Acclimation of plants to light gradients in leaf canopies: evidence for a possible role for cytokinins transported in the transpiration stream

The mechanism of response of plants to vertical light intensity gradients i n leaf canopies was investigated. Since shaded leaves transpire less than l eaves in high light, it was hypothesized that cytokinins (CKs) carried by m ass transport in the transpiration stream would be distributed over the lea f area of partially shaded plants parallel to the gradient in light intensi ty. It was also hypothesized that this causes the distribution of leaf grow th, leaf N and photosynthetic capacity, and possibly chloroplast acclimatio n as observed in plants growing in leaf canopies. In a field experiment, th e distribution of Ca, N and CKs in a bean leaf canopy of a dense and an ope n stand supported the concept of a role for CKs in the response of N alloca tion to the light gradient when a decreasing sensitivity for CKs with incre asing leaf age is assumed. Both shading of one leaf of the pair of primary bean leaves and independent reduction of its transpiration rate in a growth cabinet experiment caused lower dry mass, N and Ca per unit leaf area in c omparison to the opposite not treated leaf. Shading caused a parallel reduc tion in CK concentration, which supports the hypothesis, but independent re duction of transpiration rate failed to do the same. Application of benzyla minopurine (BA) counteracted the reduction caused by shade of leaf N, photo synthetic capacity and leaf area growth. The experiments show an important role for the transpiration stream in the response of plants to light gradie nts. Evidence is presented here that CKs carried in the transpiration strea m may be important mediators for the acclimation of plants to leaf canopy d ensity.