Responses of photosynthesis and biomass allocation of an understorey herb, Pteridophyllum racemosum, to gradual increases in irradiance

In wave-regenerating Abies forests in central Japan, light availability on the forest floor of a mature-tree stand increases gradually over 4 to 5 years during the senescence of trees. The understorey evergreen perennial Pteridophyllum racemosum Sieb. et Zucc. (Papaveraceae) is commonly found in such forests. To determine the acclimation characteristics of this species to gradual changes in light availability, leaf photosynthetic traits and biomass allocation patterns were determined along a transect running from fully shaded understorey to a canopy gap. Along the transect, the photon flux density on the forest floor relative to full sun (RPFD) varied from 2.3-36.7 %. The light-saturated net photosynthetic rate of field-grown plants increased as the light environment increased from 2.3-14.1 % RPFD. However, as light conditions increased from moderate (14.1 %) to high (36.7 %), the light-saturated net photosynthetic rate decreased to a level comparable to that exhibited by low-light grown plants. On the other hand, root mass per unit leaf area (RMA) increased with increasing light conditions. A higher RMA contributes to a higher water uptake capacity per unit leaf area which in turn supports higher rates of photosynthesis and transpiration in high light. We conclude that in wave-regenerating forests P. racemosum can survive in high-light environments by avoiding a severe water deficit and heat load by increasing RMA. (C) 2001 Annals of Botany Company.