Nitrogen allocation in response to partial shading of a plant: Possible mechanisms

The significance of photosynthetic and transpiration rates for the perception by plants of light gradients in leaf canopies has been investigated with regard to nitrogen allocation and re-allocation. A gradient of photon flux density (PFD) over a plant's foliage was simulated by shading one leaf of a pair of primary leaves of bean ( Phaseolus vulgaris L. cv. Rentegever). Photosynthetic rate was manipulated independently of PFD and, to some extent, also of transpiration, by subjecting the leaf to different CO₂ concentrations. Transpiration rate was changed independently of PFD and photosynthetic rate by subjecting the leaf to different vapour pressure differences (VPD). A reduced partial pressure of CO₂ reduced specific leaf mass (SLM) as did a decreased PFD, but did not change leaf N per unit area (N_LA) and light saturated rate of photosynthesis (A_max). A reduced VPD caused several effects consistent with the effect of PFD. It decreased N_LA and A_max and increased the chlorophyll to N ratio in old and young leaves. Furthermore, it decreased the chlorophyll a to b ratio and inhibited leaf growth in young leaves. The transpiration stream is partitioned among the leaves of a plant according to their transpiration rates. The results suggest that relative rates of import of xylem sap into leaves of a plant play an important role in the perception of partial shading of a plant, a situation normally found in dense vegetations. The possible role of cytokinin influx into leaves as controlled by transpiration rate, is discussed.