INFLUENCE OF PHOTOSYNTHETIC PHOTON FLUX-DENSITY ON GROWTH AND TRANSPIRATION IN SEEDLINGS OF FAGUS-SYLVATICA

Beech seedlings (Fagus sylvatica L.) were grown in various combination s of three photosynthetic photon flux densities (PPFD, 0.7, 7.3 or 14. 5 mol m(-2) day(-1)) for two years in a controlled environmental chamb er. Dry mass of leaves, stem and roots, leaf area and number of leaves , and unit leaf rate were affected by both previous-year and current-y ear PPFD. Number of shoots and length of the main shoot were affected by previous-year PPFD but not by current-year PPFD. Number of leaves p er shoot did not change with PPFD, whereas leaf dry mass/leaf area rat io was mainly affected by current-year PPFD. During the first 10 days that newly emerged seedlings were grown at a PPFD of 0.7 or 14.5 mol m (-2) day(-1), transpiration rate per unit leaf area declined. Thereaft er, transpiration increased to a constant new rate. Transpiration rate per seedling was closely related to leaf area but the relationship ch anged with time. In two-year-old seedlings grown at various PPFD combi nations of 0.7, 7.3 and 14.5 mol m(-2) day(-1) during Years 1 and 2, l eaf area and transpiration rate per seedling were closely correlated a t Weeks 7 and 11 after bud burst. Weak correlations were found between root dry mass and transpiration rate per seedling. During Year 2, tra nspiration rate per leaf area was higher at a particular PPFD in seedl ings grown at a previous-year PPFD of 0.7 mol m(-2) day(-1) than in se edlings grown at a previous-year PPFD of 14.5 mol m(-2) day(-1). After transfer of two-year-old seedlings at the end of the experiment to a new PPFD (7.3 or 14.5 mol m(-2) day(-1)) for one day, transpiration ra tes per leaf area, measured at the new PPFD, were correlated with leaf area and root dry mass, irrespective of former PPFD treatment.