Rice responses to rising temperatures - challenges, perspectives and future directions

Phenotypic plasticity in overcoming heat stress-induced damage across hot tropical rice-growing regions is predominantly governed by relative humidity. Expression of transpiration cooling, an effective heat-avoiding mechanism, will diminish with the transition from fully flooded paddies to water-saving technologies, such as direct-seeded and aerobic rice cultivation, thus further aggravating stress damage. This change can potentially introduce greater sensitivity to previously unaffected developmental stages such as floral meristem (panicle) initiation and spikelet differentiation, and further intensify vulnerability at the known sensitive gametogenesis and flowering stages. More than the mean temperature rise, increased variability and a more rapid increase in nighttime temperature compared with the daytime maximum present a greater challenge. This review addresses (1) the importance of vapour pressure deficit under fully flooded paddies and increased vulnerability of rice production to heat stress or intermittent occurrence of combined heat and drought stress under emerging water-saving rice technologies; (2) the major disconnect with high night temperature response between field and controlled environments in terms of spikelet sterility; (3) highlights the most important mechanisms that affect key grain quality parameters, such as chalk formation under heat stress; and finally (4), we model and estimate heat stress-induced spikelet sterility taking South Asia as a case study. In tropical rice growing regions higher temperatures know to negatively affect sensitive reproductive processes under fully flooded conditions can potentially induce greater damage under scenarios wherein water-saving technologies would be adopted. The rapid increase in night temperature compared to the day temperature both at the regional and global scale necessitates the need to understand the differential responses of rice, with the trend projected to continue in the future. Our review provides an analysis of the above aspects and in addition the need to establish a reliable and high throughput marker for estimating pollen viability; possible routes inducing yield and quality losses due to poor grain filling under increasing temperatures are highlighted. Finally we quantify the heat stress induced reduction in spikelet fertility taking South Asia as a case study and provide future research directions that would help minimize heat stress induced damage.