Role of Potassium in Carbon Dioxide Assimilation in Medicago sativa L

Abstract

Alfalfa was grown hydroponicalHy in 0, 0.6, and 4.8 millimolar K in order to determine the influence of tissue level of K on photosynthesis, dark respiration, photorespiration, stomatal and mesophyll resistance to CO2, photosystem I and II activity, and synthesis and activity of ribulose 1,5-bisphosphate carboxylase (RuBPc).A severe (0.0 millimolar) and mild (0.6 millimolar) K deficiency, compared to plants grown at 4.8 millimolar K, produced a significant decrease in photosynthesis and photorespiration, but an increase in dark respiration.Both deficient K levels increased hydrophyllic resistance to C02, but only the severe deficiency increased stomatal resistance. Photosystem I and II activity of isolated chloroplasts was not affected by K deficiency. The apparent activity of a crude RuBPc preparation was significantly reduced in severely deficient plants. Activity of the enzyme could not be restored to normal rates by the addition of K to the reaction medium.The specific activity of RuBPc isolated from severely K-deficient and K- K deficiency has been shown to reduce the rate of photosynthesis in numerous higher plants (2,10,12,13,18,19). Cooper et al. (3) attributed the decrease in photosynthesis of K-deficient alfalfa to a decrease in stomatal number and aperture size. Reduction in photosynthesis of K-deficient corn was attributed to increased stomatal resistance (4, 9). A combination of increased stomatal and mesophyll resistance to CO2 was thought to reduce photosynthesis in K-deficient sugarbeets (20, 21). Since K is the major solute in turgid guard cells (6), it is reasonable to suggest that K deficiency will result in stomatal closure. In the presence of Na, low K increased stomatal resistance less, but mesophyll resistance remained high (21). These results suggest that the role of K in regulating photosynthesis may be within mesophyll cells. (NO3)2 was 6.5, 6.2, and 4.6 and that of CaCl2 was 0, 0.3, and 2.4 mm for the 0, 0.6, and 4.8 mm K solutions, respectively. One plant per crock was established and the crocks were arranged in a completely randomized block design with eight replications. Crocks were subirrigated every 2 h during the day and every 4 h at night. Nutrient solutions were changed every 2 weeks. The temperature in the greenhouse was 25/20 ± 5 C day/night, and the average daily solar radiation was approximately 310 cal cm-2 day-'. Plants were cut back to a 5-cm stubble when they reached the 'ho bloom stage and had attained approximately 20 days of regrowth (early bud stage) at the time of measurement.Gas Exchange. Photosynthesis, transpiration, dark respiration, and stomatal and mesophyll resistance to CO2 diffusion were measured on 20 individual attached leaves (fourth fully expanded leaf from the top) per treatment using an air-sealed leaf chamber (26)

Biochemical Basis for Effects of K-Deficiency on Assimilate Export Rate and Accumulation of Soluble Sugars in Soybean Leaves

“…However, K-deficient leaves may have lower activities of certain synthetic enzymes, e.g. RuBP carboxylase (22) and SPS (Fig. 3D).…”

“…Because ofthe role of K fluxes in stomatal aperture changes (24), it has often been assumed that the influence of K on photosynthesis could be attributed to stomatal closure (17,21). However, more recent studies with sugarbeet (29), alfalfa (22), and cotton (15) have ' suggested that nonstomatal limitations may also be involved. K-nutrition affects the rate of assimilate export and the concentrations ofsugars in leaves, but the relationship between these two parameters is not clear.…”

Potassium Nutrition and Translocation in Sugar Beet

“…Although there are a number of reports that net carbon exchange goes up when K+ nutrition is increased (15,17,18,19,22) there is insufficient evidence to identify the mechanism involved. Increased rate of arrival of K+ in the xylem does not appear to enhance net carbon exchange in a source leaf within a 6-to 8-h period (Fig.…”

“…Low K+ appears to reduce net carbon exchange through an increase in both stomatal and mesophyll resistance (15,18,19,22). Two studies (6,12) showed increased net carbon exchange and a greater proportion of fixed carbon exported to the roots in K+-fertilized sugar beets.…”

Metabolic responses of guava trees irrigated withdifferent N and K levels in São Francisco Valley

“…During nitrogen turnover in plants, the conversion of inorganic N to organic N compounds of low molecular weight, such as amino acids, the synthesis of high molecular weight N compounds, such as proteins, nucleic acids and chlorophyll, and their breakdown by hydrolyzing enzymes are influenced by plant nutrition (Mengel & Kirkby, 1987). Peoples & Koch (1979) have reported a clear effect of K + the rate of CO 2 assimilation, by promoting the synthesis of Ribulose-bisphosphate carboxylase and by decreasing diffusive resistance of CO 2 in the mesophyll. Furthermore, K + has a promoting influence on phloem loading (Malek & Baker, 1977) and on translocation of newly synthesized photosynthesis (Hartt, 1969) and of mobilized stored material (Koch & Mengel, 1977;Seçer, 1978).…”