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Effects of leaf nutrient status on photosynthetic capacity in loblolly pine (Pinus taeda L.) seedlings grown in elevated atmospheric CO₂

R. B. Thomas (1, 2), J. D. Lewis (1) and B. R. Strain (1)

1. Duke University Phytotron, Botany Department, Box 90340 Science Drive, Durham, NC 27708, USA / 2. Author to whom correspondence should be addressed / Received July 19, 1993

Summary

We measured needle photosynthesis of loblolly pine seedlings grown in a factorial experiment with two CO₂ partial pressures (35 and 65 Pa) and three nutrient treatments (7 mM NH₄NO₃ + 1 mM PO₄; 7 mM NH₄NO₃ + 0.2 mM PO₄; 1 mM NH₄NO₃ + 1 mM PO₄). The data were used to parameterize a physiologically based photosynthetic model that included limitations imposed by ribulose-1,5-bisphosphate carboxylase/oxygenase activity, electron transport capacity and inorganic phosphate availability. With nonlimiting nutrients, seedlings grown at 65 Pa CO₂ had significantly higher net photosynthesis and lower stomatal conductance than seedlings grown at 35 Pa CO₂. Nutrient limitations by either N or P significantly reduced photosynthetic capacity. When either N or P was limiting, there was no effect of growth CO₂ partial pressure on photosynthesis, but stomatal conductance was significantly lower for seedlings grown at 65 Pa CO₂. Modeled biochemical parameters suggest that, in all cases, photosynthesis was co-limited by carboxylation, electron transport and phosphate regeneration. Acclimation to growth in elevated CO₂ involved a reduction in leaf N content. In the low-N and low-P treatments, modeled parameters indicated that the biochemical processes of photosynthesis were down regulated to the point that there was no effect of increasing CO₂ partial pressure. The capacity to regenerate phosphate was reduced in both low nutrient treatments, but was only reduced by elevated CO₂ when seedlings were grown under low soil P conditions. Increased photosynthetic water use efficiency and nutrient use efficiency in response to CO₂ enrichment occurred in all three nutrient treatments and have important implications for whole-plant water and nutrient balance. These data support the contention that soil nutrient status in forest ecosystems will be a critical influence on tree seedling response to increasing atmospheric CO₂ partial pressures.

Keywords: carbon dioxide enrichment, net photosynthesis, nutrient limitations.

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ISSN 0829-318X Copyright © 2002–2008 Heron Publishing

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