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Phosphorus deficiency was induced in sugar beet plants (Beta vulgaris L. var. F5855441), cultured hydroponically under standardized environmental conditions, by removal of phosphorus from the nutrient supply at the ten leaf stage 28 days after germination. CO(2) and water vapor exchange rates of individual attached leaves were determined at intervals after P cutoff. Leaves grown with an adequate nutrient supply attained net rates of photosynthetic CO(2) fixation of 125 ng CO(2) cm(-2) sec(-1) at saturating irradiance, 25 C, and an ambient CO(2) concentration of about 250 mul l(-1). After P cutoff, leaf phosphorus concentrations decreased as did net rates of photosynthetic CO(2) uptake, photorespiratory evolution of CO(2) into CO(2)-free air, and dark respiration, so that 30 days after cutoff these rates were about one-third of the control rates. The decrease in photosynthetic rates during the first 15 days after cutoff was associated with increased mesophyll resistance (r(m)) which increased from 2.4 to 4.9 sec cm(-1), while from 15 to 30 days there was an increase in leaf (mainly stomatal) diffusion resistance (r(l)') from 0.3 to 0.9 sec cm(-1), as well as further increases in r(m) to 8.5 sec cm(-1). Leaf diffusion resistance (r(l)') was increased greatly by low P at low but not at high irradiance, r(l)' for plants at low P reaching values as high as 9 sec cm(-1).

Effects of Phosphorus Deficiency on the Photosynthesis and Respiration of Leaves of Sugar Beet