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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. C02 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 CO2 fixation of 125 ng CO2 cm-2 sec-' at saturating irradiance, 25 C, and an ambient CO2 concentration of about 250 ,l I-'. After P cutoff, leaf phosphorus concentrations decreased as did net rates of photosynthetic C02 uptake, photorespiratory evolution of C02 into C02-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 (rm) which increased from 2.4 to 4.9 sec cm-', while from 15 to 30 days there was an increase in leaf (mainly stomatal) diffusion resistance (r,') from 0.3 to 0.9 sec cm-', as well as further increases in rm to 8.5 sec cm-'. Leaf diffusion resistance (ri') was increased greatly by low P at low but not at high irradiance, rz' for plants at low P reaching values as high as 9 sec cm-'.Much of the phosphorus found in the sugar beet plant is in the inorganic form and has many roles in cell metabolism (2).

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