Effects of red to far-red ratio and plant density on biomass allocation and gas exchange in Impatiens capensis

Phytochrome-mediated stem elongation in response to crowding in dense stands has been shown to be a form of adaptive phenotypic plasticity. However, increased stem elongation could affect the patterns of allocation to leaves and roots and so affect the water relations of the plants. We tested this hypothesis by measuring biomass allocation, light-saturated photosynthetic rates, and stomatal conductance of elongated and nonelongated Impatiens capensis plants in two experiments. In the first experiment, we compared elongated plants grown in high-density stands under neutral shade, which elicited normal stem elongation in response to crowding, with nonelongated plants grown in high-density stands receiving a high ratio of red:far-red (R:FR) light that suppressed the elongation response. In the second experiment, we compared elongated plants grown in high-density stands with nonelongated plants grown at low density. As expected, elongated plants had a higher allocation to stem biomass in both experiments. In addition, elongated plants had a lower proportion of root biomass to leaf area. In the light quality cue experiment, elongated plants had significantly lower photosynthetic rates and stomatal conductance and higher water use efficiency compared with nonelongated plants, but specific leaf area did not change. This result indicates that, as either a direct or indirect response to higher R:FR, stomatal conductance and therefore photosynthetic rate were increased. However, in the density experiment, we found no significant difference in photosynthetic rates between high-density and low-density plants. The high-density plants had higher stomatal conductance and higher specific leaf area. These results indicate that the lower specific leaf weight of low-density plants is more important than the light quality effects on stomatal conductance in determining the effects of density on gas exchange.