ABSTRACT Domestication of wheat (Triticum aestivum L.) in some tropical and subtropical regions may encounter both high temperature and soil salinity problems. This research aims to investigate how salt and heat stress combination affects growth and physiological characteristics of wheat seedlings to unravel their response mechanisms. The 14-d-old wheat seedlings of three cultivars were treated with salt stress (150 mM NaCl), heat stress (42 °C for 4 h) and combined stress (150 mM NaCl at 42 °C for 4 h). Growth and physiological parameters were monitored at 0 h, 24 h, 5 d and 10 d after stress exposure except for ion concentration, which was analyzed at 5 d post stress exposure. The combined stress inhibited shoot growth and compromised physiological parameters (e.g., relative water content [RWC], malondialdehyde [MDA], electrolyte leakage [EL], Na+ content and Na+/K+ ratio) to a greater extent than a single salt or heat stress, demonstrating additive effects particularly on plant water status and membrane stability. After 10 d exposure to the combined stress, the most sensitive cultivar ‘Fahng 60’ exhibited the highest reduction in shoot dry weight (34%). Moreover, ‘Fhang 60’ showed the lowest RWC (36%) and the highest EL (75%) while ‘Samerng 2’ and ‘Phrae 60’ had RWC of 70% and 64%, and EL of 54% and 60%, respectively. The effect was remarkably evidenced after 5 d of stress exposure, suggesting a late onset of stress injury. Together, the results demonstrated that the effect of salt and heat stress combination was additive, which could be manifested in an enhanced Na+ accumulation and membrane damage.

ABSTRACT Silicon (Si) is considered a beneficial element for rice (Oryza sativa L.) The objective of this work was to investigate the effects of Si in the form of nanoparticles on growth and physiology of rice under salt stress. Silica nanoparticles (SNPs) were synthesized from rice husk by sol-gel method. The prepared SNPs powders were agglomerated in semi-spherical nano- sized particles with diameters in the range of 60-135 nm. Three rice cultivars namely ‘Pokkali’, ‘KDML105’ and ‘IR29’ were grown for 30 d in plastic pots, and then divided into four groups i.e., control, SNPs, NaCl and NaCl + SNPs. Foliar spray of 120 mg L-1 SNPs was given to the SNPs and NaCl + SNPs groups for 4 d. After that the plants in NaCl and NaCl + SNPs groups were exposed to 150 mM NaCl for 17 d. The salt-stressed plants suffered significant reductions in biomass, net photosynthesis rate (PN), and maximal quantum efficiency of photosystem II (PSII) photochemistry (Fv/Fm) while three stress indicators (malondialdehyde, hydrogen peroxide [H2O2] and proline) considerably increased. The SNPs mitigated the adverse effects of salt stress by increasing PN (18% to 116% increase) and lowering H2O2 (8% to 31% reduction) in all cultivars, compared with the values under salt stress, while proline was reduced by 7% in ‘KDML105’ and 19% in ‘IR29’. The H2O2 content was regulated by the increased activities of antioxidant enzymes, notably catalase, peroxidase and ascorbate peroxidase. The application method and concentrations of SNPs used for rice plants under stress should be further optimized for the highest benefit of growth and yield in the field conditions.

ABSTRACT Drought and salinity are important abiotic stress factors negatively affecting productivity of the world-renowned jasmine rice (Oryza sativa L. subsp. indica Kato) ‘KDML105’ in the northeast Thailand. Two chromosome segment substitution lines (CSSLs) of ‘KDML105’ which were introgressed with drought tolerance quantitative trait loci (DT-QTLs), designated CSSL94 and CSSL103, and two improved ‘KDML105’ lines, designated RGD1 and RGD4, carrying the salt tolerant gene (SKC1) were used for evaluation of growth, physiological and biochemical traits in response to drought and salt stress conditions in comparison with ‘KDML105’ (sensitive parental cultivar) and DH103 (DT-QTL donor). Seedlings were grown for 21 d in hydroponic solutions and then subjected to drought stress (20% polyethylene glycol 6000) or salt stress (150 mM NaCl) for 10 d. Under both stresses, all four improved rice lines showed lower percent reduction in growth compared with ‘KDML105’. Under drought stress, CSSL94 and CSSL103 had significantly higher relative water content (88% and 89%) compared with that of ‘KDML105’ (82%). These lines accumulated 42% and 45% lower hydrogen peroxide, resulting in 25% and 32% lower malondialdehyde, and less membrane damage (24% lower electrolyte leakage). On the other hand, better growth of RGD1 and RGD4 under salt stress, compared with ‘KDML105’, was associated with an efficient maintenance of ion homeostasis (57% and 69% lower Na+/K+), 34% and 43% lower hydrogen peroxide, and 54% lower electrolyte leakage. The introgression of DT-QTL conferred not only drought but also salt tolerance to the recipient genotype. Likewise, introgression of SKC1 conferred both salt and drought tolerance.

ABSTRACT The impact of temperature on plants varies depending on the temperature level and the time of exposure. The aim of this study was evaluating different short-term temperatures to stimulate rice (Oryza sativa L.) seedling growth indicated by some physiological functions. Thus, oxidative stress, membrane stability and antioxidant activity in ‘Dular’, ‘IR64’ and ‘KDML105’ rice seedlings were evaluated in response to 25, 35 and 45 °C for 30 min. At 25 °C, ‘IR64’ showed higher electrolyte leakage (EL) percentages, protein contents and peroxidase (POX) activity but lower hydrogen peroxide (H2O2) content. At 35 °C, the highest H2O2 content was found in ‘KDML105’ followed by ‘IR64’ and ‘Dular’. In all rice seedlings the lowest EL was after 35 °C exposure and activities of catalase (CAT) in ‘IR64’ and ‘KDML105’, ascorbate peroxidase (APX) in ‘Dular’, ‘IR64’ and ‘KDML105’ and glutathione reductase (GR) in ‘Dular’, ‘IR64’ and ‘KDML105’, and superoxide dismutase (SOD) in ‘Dular’ and ‘KDML105’ were stimulated by higher H2O2 production. At 45 °C, all rice seedlings showed pronounced EL increase but reduced H2O2 production, protein content and all antioxidant enzyme activities. These results suggested that short-term temperature at 35 °C was an optimum condition for all rice seedlings as indicated by the least membrane damage (EL ≈ 45.04%) and induction of most antioxidant enzymes (CAT, APX, GR and SOD ≈ 48.29 nmol min-1 μg-1 protein, 247.49 ng ascorbic acid min-1 μg-1 protein, 0.16 μM min-1 μg-1 protein and 0.078 unit min-1 μg-1 protein, respectively). This temperature induced higher H2O2 production which plays a significant role in signaling molecules to stimulate protein function and antioxidant enzyme activities. While short-term 45 °C proved to be damaging temperature for all rice seedlings because of membrane damage and decreased reactive oxygen species defense mechanism.

ABSTRACT: Details on growth and yield for cassava planted on different dates are useful for determining suitable genotypes for particular growing seasons. Our aim was to study growth and yield of cassava planted on different dates. Four cassava genotypes (Kasetsart 50, Rayong 9, Rayong 11 and CMR38-125-77) were evaluated using a randomized complete block design (RCBD) with four replications in six growing periods (20 Apr, 25 May, 30 June, 5 Oct, 10 Nov and 15 Dec 2015-2016) at Khon Kaen, Thailand. Soil properties were determined prior to planting, and crop traits and weather data were recorded. The six planting dates had a statistically significant effect on all crop traits. Low temperatures and solar radiation related to low biomass accumulation rates and short periods of linear phases for total crop and storage root dry weights with the 30 June planting date. CMR38-125-77 is likely to be a good genotype with respect to total crop and storage root dry weights at final harvest for almost all growing dates, except for the 20 Apr. Leaf area index (LAI) at 120, 240 and 300 days after planting (DAP), specific leaf area (SLA) at 120 DAP, storage root growth rate (SRGR) during 300-360 DAP and leaf growth rate (LGR) during 60-120 and 300-360 DAP were the components for the physiological determinants of total crop and storage root dry weight. The relationship between these physiological traits and storage root could be useful for cassava breeding.

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.