Bothriochloa ischaemum L. is an important species in many temperate regions, but information about the interactive effects of water stress and fertilization on its photosynthetic characteristics was inadequate. A pot experiment was conducted to investigate the effects of three water [80% (HW), 40% (MW), and 20% (LW) of field capacity (FC)] and four fertilization regimes [nitrogen (N), phosphorus (P), nitrogen with phosphorus (NP), and no fertilization] on leaf photosynthesis. Leaf gas exchange and photosynthetic light-response curves were measured at the flowering phase of B. ischaemum. Water stress decreased not only the leaf gas-exchange parameters, such as net photosynthetic rate (PN), stomatal conductance (gs), transpiration rate (E), and water-use efficiency (WUE) of B. ischaemum, but also downregulated
PN-photosynthetically active radiation (PAR) curve parameters, such as light-saturated net photosynthetic rate (PNmax), apparent quantum efficiency (AQE), and light compensation point (LCP). Fertilization (N, P, and NP) enhanced the daily mean PN values and PNmax under the HW regime. Addition of N (either alone or with P) improved the photosynthetic capacity of B. ischaemum under the MW and LW regimes by increasing PN, PNmax, and AQE and reducing dark respiration rate and LCP, but the addition of P alone did not significantly improve the photosynthetic performance. Decline in PN under each fertilization regime occurred during the day and it was caused mainly by nonstomatal limitation. Our results indicated that water was the primary limiting factor for photosynthesis in B. ischaemum, and that appropriate levels of N fertilization improved its potential photosynthetic capacity under water-deficit conditions. and W. Z. Xu, X. P. Deng, B. C. Xu.
The exciton equilibration in the light-harvesting complex of photosystem 2 at room temperature was studied with pump-probe experiments under quasi-stationary conditions. The measurements were performed with two dye lasers, which had a pulse diuation of 10 ns and a spectral bandwidth of less than 0.01 nm. The relative probe beam transmission ffom 640-690 nm was recorded at different excitation wavelengths, varied from 645 to 685 nm. Two maxima were observed at 650 nm (Chl b absorption region) and at 680 nm (Chl a absorption region). The relative probe beam transmission in the Chl a region showed a course nearly independent of the pump beam wavelengťh in contrast to a weak dependence in the Chl b region. Measurements of the relative probe beam transmission as a function of pump beam intensity revealed a dominating exciton-exciton annihilation in the Chl a absorption region. In the Chl b region exciton-exciton annihilation was only marginal.