PHOTOSYNTHETIC ACCLIMATION AND NITROGEN PARTITIONING WITHIN A LUCERNE CANOPY .1. CANOPY CHARACTERISTICS

Acclimation by the photosynthetic system to the gradient in irradiance through a leaf canopy was investigated with a plot of lucerne (Medicago sativa L. cv. Aurora). The aims were to determine the extent to which acclimation occurred in a natural canopy and to quantify the changes in the partitioning of nitrogen within the leaf that are associated with acclimation. The canopy grew up around light sensors placed at 10 cm height increments which logged the irradiance at 1 min intervals for the 4 days that preceded sampling. Photosynthetic capacity was measured with leaf disc oxygen electrodes and the chlorophyll, soluble protein and nitrogen contents of the leaves were determined. Daily irradiance declined exponentially down through the canopy. Nitrogen content and photosynthetic capacity both declined down through the canopy. Photosynthetic acclimation by the lower leaves was evident from the lower chlorophyll a/b ratios and reduced photosynthetic capacity per unit chlorophyll. The lower photosynthetic capacity per unit of chlorophyll was offset by an increased proportion of leaf nitrogen present in the thylakoids. Consequently, the photosynthetic capacity per unit leaf nitrogen was nearly independent of irradiance. Based on a comparison of the response of many species to different irradiances during growth, it is generally the case that the proportion of thylakoid nitrogen increases for leaves grown under lower irradiance so as to maintain a constant ratio of photosynthetic capacity to total leaf nitrogen. However, the ratio of photosynthetic capacity to total leaf nitrogen varies widely between species.