P. V. Vara Prasad

Temperature variability and the yield of annual crops

Abstract Global production of annual crops will be affected by the increases in mean temperatures of 2–4°C expected towards the end of the 21st century. Within temperate regions, current cultivars of determinate annual crops will mature earlier, and hence yields will decline in response to warmer temperatures. Nevertheless, this negative effect of warmer temperatures should be countered by the increased rate of crop growth at elevated atmospheric CO2 concentrations, at least when there is sufficient water. Of more importance for the yield of annual seed crops may be changes in the frequency of hot (or cold) temperatures which are associated with warmer mean climates. The objectives of this paper are to review evidence for the importance of variability in temperature for annual crop yields, and to consider how the impacts of these events may be predicted. Evidence is presented for the importance of variability in temperature, independent of any substantial changes in mean seasonal temperature, for the yield of annual crops. Seed yields are particularly sensitive to brief episodes of hot temperatures if these coincide with critical stages of crop development. Hot temperatures at the time of flowering can reduce the potential number of seeds or grains that subsequently contribute to the crop yield. Three research needs are identified in order to provide a framework for predicting the impact of episodes of hot temperatures on the yields of annual crops: reliable seasonal weather forecasts, robust predictions of crop development, and crop simulation models which are able to quantify the effects of brief episodes of hot temperatures on seed yield.

Effects of season-long high temperature growth conditions on sugar-to-starch metabolism in developing microspores of grain sorghum (Sorghum bicolor L. Moench)

High temperature stress-induced male sterility is a critical problem in grain sorghum (Sorghum bicolor L. Moench) that significantly compromises crop yields. Grain sorghum plants were grown season-long under ambient (30/20°C, day-time maximum/night-time minimum) and high temperature (36/26°C) conditions in sunlit Soil-Plant-Atmospheric-Research (SPAR) growth chambers. We report data on the effects of high temperature on sugar levels and expression profiles of genes related to sugar-to-starch metabolism in microspore populations represented by pre- and post-meiotic “early” stages through post-mitotic “late” stages that show detectable levels of starch deposition. Microspores from high temperature stress conditions showed starch-deficiency and considerably reduced germination, translating into 27% loss in seed-set. Sugar profiles showed significant differences in hexose levels at both “early” and “late” stages at the two temperature regimes; and most notably, undetectable sucrose and ∼50% lower starch content in “late” microspores from heat-stressed plants. Northern blot, quantitative PCR, and immunolocalization data revealed a significant reduction in the steady-state transcript abundance of SbIncw1 gene and CWI proteins in both sporophytic as well as microgametophytic tissues under high temperature conditions. Northern blot analyses also indicated greatly altered temporal expression profiles of various genes involved in sugar cleavage and utilization (SbIncw1, SbIvr2, Sh1, and Sus1), transport (Mha1 and MST1) and starch biosynthesis (Bt2, SU1, GBSS1, and UGPase) in heat-stressed plants. Collectively, these data suggest that impairment of CWI-mediated sucrose hydrolysis and subsequent lack of sucrose biosynthesis may be the most upstream molecular dysfunctions leading to altered carbohydrate metabolism and starch deficiency under elevated growth temperature conditions.

INFLUENCE OF INTEGRATED USE OF FARMYARD MANURE AND INORGANIC FERTILIZERS ON YIELD AND YIELD COMPONENTS OF IRRIGATED LOWLAND RICE

ABSTRACT Integrated use of organic and inorganic fertilizers can improve crop productivity and sustain soil health and fertility. The present research was conducted with an objective to study the influence of application of farmyard manure in combination with three levels of chemical fertilizers [80 : 40 : 30, 120 : 60 : 45 and 160 : 80 : 60 kg N, P2O5 and K2O ha−1, respectively] on yield and yield components of irrigated lowland rice. The experiment was conducted in split-plot design during the rainy season with and without farmyard manure as main plots and three inorganic fertilizer levels as subplots. Data on grain and straw yield, number of tillers, panicle length, filled grains per panicle, 1000-grain weight and nutrient uptake were collected. The results showed that application of farmyard manure at 10 t ha−1 increased grain yield of rice by 25% compared to no farmyard manure control. Similar observations were also made on straw yield, tiller number, filled grains per panicle, and 1000-grain weight. There were significant interactions between farmyard manure and inorganic fertilizer treatments. The beneficial effects of application of farmyard manure were not enhanced at increased rates of application of inorganic fertilizers. However, the highest grain yield of rice was obtained with the application of farmyard manure at 10 t ha−1 and inorganic fertilizer at 120 : 60 : 45 kg N, P2O5 and K2O ha−1. The increased grain yield was due mainly to increased nutrient uptake and number of tillers, filled grains per panicle and 1000-grain weight.

Crop Responses to Elevated Carbon Dioxide and Interaction with Temperature: Grain Legumes

SUMMARY Atmospheric carbon dioxide concentration ([CO2]) and other greenhouse gases have risen over the past few decades. If this continues, it could indirectly lead to increases in global temperature. Responses of grain legume crops (soybean, dry bean, peanut and cowpea) to elevated [CO2] and interactions with temperature are summarized. Our research shows that, in the absence of biotic (pests, diseases and weeds) or abiotic (temperature, water and nutrients) stresses, elevated [CO2] will increase yield due to increased photosynthesis and growth. However, at above optimum temperatures, the beneficial effects of elevated [CO2] are more than offset by negative effects of temperature on yield and yield-components, leading to lower seed yield and poor seed quality Future research should focus on developing genetic and agronomic crop management practices to improve crop productivity under changing climates.

Effect of high air and soil temperature on dry matter production, pod yield and yield components of groundnut

Groundnuts (Arachis hypogaea L.) grown in the semi-arid tropics are commonly exposed to air and soil temperatures above 35 °C during the reproductive period causing significant yield losses. The objectives of this study were to determine: (i) whether effects of high air and/or high soil temperature in two contrasting cultivars were similar; (ii) the effects of the timing of imposition of high air and soil temperature; (iii) the effects of high air, high soil and both stresses combined on yield and yield components; and (iv) whether the effects of high air and high soil temperature were additive or multiplicative. Plants were grown at optimum and ambient soil temperature from planting until start of podding at 45 d after planting (DAP) in Experiment 1, and until start of flowering at 28 DAP in Experiment 2. Thereafter, plants of each cultivar were exposed to a factorial combination of two air temperatures (optimum: 28°/22 °C and high: 38°/22 °C) and two soil temperatures (ambient: 26°/24 °C and high: 38°/30 °C) until final harvest at 90 DAP. The effects of high air and high soil temperatures imposed from start of flowering or podding were similar. Exposure to high air and/or high soil temperature significantly reduced total dry matter production, partitioning of dry matter to pods, and pod yields in both the cultivars. High air temperature had no significant effect on total flower production but significantly reduced the proportion of flowers setting pegs (fruit-set) and hence fruit numbers. In contrast, high soil temperature significantly reduced flower production, the proportion of pegs forming pods and 100 seed weight. The effects of high air and soil temperature were mostly additive and without interaction.

High night temperature decreases leaf photosynthesis and pollen function in grain sorghum

High temperature stress is an important abiotic stress limiting sorghum (Sorghum bicolor (L.) Moench) yield in arid and semiarid regions. Climate models project greater increases in the magnitude of night temperature compared with day temperature. We hypothesise that high night temperature (HNT) during flowering will cause oxidative damage in leaves and pollen grains, leading to decreased photosynthesis and seed-set, respectively. The objectives of this research were to determine effects of HNT on (1) photochemical efficiency and photosynthesis of leaves, and (2) pollen functions and seed-set. Sorghum plants (hybrid DK-28E) were exposed to optimum night temperature (ONT; 32:22°C, day maximum: night minimum) or HNT (32:28°C, day maximum:night minimum) for 10 days after complete panicle emergence. Exposure to HNT increased thylakoid membrane damage and non-photochemical quenching. However, HNT decreased chlorophyll content, quantum yield of PSII, photochemical quenching, electron transport rate and photosynthesis of leaves as compared with ONT. Exposure to HNT increased the reactive oxygen species (ROS) level of leaves and pollen grains. Lipid molecular species analyses in pollen grains showed that HNT decreased phospholipid saturation levels and altered various phospholipid levels compared with ONT. These changes in phospholipids and greater ROS in pollen grains may be responsible for decreased pollen function, leading to lower seed-set.

Response of floret fertility and individual grain weight of wheat to high temperature stress: sensitive stages and thresholds for temperature and duration

Short episodes of high temperature (HT) stress during reproductive stages of crop development cause significant yield losses in wheat (Triticum aestivum L.). Wheat plants of cultivar Chinese Spring were grown at various temperature regimes at several stages of reproductive development for different durations. The objectives of this research were to (i) identify the stage(s) most sensitive to HT stress during reproductive development, and (ii) determine threshold temperature and duration of HT stress that decrease floret fertility and individual grain weight. Two periods (first at 8-6 days before anthesis and second at 2-0 days before anthesis) during reproductive development were most sensitive to short episodes (2 or 5 days) of HT stress, causing maximum decreases in floret fertility. Short episodes (5 days) of mean daily temperatures >24°C imposed at start of heading quadratically decreased floret fertility, with the values reaching close to 0% around mean daily temperature of 35°C; and floret fertility and individual grain weight decreased linearly with increasing duration (in the range from 2 to 30 days) of HT stress when imposed at start of heading or start of grain filling respectively. HT stress caused morphological abnormalities in pollen, stigma and style. The combination of lower floret fertility (leading to decreased grain numbers) and decreased individual grain weights can cause significant decreases in grain yield. Further research to search for genetic variability for these traits and use them in breeding programs to develop tolerant genotypes that can provide yield stability under current and future climates is warranted.

Rubisco activase and wheat productivity under heat-stress conditions

Rubisco activase (RCA) constrains the photosynthetic potential of plants at high temperatures (heat stress). Endogenous levels of RCA could serve as an important determinant of plant productivity under heat-stress conditions. Thus, in this study, the possible relationship between expression levels of RCA and plant yield in 11 European cultivars of winter wheat following prolonged exposure to heat stress was investigated. In addition, the effect of a short-term heat stress on RCA expression in four genotypes of wheat, five genotypes of maize, and one genotype of Arabidopsis thaliana was examined. Immunoblots prepared from leaf protein extracts from control plants showed three RCA cross-reacting bands in wheat and two RCA cross-reacting bands in maize and Arabidopsis. The molecular mass of the observed bands was in the range between 40 kDa and 46 kDa. Heat stress affected RCA expression in a few genotypes of wheat and maize but not in Arabidopsis. In wheat, heat stress slightly modulated the relative amounts of RCA in some cultivars. In maize, heat stress did not seem to affect the existing RCA isoforms (40 kDa and 43 kDa) but induced the accumulation of a new putative RCA of 45-46 kDa. The new putative 45-46 kDa RCA was not seen in a genotype of maize (ZPL 389) that has been shown to display an exceptional sensitivity to heat stress. A significant, positive, linear correlation was found between the expression of wheat 45-46 kDa RCA and plant productivity under heat-stress conditions. Results support the hypothesis that endogenous levels of RCA could play an important role in plant productivity under supraoptimal temperature conditions.

DORMANCY IN YAMS

SUMMARY The tubers of yam, principally those of Dioscorea rotundata (white Guinea yam) and D. alata (water or greater yam), are important staple foods and sources of carbohydrate in West Africa. Yams are grown in diverse environments ‐ from the high-rainfall forest zone on the coast to the seasonally arid savannas of West Africa, that is in situations in which the duration and the timing of the onset of the growing season vary appreciably. Dormancy in both underground and aerial tubers of the Dioscoreaceae is an important adaptive mechanism that helps to maintain organoleptic quality during storage and also ensures that tubers germinate at the start of the growing season. Plant breeders are especially keen to manipulate the duration of the dormant period in order to synchronize growth periods and, therefore, to produce more than one generation per year. The control of tuber dormancy, however, is poorly understood. This review examines critically those factors that aAect tuber initiation, dormancy and sprouting, and makes recommendations for future priorities in research.

Wheat leaf lipids during heat stress: I. High day and night temperatures result in major lipid alterations

Understanding how wheat (Triticum aestivum L.) plants under high temperature (HT) regulate lipid composition is critical to developing climate-resilient varieties. We measured 165 glycerolipids and sterol derivatives under optimum and high day and night temperatures in wheat leaves using electrospray ionization-tandem mass spectrometry. Levels of polar lipid fatty acyl chain unsaturation were lower in both heat-tolerant genotype Ventnor and susceptible genotype Karl 92 under HT, compared with optimum temperature. The lower unsaturation was predominantly because of lower levels of 18:3 acyl chains and higher levels of 18:1 and 16:0 acyl chains. Levels of 18:3-containing triacylglycerols increased threefold/more under HT, consistent with their possible role in sequestering fatty acids during membrane lipid remodelling. Phospholipids containing odd-numbered or oxidized acyl chains accumulated in leaves under HT. Sterol glycosides (SG) and 16:0-acylated sterol glycosides (ASG) were higher under HT than optimum temperatures. Ventnor had lower amounts of phospholipids with oxidized acyl chains under HT and higher amounts of SG and 16:0-ASG than Karl 92. Taken together, the data demonstrate that wheat leaf lipid composition is altered by HT, in which some lipids are particularly responsive to HT, and that two wheat genotypes, chosen for their differing physiological responses to HT, differ in lipid profile under HT.