Differential and dynamic response of rice variety uma under varying drought stress regimes

Abstract

Rice has the evolutionary advantage of being semi-aquatic, making it one of the world's most important crops. As a result, it has fewer water-scarcity responses and is especially vulnerable to drought stress. Climate change has put rice production in jeopardy today. According to recent studies, rice yield decreases as result of climate change such that when drought frequency and rainfall pattern changes, rice output is lowered. Rice is the principal staple grain grown in Kerala, with around 600 types used across the state's paddy fields. Uma (MO 16), developed by Kerala Agricultural University's Rice Research Station, Moncompu, is one of the most popular rice varieties in Kerala, which is currently planted in more than 60% rice growing areas in the state. It also has adaptability towards wide range of environmental conditions as well, even though environmental variations limit the potential yield of variety Uma. In this context, the present study on differential and dynamic response of variety Uma under drought stress at vegetative stage and its impact on stress responses of progeny, were attempted with the main objective of identifying potential drought stress specific biomarkers representing a trend in metabolite dynamics across generations. This approach was aimed to demonstrate in a simple way that a natural drought stress exposure can influence stress responses of rice variety, Uma in the present as well as future generations, with an aim of identification and selection of potential traits for inclusion in crop improvement programmes. The study was carried out at the Department of Plant Physiology, College of Agriculture, Vellanikkara, Thrissur as two experiments- a drought stress imposition study in the net house facility and a laboratory study in the next generation. As part of the first experiment, drought stress (60% field capacity; FC) was imposed to rice variety, Uma at vegetative stage by gravimetric approach, simultaneously maintaining a nonstressed control at 100% FC by regular watering. The experiment was laid out in completely randomized design in customized rain-out shelter like structures in the net house. Physiological parameterswere assessed immediately after stress exposure for assessment of stress damage and it was noted that there was a significant reduction in chlorophyll content (36.2%), membrane thermostability (78.5%) and relative water content (23.5%) under stress in comparison to non-stressed control tissues, ensuring effective stress exposure. Biochemical markers such as proline and malondialdehyde (MDA) estimated immediately after stress and after recovery across different phenophases, revealed a differential pattern in their regulation. The levels of both the biomarkers were significantly higher (14% and 70.7% respectively) under stress which dropped drastically at panicle initiation stage, followed by an increase in the grain filling stage. However, it was interesting to note that correspondingly both the biomarker metabolites exhibited lesser accumulation (95.1% and 25.2% respectively) in comparison to the non-stressed control plants in grain filling stage. The differential dynamics of proline as well as MDA over different phenophases when related to the protein levels estimated at panicle initiation stage, revealed that there was no concomitant recovery in protein levels even after stress recovery. The non-stressed control and stressed plants were raised to maturity and harvested seeds were used for the second experiment for assessing the stress memory response in the next generation. Yield attributes assessed also showed significant difference between the stressed and non-stressed plants. The second experiment to assess the stress memory response aimed at comparative stress response assessment under salinity as well as osmotic stresses at seedling stage in the next generation. In this context, a standardization experiment was carried out using different concentrations of NaCl and polyethylene glycol (PEG 6000) to impose salinity and osmotic stresses respectively, to uniformly germinated seedlings of rice variety, Uma, to ascertain sub-lethal stress levels. From the standardization experiment, sub-lethal levels of salt (100 mM NaCl) and osmotic (-4 bar) stresses were arrived at based on the percent reduction in growth over control. The progenies of the first experiment along with a lot of fresh seeds were exposed to stress using the standardized doses of salt as well as osmotic stresses. Priorto stress exposure, germination speed and the vigour index of the seedlings were estimated, which led to the discovery of two classes of stress exposed plants of first experiment with their seeds differing in vigour viz. moderate and high vigour. Four classes of seed materials- control and stressed (high and moderate vigour) seed sets from first generation and a fresh set of seeds of rice variety, Uma were used for the stress response experiment using the sub-lethal stress levels ascertained. The high vigour seedlings from the stressed plants showed better growth under salt stress compared to control set of seedlings from first generation as evidenced by lesser percent reduction in growth as well as recovery responses. Subsequently, biochemical marker MDA was estimated in the different treatments revealing that moderate as well as high vigour seedlings of the stress class accumulated lesser (64.5% and 18.6% respectively) MDA levels in comparison to the non-stressed control, with a consistent pattern under salinity stress. GC-MS/MS analysis was performed using shoot tissues of the seedlings across the different treatments, with an attempt to unravel any possible stress responsemechanism. GC spectra displayed a number of highly abundant as well as uniquely present compounds in the plants. Diphenyl sulfone, a potent sulfone derivative with antioxidant activity was differentially detected in all classes of plant materials used for the stress response study. There were corroborative inferences from the fold changes in diphenyl sulfone as well as the MDA contents, wherein there were proportional variations in the two especially in NaCl induced salinity stress in all treatments, indicating its probable role in maintaining lower MDA levels in the progenies of plants that were subjected to a prior drought stress exposure in the previous generation. A round of stress exposure at vegetative stage in rice variety, Uma influenced several morpho-physiological and biochemical parameters at different stages of the crop. There was an indication of the transmission of stress induced signal even in the next generation as evidenced by profuse growth and lower MDA contents under different stresses at seedling stage. It will be rewarding to observe the stress response/acclimation behaviour of Uma, under different abiotic stress exposure regimes across different phenophases.