Chlorophyll fluorescence (CF), growth parameters, phytochemical contents [proline, chlorophyll, ascorbic acid, total phenol content (TPC), total flavonoid content (TFC)], and antioxidant activities were investigated in lettuce (Lactuca sativa L.) seedlings grown under different sodium chloride (NaCl) concentrations (0, 50, 100, 200, 300, and 400 mM) in a controlled environment for eight days. The parameters were evaluated at two days intervals. Almost of the CF and growth parameters as well as phytochemicals were significantly affected by both NaCl concentrations and progressive treatment schedule. The maximum quantum yield (Fv/Fm), effective quantum yield of photochemical energy conversion in PSII [Y(PSII)], coefficient of photochemical quenching (qP), coefficient of non-photochemical quenching (qN), and ratio of fluorescence decline (Rfd) showed decrements only at the highest saline concentration (400 mM), whereas the quantum yield of non-regulated energy dissipation in PSII [Y(NO)] exhibited a dissipation trend. All the growth parameters decreased with increasing NaCl concentrations, showing the highest decrease (~8 fold) in shoot fresh weight, compared to control seedlings. Proline significantly increased with increasing NaCl concentration and treatment time. Other phytochemicals decreased with the increase in NaCl concentration and reached their lowest at 400 mM. Overall, the results showed major changes in all parameters when the seedlings were grown at a NaCl concentration of 400 mM. The present findings will be useful for understanding the differential effect of NaCl concentrations in lettuce seedlings, and also might be useful to optimize the NaCl concentrations in other crops grown in controlled environmental conditions.

Plants are very sensitive to environmental stresses, particularly to high temperature (HT) which is predicted to increase. This study was conducted to investigate the response to HT for identifying physiological traits associated with HT tolerance in tomato. Thirty-eight tomato accessions with three fruit sizes (cherry, medium and large) were grown in greenhouses where temperature set-point for ventilation was 25 °C and 40 °C for normal temperature (NT) and HT, respectively. HT increases plant height and stem diameter but little effect on leaf length and width. For reproductive traits, HT has positive effect on the number of flowers (NFL) but negative on the number of fruits (NFR), fruit set (FS), weight (FW) and yield (FY), and pollen germination (PG) and tube length. In the correlation analysis, except for accession T37 with extreme value causing spurious results, PG did not show significant correlation with FS, FW and FY, but with NFL and NFR in HT condition. NFR (HT), FS (HT) and FY (NT) are identified as physiological traits associated with HT tolerance based on correlation with FY (HT). However, their association to HT tolerance differ by fruit sizes. NFR (HT) was significantly correlated with FY (HT) in sub-population of cherry (r = 0.859**, n = 14), medium (r = 0.848**, n = 7) and large fruit types (r = 0.769**, n = 16) but not in total population (r = 0.302NS, n = 37). FS (HT) and, interestingly, FY (NT) were significantly correlated with fruit yield (HT) in total population and a cherry fruit sub-population but not in medium and large fruit sub-population. Results suggest that physiological traits associated with HT tolerance differ by fruit size and breeding programs should consider different selection criteria for different fruit types when develop cultivars with HT tolerance.

High temperature (HT) significantly affects crop physiological traits and reduces productivity in plants. To increase yields as well as survival of crops under HT, developing heat-tolerant plants is one of the main targets in crop breeding programs. The present study attempted to investigate the linkage of the heat tolerance between the seedling and reproductive growth stages of tomato cultivars ’Dafnis‘ and ’Minichal.’ This research was undertaken to evaluate heat tolerance under two experimental designs such as screening at seedling stage and screening from reproductive traits in greenhouses. Survival rate and physiological responses in seedlings of tomatoes with 4-5 true leaves were estimated under HT (40 °C, RH 70%, day/night, respectively) and under two control and HT greenhouse conditions (day time 28 °C and 40 °C, respectively). Heat stress significantly affected physiological–chemical (photosynthesis, electrolyte conductivity, proline) and vegetative parameters (plant height, shoot fresh weight, root fresh weight) in all tomato seedlings. The findings revealed that regardless of tomato cultivars the photosynthesis, chlorophyll, total proline and electrical conductivity parameters were varied in seedlings during the heat stress period. The heat tolerance rate of tomatoes in the seedling stage might not always be associated with reproductive parameters. HT reduced fruit parameters such as fruit weight (31.9%), fruit length (14.1%), fruit diameter (19.1%), and fruit hardness (9.1%) compared to NT under HT in heat-susceptible tomato cultivar ‘Dafnis’, while in heat-tolerant cultivar ‘Minichal’ fruit length (7.1%) and fruit diameter (12.1%) was decreased by the effects of HT, but on the contrary fruit weight (3.6%) and fruit hardness (8.3%) were increased. In conclusion, screening and selection for tomatoes should be evaluated at the vegetative and reproductive stages with consideration of reproductive parameters.

Understanding the mechanism for heat tolerance is important for the hot pepper breeding program to develop heat-tolerant cultivars in changing climate. This study was conducted to investigate physiological and biochemical parameters related to heat tolerance and to determine leaf heat damage levels critical for selecting heat-tolerant genotypes. Seedlings of two commercial cultivars, heat-tolerant ‘NW Bigarim’ (NB) and susceptible ‘Chyung Yang’ (CY), were grown in 42 °C for ten days. Photosynthesis, electrolyte conductivity, proline content were measured among seedlings during heat treatment. Photosynthetic rate was significantly reduced in ‘CY’ but not in ‘NB’ seedlings in 42 °C. Stomatal conductivity and transpiration rate was significantly higher in ‘NB’ than ‘CY’. Proline content was also significantly higher in ‘NB’. After heat treatment, leaf heat damages were determined as 0, 25, 50 and 75% and plants with different leaf heat damages were moved to a glasshouse (30–32/22–24 °C in day/night). The growth and developmental parameters were investigated until 70 days. ‘NB’ was significantly affected by leaf heat damages only in fruit yield while ‘CY’ was in fruit set, number and yield. ‘NB’ showed fast recovery after heat stress compared to ‘CY’. These results suggest that constant photosynthetic rate via increased transpiration rate as well as high proline content in heat stress condition confer faster recovery from heat damage of heat-tolerant cultivars in seedlings stages.

No abstract