Plant Physiology 76:424-430 (1984)
© 1984 American Society of Plant Biologists
Articles
Biochemical Basis for Effects of K-Deficiency on Assimilate Export Rate and Accumulation of Soluble Sugars in Soybean Leaves 1
Steven C. Huber
United States Department of Agriculture, Agricultural Research Service, North Carolina State University, Raleigh, North Carolina 27695-7631,
Department of Crop Science, North Carolina State University, Raleigh, North Carolina 27695-7631,
Department of Botany, North Carolina State University, Raleigh, North Carolina 27695-7631
The effects of K-deficiency on carbon exchange rates (CER), photosynthate partitioning, export rate, and activities of key enzymes involved in sucrose metabolism were studied in soybean (Glycine max [L.] Merr.) leaves. The different parameters were monitored in mature leaves that had expanded prior to, or during, imposition of a complete K-deficiency (plants received K-free nutrition solution). In general, recently expanded leaves had the highest concentration of K, and imposition of K-stress at any stage of leaf expansion resulted in decreased K concentrations relative to control plants (10 millimolar K). A reduction in CER, relative to control plants, was only observed in leaves that expanded during the K-stress. Stomatal conductance also declined, but this was not the primary cause of the decrease in carbon fixation because internal CO2 concentration was unaffected by K-stress. Assimilate export rate from K-deficient leaves was reduced but relative export, calculated as a percentage of CER, was similar to control leaves. Over all the data, export rate was correlated positively with both CER and activity of sucrose phosphate synthase in leaf extracts. K-deficient leaves had higher concentrations of sucrose and hexose sugars. Accumulation of hexose sugars was associated with increased activities of acid invertase. Neutral invertase activity was low and unaffected by K-nutrition. It is concluded that decreased rates of assimilate export are associated with decreased activities of sucrose phosphate synthase, a key enzyme involved in sucrose formation, and that accumulation of hexose sugars may occur because of increased hydrolysis of sucrose in K-deficient leaves.
1 Cooperative investigations of the Agricultural Research Service, United States Department of Agriculture and the North Carolina Agricultural Research Service, Raleigh, NC. Paper No. 9213 of the Journal Series of the North Carolina Agricultural Research Service, Raleigh, NC.
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