Partitioning of Nitrogen Between and Within Leaves Grown Under Different Irradiances

Abstract

The distribution of nitrogen between leaves on individual plants of Phaseolus vulgaris and Cucumis sativus which were grown under different irradiances was examined. For Phaseolus, shading treatments were imposed on individual leaflets when they had reached one-third of full expansion. Adjacent leaflets were either grown under the same irradiance or had different irradiances imposed on them. The nitrogen content of leaves depended on their growth irradiance and not on the growth irradiance of adjacent leaflets, with more nitrogen being found in leaves grown under higher irradiance compared to those grown in shade. For Cucumis, the nitrogen contents of the leaves changed following the imposition of shading treatments. The experiment was repeated four times with different nitrate nutrient treatments, twice in combination with a pretreatment growth irradiance of 40% sunlight. The relative changes in leaf nitrogen content for each irradiance treatment were independent of changes to the leaf nitrogen content of the plant and of the growth irradiance prior to the shading treatments. Again, nitrogen contents were highest in leaves grown at high irradiance.

Acclimation of individual leaves to their irradiance treatment was seen for both Phaseolus and Cucumis. Growth under shade resulted in lower rates of oxygen evolution per unit of chlorophyll, when measured at high irradiance, and increased partitioning of nitrogen into pigment-protein complexes. These two changes working in opposition to each other meant that for Cucumis, the relationship between photosynthetic capacity and nitrogen content was similar between irradiance treatments. For Phaseolus, the increased partitioning of nitrogen into pigment-protein complexes at low irradiance was not as great as the reduction in photosynthetic rate per unit of chlorophyll, so that the photosynthetic rate per unit leaf nitrogen was less for leaves grown under low irradiance compared to those grown under high irradiance. It is shown that acclimation to lower irradiance can increase the potential daily photosynthesis for a given leaf nitrogen content.