No abstract

In the tropical canopy tree, Dryobalanops aromatica Gaertn. f., upper-canopy leaves (UL) develop under sunlit conditions but are subjected to self-shading within the crown as they age. In contrast, lower-canopy leaves (LL) are exposed to uniform dim light conditions throughout their life span. By comparing leaf morphology and physiology of UL and LL, variations in leaf characteristics were related to leaf age and self-shading. Mass-based chlorophyll (chl) concentration and the chlorophyll/nitrogen (chl/N) ratio were lower and the chl a/b ratio was higher in UL than in LL. In UL, the chl/N ratio gradually increased and the chl a/b ratio gradually decreased with leaf aging, whereas these ratios remained unchanged with leaf age in LL. The effective quantum yield of photosystem II (PSII) (DeltaF/F(m)') at a given irradiance remained unchanged with leaf age in LL, whereas DeltaF/F(m)' changed with leaf age in UL. These data indicate N reallocation within the leaves from carbon fixation components to light harvesting components and a dynamic regulation of photochemical processes of PSII in response to increased self-shading of UL. Despite the difference in light environment with leaf age between UL and LL, maximum photosynthetic rates and nitrogen-use efficiency decreased with leaf aging in both UL and LL. Constancy in the chl/N ratio with leaf age in LL indicated that the decrease in photosynthetic capacity was caused by effects other than shading, such as leaf aging. We conclude that N reallocation and acclimation of PSII to self-shading occurred even in mature leaves, whereas the change in photosynthetic capacity with leaf age was more conservative.

To examine the effects of different solar irradiances on leaf characteristics at the leaf primordium and expansion stages, we shaded parts of branches in the upper canopies of two adult beech trees, Fagus crenata Blume and Fagus japonica Maxim., for 4 years. The treatments during the leaf primordium and leaf expansion stages, respectively, were: (1) high light and high light (H, control), (2) high light and low light (HL), (3) low light and low light (LL), and (4) low light and high light (LH). Both number of cell layers in palisade tissue and individual leaf area were affected by the previous-year irradiance, whereas cell length of palisade tissue was larger in LH leaves than in LL leaves, suggesting determination by current-year irradiance. Lamina chlorophyll/nitrogen ratio was higher in HL and LL leaves than in LH leaves, suggesting determination by current-year irradiance. Diurnal minimum values of leaf water potential measured under sunlit conditions were lower in H and LH leaves than in HL and LL leaves. Effective osmotic adjustment was found in H and LH leaves, suggesting that leaf water relations were affected by current-year irradiance. Net photosynthetic rate and stomatal conductance measured under sunlight conditions were higher in H and LH leaves than in HL and LL leaves. Thus, effects of current-year irradiance had a greater effect on leaf-area-based daily carbon gain than previous-year irradiance.

were measured at the photosynthetic steady state at least 10 The effects of strong light in combination with elevated tempermin after the light exposure (180 mmol m − 2 s − 1 PFD). Stable atures on the photosynthetic system were examined in 4 dipterocarp tree species with ecologically different habitats. temperature responses of DF/F% m and q P were observed in S. The 4 dipterocarp tree species were: Shorea platyclados origi-platyclados and D. oblongifolius, while those in S. par7ifolia and S. assamica were more temperature-dependent and severely nated from upper dipterocarp forests, Shorea par7ifoliaaffected at 45°C. Little difference was observed in temperature-lowland and hill dipterocarp forests, Shorea assamica -lowland dipterocarp forests, and Dipterocarpus oblongifolius -dependent F% v /F% m among species. Photoinhibitory light exposure riparian fringes. S. platyclados and D. oblongifolius have (1600 mmol m − 2 s − 1 PFD) for 2 h at 40°C had little effect on higher growth and survival rates in open sites than S. par7ifolia the recovery kinetics from photoinhibition of S. platyclados and D. oblongifolius compared with those at 35°C. In contrast, the and S. assamica. Tolerance of high temperature among the recovery from photoinhibition was retarded in S. par7ifolia and species was assessed by determining the critical temperatures (T c ) at which the minimal fluorescence (F o ) began to rise S. assamica. These findings suggest that even at 40°C, a sharply. This was measured by exposing plants to an increasing temperature below T c , an exposure to strong light exacerbated photoinhibition in S. par7ifolia and S. assamica corresponding temperature of about 1°C min − 1 . The intrinsic thermotolerto the closure of PSII reaction centers, as indicated by the ance of the thylakoid membrane appears to be the highest for decrease in q P at this temperature. Thus, S. platyclados and D. D. oblongifolius (T c =46.4°C), intermediate for S. platyclados (45.7°C), and lowest for S. par7ifolia and S. assamica (45.2 and oblongifolius, which occur at uplands and riparian fringes with 45.3°C, respectively). The temperature-dependent efficiency of frequent disturbances, are suggested to have higher photosynthetic tolerance to elevated temperatures contributing to a PSII electron transport (DF/F% m ), photochemical quenching circumvention of photoinhibition. (q P ), and the efficiency of light capture of open PSII (F% v /F% m )

We investigated how shoot gross morphology and leaf properties are determined in Fagus japonica Maxim., a deciduous species with flush-type shoot phenology, in which all leaves are produced in a single flush at the start of each season. We examined relationships between current-year shoot properties and local light environment in a 14-m tall beech tree growing in a deciduous forest. Leaf number (LN), total leaf area (TLA), and total leaf length (SL) of the current-year shoot increased with increasing photosynthetic photon flux density (PPFD). Leaf thickness, dry mass per leaf area and nitrogen content on a leaf area basis increased, whereas the chlorophyll/N ratio decreased with increasing PPFD. To separate the effects of current-year PPFD from those of previous year(s), we artificially shaded a part of the uppermost leaf tier. Reciprocal transfers of beech seedlings between controlled PPFD regimes were also made. Characteristics of shoot gross morphology such as LN, TLA and SL were largely determined by the PPFD of the previous year. The exception was the length of the longest "long shoots" with many leaves, in which elongation appeared to be influenced by both previous-year and current-year PPFD. In contrast, leaf properties were determined by current-year PPFD. The ecological implications of our findings are discussed.