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Effects of Variation in Leaf Area Index on Growth of Maize and Soybeans¹

Maize and soybean crops were grown at high density in separate plots in the same field experiment; when a thick canopy had developed, a range of leaf area indexes (LAI) was created by thinning. Samples were taken at time of thinning and 2 weeks later. The experiment was repeated twice yearly for 3 years.

Mean relative growth rate (RGR) and mean net assimilation rate (NAR) increased with increasing density, while mean crop growth rate (CGR) and leaf area ratio (LAR) decreased. In four maize and two soybean experiments the best fitting regression of NAR on LAI was a straight line; in the other six experiments the relationship was curvilinear. NAR of maize declined more rapidly with increase in LAI than did that of soybeans, but if the regression values at LAI = 4 were expressed as percentages of the values at LAI = 1, the NAR of maize and soybeans decreased at the same rate. Thus, the adverse effects of self-shading with increase in LAI are at least as pronounced in maize as in soybeans. Despite this, NAR of soybeans was only 57% of that of maize, and CGR of maize was correspondingly greater than that of soybeans. In only 4 out of the 12 experiments was an optimum value of LAI indicated.

Key Words: Growth analysis • Photosynthesis • Plant population • Thinning • Glycine max • Zea mays

² Research scientist in Plant Physiology.

Received for publication February 3, 1969.