C. Vijayalakshmi’s research while affiliated with Tamil Nadu Agricultural University and other places

Photosynthesis is fundamental to biomass production, but sensitive to high temperature stress. To understand the genetics of leaf photosynthesis, especially under heat, RILs of rice cv. Improved White Ponni (IWP) introgressed with QTLs controlling spikelet fertility (qHTSF 1.1 and qHTSF 4.1) were grown in temperature controlled chambers (TCCs). The temperature inside the TCCs were maintained at ambient and elevated temperatures. Gas exchange and chlorophyll fluorescence were measured to evaluate introgressed lines for high temperature stress tolerance. It was observed that, the lines introgressed with both the QTLs (# 246, # 295, # 296, # 277) had higher photosynthetic rate coupled with higher stomatal conductance and lower transpiration rate. These lines also exhibited higher PSII quantum yield. The lines with one positive QTL either qHTSF 1.1 or qHTSF 4.1 showed better gas exchange and PSII quantum yield as compared to negative lines under high temperature stress. This shows that the physiological basis of the introgressed QTLs controls the spikelet fertility by maintaining the photosynthetic rate and chlorophyll fluorescence and minimizing the transpiration rate under high temperature stress.

The responses of soybean genotypes to high temperature for intrinsic tolerance was studied using Temperature Induction Response (TIR) technique in order identify the genotypes tolerant to high temperature stress. Seven soybean genotypes subjected to lethal and sub lethal temperatures showed significant variation for acquired thermotolerance. Thermotolerant genotypes ADT 1 and CoSoy1 identified by the TIR technique, demonstrated higher survival percentage, and lower growth reduction. Further the tolerant genotypes identified based on TIR also showed higher antioxidant enzymes activity implying the critical role of antioxidant in cellular thermotolerance. This clearly demonstrated that TIR can be effectively used for screening high temperature tolerance genotypes in soybean. This study also established the fact that the alternations in antioxidants during induction are vital for imparting tolerance to high temperature stress.

Agricultural College and Research Institute, Madurai – 625 104.

The effect of combined heat and drought stress on proline, chlorophyll stability index, membrane thermal stability and abscisic acid (ABA) contents were investigated to screen and study the physiological basis of heat and drought tolerance in three rice genotypes (ADT 43, TKM 9 and N 22). The stresses were imposed at panicle initiation (PI) and anthesis stages of crop growth. The genotype N 22 was found to be the tolerant followed by TKM 9 while ADT 43 was observed to the susceptible genotype based on the above physiological and biochemical traits. Irrespective of genotypes, anthesis stage stress brought about accumulation of osmolyte and ABA contents with membrane stability changes compared to stress at PI stage. ADT 43 recorded low proline content of (201 μg g⁻¹), CSI of (52.5%), MTS of (24.39%) and ABA content of (0.587 μg g⁻¹). While, N 22 recorded higher proline content of (252.6 μg g⁻¹), CSI of (89.8%), MTS of (33.08%) and ABA content of (0.685 μg g⁻¹). Higher proline and ABA contents with improved membrane stability was clearly demonstrated in the tolerant genotype (N 22) compared to susceptible genotype (ADT 43). This conforms that these biochemical traits can be used to screen rice genotypes to combined stresses and understand the mechanism underlying stress tolerance.

Salinity is a serious problem all over the world with an average of 830 M ha being affected. In India, it's about 13.3 M ha with coastal and inland salinity. The present study focused on effects of salinity on chlorophyll content, chlorophyll fluorescence, gas exchange parameters and osmotic potential in different salt concentration. The chlorophyll content reduced as salinity level increases and chlorophyll a and b concentration coincides with decrease in Fv/Fm ratio. The genotype IR72593 exhibited better survival response than the susceptible genotype IR29. The sensitive cultivar IR29 responds to salinity stress quickly and did not recover after one week of salinity stress which led to complete plant death. Tolerant genotypes showed lower reduction in gas exchange parameters, while IR29 showed sudden reduction during the initial hours of salt stress and decreased at 312 hours due to complete death of plants. Osmotic potential becomes negative with increase in salinity level irrespective of genotypes; however tolerant genotype responds lower than sensitive genotype. This differential response of tolerant genotypes to salinity stress is due to the reduced transpiration rate and closure of stomatal openings during stress period, with no significant change in chlorophyll content. The transpiration rate and stomatal conductance is positively associated with intake of Na ions into the plant system.

The increasing CO2 concentration in the atmosphere levels often stimulates the photosynthesis and biomass. However, the duration and magnitude of this stimulation on physiological processes of tropical tree species is unknown. The objective of this experiment was to examine the physiological response of four commercially important tropical tree species to elevated CO2 levels. Tectona grandis (teak), Azadirachta indica (neem), Ailanthus excelsa (maharukh) and Bambusa bambos (bamboo) seedlings were exposed to ambience (380 ppm) or elevated CO2 (600 and 900 ppm) levels using an automated open top chamber (AOTC). The seedlings were exposed to elevated CO2 levels for 180 days. Leaf gas exchange characters were measured in the second or third fully expanded leaves of the seedlings with a portable infra-red gas analyser (Li-Cor 6400XT) at the end of the study period. The elevated CO2 levels significantly affected the physiological processes and did not show identical response in the studied tree species. Maharukh and bamboo plants were recorded optimistic response in terms of photosynthesis under elevated CO2 even up to 900 ppm. Teak also registered a high photosynthesis up to 600 ppm level of CO2, but it showed a decreased photosynthetic rate under 900 ppm of CO2. Contrary to these three species, neem showed a negative response to the elevated CO2 at both 600 and 900 ppm levels. The response of the species on transpiration (E) mmol m-2s-1 was similar to that of the photosynthetic rate (Pn). This study recommend that we should not generalise the response of tropical tree species to elevated CO2. However, the commercially important tropical tree species should be assessed individually for the physiological functions to elevated CO2.

Rice is staple food for Asian people and there is urgent need to produce more rice per unit area, which can be enhanced by possibly utilizing the unexplored germplasm for adverse climatic and soil conditions. Among the stress, salinity is profound and gaining prime importance in upcoming years; only possible way is breeding for salt tolerance by utilizing available genetic resources. This study was undertaken to identify promising genotypes based on its genetic diversity evaluated under saline and normal conditions. In the present study, Pokkali showed better visual salt injury score and IR 72593 showed highest grain yield under salinity condition. Cluster analysis is effective in identifying selection of better parents to harness heterosis in germplasm. Among 127 genotypes, cluster analysis resulted in 28 clusters following Tochers method. Cluster VII comprising of 10 genotypes had the highest intra cluster distance indicating the high genetic divergence among the genotypes. The genotypes in Cluster VIII and IX showed minimum inter cluster distance, which is obvious that, these clusters would have been evolved by similar evolutionary procedure. Maximum inter cluster distance was noticed between Cluster XXIV and XXVII. Hybridization among divergent clusters in all possible combinations will exhibit good heterosis for yield and tolerance to salinity.

The tolerant genotypes have compensating mechanism of Na+ exclusion and K+ absorption which buffers to excessive salt stress, whereas susceptible genotypes respond quickly to salinity and did not recover after a week of salinity leading to complete death of whole plants. The reduced level of CO2 assimilation, stomatal closure, transpiration rate, exclusion of Na+ ion and K+ absorption and increased level of antioxidant enzymes serve as a potential indicator in identification of tolerant genotypes at early seedling stage. Therefore, screening of genotypes at seedling stage by using these criteria would be quite effective and time saving method.

Three rice genotypes, IR 74802, IR 73104 and IR 72593, along with FL 478 and IR 29 as resistant and susceptible controls, respectively, were subjected to 21 days' salinity stress at the seedling stage in modified yoshida solution with two salt levels (60 and 120 mM NaCl). The results indicated that there was a profound increase in proline and ascorbic acid levels, and in the activity of nitrate reductase and antioxidant enzymes, i.e. catalase, peroxidase and ascorbate peroxidase, as well as malondialdhyde and membrane stability index, which were associated with salt tolerance. Salt stress had a significant and drastic effect on all parameters when the salinity level increased to 120 mM NaCl. The increased enzyme activity was directly related to an increased membrane stability index, as in IR 72593, which is identified as the most tolerant among the genotypes tested. It is clearly confirmed that predicting tolerance at the early seedling stage is the best way to assess the salinity tolerance level by utilizing physiological parameters, especially antioxidant enzyme activities which are found to be closely associated with salinity tolerance. Physiological adaptation of the plant to NaCl salt stress resulted in enhanced activity of stress-related enzymes and low sodium uptake in tolerant genotypes.