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Photosynthetic Efficiency of Alfalfa with Increasing Potassium Nutrition¹

Potassium concentration in alfalfa (Medicago sativa L.) herbage has been used to indicate sufficient levels to attain maximum dry matter productions. Since photosynthesis is the source of dry matter, the relationships of potassium concentration to CO₂ exchange rate (CER) was studied in an attempt to find an optimum level. Several soil K rates were applied to four alfalfa plant populations. CER measurements were made for single leaves and canopies of alfalfa during early vegetative growth. CER increase due to added increments of K could be camouflaged by differential growth rates of canopies and mutual shading of leaves that result from increased fertility. The confounding was avoided by adjusting CER to similar leaf area index values across K rates.

CER increased 22% as the K concentration in prebud herbage increased by 133% (1.62 to 3.77). A herbage K concentration of about 2.0% resulted in 90% of maximum observed CER. K concentration of 2.0% for prebud herbage relates to a concentration of 1.3% for one-tenth bloom herbage. The CER and K concentrations of individual young and old leaves increased with added soil K increments. High K rates prolonged CER as leaves aged. These data indicate a lower critical K percentage for maximum CER than is reported in literature for maximum yields; thus, other growth processes may be more limiting by low K concentrations than photosynthesis. Plant development, plant part sampled, leafiness, and geographic locality may influence the K sufficiency concentration.

Key Words: Carbon dioxide exchange rate • Medicago sativa L. • Specific leaf weight

² Associate professor, Virginia Polytechnic Institute and State Univ., associate professor of agronomy, North Carolina State Univ., Raleigh. N.C., and university professor of agronomy, Virginia Polytechnic Institute and State Univ., respectively.

Received for publication May 19, 1975.