Growth and CO2 Assimilation by Corn as Related to Potassium Nutrition and Simulated Canopy Shading1

Abstract

AbstractRadiant flux density and K are potentially limiting factors in CO2 assimilation by corn leaves having the common property of being more accessible to the upper leaves than to lower leaves of the plant canopy. In this experiment, the effects of K nutrition and shading on growth and CO2 assimilation were evaluated in the absence of confounding factors such as differentials in moisture supply, rooting volume, and availability of other nutrients that often accompany studies having variable plant populations. Corn was grown outdoors in sand cultures receiving solutions containing 15, 45, 135, or 400 μg of K/ml. Plant populations of 33,000, 98,800, and 118,000 plants/ha were simulated by progressive artificial shading of each plant with layers of shade cloth during growth. Rates of CO2 assimilation by selected leaves, yield of dry matter, and leaf concentrations of K were measured at the silking stage. In general, CO2 assimilation rates increased with K levels, with simulated population, and with proximity to the plant top. However, these relationships were complex as evidenced by a significant interaction of K ✕ leaf location and K ✕ population ✕ leaf location. The interactions apparently were associated with increases in leaf K concentrations due to less growth with shading and to movement of K from lower into upper leaves under conditions of K stress. Across all treatment combinations, CO2 assimilation was highly correlated (r = 0.89) with tissue levels of K. Thus, the effects of shading and leaf position were indirect through their influence on the concentrations of K in leaf tissue. Measured assimlation rates (mg/hour) at canopy illumination levels for leaves 5, 7, and 9, when the entire leaf surface was considered uniformly photosynthetic provided indices that were highly correlated (r=0.96, significant at the 0.01 level) with yields of dry matter per plant.