RESPONSES OF GROWTH, PHOTOSYNTHESIS, AND LEAF CONDUCTANCE TO WHITE-LIGHT IRRADIANCE AND END-OF-DAY RED AND FAR-RED PULSES IN FUCHSIA-MAGELLANICA LAM

The response of Fuchsia magellanica Lam. - a shade-tolerant perennial - to white light photon flux density and end-of-day red and far-red light irradiation was studied to evaluate the role that different morphological and functional alterations play in whole-plant acclimation to shade. The estimated relative growth rate of plants growing under a photon flux density of 30-mu-mol m-2 s-1 was 71% of that of those growing under 450-mu-mol m-2 s-1. The proportion of total dry weight allocated to shoots was higher in low light plants, but the proportion of shoot dry weight allocated to leaf blades was not affected by any of the treatments. The estimated relative growth rate, specific leaf area and leaf conductance were not affected by end-of-day irradiation. In low light plants the specific leaf area was higher, and at saturating photon flux density photosynthesis was lower on a leaf area basis, but was not on a leaf dry weight basis. Dark respiration was higher in high light plants when expressed on an area basis, but not when expressed on a dry weight basis. During the normal photoperiod, leaf conductance measured in darkness was higher in high light plants than in low light ones, which is consistent with the higher stomatal density observed in these plants. The cross-sectional area of the petioles was slightly reduced in low light in spite of a large increase in the area of individual leaves. Low irradiance affected the display of leaves, decreased leaf thickness and increased chlorophyll content per unit dry weight. End-of-day far-red resulted in plants with more erect branches. Both low photon flux density and far-red enhanced internode elongation. The effect of end-of-day irradiations was bigger in low light than in high light, which is the opposite to what is observed in shade-avoiders. The relatively small difference in mean growth rate can be explained by changes in specific leaf area, shoot:root dry weight partition ratio, and by the shape of the photosynthetic light response curve. However when comparing photosynthetic rates on a dry weight basis low light plants do not seem to be at a disadvantage under high light.