Improvement of polyamine synthesis maintains photosynthetic function in wheat during drought stress and rewatering at the grain filling stage

Drought stress during the grain-filling stage damages the photosynthetic physiology of wheat ( Triticum aestivum L.) and causes significant yield losses. To investigate the responses of polyamine synthesis of different wheat cultivars to drought stress and rewatering in the grain filling stage, a new cultivar, Zhengmai 1860, with high yields and high resistance characteristics, was selected as cultivar. The results showed that the net photosynthetic rate ( P n ), intercellular CO 2 concentration ( C i ), stomatal conductance ( g s ) and water use efficiency (WUE) of Zhengmai 1860 were significantly higher than those of Zhoumai 18 under drought stress. Compared with drought sensitive Zhoumai 18 cultivar, Zhengmai 1860 could induce the expression of polyamine synthesis-related enzyme genes, promote polyamine synthesis through both the directly and ornithine metabolism pathway of arginine, and the methionine metabolism pathway, maintain the balance of substances involved in osmotic regulation. Meanwhile, transcript levels of rbcL and rbcS were maintained, which thus protected the integrity of chloroplasts and thylakoid membrane ultrastructure as observed by transmission electron microscopy. After rewatering following drought exposure, the thylakoid lamellae of Zhengmai 1860 were arranged in an orderly manner, and the actual photochemical efficiency and net photosynthetic rate recovered more significantly. Therefore, Zhengmai 1860 maintained high grain yields when rewatering was applied after 7 days of drought stress. The results of this study indicated that the synthesis ability of endogenous polyamines was closely related to the drought stress tolerance in the grain filling stage, which could provide a reference for breeding drought-tolerant wheat cultivars.