Interactive effects of phosphorus and light availability on early growth of maritime pine seedlings

Abstract

We examined the response of early growth of maritime pine seedlings to combined levels of light and phosphorus. Seedlings were grown under three levels of phosphorous availability, i.e., two relative addition rates (RAR = 2 and 4 g P (100 g -1 )P d -1 ) and a free-access to P, crossed with two light levels (photosynthetic photon flux densities of 150 and 450 mol m -2 s -1 , respectively). Relative growth rate (RGR) and relative uptake rate of phosphorus (RUR) were computed, as well as the amount of light absorbed per seedling. We found that phosphorus and light acted as limiting factors with a complex interaction. Under low light and at the lowest P level, P and light were co-limiting, i.e., growth was enhanced only when P and light were increased together. Light was the limiting factor for growth under low light conditions at all other levels of P availability. P was the limiting factor at a RAR of 2% under high light. Enhancing P from 4% to free access did not significantly improve growth under high light. RUR was controlled systematically by P availability at 2 and 4% RAR. RGR values were close to RUR values except under free access to P. Therefore, growth-independent accumulation of P was observed under high P conditions. The differences in biomass production among P treatments were explained primarily by the reduced amount of radiation intercepted by the seedlings as a consequence of their reduced leaf area. No effect of P treatments were observed on the calculated radiation-use efficiency (RUE), which was found to be larger under low light. This confirms that pine seedlings adjust to moderate phosphorus deficiency mainly by changing their morphology (leaf area, dry-mass partitioning) while biochemical and photochemical limitations of photosynthesis play only a very secondary role.