Potassium has been implicated functionally in numerous roles within the plant (7,17,25,26), and effects of its deficiency are manifested in many ways (4,12,13,24,25). Rates of photosynthetic CO2 uptake have been shown to be diminished by K deficiency (2,4,12,20,23). Peaslee and Moss (21) examined the effects of low K on the gas diffusion resistances of leaves and concluded that low K primarily affected photosynthetic CO2 uptake by increasing stomatal diffusion resistance, although mesophyll resistance to CO2 also increased. Respiration rates have been shown to increase with K deficiency in some instances (2,11,12) and to decrease in others (18,23 NaH2PO, H2O in place of the 2.5 Ca(NO.)2-4 H20, 0.5 KH2PO4, and 2.5 KNO2, of the control culture solution.The seeds were planted on day 0, and the germinated seedlings were transplanted at the two-leaf stage on day 14. Potassium deficiency was induced on day 28 (cut-off) by rinsing the plant roots with distilled water and transferring the plants individually to pots containing the K-deficient culture solution. Comparable plants serving as controls were transferred to solutions containing K. Culture solutions were replenished by the addition of stock solutions on day 35 and were replaced on day 42. The experiment terminated on day 49. Determination of Gas Exchange Parameters. The main parameters of leaf gas exchange considered below are: (a) the net rates of CO2 exchange in oxygen-free air, F*, and in normal air, i.e., containing about 21% oxygen, F; (b) the rate of respiratory CO2 evolution in the dark, RD; (c) the rate of respiratory evolution of CO2 in the light into CO2-free air, RL,

ABSTRACI'The effects of Mg deficiency on the photosynthesis and respiration of sugar beets (Beta vulgaris L. cv. F58-554H1) were studied by withholding Mg from the culture solution and by following changes in C02 and water vapor exchange of attached leaves. Leaf blade Mg concentration decreased from about 1200 to less than 200 meq kg-' dry matter without change in the rate of photosynthetic C02 uptake per unit leaf area, while from 200 to 50 meq kg-' the rate decreased to onethird. Rates of photorespiratory evolution of CO2 into C02-free air responded to Mg like those of photosynthetic C02 uptake, the rates decreasing to one-half, below 200 meq kg-'. Respiratory C02 evolution in the dark increased almost 2-fold in low Mg leaves. Magnesium deficiency had less effect on leaf (mainly stomatal) diffusion resistance (ri) than oni mesophyli resistance (rm); in Mg-deficient plants rm increased from 2.9 to 7.1 sec cm-', whereas ri became significantly greater than the control value only in the most severe instances of Mg deficiency.Magnesium is required by a large number of enzymes involved in energy transfer, particularly those utilizing ATP (2). It is a constituent of the Chl molecule and is required for the normal structural development of the chloroplast (4, 11), as well as other organelles such as the mitochondrion (5). Thus, it is to be expected that Mg deficiency would have damaging effects on photosynthesis and respiration. This has been shown for both processes in spinach (1), and for photosynthesis in maize (6). Magnesium deficiency has a pronounced effect on sugar beet plants, causing chlorosis, yellowing, scorching of the interveinal tissues of the leaf blade, and eventually, necrosis (13). In the present investigation, our objective was to assess the effects of Mg deficiency on the photosynthetic and respiratory CO2 exchange of sugar beet leaves and to determine whether Mg has any effect on leaf CO2 uptake attributable to changes in stomatal diffusion resistance. attached leaves, and in the estimation of the contents of leaf minerals, are as described in earlier papers (8 to 10). The culture solution employed to obtain Mg deficient plants contained, in mmole/l, 2.5 Ca(NO3)2*4 H20, 0.5 KH2PO4, 2.5 KNO3, 1.0 K2SO4, and 0.5 NaCl, and, in mg/l, 0.25 B, 0.25 Mn, 0.025 Zn, 0.01 Cu, and 0.005 Mo. Iron was added as ferric-sodium ethylene diamine tetraacetate complex to give 2.5 mg of Fe/l. The culture solution for the control plants was the same except for 1 mmole of MgSO/l which was added in place of the K2S04. MATERIALS AND METHODS DetailsThe gas-exchange measurements were begun at Mg cutoff, 28 days from planting, at which time three control plants were supplied with the complete culture solution and eight plants with the culture solution containing no added Mg. Measurements were made on control plants at the beginning and end of each period of deficiency.In addition to the information concerning gas-diffusion resistances, provided in earlier work (8, 9), we wish to make the following points. Assuming a cuticula...