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Photosynthetic and Respiratory CO₂ Exchange of Sugar Beet Leaves as Influenced by Manganese Deficiency¹

Photosynthetic and respiratory CO₂ exchange rates were determined for attached leaves of manganese-deficient or -sufficient sugar beets (Beta vulgaris L. cv. ‘F58-554H1’) cultured hydroponkally in a growth chamber. On withholding Mn from the culture solution, the Mn concentration of the leaf blades decreased from 8 to 10 to about 0.3 meq Mn²⁺ kg^-1 dry matter and rates of photosynthetic CO₂ uptake per unit leaf area decreased to about onethird of the control rates. There was no apparent effect of Mn deficiency on stomatal opening since leaf diffusion resistance (r₁’) was unchanged, but mesophyll resistance to CO₂ transfer (r_m) increased from 2.5 to 10 sec cm^-2 in low Mn leaves. Manganese deficiency decreased rates of photorespiratory evolution of CO₂ into CO₂-free air but had no effect on respiration in the dark.

Key Words: Stomatal resistance • Mesophyll resistance • Photorespiration

² Assistant Professor of Environmental Plant Physiology and Plant Physiologist, University of California, Berkeley, CA 94720.

Received for publication November 14, 1973.