Drought‐Induced Changes in Physiological Responses and Starch Properties at the Booting Stage in Thai Rice

ABSTRACT

Rice ( Oryza sativa L. ) is a staple food for half of the world's population, and increasing attention has been given to its starch properties in relation to health concerns such as diabetes. Drought stress can limit rice production, particularly in key producing countries including China, India, and Thailand. This study examined the effects of drought on physiological traits and starch properties in five Thai rice varieties that differ in apparent amylose content (AAC): “RD43” (intermediate AAC), “RD69” (low AAC), “Suphanburi 1 (SPR1)” (high AAC), “Hawm Daeng (HD; photoperiod insensitive)” (high AAC), and “Pathumthani 1 (PTT1)” (low AAC). Plants were subjected to 6 days of drought stress at the booting stage, followed by recovery. Drought stress negatively affects physiological responses, such as Soil–Plant Analysis Development (SPAD) values and vegetation indices. Principal component analysis revealed distinct variety‐specific responses and recovery capacities, indicating clear genotype‐dependent differences in drought tolerance. Yield components, including grain weight, total grain weight per plant, and the number of filled grains, were also negatively affected. In addition, drought altered starch properties, particularly AAC. RD43 showed the greatest tolerance to drought stress based on physiological responses, but exhibited the largest change in AAC, suggesting strong drought sensitivity of starch traits. By contrast, SPR1 was more sensitive in physiological responses but maintained relatively stable AAC. We also found a significant correlation between physiological parameters (SPAD, NDVI, SR, G, and SRPI) and AAC. Among these, SRPI showed the strongest relationship with AAC, and a significant correlation coefficient was detected in all cultivars tested ( r  = 0.496–0.685, depending on genotype). Lower SPAD values and reflectance indices (NDVI, SR, G, and SRPI) measured during the booting stage were consistently associated with reduced AAC in the final grain, indicating that declines in these parameters correspond to reductions in grain quality. These findings demonstrate how Thai rice varieties differ in their physiological and starch responses to drought and highlight the potential of measuring SPAD and reflectance indices at the booting stage as indicators associated with AAC under drought stress.