Alterations in photosynthetic capacity and morpho-histological features of leaf in alfalfa plants subjected to water deficit-stress in different soil types

Abstract

Water deficit can greatly influence plant growth and development. A pot trial was conducted to study the impact of various levels of water deficit on alfalfa (Medicago sativa L.) plants grown in different soil types. The results elucidated that increasing drought level and/or sand proportion in the cultivation soil reduced leaf growth vigor as indicated by declined number of leaves per plant as well as less cumulative leaf biomass, area, specific area, relative water content and degree of succulence with corresponding increase in the cumulative degree of leaf sclerophylly. Leaf anatomical analysis revealed that drought increased leaf thickness and ground tissue thickness but reduced phloem, xylem and whole vascular bundle areas, as well as the number of xylem vessels and rays. In addition, transmission electron microscopic examination of leaves cleared that the shape of chloroplasts was irregular when droughted. Also, stress reduced the number of both chloroplasts per cell and starch grains per chloroplast as well as the area of individual chloroplasts and starch grains. Moreover, severe water deficit resulted in the appearance of plastoglobules within chloroplasts. Concerning pigment fractions, chlorophyll a, b and a + b as well as total pigments and photosystem II activity were all declined in water-unsatisfied plants. Meanwhile, stress enhanced the production of carotenoids and also increased the calculated chlorophyll a/b ratio. The results also showed that water deficit increased glucose, fructose, trehalose, sucrose and total soluble sugars but reduced the amount of polysaccharides and total carbohydrates in alfalfa plants.