Effects of light quality on growth and N accumulation in birch seedlings.

We studied the effects of light quality and nutrient supply on growth and nitrogen accumulation in silver birch (Betula pendula Roth) seedlings to test three hypotheses: (1) growth of birch seedlings is sensitive to changes in light quality; (2) the response of birch seedling growth to light quality depends on nutrient supply; and (3) assimilation and allocation of nitrogen by birch seedlings are affected by light quality. The two light regimes simulated the spectral quality of sunlight and shadelight, but did not differ in photosynthetic photon flux density, and the two nutrient supply regimes differed in the rate of supply, but not in the composition, of mineral nutrients. Accumulation and allocation of dry weight and nitrogen were strongly affected by nutrient supply regime, but light quality had little effect. During the first 15 days of the experiment, the largest effect of light quality was on height growth, which was greater in seedlings in simulated shadelight than in seedlings in simulated sunlight. Light quality had little effect on dry weight and nitrogen allocation to the stem during this period. However, at the end of the experiment (Day 29), there was an increase in N concentration per unit dry weight in leaves and stems of seedlings in the simulated shadelight plus high nutrient supply treatment.