Photosynthetic Acclimation and Nitrogen Partitioning Within a Lucerne Canopy. II. Stability Through Time and Comparison With a Theoretical Optimum

Abstract

Nitrogen redistribution between and within leaves was examined in a plot of lucerne (Medicago sativa L. cv. Aurora) in relation to potential canopy photosynthesis. The canopy was sampled during regrowth after cutting and just prior to flowering. As leaves were progressively shaded by the newly produced leaves, nitrogen content fell and photosynthetic acclimation occurred. The rate of acclimation in the canopy was the same as occurred following a step change to 23 or 6% sunlight. The profile of leaf nitrogen content was stable with respect to leaf area index and independent of time of sampling. Optimal profiles of nitrogen distribution between leaves, photosynthetic rate per unit chlorophyll and nitrogen partitioning within leaves were calculated from the relationships between photosynthesis and nitrogen in conjunction with the light environment of the preceding 3 days. The optimal nitrogen content of the leaves should vary in proportion to the relative daily irradiance at each leaf. The observed distribution achieved 88% of the potential daily photosynthesis, while a uniform nitrogen distribution yielded only 80%. Photosynthetic acclimation and nitrogen partitioning within each leaf both responded to daily irradiance similarly to the calculated optimum except at the two extremes. At the top of the canopy, photosynthetic rate per unit of chlorophyll did not increase as much as the calculated optimum, while at the base of the canopy, nitrogen partitioning failed to fall as much as the calculated optimum. This may reflect the constraints on the flexibility of the photosynthetic system. Nitrogen redistribution between leaves made a dramatic contribution to increasing the potential photosynthesis by the canopy. Although acclimation to low irradiance reduced the photosynthetic capacity per unit nitrogen by 12%, the considerable reorganisation of proteins within the thylakoids increased potential daily photosynthesis by 20% over that which would have been gained by a 'sun' leaf. However, in terms of canopy photosynthesis, which is dominated by leaves intercepting most of the light, acclimation contributed only a few per cent to the potential daily canopy photosynthesis.