Effects of exogenous putrescine on the ultrastructure of and calcium ion flow rate in lettuce leaf epidermal cells under drought stress

Abstract

The alleviation effects of exogenous putrescine treatment on the ultrastructure of and calcium ion flow rate in lettuce (Lactuca sativa L.) leaf cells under drought stress were studied. Lettuce seedlings were treated with foliar sprays of 0.1 mM putrescine for 8 days, after which drought stress was simulated by using 10% polyethylene glycol 6000. The morphological characteristics of the seedlings and the calcium ion flow rate across stomatal guard cells were subsequently determined, and the leaf cell ultrastructure was observed via transmission electron microscopy. Under drought stress, the morphological characteristics of the seedlings decreased, and calcium ion influx was predominant in the guard cells. In addition, compared to that under control conditions, the stomatal density under drought stress conditions increased significantly, the open/closed stoma ratio was lower, and the degree of stomatal opening was smaller. Exogenous putrescine sprays effectively reduced the stomatal density, increased the degree of stomatal opening, and increased the proportion of open stomata. In addition, the chloroplasts became round in shape, the thylakoid structure became blurry in appearance, the number of starch grains decreased, many osmium granules were produced, and plasmolysis occurred in the mesophyll cells. However, the chloroplasts were elongated, the thylakoid structure was clear, the starch grains were abundant, few osmium granules were produced, and plasmolysis did not occur. The above results show that, by altering the leaf cell ultrastructure as well as the flow rate and direction of calcium ions in guard cells, exogenous putrescine effectively improves the drought tolerance of lettuce.