Studies On The Effects Of High Temperature Stress And Growth Regulators Abscisic Acid Cytokinins And Brassinosteroids On Photosynthetic Apparatus Of Tomato (Lycopersicon Esculentum Mill Cv 9021)

Abiotic stresses such as drought, salinity and extreme temperatures cause significant losses of crop productivity and quality. In abiotic stress prone areas, vegetable production is reduced, which leads to malnutrition and associated health disorders in these regions. One form of abiotic stress synonymous with reduced yields is high temperature stress. Alternatives currently available for alleviating losses in crop productivity and quality due to the effects of high temperature stress includes development of crops with an inherent capacity to withstand high temperature stress through plant breeding and biotechnology. Plant breeding though effective is time consuming and too demanding in terms of the procedures and methodology. Therefore biotechnology which includes the use of hormones to modulate stress responses and control plant growth responses becomes handy. The growth regulators such as Abscisic acid (ABA), cytokinins (CTK) and brassinosteroids (BRs) are reputed to have some adaptive responses to environmental stresses.Of these three growth regulators brassinosteroids has been intensively studied in the last few years. They have a potential not only to increase yield and quality of crops but also to modulate abiotic stresses such as high temperature. Although BRs have been widely used in increasing yields and stress tolerance, the mechanism, however, is still not clear. Therefore, it is important to study the role of BRs in the regulation of high temperature induced inhibition of photosynthesis and the subsequent role of antioxidant enzymes in the process. In this study, we investigated the effects of optimum and non optimum temperatures on photosynthesis, the effects of growth regulators ABA and CTK on photosynthesis and proceeded to establish the role EBR on photosynthesis of tomato plants (Lycopersicon esculentum Mill) subjected to different temperature and discussed mechanism used by EBR to regulate the detrimental effects of high temperature on photosynthesis. Experiments were also conducted to examine the effect of EBR on antioxidant system activities and photosynthetic apparatus under high temperature stress conditions. The results are as follows:High temperature stress at 35℃ reduced the net photosynthetic rate, the maximal photochemical efficiency (Fv/Fm), the quantum yield of the photosystem â…¡ (ΦS â…¡) reaction centers, and the efficiency of excitation capture of open PS â…¡ centre (Fv'/Fm'), which caused destruction of the photosynthetic apparatus in detached tomato leaves. The destruction was through a damaged portion of PS â…¡ reaction centre as a result of the physical dissociation of the light harvesting complex from the PS â…¡ core, a decrease in the number of open PS â…¡ reaction centre, impaired capacity for QA reduction, decreased non-radiative energy dissipation under high temperature and the reduction in the efficiency of energy capture by these open centers, which finally led to a reduced rate of electron transfer.High temperature stress caused a significant increase in the activities of APX, SOD, G-POD, and the contents of MDA but no significant increase in the activities of CAT. Photorespiration, Mehler reaction and cyclic electron flow are potential candidates for dealing with the increase in electron flux relative to CO₂ assimilation during high or low temperature. Mehler reaction was identified as the electron sink operating to sustain a high Φ_PSâ…¡/Φ_CO2, under high temperatures stress, whereas photorespiration acted as a protective mechanism at high temperatures and cyclic electron transport probably contributed as an alternative electron sink under low temperatures.Growth regulators Abscisic acid (ABA), Cytokikins And 24-epibrassinolide (EBR) showed a dose dependent manner with respect to gas exchange, chlorophyll fluorescence and activities of antioxidant enzymes, lower concentrations of the growth regulators protected the photosynthetic apparatus more efficiently. This was as a result of increase of Fv/Fm, ΦPSâ…¡, and qP that kept more PS 11 reaction centers in an open state and the increased energy capture by these open centers in addition, the excitation energy was used for electron transport, under slightly elevated temperature's conditions. The less in Fv'/fm' implies that the low concentration of the growth regulators resulted in less dissipation of excitation energy as heat in the PS 11 antennae. The higher concentration not only became cytotoxic but also induced a severe oxidative stress that could not be controlled by the enhanced antioxidant defense systems. In my studies the concentration of 0.1 and 1.0 mg l^-1 CTK maintained a high photosynthetic rate by inhibiting oxidativedegradation, reduction of MDA levels, acting as membrane protectant and thus delaying senescence in the detached leaves of tomato at slightly elevated temperatures.The effects of 24-epibrassinolide (EBR) spray application on gas-exchange, chlorophyll fluorescence characteristics, were investigated in tomato (Lycopersicon esculentum Mill. Cv 9021) plants grown in a*greenhouse and exposed to 40/30 C for 8d and then returned to optimal conditions for 4d. EBR significantly increased the net photosynthetic rate (Pn), stomatal conductance (gs), maximum carboxylation rate of Rubisco (Vcmax), the maximum potential rate of electron transport contributed to ribulose -1,5-bisphosphate (RuBP), (Jmax)> as well as to the relative quantum efficiency of PS II photochemistry (OPSII) and photochemical quenching while decreasing non-photochemical quenching (NPQ). Generally, spraying with 0.1 mg I"1 EBR proved to be most effective by significantly alleviating high temperature induced inhibition of photosynthesis. EBR-treated leaves also had a higher quantum yield of PSII electron transport (PSH) than the controls, which was mainly due to a significant increase in the photochemical quenching (qP), with no change in the efficiency of energy capture by open PSII reaction centers (Fv/Fm). EBR did not influence photorespiration.Photosynthesis measured under different temperatures was significantly increased in plants after spraying with 0. lmg I'1 EBR for 8d. High temperature stress significantly decreased Fv/Fm and OPSII after 8d. However, this reduction was significantly alleviated by EBR treatment. Further more, higher photochemical activity in EBR plants was accompanied by higher Vmax> RuBP and low heat dissipation rate in PSII which contributed to alleviating photoinhibition of PSII during high temperature stress. Moreover, EBR treatment also significantly increased the activities of antioxidative enzymes (APX, SOD, CAT, G-POD), the plant biomass, as well as the decrease in the contents of MDA and H2O2. It was concluded that EBR could alleviate the detrimental effects of high temperature on plant growth by increasing carboxylation efficiency.