Influence of drought stress on leaf anatomical structure and micro-morphology traits and chloroplast ultrastructure of three Malus species

The changes of leaf anatomical structure and several micro-morphology features(stomatal density and size,and the upper and lower cuticle thickness)as well as chloroplast ultrastructure were observed for three Malus plants,drought-tolerant M.prunifolia and M.sieversii,and drought-sensitive M.hupehensis,grown under mild,moderate and severe water deficit,using the semi-thin/ultrathin section,light microscope,scanning electron microscopy(SEM),and transmission electron microscopy(TEM)methods.For the three genotypes,compared with well watered plants,the leaf mesophyll contents and palisade tissue thickness as well as cell tightness rate(CTR)in droughtstressed plants significantly decreased(P0.05),while the sponge tissue thickness and scattered rate(SR)significantly increased(P0.05).SEM photos indicated that the stomatal density in young leaves of drought-stressed plants increased(P0.05),while the stomata width,stomatal opening rate,and the relative opening degree of stomata decreased.TEM analysis showed that the upper and lower cuticle thickness of M.prunifoliaand M.sieversii increased,whereas those of M.hupehensisincreased first and then decreased with prolonged drought treatment,swelling chloroplast,decreased the number of larger starch granules,and loosing grana and thylakoid were the typical leaf ultrastructure for medium water stress.Under severe water deficit,the chloroplasts were round in a shape,with more damagedstructure of membranes,and an extensive vacuolization and disorganization of thylakoid.However,the degree of drought-induced damage was smaller in M.prunifoliaand M.sieversii plants as compared to M.hupehensis plants,duo to the ability to maintain cell integrity.