Chiang Mai Journal of Science

     Khao Da wk Mali 105 (KDML105) and Pathum Thani 1 (PTT1) are two popular rice cultivars in Thailand; KDML105 is known for its unique texture and fragrance and PTT1 for its high yield and ability to be grown year-round in various conditions. This research aimed to (1) study the morphological and physiological processes of two rice cultivars KDML105 and PTT1 under six levels of soil moisture including 20, 30, 40, 50, 60, and 70% available water holding capacity (AWHC) in clay soil and sandy clay loam soil (SCL); (2) identify the critical soil moisture content (CMC) using the physiological response to drought. All physiological traits decreased with reduced soil moisture levels across all growth stages and soil types. The 20% AWHC maximum reduced plant height (1.61-folds), predawn leaf water potential (LWP_pd, 3.07-folds), net photosynthesis rate (3.26-folds), stomatal conductance (17.00-folds), chlorophyll fluorescence (1.28-folds), dry matter (2.70-folds), grain yield (3.91-folds) in flowering stage. All physiological traits positively correlated with dry matter and grain yield in both cultivars and both soils, but the highest correlation was found between LWP_pd with dry matter (0.884–0.961**) and grain yield (0.790–0.936**) in PTT1, or dry matter (0.883-0.973**) and grain yield (0.818–0.945**) in KDML105. Therefore, LWP_pd was used to evaluate the CMC from the linear regression between LWPpd and soil moisture contents. At the seedling stage, the CMCs of PTT1 were at 60% AWHC in both soils, while KDML105 was at 60 and 70% AWHC in clay and SCL soils, respectively. At the tillering and flowering stages, the CMCs of both cultivars were at 70% AWHC in both soils. The results indicate that CMC values vary with soil texture, rice cultivar, and growth stage. This information is crucial for optimizing irrigation practices, such as implementing alternate wetting and drying systems tailored to specific soil textures and rice cultivars.