Physiological and Metabolic Responses of Rice to Reduced Soil Moisture: Relationship of Water Stress Tolerance and Grain Production

Abstract

Access to adequate irrigation resources is critical for sustained agricultural production, and rice, a staple cereal grain for half of the world population, is one of the biggest users of irrigation. To reduce water use, several water saving irrigation systems have been developed for rice production, but a reliable system to evaluate cultivars for water stress tolerance is still lacking. Here, seven rice cultivars that have diverse yield potential under water stress were evaluated in a field study using four continuous irrigation regimes varying from saturation to wilting point. To understand the relationship between water stress and yield potential, the physiological and leaf metabolic responses were investigated at the critical transition between vegetative and reproductive growth stages. Twenty-nine metabolite markers including carbohydrates, amino acids and organic acids were found to significantly differ among the seven cultivars in response to increasing water stress levels with amino acids increasing but organic acids and carbohydrates showing mixed responses. Overall, our data suggest that, in response to increasing water stress, rice cultivars that do not show a significant yield loss accumulate carbohydrates (fructose, glucose, and myo-inositol), and this is associated with a moderate reduction in stomatal conductance (gs), particularly under milder stress conditions. In contrast, cultivars that had significant yield loss due to water stress had the greatest reduction in gs, relatively lower accumulation of carbohydrates, and relatively high increases in relative chlorophyll content (SPAD) and leaf temperature (Tm). These data demonstrate the existence of genetic variation in yield under different water stress levels which results from a suite of physiological and biochemical responses to water stress. Our study, therefore, suggests that in rice there are different physiological and metabolic strategies that result in tolerance to water stress that should be considered in developing new cultivars for deficit irrigation production systems that use less water.

Response of U.S. Rice Cultivars Grown under Non-Flooded Irrigation Management

“…In 2016, canopy temperature was measured in the same method as in 2014 but at 85 DAE. On this same date in 2016, a SPAD 502 Pulse (Spectrum Technologies, Aurora, IL, USA) was used to measure the chlorophyll content of the most recent fully emerged leaf as explained in Barnaby et al [26].…”

“…There has been worldwide interest in evaluating rice germplasm for production under reduced irrigation systems, particularly in areas where access to irrigation resources is limited and/or incidence of drought is expected to increase [29][30][31][32]. However, such efforts have been more limited in the U.S. where rice research has historically been focused on achieving higher yield potential without production constraints [26,30].With the availability of whole genome sequencing there is a greater understanding of the genetic population structure found in cultivated rice [33,34]. Most U.S. cultivars trace to the tropical japonica (TRJ) subpopulation [35], whereas most global rice acreage is planted with cultivars derived from the indica sub-population [36].…”

“…In addition, germplasm identified to be tolerant to drought at a particular stage may be different from that which is tolerant to a range of drought timings and severities [38]. In general, biochemical, physiological, and biomass production traits were found to be related to higher grain yield under drought stress, and a few genebank accessions were identified as being suitable for rice breeding for drought tolerance [26,29,[40][41][42]. However, no consistent relationship was observed between component traits (such as root dry weight, drought response index, or stomatal conductance) and potential GY.…”

“…This may be due to the complexity of traits that respond to drought, such as plant height, flowering time, flowering duration, loading and unloading rates of xylem and phloem, that may have positive or negative effects on GY depending on the timing, severity, and duration of the drought [43]. Recently it has been demonstrated that crops which accumulate carbohydrates in leaves and maintain stomatal conductance under water stress conditions sustain GY production [26,44,45]. In other reports, robust quantitative trait loci (QTL) and traits with high heritability under water stress conditions have been identified, indicating that improvement of such a complex trait is possible through breeding [46,47].…”

“…There has been worldwide interest in evaluating rice germplasm for production under reduced irrigation systems, particularly in areas where access to irrigation resources is limited and/or incidence of drought is expected to increase [29][30][31][32]. However, such efforts have been more limited in the U.S. where rice research has historically been focused on achieving higher yield potential without production constraints [26,30].…”