Impact of Water Stress on Leaf Anatomy and Ultrastructure in Cynodon dactylon (L.) Pers. under Natural Conditions

Abstract

Seasonal changes in leaf anatomy and ultrastructure were studied in a sward of Cynodon dactylon (L.) Pers. grown under Mediterranean field conditions and under water stress. Relative water content (RWC), leaf water potential (ψ), and specific leaf weight were determined. Anatomical measurements included leaf thickness and number and area of bundle sheath and mesophyll cells. Quantitative measurements from electron micrographs were used to evaluate the subcellular structure of mesophyll and bundle sheath cells from well-watered and water-stressed plants. In well-watered swards leaf mesophyll cell number was highest at the beginning of June, but in stressed plants the maximum was reached in the middle of July. Water stress decreased both mesophyll and bundle sheath cell areas during the experimental period, although there were no significant differences in plant water relations in autumn. Ultrastructural changes in bundle sheath cells under stress included increase in starch deposition in chloroplasts, changes in the orientation of thylakoids, and reduction in chloroplast area. Furthermore, water stress increased the cell wall thickness by 20%. Stressed mesophyll chloroplasts were characterized by an increase in the peripheral reticulum and in starch granules and a decrease in the amount of grana stacking related to a decrease in leaf sodium concentration. The number of mitochondria per mesophyll cell was increased by water stress. Longer periods of stress induced folds in the outer tangential walls in bundle sheath cells. In mesophyll cells alterations in cellular shape and in plasmalemma were observed, and the cytosol appeared to be markedly heterogeneous. In late summer the chloroplast envelope was swollen and distorted; undulating dilated thylakoids were observed and the stroma appeared less electron dense. In mitochondria, the matrix became progressively clearer. Environmental stresses influenced leaf anatomy and cell ultrastructure. Furthermore, ultrastructural changes could be related to variations in potassium and sodium contents.