Salt Tolerance and Physiological Responses at the Seedling, Vegetative, and Reproductive Stages of Thai Jasmine Rice KDML105 and Its Genetically Improved Variety (RD73) and Line (TSKC1-144)

The level of salt tolerance of rice varies with the developmental stage. The information on the tolerance of rice at each stage is valuable for breeding and planning in cultural management to obtain the optimal growth and yield in salt-affected areas. The objectives of this study were to compare the salt tolerance and physiological- and yield-related responses at three growth stages of the salt-sensitive Thai jasmine rice, KDML105, with its genetically improved variety RD73 (a registered commercial variety) and TSKC1-144 (a breeding line), both containing Pokkali-derived salt-tolerant QTL. The young hydroponically grown seedlings of TSKC1-144 treated with 150 mM NaCl were highly tolerant, while KDML105 was highly sensitive and RD73 was moderately tolerant. At the vegetative stage, KDML105 exhibited more growth and leaf physiological damage, showing the highest percentage reductions in the net photosynthesis rate (Pn), leaf relative water content (RWC), and shoot and total plant dry weight, but the highest increase in leaf electrolyte leakage (EL) and the highest leaf Na+/K+ ratio. During the reproductive phase, salt stress did not significantly induce physiological damage to the flag leaves, except for Pn, which was significantly reduced, particularly for KDML105. Both RD73 and TSKC1-144 exhibited lower biomass reductions and higher yields and yield components than KDML105. Compared with TSKC1-144, RD73 produced a lower grain number panicle−1, grain weight panicle−1, and lower 100-grain weight but 34% more panicles; therefore, it yielded a higher grain weight plant−1 (27.57 cf. 20.93 g). The most prominent trait that conferred a greater salt tolerance to RD73 and TSKC1-144 compared with KDML105 was the more efficient Na+ exclusion. Taking their tolerance at all growth stages into consideration, RD73 and TSKC1-144 are deemed suitable for growing under rain-fed conditions where the intensity of the soil salinity fluctuates throughout the growing season.