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Effects of Potassium Deficiency on the Photosynthesis and Respiration of Leaves of Sugar Beet under Conditions of Low Sodium Supply ¹

^a Department of Soils and Plant Nutrition, University of California, Berkeley, California 94720

Sugar beet plants (Beta vulgaris L. cv. F58-554H1) were germinated and cultured under standardized environmental conditions. The effects of K deficiency on photosynthetic and respiratory CO₂ exchange rates of attached leaves were studied under conditions of low Na supply by withholding both Na and K from the culture medium at cut-off (28 days after planting). Potassium and Na concentrations in the leaf blade and petiole decreased rapidly during the 8 days after cut-off, then more slowly.

Photosynthetic CO₂ uptake per unit leaf area decreased rapidly with time after cut-off to 23% of the control rate in 17 days. Mesophyll resistance to CO₂ (r_m) increased sharply after cut-off, r_m eventually attaining 8.3 sec cm^–1. Leaf (mainly stomatal) diffusion resistance, r₁', also increased rapidly from 4 days after cut-off, reaching 1.9 sec cm^–1 13 days later. The photorespiratory evolution of CO₂ into CO₂-free air decreased progressively after cut-off, but the rate of dark respiratory CO₂ evolution increased. It was concluded that withholding Na as well as K at cut-off increased the deleterious effects of K deficiency on photosynthesis and stomatal opening.