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The potential of seed priming by different chemicals on alleviation of growth inhibition of rice (Oryza sativa L.) seedlings under salt stress was investigated. A preliminary experiment using 10 seed-priming chemicals including NaCl, KCl, CaCl 2 , KNO 3 , ascorbic acid (AsA), mannitol, polyethylene glycol (PEG 6000 ), sorbitol, wood vinegar and distilled water revealed that mannitol, KNO 3 and wood vinegar were more effective than the others in alleviating salt-induced growth inhibition of 10-day-old seedlings. Various concentrations of mannitol (1, 2 and 3%), KNO 3 (0.25, 0.5 and 0.75%) and wood vinegar (1:1000, 1:300 and 1:100 dilutions) were subsequently used to prime rice seeds to investigate the effects on mitigation of salt-induced growth inhibition and modulation of physiological responses of 4-week-old rice plants grown in a hydroponic solution. All tested concentrations of mannitol, KNO 3 and wood vinegar resulted in seedlings with significantly higher dry weights than those grown from non-primed and hydroprimed seeds under both controlled and saltstressed (150 mM NaCl, 7 days) conditions. Under salt stress, enhanced growth of seedlings raised from seeds primed with all three chemicals was attributable to greater membrane stability, higher chlorophyll content and lower Na + /K + ratio.

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Thirty rice cultivars were evaluated for salinity tolerance during the seedling stage and were divided into five tolerance groups including tolerant (T), moderately tolerant (MT), moderately susceptible (MS), susceptible (S) and highly susceptible (HS) which comprised 5, 10, 9, 4 and 2 cultivars respectively. Genetic diversity of all rice cultivars was evaluated using random amplified polymorphic DNA (RAPD) and simple sequence repeats (SSR) markers. The cultivars were evaluated for polymorphisms after amplification with 20 random decamer primers and 20 SSR primer pairs. A total of 161 RAPD markers and 190 SSR alleles were produced which revealed 68.94% and 89.47% polymorphism respectively. Mean genetic similarity coefficient was 0.82 for RAPD and 0.70 for SSR. Cluster analysis based on RAPD markers was effective in grouping cultivars based on their salt tolerance ability. Group IA1, IB and IV contained three T, three S and two HS rice cultivars respectively. The MT and MS cultivars which showed similar physiological responses to salinity were resolved into two groups: Group IA2 and Group II comprising ten and eight MT/MS cultivars respectively. Cluster analysis based on SSR markers separated rice cultivars into groups based on genetic relatedness which did not correspond to salinity tolerance level. The results from this study provided some useful implications for salt tolerance breeding programs. The evaluation of genetic similarity and cluster analysis together with salt tolerance ability provides some useful guides for assisting plant breeders in selecting suitable genetically diverse parents for the crossing program.

Leaf rolling is a common response to drought among members of the grass family. A detailed understanding of the structural changes that occur when rice leaves roll in response to drought may assist in determining the physiological mechanisms underlying this feature and determining its potential utility in plant breeding. The anatomical characteristics of rice cultivars native to northeastern Thailand in response to drought stress were evaluated in this study. Polyethylene glycol (PEG) treatments were applied to seedlings of 28 native rice cultivars and two rice breeding cultivars for 10 d. When cultivated in drought-prone environments, the native rice cultivars had a leaf rolling index between 4 and 7, which was classified as moderate to high leaf rolling. Rice leaf anatomy exhibited profound changes in response to drought, with smaller bulliform cells (36.8%), thicker epidermis (10.98%), smaller vascular bundles (7.41%), and smaller bundle sheath cells (11.95%). Decision trees were used to explore the association between the degree of leaf rolling and anatomical traits. According to the decision tree models generated on the basis of the gain ratio, information gain, and Gini index, the epidermis, leaf thickness, and bulliform cells were the major factors of the root nodes, and the maximum accuracy of the models was 70.90%. Overall, the models indicated that rice leaves with a thin epidermis, large bulliform cells, thin leaves, and small vascular bundles are more likely to display high leaf rolling adaptations under drought stress conditions.