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 stand s has been shown to be a form of adaptive phenotypic plasticity. However, i ncreased stem elongation could affect the patterns of allocation to leaves and roots and so affect the water relations of the plants. We tested this h ypothesis by measuring biomass allocation, light-saturated photosynthetic r ates, and stomatal conductance of elongated and nonelongated Impatiens cape nsis plants in two experiments. In the first experiment, we compared elonga ted plants grown in high-density stands under neutral shade, which elicited normal stem elongation in response to crowding, with nonelongated plants g rown in high-density stands receiving a high ratio of red:far-red (R:FR) li ght that suppressed the elongation response. In the second experiment, we c ompared elongated plants grown in high-density stands with nonelongated pla nts grown at low density. As expected, elongated plants had a higher alloca tion 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 a nd stomatal conductance and higher water use efficiency compared with nonel ongated plants, but specific leaf area did not change. This result indicate s that, as either a direct or indirect response to higher R:FR, stomatal co nductance and therefore photosynthetic rate were increased. However, in the density experiment, we found no significant difference in photosynthetic r ates between high-density and low-density plants. The high-density plants h ad 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 determ ining the effects of density on gas exchange.