Is plasticity in partitioning of photosynthetic resources between and within leaves important for whole-plant carbon gain in canopies?

1. The significance for whole-plant carbon gain of plasticity in between-leaf and within-leaf partitioning of photosynthetic resources was investigated using an experimental and modelling approach. Lysimachia vulgaris L. was grown at two contrasting stand densities and two levels of nutrient availability in a glasshouse. Whole-plant daily C gain was calculated for the four treatments. The importance of the two forms of plasticity in photosynthetic resource partitioning was investigated by switching distribution patterns of leaf nitrogen and photosynthetic capacity per unit chlorophyll between plants in the two stand densities and recalculating whole-plant C gain, which was used as a measure of fitness. 2. The plants had a high photosynthetic capacity per unit leaf area in top leaves in the dense stands, and a low capacity in bottom leaves. The distribution over plant height was more homogeneous in the open stands. This plasticity in between-leaf resource partitioning was not very important for whole-plant C gain, provided the plants had a dense-canopy type of partitioning. It is argued, however, that this result of the model calculations is valid only for low, but not high, nutrient availability. 3. Photosynthetic capacity per unit chlorophyll, a parameter representative of within-leaf partitioning of photosynthetic resources between capacity and light harvesting, also showed plasticity in response to stand density. High values were found in open stands and in the top leaves in dense stands, whereas a low capacity per unit chlorophyll was found in shaded bottom leaves in dense stands. Plasticity in this trait was also not very important for C gain of plants in stands of contrasting densities. Here the condition was that plants have a distribution pattern found for open-stand plants. However, in the case of subordinate plants that have all their leaves in the shade cast by their taller neighbours, adjustment of photosynthetic capacity per unit chlorophyll to the level of irradiance is much more important for whole-plant C gain.