Effects of Exogenous Putrescine on Leaf Anatomy and Carbohydrate Metabolism in Cucumber (Cucumis sativus L.) Under Salt Stress

The objective of this study was to identify the effects of exogenous putrescine (Put) on leaf traits, hormones levels, and carbohydrate metabolism in cucumber seedlings under salt stress. The stress of 75 mM NaCl decreased plant growth and photosynthetic carboxylation efficiency. NaCl also increased the leaf thickness but destroyed the leaf internal structure. The contents of indoleacetic acid, zeatin, and zeatin riboside were decreased and the abscisic acid level was increased by NaCl. In addition, NaCl stress caused the accumulation of sucrose and starch in leaves and led to the feedback inhibition of photosynthesis, which can be inferred from the lower maximum rate of RuBisCo-mediated carboxylation (V cmax). The analysis of starch-metabolizing enzyme activities suggested that the largely improved amylopectin contents contributed to the starch accumulation. Put regulated the levels and ratios of endogenous hormones, lowered the leaf thickness, and protected the leaf structure from damage caused by NaCl. Furthermore, Put decreased the accumulation of sucrose and starch caused by NaCl in leaves through regulating the activities of enzymes involved in carbohydrate metabolism. In conclusion, Put regulates the hormones levels, leaf structure, and carbohydrate metabolism and therefore improves the photosynthesis and growth of NaCl-stressed cucumber plants.