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

Abstract

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 (rm) increased from 2.5 to 10 sec cm‐1 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.