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Environmental and Stress Physiology

Leaf Magnesium Alters Photosynthetic Response to Low Water Potentials in Sunflower ¹

Department of Plant Biology, University of Illinois, Urbana, Illinois 61801, Department of Agronomy, University of Illinois, Urbana, Illinois 61801, United States Department of Agriculture, Agricultural Research Service, Urbana, Illinois 61801

We grew sunflower (Helianthus annuus L.) plants in nutrient solutions having nutritionally adequate but low or high Mg²⁺ concentrations and determined whether photosynthesis was effected as leaf water potentials (_w) decreased. Leaf Mg contents were 3- to 4-fold higher in the plants grown in high Mg²⁺ concentrations (10 millimolar) than in those grown in low concentrations (0.25 millimolar). These contents were sufficient to support maximum growth, plant dry weight, and photosynthesis, and the plants appeared normal. As low _w developed, photosynthesis was inhibited but moreso in high Mg leaves than in low Mg leaves. The effect was particularly apparent under conditions of light- and CO₂-saturation, indicating that the chloroplast capacity to fix CO₂ was altered. The differential inhibition observed in leaves of differing Mg contents was not observed in leaves having differing K contents, suggesting that the effect may have been specific for Mg. Because Mg²⁺ inhibits photophosphorylation and coupling factor activities at concentrations likely to occur as leaves dehydrate, Mg may play a role in the inhibition of chloroplast reactions at low _w, especially in leaves such as sunflower that markedly decrease in water content as _w decreases.

³ Present address: Department of Horticulture, University of Missouri, Columbia, MO 65211.

⁴ Present address: Department of Soil and Crop Sciences, Texas A&M University, College Station, TX 77843.

¹ Supported in part by National Science Foundation Grant PCM 79-09790 and Department of Energy Grant DE-FG05-84ER13273 to J. S. B.

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