VARIATON OF GROWTH AND YIELD PLASITICITY TO DROUGHT IN HIGHYIELDING THAI CASSAVA VARIETIES

Cassava shows ample genotype variability in morphological traits under varying soil moisture. Understanding these responses is key to improving drought tolerance. We investigated the responses of seven high-yielding Thai cassava varieties by aiming to study their plasticity and acclimation to drought. These genotypes include Rayong 11 (R11), Kasetsart 50 (KU50), Huay Bong 60 (HB60), Kasetsart 72 (KU72), Rayong 72 (R72), Rayong 7 (R7), and Rayong 9 (R9). Herein, cassava plants were planted under various levels of drought stress, from well-watered (80% field capacity [FC], estimated at 72% w/w) to severely stressed levels (70%–20% FC, corresponding to 63% - 18% w/w, respectively) for 22 days, released to re-watering condition (72% w/w) for 15 days. At 30 days after planting under well-watered conditions, R11 had a larger canopy, while KU50 and HB60 had fewer leaves. To account for early differences, relative growth rates across moisture levels were used to build dose-response curves. The results showed distinct genotype responses in leaf number, plant height, stem length, and leaf abscission. KU72 and R72 displayed complex trends, with steady leaf production and stem elongation until growth plateaued. HB60 and KU50 maintained leaf numbers even under drought. R11 was less responsive to moisture changes, while R9 showed steep declines at low %FC, confirming susceptibility. KU72 and R72 slowed growth only at supra-optimal levels (70–80% FC), indicating saturation limits. Upon rewatering, all genotypes except R11 recovered, reinforcing its drought sensitivity. Key plastic traits were root dry mass, stem dry mass, and root–leaf ratio. Overall, KU50, R72, HB60, and KU72 were drought-tolerant, while R9, R11, and R7 were less tolerant. These findings identify critical moisture thresholds and the limited benefit of excess water, guiding cassava breeding for drought resilience.