Effects of potassium deficiency on the photosynthesis and respiration of leaves of sugar beet

Sugar beet plants (Beta vulgaris L. var. F5855441) were germinated and cultured under standardized environmental conditions for 28 days. Potassium deficiency was then induced by withholding K from the culture solution. Changes in CO₂ and water vapor exchange rates and surface temperatures of individual attached leaves were measured with time after K cut-off, along with changes in the concentrations of the leaf minerals K, Na, Ca, Mg, Fe, Mn, Cu, and Zn. During the 1st week after K cut-off the concentration of Na in the leaf blade increased from 200 to 1000 milliequivalents per kilogram dry matter while K decreased from 1500 to 300 milliequivalents per kilogram. During the subsequent 2 weeks, both Na and K concentrations decreased. The concentrations of other leaf minerals, except Mn, were little affected by K cut-off. Photosynthetic CO₂ uptake per unit area decreased linearly with time after cut-off and attained one-third of the control rate after 21 days. Low K apparently decreased photosynthesis through an increase in mesophyll resistance to CO₂ (r_m) from 2.8 to 5.3 seconds per centimeter in 21 days. Leaf (mainly stomatal) diffusion resistance (r′₁) increased only slowly during the first 15 days from 0.3 to 0.5 second per centimeter, eventually reaching 1.6 seconds per centimeter at 21 days. Low K progressively decreased the photorespiratory evolution of CO₂ into CO₂-free air, but steadily increased the rate of CO₂ evolution in dark.