Disaggregating the Effect of Drought and Heat Stress During Flowering on Spikelet Fertility in Rice

Due to rice's wide geographic distribution, extending from 50°N to 35°S, rice is forecasted to be the most vulnerable crop to warming global climates. Previous studies have predicted lower rice yields and increasing rice yield variability due to higher frequencies of heat stress events, and a higher variability in precipitation patterns due to global warming. As such, understanding the effects of drought and heat stress intensity and frequency on rice yields is of upmost importance to feeding the growing global population.;Given that drought and high-temperature stress often occur together, it is essential to disaggregate the two individual stressors and examine possible interactions by modeling them simultaneously. A reliable and robust temperature and drought threshold inducing rice spikelet sterility under field conditions involving cultivars with highly varying phenology has been a major limitation for devising adaptation strategies for rice breeders and to estimate heat stress and drought impacts by the climate and crop modeling communities. It is in this spirit that this study was designed. This study examines and quantifies the individual marginal effects of drought and heat stress occurring simultaneously under field conditions by means of a regression analysis. Moreover, a cardinal threshold is found for drought as well as for heat in relation to spikelet fertility. This study utilizes canopy temperature threshold instead of the more commonly found ambient temperature thresholds in the literature. The canopy temperature threshold is more precise since canopy temperature has a more direct connection to spikelet temperature and therefore spikelet fertility than ambient temperature. Another contribution of this thesis is from modeling the relationship between ambient temperature, drought and canopy temperature as a recursive system which will allow future research to estimate the effects of changes in global ambient temperature to spikelet fertility. The results of this study found that exposure to a canopy temperature over a threshold of 33°C causes a severe increase in rice spikelet sterility. An estimated drought threshold of 12 kPa (kilopascal) was found to be the most detrimental to spikelet fertility. This is important given two of the largest pressures facing future rice production are heat and drought stress. Another related finding is that in the presence of heat stress the availability of adequate water during flowering can decrease sterility by 14.16 %. The results of this study, which are variety specific, can allow for understanding the properties of combined heat and drought stress, which can provide information to rice breeders on how to promote reproductive-stage drought tolerance through improved germplasm and attempt to help mitigate the effects of a global climate change.