DISTRIBUTION OF NITROGEN AMONG LEAVES IN SOYBEAN CANOPIES

The N distribution among leaves in a plant canopy can theoretically ha ve an important influence on crop photosynthetic activity. However, li ttle detailed information is available on the specific leaf nitrogen ( SLN, g N m-2) distribution in soybean [Glycine max (L.) Merr.] canopie s. Experiments were established at Gainesville, FL and in Japan to exa mine the effects of cultivar, canopy maturity, plant density, and N fe rtility on SLN distribution. The SLN distribution was measured by divi ding canopies by height or node number into relatively thin layers and determining SLN for each layer. Canopies with expanding leaves were f ound to have a fairly uniform SLN in the top 1.5 to 2.0 LAI. Below thi s top zone there was a linear decline in SLN with increased cumulative LAI. In contrast, mature canopies with fully expanded leaves only wer e found to have a continuous linear decrease in SLN with cumulative LA I 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 ex panding leaves. Neither plant density nor N fertility substantially al tered the linear decline in SLN with cumulative LAI.