Effect of soil moisture on leaf ecophysiology of Parasenecio yatabei, a summer-green herb in a cool-temperate forest understory in Japan.

Leaf physiological and gas-exchange traits of a summer-green herbaceous perennial, Parasenecio yatabei, growing along a stream were examined in relation to leaf age. In its vegetative phase, the aerial part of this plant consists of only one leaf and provides an ideal system for the study of leaf longevity. Volumetric soil water content (SWC) decreased with increasing distance from the stream, whereas relative light intensity was nearly constant. The light-saturated net CO2 assimilation rate (Asat) and leaf stomatal conductance (gs) were approximately 1.5-fold and 1.4-fold higher, respectively, in the lower slope near the mountain stream than in the upper slope far from the mountain stream. The lifespan of aerial parts of vegetative plants significantly increased with decreasing SWC. The leaf mass-based nitrogen content of the leaves (N mass) was almost constant (ca. 2.2%); however, the maximum carboxylation rate by ribulose-1,5-biphosphate carboxylase/oxygenase (rubisco) (V cmax) and photosynthetic nitrogen use efficiency (PNUE, A sat/N area) decreased more slowly in the upper slope than in the lower slope. The higher leaf photosynthetic activity of P. yatabei plants growing lower on the slope leads to a decrease in V (cmax) and PNUE in the early growing season, and to a shorter leaf lifespan.