Acclimation of Myrtus communis to contrasting Mediterranean light environments - effects on structure and chemical composition of foliage and plant water relations

Leaf anatomical and chemical characteristics. water relations and stomatal regulation were studied in the shrub Myrtus communis growing under two cont rasting Mediterranean light environments (full light versus 30% of full lig ht) during the spring-summer period. These studies aimed to assess plant re sponse to the combined effects of light and water availability. Foliar morp hology, anatomy and chemistry composition acclimated positively to light co nditions. Leaves of sun-exposed plants were thicker (38.7%) than those of s haded plants. mainly due to increased palisade parenchyma thickness. had a higher nitrogen concentration and stomatal density than the shade ones, whi ch maximized foliar area (> SLA) and Chl/N molar ratio to improve light int erception. Chlorophyll concentration per leaf area (Chl(a)) was always high er in jun Leaves while. as expressed on dry mass (Chl(m)), significant diff erences were only apparent in September. shade leaves presenting higher val ues. During the summer period Chl(a). and Chl(m), markedly declined in sun leaves and remained unchanged in shade ones. The ratio of chlorophyll alb w as not affected either by the light intensity or by the season. Shade leave s presented generally a higher concentration of soluble carbohydrates per d ry mass. No significant differences in starch concentration were apparent b etween sun and shade leaves and a gradual depiction occurred during the wat er stress period. Maximum stomatal conductances correlated positively with predawn water potential. Throughout the season. sun plants always presented higher leaf conductance to water vapour and lower minimum leaf water poten tials. indicating an interaction of light-environment on these water relati on parameters. Stomatal closure constitutes a mechanism to cope with diurna l and seasonal water deficits. sun plants presenting a more efficient contr ol of water losses during water deficiency period. In addition, both sun an d shade plants evidenced leaf osmotic adjustment ability in response to wat er stress, which was greater in sunones. (C) 2001 Elsevier Science B.V. All rights reserved.