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 yea rs during the senescence of trees. The understorey evergreen perennial Pter idophyllum racemosum Sieb. et Zucc. (Papaveraceae) is commonly found in suc h forests. To determine the acclimation characteristics of this species to gradual changes in light availability, leaf photosynthetic traits and bioma ss allocation patterns were determined along a transect running from fully shaded understorey to a canopy gap. Along the transect, the photon flux den sity 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 increas ed as the light environment increased from 2.3-14.1 % RPFD. However, as lig ht conditions increased from moderate (14.1 %) to high (36.7 %), the light- saturated net photosynthetic rate decreased to a level comparable to that e xhibited by low-light grown plants. On the other hand, root mass per unit l eaf area (RMA) increased with increasing light conditions. A higher RMA con tributes to a higher water uptake capacity per unit leaf area which in turn supports higher rates of photosynthesis and transpiration in high light. W e conclude that in wave-regenerating forests P. racemosum can survive in hi gh-light environments by avoiding a severe water deficit and heat load by i ncreasing RMA. (C) 2001 Annals of Botany Company.