Photosynthesis and micro-environmental factors in a spring ephemeral,Erythronium japonicum, from native and open habitats

Abstract

Environmental factors affecting photosynthetic activity of the typical vernal speciesErythronium japonicum Decne were examined on the floor of a deciduous broad-leavedQuercus mongolica forest (Q.m. stand) and on bare land left undisturbed for 4 years after forest clearing (bare stand). Daytime solar irradiation and air and leaf temperatures at the bare stand were significantly higher than those at theQ.m. stand. The relative air humidity was very low and did not differ much between the stands, although the leaf-air vapor pressure differences (VPD) at the bare stand were about twice as high as those at theQ.m. stand. The plants at both stands were supplied with sufficient soil water throughout their growing season by a large snowmelt. However, the aboveground parts of the plants at the bare stand were subjected to much more severe heat stress, caused by the strong radiations and high leaf temperatures, and water stress, caused by the highly transpiring conditions, than those at theQ.m. stand. When the radiation on leaves, leaf temperatures and VPD in the assimilation chamber were changed from those observed at theQ.m. stand to those at the bare stand, the photosynthetic rate and stomatal conductance fell significantly. However, the rate and conductance were immediately restored to the respective values near those measured under the conditions at theQ.m. stand when only VPD was dropped to the value similar to that observed at theQ.m. stand. These results indicate that the photosynthetic rate of the plants at the bare stand was lowered largely by a decrease in stomatal conductance. The internal CO₂ partial pressure$(P_{CO_2} )$ was considerably greater in leaves placed under environmental conditions similar to those at the bare stand.