a major role in achieving the required balance between food-energy intake and ‘net output’ of energy in its various forms such as activity, growth, lactation and change in body reserves. The development of such a system, through evolution, to achieve optimum reproductive fitness in a given environment, is consistent with the theory of optimal foraging which is characteristic of many animals in the wild. Newman (1991) has used such an approach in modelling diet selection by sheep.

Domesticated animals have inherited the complex genome of their progenitors, albeit modified in certain respects through natural and artificial selection, in their new circumstances. Many aspects of the physiology and behaviour of the domesticated animal still relate to the environment in which they evolved. For example Jensen & Redbo (1987) have demonstrated the complex nesting behaviour retained by the modern domestic pig, under free ranging conditions similar to those of its wild progenitor.

Two major non-genetic factors have emerged as major influences on the appetite of domesticated animals and man in modern times.

Nuclear magnetic resonance (NMR) micro-imaging techniques have been employed to study noninvasively the spatial distribution of mobile protons (1H) around the cotyledonary node of flax (Linum usitatissimum) plants of two differing growth morphologies. The gross anatomy of the tissues of the stem can be discerned as a result of differences in their mobile 1H contents. The technique produced excellent images of the complex changes in stem structure that occur at the point of origin of side shoots. Detailed structure within the xylem could be visualized and the presence of fibre bundles deduced as dark areas amongst tissues of higher 1H signal intensity.

As a result of the non-invasive and non-destructive nature of NMR-imaging, the images obtained have been compared to micrographs obtained by conventional histological techniques on the same plant tissue. In general, the two approaches produce comparable results, but the NMR images are influenced by the relaxation properties of the protons as well as their concentration. Paramagnetic species, such as Mn2+ ions, produce enhanced relaxation rates of protons in their vicinity and an apparent increase in proton density at short recycle times. Thus an NMR image can yield both chemical and structural information. Some of the advantages and disadvantages of this technique over conventional histological methods are discussed.

The effects of a preceding (1986/87) season's crop of vetch (Vicia saliva) or oats (Avena sativa), either singly or as two different mixtures, on the nitrogen uptake and growth of a succeeding barley crop (Hordeum vulgare) were examined in Cyprus in 1987/88, on field plots previously labelled (1984/85) with 15N enriched organic matter. Grain as well as the vegetative material were harvested before planting the barley. Barley after vetch accumulated 61 % more nitrogen than barley after oats, while, for the mixtures, the nitrogen yields in the succeeding barley crop did not differ significantly from those of barley grown after oats. The 15N enrichment in barley indicated that fixed N2 from vetch contributed little nitrogen to the available soil nitrogen pool and to the nitrogen in barley. Most of the increased assimilation of nitrogen in barley following vetch (compared to barley after oats) was attributable to a greater availability of soil nitrogen arising from the lower soil nitrogen uptake by the preceding vetch than oats. The dry matter yield of barley following vetch was 84% higher than for barley after oats, while barley after the vetch-oats mixtures yielded 38–54% higher than barley following oats. It was concluded that greater nitrogen availability to the cereal following the legume was not the sole cause of the dry matter yield responses observed.

Maize was grown al the Regional Agricultural Research Station, Hathazari, Bangladesh during 1988/89 and 1989/90 to study fodder yield, cob growth and grain yield together with the contribution of pre-silking stem reserves to grain. Population densities were 33300, 44400 and 66600 plants/ha. Plants were either detopped after silking and pollen shedding, keeping 0, 2 or 3 leaves above the cob, or were left entire.

The results showed that the maize crop could successfully be detopped for fodder with little or no adverse effect on grain yield. Fodder yield increased with increased plant density and among the detopping treatments the highest fodder yield was obtained when the plants were detopped just above the cob. Cob growth followed a sigmoid pattern and the highest dry weight per cob was obtained from the lowest plant density and from entire plants. The number of cobs/m2 increased with increased plant density but detopping treatments did not give any significant difference in relation to densities. The number of grains/cob was highest with 33300 plants/ha but, among the detopping treatments, plants detopped just above the cob had the lowest number of grains/cob in both years. Weight of 1000-grain decreased with increasing plant density but it was increased by detopping plants just above the cob during 1988/89, although it was decreased in 1989/90. The highest apparent translocation of pre-silking reserves was obtained using densities of 44400 plants/ha but detopping treatments did not show any consistent effect, although the highest apparent translocation (20%) and harvest index (58%) were obtained from plants detopped just above the cob.

In experiments at Rothamsted in 1984–86, seven factors, each at two levels, were tested in factorial combination on winter barley (cv. Panda) grown as a third take-all susceptible crop. The factors were seed rate, a growth regulator prior to stem extension, amounts of N in spring, ‘winter’ nitrogen, an autumn insecticide, a fungicide applied to the seed (‘Baytan’) and a programme of fungicide sprays in spring and summer. Sowing 50% more seeds than normal increased the number of ears/unit area but had no effect on mean grain yield because grains were smaller. There were, however, significant, but unexplained, interactions between seed rate and the fungicide ‘Baytan’ applied to the seed. A growth regulator applied prior to stem extension had little effect on crop growth and no significant effect on grain yield. If sufficient N was applied in April there was little benefit from applying ‘winter’ N (30 kg/ha in November and again in February/March) except in 1985 when the amount of NO3-N in the soil, measured in the previous October, was lowest. Insecticide sprays applied in autumn to control the aphid vectors of barley yellow dwarf virus (BYDV) had no significant effect on grain yield but infectivity indices were below the threshold needed for treatment in each year. On average, ‘Baytan’ applied to the seed increased grain yield by 0·28 t/ha and this was associated with decreases in the severity of take-all. Over the three years, programmes of fungicide sprays, applied during spring and summer, increased grain yield by 0·92 t/ha but the mean response was largest where most N was applied.

The experiments also allowed the importance of interactions between different agronomic factors to be examined. A combined analysis of grain yields for all three years (based on 192 plot values) showed that only six 2- or 3-factor interactions, out of the 73 estimated, were significant (P < 0·05). Two of these interactions reflected variable responses to ‘winter’ N and fungicide sprays in the three years and three of the remaining four involved ‘Baytan’.

Additional plots of barley grown after oats had little take-all and yielded 1·14 t/ha more grain than similarly treated plots grown after barley. These responses were obtained despite evidence that oat residues had adverse effects on the growth of barley seedlings. Additional plots of barley grown after a bare fallow also had little take-all and gave even larger total yields (grain plus straw) than did barley after oats but the mean yield of grain was less than after oats because more of the dry matter after a fallow was straw. In 1984, when take-all was relatively slight, plots after a fallow gave even less grain than plots after barley (−0·77 t/ha) despite producing 2·12 t/ha more dry matter in grain plus straw.

Field studies were conducted in 1989 and 1990 to investigate the effect of planting density on growth and yield of maize under irrigation in the Batinah Coast region, an arid zone in the Sultanate of Oman. Two maize cultivars, Eperon and Challenger, were grown at three densities (24000, 48000 and 74000 plants/ha). Grain yield was highest at 48000 plants/ha. Leaf area index (LAI) increased with increase in plant density but was not related to grain yield. Lodging was highly correlated with shelling percentage.

Experiments on winter barley at Rothamsted, testing different sowing dates, were sampled in 1987–89 to measure effects on take-all caused by the fungus Gaeumannomyces graminis var. tritici. The experiments used the same plots in each year, and in 1988 and 1989 the randomization was restricted so that sowing dates were balanced for sowing dates in the previous year.

The site had been used to grow barley since at least 1979 (spring barley from 1979 until 1985), and take-all was much more severe than expected. It was usually most severe in the earliest-sown plots and decreased almost linearly as sowing was delayed. There was also evidence that the sowing dates of the preceding crop had a continuing, residual effect. In the two years that the effects of previous sowing dates were tested, there was usually least take-all in plots where the crop followed one sown very early or very late in the previous year, and most where it followed one sown at the end of September or in early October. These effects probably reflect differences in amounts of inoculum and in the rate of development of take-all decline.

Under intermediate deepwater conditions (0–80 cm) at Cuttack, India, direct sowing of the semi-tall rice variety Utkalprabha with 200–800 seeds/m2 produced similar yields in 1989 when heavy rainfall (105 mm) after sowing resulted in more than 50% germination, followed by the gradual accumulation of water in the field. However, in 1990, which was characterized by drought in the initial stages, leading to poor germination (35–40%), followed by a sudden accumulation of water, the crops sown at 200 seeds/m2 produced a significantly lower grain yield than those sown at higher seed rates. Higher seed rates increased plant height, tillers/m2 and panicles/m2 but the associated decrease in panicle weight resulted in similar grain yield production. There was no decrease in the yield of crops sown at all seed rates in 1989 and at > 400 seeds/m2 in 1990 when clonal tillers were removed after 60 days to transplant either the equivalent of or double the uprooted plot area. The loss due to decrease in panicles/m2 with the removal of clonal tillers was compensated for by the resulting increase in panicle weight. The crop planted from clonal tillers produced a significantly higher grain yield than that planted from seedlings of equivalent age raised in a nursery seed-bed with or without fertilizer application. Furthermore, the clonal-propagated crop tolerated simulated flash-flooding better at the early vegetative stage, measured by less tiller mortality and relatively higher dry matter production than in the crop raised from nursery seedlings. Therefore, planting with clonal tillers uprooted from a well-established direct-sown crop is recommended under excess water conditions.

Nitrate leaching from sheep-grazed pastures was measured over three winters between 1987 and 1990 at a lowland site in Wales. Losses from replicated plots of ryegrass/white clover (Lolium perenne/Trifoliwn repens) pasture that received no N fertilizer and from pure ryegrass pasture receiving 150–200 kg fertilizer-N/ha per year were determined using suction cup samplers. In Year 1, the average clover content of the grass/clover swards (April-September) was equivalent to 35% of total dry matter but declined during the course of the experiment and was only 4% in Year 3. Concentrations of nitrate and quantities of N leached from separate sampling positions exhibited a wide range of values and conformed to a log-normal distribution. In the first two winters, mean nitrate concentrations in soil water and quantities leached from grass/clover plots were greater than those from fertilized grass plots. In Year 3 the loss from the N-fertilized plots was higher and exceeded that from the grass/clover treatment. Total quantities of N leached during the three winters ranged from 6 to 33 kg N/ha from grass/clover plots and from 2 to 24 kg N/ha from fertilized grass plots. At no time did mean nitrate concentrations from either treatment exceed 11 mg N/litre. These values probably underestimate the true loss as the distribution of the main sets of water samplers did not provide adequate coverage of those areas of the plots where sheep congregated and which received high returns of excreta. These positions were characterized by particularly high leaching losses. Differences between losses from the two treatments and between years could only be partly explained by the decline in clover content and differences in stocking rates.

Four varieties of rice straw, MRl, MR71, MR84 and MR27, were separated into the following botanical fractions; inflorescence, stem, leaf blade and leaf sheath. The relative proportion of each fraction in whole straw was estimated and analysed for its chemical composition and in vitro digestibility (IVD). The composition and IVD of whole straw was then calculated. Cluster analysis showed that MRl was distinct from the other varieties. This could be attributed to a high proportion of stem (32·3%), high IVD (36·9%), low insoluble ash (4·0%) and a low silica content (2·7%) in the whole straw. The genetic distance between any two varieties ranged from 5·4 × 10-3 to 47·8 × 10-3 and was largest between MRl and the other varieties. Cluster analysis also showed that within a variety, the chemical composition and IVD of the stem, leaf blade and leaf sheath were similar, suggesting that sampling errors may be reduced if the inflorescence, which comprises 7–13% of the whole straw, is excluded when the material is analysed. Stepwise and canonical discriminant analyses indicated that the important variables to be measured when comparing straw varieties were IVD, crude protein and insoluble ash.

An assay was developed for assessing the potential for growth of shoot and root buds of white clover. This paper describes the technique and the experimental results on which it was based. The assay involved the incubation of nodes under standard conditions and the monitoring of bud activity at the nodes. Shoot buds were of primary interest but the assay was also used to investigate the activity of root primordia. Correlative influences were reduced by separating individual nodes and by removal of their subtending leaves and petioles. The optimal environmental conditions for incubation were: a constant temperature of 20 °C, a 24 h photoperiod of low irradiance (4 W/m2) and a high relative humidity (≥ 90%). The technique proved suitable for the monthly screening of 1000–2000 nodes sampled from field populations of white clover.

Thirty-two experiments examining the effects of the weight and within-row spacing of potato seed tubers on graded tuber yields of five varieties were conducted on eight sites from 1980 to 1985. A complex analysis technique was used to combine these data and estimate the optimum tuber planting densities for different ratios of seed cost to small (40–60 mm) and large (60–80 mm) ware value. The same technique could be applied to any other combination of seed cost, ware size and ware value.

The optimum tuber planting density decreased with increasing seed-tuber weight. Differences in optimum planting density between varieties were much greater with small (35 g) than with large (105 g) seed tubers and decreased as the cost of seed increased relative to the value of ware. As large ware became worth more than small ware the influence of increasing seed cost on the optimum density was reduced. As the value of large ware increased, net returns increased and the effect of seed cost on net returns was reduced. Mean tuber size decreased with increasing stem density at harvest and at the same stem density was lower in varieties producing more daughter tubers/stem. Changes of mean tuber size (μ) and the spread of yield across size grades (σ) with time were well described by parallel curves in different varieties. It is suggested that in future it may not be necessary to determine optimum tuber planting densities by complex experiments involving several seed-tuber weights and spacings. Instead μ and σ could be estimated from simple experiments and tuber spacings determined by comparison with control varieties.

Stolon nodes of white clover were sampled monthly for 18 months from continuously grazed (set stocked) and rotationally grazed pastures in New Zealand. Both pastures were stocked at 22·5 ewes plus lambs/ha. Axillary buds were classified into viability categories using an incubation technique. On average, 54% of nodes had non-viable or dormant buds, 25% had axillary buds that were viable but non-active and 0·1% bore reproductive buds. Although 21% of buds emerged, only 8·5% of nodes bore live branches. Bud activity was strongly inhibited during the spring period of both years and this inhibition could not be removed by incubation.

The greatest bud viability occurred at node 4 (from the apex) although 30–40% of the buds at this position were non-viable. This pattern of variation changed somewhat with season. Some viable buds were present at nodes more than 20 from the apex but these were of low vigour. Secondary stolons had different patterns of bud activity from those of primary stolons until they comprised more than 16 nodes.

A series of experiments to quantify the response of diverse potato genotypes to production and storage conditions in hot climates was carried out in 1984–87 at three sites in Peru, two of which were characterized by at least one hot season per year. Yield stability and field performance of seed tubers of six clones produced in situ, and stored in diffused light (DLS) or cold stored (CS), were compared with those of good quality seed tubers produced and stored in cool conditions. The yield potential of tubers produced in hot climates and grown as (i) a double crop per year with short DLS (1–4 months) (ii) a single crop per year with long DLS (c. 8 months) and (iii) a single crop per year with long CS (c. 8 months) was compared with that of seed tubers produced in cool conditions and introduced in successive crops.

The first replanting of tubers produced in hot climates resulted in lower yield than that of coolproduced tubers. Yield reductions in clones ranged from 30 to 93% when replanted within 4 months, from 10 to 100% when replanted after 7–10·5 months of DLS and from 0 to 82% after similar periods of CS. Reduced yield was often due to poor plant vigour and crop growth leading to inefficient interception of solar radiation. There was, however, no evidence that the efficiency of utilization of intercepted radiation for tuber dry matter production was influenced by storage treatment. Poor sprout growth at planting reduced yield considerably for late clones in double cropping. Low dry matter content of tubers produced in hot climates did not influence emergence rate, but was related to poor plant vigour in the first replant under warmer conditions. Replanting and storage in hot climates did not enhance the initial reduction of tuber dry matter, which was evident after the first hot season. Tuber yields continued to decline, particularly under single cropping with long DLS.

For the clones studied, CS would be a feasible storage method for potato production in hot climates if replanted tubers were used on an annual basis. For double cropping, short dormancy is essential; however, poor plant vigour results in additional yield reductions. Our data indicate that clonal selection for maximum production in hot climates should take into account the proposed or existing production and storage schemes.

The results of 20 experiments which compared seed from July-planted crops with seed from earlier planted (April-May) crops either grown locally or from certified seed-producing areas are reported. The effects of dates of planting and harvesting of late-planted seed crops and seed storage temperature were examined in eight experiments. In five experiments, the effect of seed rate was studied on two weights of seed-tubers in the July-planted crops.

Tubers of several cultivars from the July-planted crops ended their dormancy close to the normal planting time of ware crops, 2–4 months later than tubers from spring-planted crops grown either locally or in areas certified for seed production. Tubers from seed crops planted in July produced fewer mainstems in ware crops than tubers from spring-planted crops, but effects on stem density were usually too small to have any significant effect on foliar ground cover and consequently on tuber yield. Date of planting the seed crops in July and date of harvest of the seed-tubers had no effect on sprout or ware crop growth in Record or Wilja. Ware tuber yields were similar for the different seed sources in 12 out of 15 experiments, suggesting little difference in the performance of tubers from seed crops planted early and late in the season.

At equivalent seed rates, small seed-tubers (12·5 ± 2·5 g) from July-planted seed crops usually produced more stems and tubers and greater ware yields than larger tubers (37·5 ± 2·5 g). For the smaller seed, the results suggested optimum seed rates for ware tuber yields of c. 0·88, 0·88 and 0·59 t/ha for Wilja, Record and Saturna, respectively. These are substantially lower than those currently used in the UK. For the larger seed, rates of c. 2·64, 2·64 and 1·76 t/ha were adequate for Wilja, Record and Saturna, respectively.

Factors influencing potato virus X (PVX)-induced resistance to Alternaria solani in potato leaves are reported. The inhibition of fungal infection was dependent on the time interval between viral and fungal inoculations, fungal inoculum load, age of the host, virus strain and host cultivar. The maximum reduction in fungal infection was observed when a gap of 120 h occurred between PVX and A. solani inoculations. The clearest interactive effect in terms of reduced susceptibility to A. solani infection was seen at a fungal spore concentration of 1000 spores/ml in 20-day-old plants. This effect was gradually reduced following an increase in inoculum load and age of the host.

The extent of pathogen interaction was also affected by different virus strains and host cultivars. Maximum inhibition of fungal disease occurred with the PVX-ring spot strain. Of five potato cultivar tested, the maximum inhibition occurred in Kufri Chandramukhi and Kufri Sindhuri, the cultivars most susceptible to A. solani.

Twenty genotypes of chickpea, selected according to their performance in trials conducted during 1988/89 in saline fields having equal concentrations of SO4 and Cl, were studied during 1989/90 at Karnal, India. The Cl-dominated soils were divided into microplots with ECe values of 2, 4, 6 and 8 dS/m. Germination, although delayed at ECe 6 and 8 dS/m, was not inhibited up to ECe 8 dS/m. The sensitivity of all genotypes increased with plant growth and greater salinity. Plant growth was most adversely affected by a salinity of ECe 8 dSm. 100-seed weight was less affected by salinity than other yield components, namely, number of pods/plant, number of seeds/plant and seed yield/plant. Chlorides had a more severe toxic effect when present in domination than when they were balanced by an equal concentration of sulphates. Genotypes varied in their tolerance of Cl-dominated salinity. Cultivars ICCC32 and 1CCL86446 showed the most tolerance to Cl-salinity, having > 0·75 Mean Tolerance Index values for all eight characters, followed by ICC5003, ICC10575, ICC12908 and ICC12926. ICC12928 had the highest mean tolerance index value for number of pods/plant and highest threshold value and ICC4953 the highest mean tolerance index for 100-seed weight, indicating their better tolerance at flowering and maturity respectively. Therefore, crossing genotypes of the latter with the former group should produce some recombinants with improved tolerance to salinity at all growth stages and at maturity.

Results are reported from four carrot experiments (1987/88) to measure edge effects in crops of different ages grown in beds. A wide range of cultivars, densities and contrasting row-spacing systems was used to determine how effectively these edge effects could be eliminated by increasing the seeding rate in the outer rows relative to that in the inner ones.

Edge effects were first significant between 50 and 100 days after sowing. Where each row was sown with the same quantity of seed, carrots in the outer rows were 1·5 to 2·0 times heavier than those in the inner rows.

Increasing the seeding rate in the outer rows to between 1·5 to 3·0 times that in the inner rows, whilst holding the overall density constant, resulted in equal mean root weight in all rows. There was no evidence of alternate large and small plants in successive rows across the bed, as has been suggested in the literature.

A previously published spacing model gave large systematic deviations when fitted to these data, but this was rectified by a simple modification to the model, which did not entail the incorporation of more parameters.