Sun and shade ecotypes of Solanum dulcamara L.: Photosynthetic light dependence characteristics in relation to mild water stress.

This work extends previous studies on ecotypic differentiation in Solanum dulcamara to include interactions between irradiance and plant water status, in particular the effect of mild water stress on the responses of the photosynthetic apparatus of sun and shade ecotypes to high irradiance levels. The tolerance of shade ecotypes to high irradiance was markedly decreased by even a mild water stress, as evidenced by an aggravation of the degree of inhibition of the quantum efficiency of photosynthesis, and of other symptoms of photoinhibition that are commonly seen when shade plants are grown at high irradiances. In contrast, the photosynthetic apparatuses of ecotypes native to dry, sunny habitats were completely stable at the same water stress and irradiance levels that caused severe inhibitory effects in shade ecotypes. Growing the sun ecotypes under high irradiances resulted in a markedly higher photosynthetic capacity than when they were grown under a low irradiance level, irrespective of the water potential of the root medium.Surprisingly, stomatal factors are not responsible for the increase in sensitivity to photoinhibition that takes place when shade ecotypes are subjected to water stress, nor can they account for the differences between sun and shade ecotypes in their photosynthetic responses to high irradiance and water stress. Possible underlying causes for the observed effects of water stress and ecotypic differences in sensitivity to high light damage of the photosynthetic apparatus are discussed.