Conferment of thermotolerance : Physiology, productivity, quality and post-harvest quality of subtropical and temperate vegetable crops exposed to sub-lethal heat stress
Authors:   , Cheng Hsiang Lai 
Published: November 2024

In face of imminent climate change and its catastrophic effects on crop yield and food security, it is imperative that the thermal limits of food crops should be explored to elucidate their optimal physiological thermal window and thermal limits. This research was carried out in 3 phases: 1) to inaugurate a dose-response of temperate vegetables to heat stress and therein determine the thermal biology, optimal thermal window for physiological and photosynthetic performances, and the critical temperature at which irreparable failure of physiological processes occur; 2) to determine the kinetics and magnitude of heat shock responses of the vegetables via the heat shock protein (hsp70) quantification and 3) to design sub-lethal heat priming regimes to boost productivity and/or qualities of food crops in response to heat stress. Six temperate vegetables (Lacluca saliva 'Canasta', Eruca saliva "Arugula", L. serriola, L. saliva L. 'Salinas' and RILs 141 and 192) were stressed with high temperature and their physiological and photosynthetic responses were recorded. Dose relationships were observed in shoot FW, Asat, gs sat, Pmax, ETR, qP & NPQ with significant decrease in performance at 28°C (versus 25°C) for selected species. Through these relationships of physiological performances with response to temperature, LT50, for each stress index was calculated. This is the temperature at which the measured parameter would differ from that of control plants (25°C RZT) by 50%. It is expected that LT50 value of maximum quantum yield, of photosystem II, Fv/Fm ratio (-46°C) will approximate the critical (pessimum) temperature of the plant species. Heat shock protein (hsp70) analysis of plants grown in constant 25, 28, 32, and 36°C RZT showed that hsp70 level in the shoot tissue increased with increasing RZT when analysed at noon time. Acute heat shock experiment showed that Canasta and RIL192 had peak hsp70 at 38°C while Arugula and RIL 142 at 42°C with hsp70 attenuated at 46°C for all four vegetables. Temporal kinetics of hsp70 synthesis showed that 30 min heating at 38°C was enough to induce hsp70 synthesis in all four vegetables. From the hsp70 kinetic experiment, it was determined that heat shock response could be elucidated at -38°C for most of the vegetables. From these two findings, heat priming regime at 38°C was designed, and both control and heat primed plants were later subjected to 46°C heat shock to test the effects of heat priming. It was discovered that heat primed vegetables could have productivity comparable to control plants that were kept constantly in cool RZTs and yet have higher nutritional value in terms of ascorbic acid content and phenolic compound levels. Conversely, plants that were not heat primed and constantly kept in 25°C RZT had much lower shoot productivity when subjected to 46°C heat shock 21-30 DAT compared to both heat primed vegetables and plants kept in constant 25°C RZT. This study further suggested that aeroponic plants could be kept in ambient RZT after 11 DAT, thus conserving greenhouse energy expenditure.