Relationships Between Nitrogen and Water Concentration in Shoot Tissue ofMolinia caeruleaDuring Shoot Development

Plants of Molinia caerulea were supplied with either a low (0.2 mol m -3 ) or high (10 mol m -3 ) supply of nitrogen over two growing seasons. A total of 14 destructive plant harvests were made: when plants were in an over-wintering state prior to the second season; immediately following bud burst; and on 12 further occasions throughout the second season. The relationships between shoot nitrogen concentration on a dry mass basis, shoot water content and plant developmental stage were investigated. Shoot nitrogen concentration on a dry mass basis fell as the growing season progressed. In contrast, the concentration of nitrogen in tissue water after bud burst showed only a slight reduction. The concentration of nitrogen both on a dry mass basis and in tissue water was greater for plants receiving the higher supply of nitrogen. Shoot water content was highest immediately following bud burst then declined as the season progressed, with plants receiving the low nitrogen supply having slightly greater shoot water contents. It was concluded that the decline in shoot nitrogen concentration of M. caerulea on a dry mass basis as the mass increased was mainly explained by changes in shoot water content. The observed increase in the rate of decline of both shoot nitrogen concentration and water content with increased shoot mass coincided with the cessation of leaf tissue production and was therefore due to a switch from the production of leaves to other tissues. Copyright 1999 Annals of Botany Company