Natural plant inducer 2-Amino-3-Methylhexanoic acid protects physiological activity against high-temperature damage to tea (Camellia sinensis)

Increasing frequency of extreme summer high-temperature (HT) has become one of the most important limiting factors affecting tea production in China. 2-Amino-3-methylhexanoic acid (AMHA) is a unique natural plant inducer against HT stresses. In this study, the field efficacy and physiological mechanism of AMHA for improving tea plant resistance against HT in the summer was studied using the temperature-sensitive tea cultivar "Longjing 43″. It was observed that field-grown tea plants treated with 100 or 1000 nM AMHA maintained erect and normal stems and foliage under HT stress. The optimal concentration of AMHA for counteracting HT in field-grown tea plants was 100 nM. At three days after treatment (DAT), AMHA at 100 nM increased net photosynthetic rate Pn by 199% and stomatal conductance Gs of tea plants by 215% of tea plants. The fast chlorophyll a rise kinetics and JIP-test analysis further confirmed that AMHA improved the overall photosynthetic activity of photosystem II (PSII) mainly through enhancing the fraction of active oxygen evolving complex (OEC) centers, PSII energetic connectivity and electron transport efficiency. PIABS reflecting PSII overall activity in 100 nM AMHA-treated tea plants was increased by 95%, 51%, 39%, and 48% at 3, 5, 7, and 14 DAT, respectively, compared to mock. At the same dose, AMHA also increased osmotic adjustments for the rapid accumulation of amino acids and soluble sugars, which increased by 21% and 145% at 7 DAT, respectively. In addition, AMHA significantly enhanced the activities of peroxidase (POD), catalse (CAT) and superoxide dismutase (SOD), reducing oxidative damage on membrane lipids. Even at 14 DAT, 100 nM AMHA led to 50%, 75% and 19% increase in POD, CAT and SOD activity in tea plants relative to mock, respectively. Clearly, exogenous application of AMHA facilitated HT resistance by alleviating physiological damage in field-grown tea plants due to improved photosynthetic performance, osmotic adjustments and antioxidant enzyme activities.