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Paraheliotropic Leaf Movements in Common Bean under Different Soil Nutrient Levels

Dep. of Horticultural Sciences, Texas A&M Univ., College Station, TX 77843
Dep. of Biology, Univ. of Utah, Salt Lake City, UT 84112

^* Corresponding author.

The primary objective of this study was to investigate the possible physiological consequences of paraheliotropic leaf movements in response to soil nutrient levels in well-watered common bean (Phaseolus vulgaris L. cv. Cahone). In pot-grown plants, leaflets from low-fertilizer treatment (LF) plants oriented away from the sun's direct rays, receiving a significantly lower photosynthetic photon flux density (PPFD) than leaflets from high-fertilizer treatment (HF) plants. Leaf conductance and transpiration rate were significantly lower in LF treatment plants than in HF plants. Interactions among incident PPFD, transpiration rate, and leaf area resulted in equivalent leaf temperatures under different fertilizer levels. Leaf water potential was not significantly different between these treatments Carbon isotope discrimination of HF plants was significantly lower than that of LF plants, indicating that intercellular CO₂ concentrations were lower and long-term water-use efficiency higher for HF plants. The variations in water-use efficiency most likely resulted from a lower ratio of photosynthesis to leaf conductance in LF plants than in HF plants. Similar tendencies were observed in field-grown plants. Photosynthetic light-saturation point was lower for LF plants than for HF plants. As a result of paraheliotropic leaf movements, incident PPFD on leaflets remained close to the photosynthetic light-saturation points in both treatments.

Received for publication September 26, 1991.