A mathematical model for photoinhibition of leaf photosynthesis was developed by formalising the assumptions that (1) the rate of photoinhibition is proportional to irradiance; and (2) the rate of recovery, derived from the formulae for a pseudo first-order process, is proportional to the extent of inhibition. The photoinhibition model to calculate initial photo yield is integrated into a photosynthesis-stomatal conductance (gs) model that combines net photosynthetic rate (PN), transpiration rate (E), and gs, and also the leaf energy balance. The model was run to simulate the diurnal courses of PN, E, gs, photochemical efficiency, i.e., ratio of intercellular CO2 concentration and CO2 concentration over leaf surface (Ci/Cs), and leaf temperature (T1) under different irradiances, air temperature, and humidity separately with fixed time courses of others. When midday depression occurred under high temperature, gs decreased the most and E the least. The duration of midday depression of gs was the longest and that in E the shortest. E increased with increasing vapour pressure deficit (VPD) initially, but when VPD exceeded a certain value, it decreased with increasing VPD; this was caused by a rapid decrease in gs. When air temperature exceeded a certain value, an increase in solar irradiance raised T1 and the degree of midday depression. High solar radiation caused large decrease in initial photon efficiency (α). PN, E, and gs showed reasonable decreases under conditions causing photoinhibition compared with non-photoinhibition condition under high irradiance. The T1 under photoinhibition was higher than that under non-photoinhibition conditions, which was evident under high solar irradiance around noon. The decrease in Ci/Cs at midday implies that stomatal closure is a factor causing midday depression of photosynthesis. and Qiang Yu, J. Goudriaan, Tian-Duo Wang.
Changes in pigment composition and chlorophyll (Chl) fluorescence parameters were studied in 20 year-old Scots pine (Pinus sylvestris L.) trees grown in environment-controlled chambers and subjected to ambient conditions (CON), doubled ambient CO2 concentration (EC), elevated temperature (ambient +2-6 °C, ET), or a combination of EC and ET (ECT) for four years. EC did not significantly alter the optimal photochemical efficiency of photosystem 2 (PS2; Fv/Fm), or Chl a+b content during the main growth season (days 150-240) but it reduced Fv/Fm and the Chl a+b content and increased the ratio of total carotenoids to Chl a+b during the 'off season'. By contrast, ET significantly enhanced the efficiency of PS2 in terms of increases in Fv/Fm and Chl a+b content throughout the year, but with more pronounced enhancement in the 'off season'. The reduction in Fv/Fm during autumn could be associated with the CO2-induced earlier yellowing of the leaves, whereas the temperature-stimulated increase in the photochemical efficiency of PS2 during the 'off season' could be attributed to the maintenance of a high sink capacity. The pigment and fluorescence responses in the case of ECT showed a similar pattern to that for ET, implying the importance of the temperature factor in future climate changes in the boreal zone. and K. Y. Wang, S. Kellomäki, T. Zha.
Diurnal patterns of gas exchange and chlorophyll (Chl) fluorescence parameters of photosystem 2 (PS2) as well as H2O2 content were analyzed in Reaumuria soongorica (Pall.) Maxim., a perennial semi-shrub. The rate of photorespiration was estimated by combined measurement of gas exchange and Chl fluorescence. The rate of photorespiration increased with the increasing drought stress (DS). The ratio of carboxylation electron flow to oxygenation electron flow (Jc/Jo) and the maximal photochemical efficiency of PS2 (variable to maximum fluorescence ratio, Fv/Fm) decreased with the increasing DS. Fv/Fm in isonicotinic acid hydrazide (INH)-sprayed plants was lower than that in normal plants under moderate DS, but no significant difference was observed under severe DS. H2O2 content in INH-sprayed plants was significantly lower than that in normal plants under severe DS. Taken together, photorespiration in R. soongorica consumed excess electrons and protected photosynthetic apparatus under moderate DS, whereas it accelerated H2O2 accumulation markedly and induced the leaf abscission under severe DS. and J. Bai ... [et al.].
The southernmost presence of Rhizophora mangle in the western Atlantic coast occurs in coastal wetlands between 27 and 28°S in the State of Santa Catarina, Brazil. We selected mangrove communities at the estuary of the River Tavares, Florianopolis, and the Sonho Beach, Palhosa, for measurement of photosynthetic performance and intrinsic water-use efficiency of R. mangle and coexisting individuals of Avicennia schaueriana and Laguncularia racemosa, during the spring season. We used gas-exchange techniques and isotopic signatures of C and N to estimate instantaneous water-use and carboxylation efficiency (CE), long-term water-use efficiency, and potential N limitation. Results showed that R. mangle had significantly lower photosynthetic rates but similar conductance values as the other two species resulting in lower intrinsic water use (WUEi) and CE. WUEi and CE were positively correlated in L. racemosa and A. schaueriana, but not in R. mangle. At each site, δ13C values of A. schaueriana were consistently higher than those for the other species, indicating that these species are subjected to contrasting water stress conditions. Leaf concentrations of C were lower, whilst those of N were always higher in A. schaueriana, indicating accumulation of salts and nonprotein N-compounds in leaves. Nitrogen concentrations and moderate positive δ15N values indicated that plant growth at the study sites was not inhibited by nutrient deficiency, and was not influenced by urban residual waters. Lower photosynthetic rates and values of CE of R. mangle compared to the other two species may constitute constraining factors preventing this species from establishing at higher latitudes., M. L. Gomes Soares, M. M. Pereira Tognella, E. Cuevas, E. Medina., and Obsahuje seznam literatury
Chlorophyll fluorescence parameter Fv/Fm, an indicator of the maximum efficiency of PS2, is routinely measured in the field with plant leaves darkened by leaf clips. I found that on a sunny day of subtropical summer, the Fv/Fm ratio was often underestimated because of a large F0 value resulted from a high leaf temperature caused by clipping the leaf under high irradiance, especially for long (e.g. 20 min) duration. This phenomenon may overestimate the down-regulation of PS2 efficiency under high irradiance. When leaf temperature was lower than 40 °C, the F0 level of rice leaves under clipping remained practically unchanged. However, F0 increased drastically with leaf temperature rising over 40 °C. In most measurements, no significant difference in Fm was found between rice leaves dark-adapted by leaf clips for 10 min and for 20 min. Therefore, shading leaf clips to prevent a drastic increase of leaf temperature, using F0 measured immediately after the leaf being darkened to calculate Fv/Fm, as well as shortening the duration of leaf clipping are useful means to avoid an underestimate of Fv/Fm.