DISTRIBUTION OF NITROGEN AMONG LEAVES IN SOYBEAN CANOPIES

The N distribution among leaves in a plant canopy can theoretically have an important influence on crop photosynthetic activity. However, little detailed information is available on the specific leaf nitrogen (SLN, g N m-2) distribution in soybean [Glycine max (L.) Merr.] canopies. Experiments were established at Gainesville, FL and in Japan to examine the effects of cultivar, canopy maturity, plant density, and N fertility on SLN distribution. The SLN distribution was measured by dividing canopies by height or node number into relatively thin layers and determining SLN for each layer. Canopies with expanding leaves were found to have a fairly uniform SLN in the top 1.5 to 2.0 LAI. Below this top zone there was a linear decline in SLN with increased cumulative LAI. In contrast, mature canopies with fully expanded leaves only were found to have a continuous linear decrease in SLN with cumulative LAI from the top of the canopy. The SLN of the leaves at the top of the mature canopies were substantially greater than those canopies with expanding leaves. Neither plant density nor N fertility substantially altered the linear decline in SLN with cumulative LAI.