NITRATE SUPPLY AND THE BIOPHYSICS OF LEAF GROWTH IN SALIX-VIMINALIS

The influence of nitrogen on leaf area development and the biophysics of leaf growth was studied using clonal plants of the shrub willow, Salix viminalis grown with either optimal (High N) or sub-optimal (Low N) supplies of nitrate. Leaf growth rate and final leaf size were reduced in the sub-optimal treatment and the data suggest that in young rapidly growing leaves, this was primarily due to changes in cell wall properties, since cell wall extensibility (% plasticity) was reduced in the Low N plants. The biophysical regulation of leaf cell expansion also differed with nitrogen treatment as leaves aged. In the High N leaves, leaf cell turgor pressure (P) increased with age whilst in the Low N leaves P declined with age, again suggesting that for young leaves, cell wall plasticity limited expansion in the Low N plants. Measurements of cell wall properties showed that cell wall elasticity (% E) was not influenced by nitrogen treatment and remained constant regardless of leaf age.