Two tropical species of North Australia, Acacia crassicarpa and Eucalyptus pellita, have similar leaf size and leaf structure but different leaf angles. A. crassicarpa with near vertical leaf angle directly reduced photon absorption and leaf temperature (Tl) and had relatively high photosynthetic activity (Pmax) and low xanthophyll cycle activity. In contrast, E. pellita with a small leaf angle exhibited high Tl, low Pmax, and high activity of xanthophyll cycle which was useful for the dissipation of excessive energy and reduction of photoinhibition. In the dry season, contents of soluble sugars including pinitol, sucrose, fructose, and glucose in A. crassicarpa increased whereas larger amounts of only fructose and glucose were accumulated in E. pellita. Different sugar accumulation may be involved in osmotic adjustment of leaves during water stress that makes photosynthesis more efficient. The leaf angle may be critical for developing different protective mechanisms in these two tropical tree species that ensure optimal growth in the high irradiance and drought stress environment in North Australia. and S.-M. Xu ... [et al.].
Alhagi sparsifolia Shap. is exposed to a high-irradiance environment as the main vegetation found in the forelands of the Taklamakan Desert. We investigated chlorophyll a fluorescence emission of A. sparsifolia seedlings grown under ambient (HL) and shade (LL) conditions. Our results indicated that the fluorescence intensity in the leaves was significantly higher for LL-grown plants than that under HL. High values of the maximum quantum yield of PSII for primary photochemistry (φPo) and the quantum yield that an electron moves further than QA - (φEo) in the plants under LL conditions suggested that the electron flow from QA - (primary quinone electron acceptors of PSII) to QB (secondary quinone acceptor of PSII) or QB - was enhanced at LL compared to natural HL conditions. The efficiency/probability with which an electron from the intersystem electron carriers was transferred to reduce end electron acceptors at the PSI acceptor side and the quantum yield for the reduction of end electron acceptors at the PSI acceptor side were opposite to φPo, and φEo. Thus, we concluded that the electron transport on the donor side of PSII was blocked under LL conditions, while acceptor side was inhibited at the HL conditions. The PSII activity of electron transport in the plants grown in shade was enhanced, while the energy transport from PSII to PSI was blocked compared to the plants grown at HL conditions. Furthermore, PSII activity under HL was seriously affected in midday, while the plants grown in shade enhanced their energy transport., L. Li, X. Y. Li, F. J. Zeng, L. S. Lin., and Seznam literatury
The photochemical activity of native Central Siberian Scots pine trees (Pinus sylvestris L) was estimated from the middle of February to the middle of March 2001. We measured chlorophyll (Chl) fluorescence in attached intact needles from trees located approx. 30 km west of the Yenisey river (60°44'N, 89°09'E) near the village of Zotino. In this period, the air temperature varied between -39 °C and +7 °C. At temperatures below -10 °C, P. sylvestris needles did not exhibit any variable Chl fluorescence during the daylight period. During the night, however, the effective quantum yield of photosystem 2 (PS2) photochemistry, Φ2 [Φ2 = (Fm' - Ft)/Fm'), increased from values near zero to values between 0.05 and 0.20 depending on the needle temperature and sample investigated. The increase started soon after dusk and lasted for 3-6 h depending on the temperature. A faster increase of Φ2 was found for temperatures around -16 °C, and lower rates occurred at lower temperatures. Irrespective of the temperature, Φ2 decreased rapidly to near zero values at dawn, when the photosynthetic photon flux density increased to about 1-5 µmol m-2 s-1, and remained near zero throughout the day. At temperatures higher than -10 °C, the diurnal decrease and the nocturnal increase of Φ2 were less distinct or disappeared completely. Hence the winter-adapted Scots pine maintains some photochemical activity of PS2 even at extremely cold temperatures. The capacity of photochemical reactions below -10 °C is, however, very limited and PS2 photochemistry is saturated by an extremely low irradiance (less than 5 µmol m-2 s-1). and P. Šiffel, J. Šantrůček.
Cuttings of P. przewalski were exposed to two different watering regimes which were watered to 100 and 25 % of field capacity (WW and WS, respectively). Drought stress not only significantly decreased net photosynthetic rate (PN), transpiration rate (E), stomatal conductance (gs), efficiency of photosystem 2 (PS2) (Fv/Fm and yield), and increased intrinsic water use efficiency (WUEi) under controlled optimal conditions, but also altered the diurnal changes of gas exchange, chlorophyll fluorescence, and WUEi. On the other hand, WS also affected the
PN-photosynthetically active radiation (PAR) response curve. Under drought stress, PN peak appeared earlier (at about 10:30 of local time) than under WW condition (at about 12:30). At midday, there was a depression in PN for WS plants, but not for WW plants, and it could be caused by the whole microclimate, especially high temperature, low relative humidity, and high PAR. There were stomatal and non-stomatal limitations to photosynthesis. Stomatal limitation dominated in the morning, and low PN at midday was caused by both stomatal and non-stomatal limitations, whereas non-stomatal limitation dominated in the afternoon. In addition, drought stress also increased compensation irradiance and dark respiration rate, and decreased saturation irradiance and maximum net photosynthetic rate. Thus drought stress decreased plant assimilation and increased dissimilation through affected gas exchange, the diurnal pattern of gas exchange, and photosynthesis-PAR response curve, thereby reducing plant growth and productivity. and C. Y. Yin, F. Berninger, C. Y. Li.