The physiology and genetic of high temperature effects on growth and development of sorghum

Abstract

Sorghum (Sorghum bicolor L. Moench) is a major cereal crop in (sub)-tropical and semi-arid dryland production areas. Crops are often exposed to high temperatures throughout the growing season, which can have detrimental effects on grain yield. Climate change is likely to increase the frequency, duration, and intensity of high temperature stress. However, there is currently limited understanding of the presence of genotypic differences in high temperature tolerance in sorghum, the physiological processes that underpin high temperature tolerance, and the risk to production of high temperature occurrence in the sorghum belt of north-east Australia. Such information is essential if crop improvement programs are to develop genotypes that are resilient to future climates. Therefore, the objectives of this study were to (i) explore genetic variability for the effects of high temperature on crop growth and development, (ii) quantify the response of in vitro pollen germination and seed set to high temperatures, (iii) determine the development stage that is most sensitive to high temperature stress, and (iv) quantify the potential role of management and genetics in minimising the adverse effects of high temperature stress on grain yield in NE Australia. A set of 18 diverse sorghum genotypes was used to explore genotypic variation in high temperature tolerance. Plants were grown in a controlled environment facility under four conditions that ranged in maximum temperatures from 32˚C to 38˚C, but had similar minimum temperature at 21˚C and relative humidity throughout the day ranged between 52 to 94%. Results showed that high temperature significantly accelerated development and reduced plant height, but had no consistent effect on leaf area per plant. In contrast, it significantly reduced in vitro pollen germination percentage and seed set percentage of all genotypes. Pollen germination percentage and seed set percentage were highly correlated, for subset of six genotypes, seed set percentage in controlled environments correlated well with that in field experiments that experienced similar temperatures around anthesis. These results indicated that seed set percentage could be a useful phenotypic screen for high temperature tolerance. Genotypic differences in pollen germination and seed set under high temperature were associated with differences in the threshold temperature and in the responsiveness of pollen germination and seed set to temperatures above that threshold. However, an experiment on a temperature gradient plate found little evidence of genotypic differences in cardinal temperatures for pollen germination. To identify whether observed genotypic differences in high temperature tolerance were associated with differences in the timing or the duration of the most sensitive stage for high temperature stress, two genotypes with contrasting high temperature tolerance were grown at high and optimum temperature in a controlled environment, with some plants transferred between temperature regimes for a period of five days at regular intervals between the stages of 18 fully expanded leaves and start of grain filling, both genotypes took on average 17 days to anthesis from the start of the transfers between the two rooms. Results indicated that pollen germination and seed set were most affected by high temperature stress during a 15-day period around anthesis that started soon after the flag leaf stage. Reduced tolerance to high temperature stress was predominantly associated with an increased sensitivity of genotype to high temperature during the critical period around flowering rather than duration of heat stress period. In addition, results implied that the effect of high temperature stress on seed set and pollen germination was cumulative. The insights gained from all these experiments were used to develop a high temperature stress module within the APSIM-Sorghum crop growth simulation model. The model was used to simulate the effects of genotypic differences in high temperature tolerance on grain yield of sorghum at six locations in the sorghum belt of NE Australia, using 59 years of historical weather data at each location. In general, the most prevalent occurrence of high temperature stress around anthesis was occasional individual days with maximum temperatures between 36-38˚C. For genotypes with a threshold temperature of 36˚C, an increase in high temperature tolerance above the threshold was important to minimise adverse effects of high temperatures on grain yield. In contrast, a threshold of 38˚C was sufficient in itself to minimise negative effects of high temperature stress on grain yield. Because an increase in temperature in the sorghum belt, associated with climate change, could negate the effects of an increased threshold temperature, selection for increased tolerance above the threshold is important. In conclusion, genotypic differences in the response of pollen germination and seed set to high temperatures were identified for sorghum, but expected increases in temperature associate with climate change could negate the advantages of the most tolerant genotypes. Screening for seed set percentage in plants subjected to high temperatures around anthesis could provide a phenotypic screen for high temperature tolerance that could be implementation in crop improvement programs.