Salinity tolerance levels and physiological changes were evaluated for twelve rice cultivars, including four white rice and eight black glutinous rice cultivars, during their seedling stage in response to salinity stress at 100 mM NaCl. All the rice cultivars evaluated showed an apparent decrease in growth characteristics and chlorophyll accumulation under salinity stress. By contrast an increase in proline, hydrogen peroxide, peroxidase (POX) activity and anthocyanins were observed for all cultivars. The K(+)/Na(+) ratios evaluated for all rice cultivars were noted to be highly correlated with the salinity scores thus indicating that the K(+)/Na(+) ratio serves as a reliable indicator of salt stress tolerance in rice. Principal component analysis (PCA) based on physiological salt tolerance indexes could clearly distinguish rice cultivars into 4 salt tolerance clusters. Noteworthy, in comparison to the salt-sensitive ones, rice cultivars that possessed higher degrees of salt tolerance displayed more enhanced activity of catalase (CAT), a smaller increase in anthocyanin, hydrogen peroxide and proline content but a smaller drop in the K(+)/Na(+) ratio and chlorophyll accumulation.

Rice cultivated in the tropics is exposed to high temperature (HT) stress which threatens its growth and survival. This study aimed at characterizing the HT response in terms of PSII efficiency and membrane stability, and to identify leaf fatty acid changes that may be associated with HT tolerance or sensitivity of rice genotypes. Twenty-eight-day-old seedlings of two Thai rice cultivars (CN1 and KDML105), a standard heat tolerance (N22), and a heat sensitive (IR64) rice genotype were treated at 42 °C for 7 days. Under HT, N22 showed the highest heat tolerance displaying the lowest increase in electrolyte leakage (EL), no increments in malondialdehyde (MDA) and stable maximum quantum yield of PSII efficiency (F_v/F_m). Compared to KDML105 and IR64, CN1 was more tolerant of HT, showing a lower increase in EL and MDA, and less reduction in F_v/F_m. N22 and CN1 showed a higher percentage reduction of unsaturated fatty acids (C18:2 and C18:3), which are the major components of the thylakoid membrane, rendering the optimum thylakoid membrane fluidity and intactness of PSII complex. Moreover, they exhibited sharp increases in long-chain fatty acids, particularly C22:1, while the heat sensitive IR64 and KDML105 showed significant reductions. Dramatic increases in long-chain fatty acids may lead to cuticular wax synthesis which provides protective roles for heat tolerance. Thus, the reduction in unsaturated fatty acid composition of the thylakoid membrane and dramatic increases in long-chain fatty acids may lead to high photosynthetic performance and an enhanced synthesis of cuticular wax which further provided additional protective roles for heat tolerance ability in rice.

Rice is an important and main staple food crop. Rice in Thailand grows in both the on- and off-seasons. The problem of growing rice in the off-season is that it is dry and the temperature tends to be high. To evaluate the effects of high temperatures on their phenology, yield and seed quality, three rice varieties were cultivated off-season in 2018 and 2019. Rice plants were grown in cement pots on planting date I (PDI; off-season; mid-January) and planting date II (PDII; late off-season; beginning of February). The results showed that rice plants were exposed to higher temperatures in 2019 (than 2018), as indicated by a higher accumulated growing degree day (AGDD). The high AGDD affected the phenology of the rice by shortening the duration of its development from sowing to physiological maturity (PM) from 106.8 DAS in 2018 to 86.0 DAS in 2019. The high AGDD shortened the development duration of the embryo and endosperm, resulting in reductions in the size and growth rates of the embryo and endosperm, and eventually reduced the yield and the yield components. Moreover, the high AGDD reduced the seed quality, as indicated by a decline in the seedling growth rate (SGR) and an increase in chalkiness. Among the varieties, the high temperature in 2019 caused the smallest phenological shift in Chai Nat 1 (CN1), while the shift was largest in Pathum Thani 1 (PTT1). In addition, CN1 exhibited a significantly higher total seed weight/panicle, 1000-seed weight and percentage of filled seed/pot than SP1 and PPT1. It was suggested that CN1 could be described as heat tolerant, and PTT1 as heat sensitive. It was also suggested that farmers should select appropriate rice varieties to grow in the off-season due to the risk of a high-temperature-induced reduction in the seed yield and quality.

Global warming is a serious problem, with significant negative impacts on agricultural productivity. To better understand plant anatomical adaptation mechanisms as responses to heat stress, improved basic knowledge is required. This research studied the physiological and anatomical responses of Khao Dawk Mali 105 (KDML105) to artificial heat stress. Dehusked seeds were sterilized and cultured on Murashige and Skoog (MS) medium, supplemented with 3 mg/L 2,4-Dichlorophenoxyacetic acid (2,4-D) for callus induction. The cultures were maintained at 25 °C and 35 °C for 4 weeks, while the other culture was treated with heat shock at 42 °C for 1 week before further incubation at 25 °C for 3 weeks. Results revealed that elevated temperatures (35 °C and 42 °C) adversely impacted seedling growth. Plant height, root length, leaf number per plant, fresh and dry weight, chlorophyll a, chlorophyll b and total chlorophyll content decreased after heat stress treatment, while malondialdehyde (MDA) and electrolyte leakage percentage significantly increased, compared to the control. Heat stress induced ROS accumulation, leading to lipid peroxidation and membrane instability. Principal component analysis (PCA) and hierarchical cluster analysis (HCA) results also confirmed negative correlations between MDA, electrolyte leakage and other parameters. MDA content and electrolyte leakage are effective indicators of heat stress in rice. Surface anatomical responses of rice seedlings to heat stress were studied but significant alterations were not observed, and heat stress had no significant negative effects on KDML105 calli. Size and mass of calli increased because heat stress stimulated gene expression that induced thermotolerance. Our results provide useful information for rice breeding and heat stress tolerance programs to benefit long-term global food security.

Photosynthetic performance and biomass at different growth stages of the salt-sensitive KDML105 rice cultivar, three improved lines (RD73, CSSL8-94, and TSKC1-144), and the salt-tolerant standard genotype (Pokkali) were investigated under non-saline, semi-saline, and the heavy-saline field conditions in the northeast of Thailand. In the non-saline field, net photosynthesis rates (P_n) of all genotypes remained high from the early vegetative stage to the milky stage and then dramatically reduced at maturity. In contrast, in both saline fields, P_n was the highest at the early vegetative stage and continuously declining until maturity. Leaf chlorophyll content remained high from the early vegetative to milky stage then reduced at maturity for all three field conditions. During the reproductive phase, P_n of KDML105 and the improved lines were reduced by 4–17% in the heavy-saline field, while that of Pokkali was increased (11–19% increase over that of the non-saline). Pokkali also showed a prominent increase in water use efficiency (WUE) under salinity. Nevertheless, rice leaves under saline conditions maintained the PSII integrity, as indicated by the pre-dawn values of maximum quantum yield of PSII photochemistry (F_v/F_m) of higher than 0.8. Pokkali under the semi-saline and the heavy-saline conditions exhibited 51% and 27% increases in final biomass, and 64% and 42% increases in filled grain weight plant⁻¹, respectively. In the semi-saline condition, RD73, TSKC1-144, CSSL8-94, and KDML105 showed moderate salt tolerance by displaying 24%, 18.6%, 15%, and 11.3% increases in final biomass, and 24%, 4%, 13%, and 6% increases in filled grain weight plant⁻¹, respectively. In contrast, in the heavy-saline field, final biomass of RD73, KDML105, CSSL8-94, and TSKC1-144 showed 48%, 45%, 38%, and 36% reductions from that in the non-saline field, while the filled grain weight plant⁻¹ were reduced by 45%, 58%, 35%, and 32%, respectively. This indicated that the improved lines carrying drought- and/or salt-tolerance genes achieved an increased salt tolerance level than the parental elite cultivar, KDML105.