Improving rice productivity under water deficit through a comprehensive assessment of adaptive physiological traits

Global climate change, especially in the view of severe changes in precipitation pattern, is posing a great threat for the agricultural sector, mainly in crops with large water consumption like rice. Major advances have occurred in rice cultivation system to grow rice with lower water input. But the lower yields compared with traditional puddle cultivation is the major disadvantage. It is estimated that there is a need to produce 50% more rice by 2050. To meet this challenge, we need rice varieties with higher yield potential under limited water supply. Therefore, it is important to identify physiological traits to improve drought tolerance and productivity of rice under water limitation. The main goal of this thesis is to compare the physiological basis of drought adaptation among rice genotypes and between rice and wheat.

In an initial experiment, rice cultivars adapted to puddle, aerobic and upland ecosystems were compared for their response to drought stress imposed at critical growth stages of the crop.  All cultivars investigated showed significant reduction in yield under drought and the largest reduction was noticed when drought stress coincided with flowering. Comparatively, the aerobic rice cultivar showed higher tolerance irrespective of stage of drought occurrence than lowland and upland cultivars. Upon analysis of the results, reduction in leaf area, increased spikelet sterility and reduced source capacity were seen as the major reasons for the observed yield loss when drought occurred during vegetative, reproductive and grain filling phases, respectively.  At all growth stages, maintenance of photosynthesis contributed to higher yield in the drought tolerant aerobic cultivar.

For a specific understanding of drought tolerance at one particular stage, two contrasting rice genotypes were again compared to capture the drought response during grain filling by using a high-throughput phenomics platform and a metabolomics approach. The outcome revealed the relevance of maintaining a high source and its transport efficiency for a high productivity. The maintenance of transport of assimilates facilitated an enhanced accumulation of carbohydrates and secondary metabolites in the tolerant rice cultivar. Phenylproponoid pathway metabolites which are involved in protective mechanisms against reactive oxygen species (ROS) were the most prominent changes among differentially expressed metabolites.

Besides within rice, comparison was also made between two C3 cereals, rice and wheat. Wheat is known to be drought adaptive and consumes less water than rice. Rice was more susceptible to drought-induced oxidative damage than rice. Although scavenging capabilities were similar between species, rice was not able to detoxify the ROS sufficiently. In contrast, wheat possessed better activation of a photo-protection mechanism which prevented ROS production itself. The wheat cultivar also exhibited a significant increase in osmotic adjustment, epicuticular wax content, with a lower leaf temperature. These protection mechanisms in wheat led to a higher photosynthetic efficiency and capacity than in rice. In line with this, there was a higher expression of photosynthetic genes in wheat than in rice.

Overall findings in this thesis suggest that although certain mechanisms are valuable in the aerobic rice cultivar, it is not sufficient to withstand drought. The outcome of the comparative analysis between species clearly indicate the need for introgressing a few important physiological traits to grow rice like wheat.