Diurial variations in leaf conductance for water vapour (g^), and in rates of net photosynthesis (P^) transpiration (P) were investigated for individual Fagus crenata, Ginkgo biloba and Alnus firma trees during the growing season (May 12, June 3, August 19 and October 22, 1992), to defme the effects of main climatic factors limiting the photosynthetíc capacity of leaves. Measurements were undertaken at 1 h intervals in fully expanded leaves from 04.00 to 20.00 under sunny day and favourable water supply. Diumal patterns of gg and P^ in F. crenata were similar to G. biloba, showing strong dependence on irradiance in the early moraing and early evening, in May, June and August. The maximum values of P^, gg and water-use eíTiciency (WLIE) were recorded at 07.00 to 08.00 when photosynthetícally active radiation (PAR) and leaf temperature (Zj) were approximately 1200 pmol m'^ s"' and below 25 °C, respectively. P^, gg and WUE decreased from 08.00 to 13.00 contínuously, followed by a slight recovery at about 17.00 and a steep dechne until darkness. A. firma remained at maximum P^ from 07.00 to 14.00, and P^, gg and E were much higher than for the other two species. The peak of E in all three species always occurred at midday, coincident with maximum PAR and highest Ty But in October, P^ and E in all three species were highest around noon, also parallel to the maximum PAR and Ty
Acclimation to irradiance was investigated in seedlings of Japanese beech {Fagus crenata Bl.) grown under one of five irradiance regimes for three years. Inadiance (/) regimes covered the range of environmental conditions which this species normally would experience in Japanese beech forests: exposed [22.1 mol(quantum) m‘2 d'*, matching mean daily irradiance of exposed canopies], partially shaded [8.8, 2.7 and 1.5 mol(quantum) d'*, respectively, comparable to various midcanopies], and deeply shaded [0.5 mol(quantum) nu^ d"', equivalent to forest floor], There were consistent strong relationships between irradiance, growth performance and net photosynthetic rate (P^) for this species. Greater growth performance and were achieved as / increased, with aboveground height growth increasing as rapidly as basal diameter growth. This tiend was also observed in the leaf specific mass and leaf nitrogen content. Exposed plants had higher saturation I for (> 1400 |amol m'^ s‘*) than partially shaded plants (1180, 1100 and 753 pmol m‘2 s'', respectively). Higher leaf conductance, leaf specific mass and leaf nitrogen content correlated positively with higher Nevertheless, the beech seedlings adapted to deep shade by producing more total chlorophyll and requiring lower compensation / (Qq) at lower growth irradiances. Seedlings of Japanese beech were very tolerant to shade but capable of acclimating to strong I. Thus / is an important factor influencing growth and photosynthetic capacity of Japanese beech.
The effect of CO2 increase on gas exchange and water-use effíciency (WUE) in three temperate deciduous species (Fagus crenata, Ginkgo biloba and Alnus firma) under gradually-developing drought-stress was assessed. Seedlings were grown within transparent open-top cabinets and maintained for 4 monťhs at mean CO2 concentrations of either 350 (ambient; C350) or 700 pmol moT’ (elevated; C700) and combined with five water regimes [leaf water potential, higher than -0.3 (well- watered), -0.5 and -0.8 (moderate drought), -1.0 and lower than -1.2 MPa (serious drought-stress)]. Increase in CO2 concentration induced a 60 % average increase in net photosynthetic rate (P^) under well-watered conditions. The effect of C700 became more pronounced with drought stress established, with an 80 % average increase in at as low as -0.8 MPa; leaf conductance to water vapour transfer (gs) and transpiration rate (£), however, were significantly decreased. Consequently, WUE increased under drought, through drought stress affected potential E sooner than potential P^. The interaction of CO2 x drought stress on WUE was significant in that Pn was stimulated while E in C700 enriched plants resembled that of C350 plants under drought. Hence if a doubling of atmospheric CO2 concentration occurs by the mid 2U* century, then greater P^ in F. crenata, G. biloba and^. firma may be expected and the drought susceptibility of these species will be substantially enhanced.