Responses of two provenances of Fagus sylvatica seedlings to a combination of four temperature and two CO2 treatments during their first growing season: gas exchange of leaves and roots.

Physiological responses of two provenances of European beech (Fagus sylvatica) were studied in seedlings grown at two [CO2 ] in combination with four temperature treatments. For the local Danish provenance, the average effect of elevated [CO2 ] during growth was to increase light-saturated net photosynthesis (An ) and instantaneous water-use efficiency or transpiration efficiency (ITE). These increases were strongly related to the temperature treatment. Stomatal conductance (gs ) was reduced in seedlings in high [CO2 ], but there was no statistically significant effect of temperature treatment. Stomatal conductance was 13-26% lower at elevated [CO2 ] and ITE was 89-156% higher, depending on growth temperature. The effects of [CO2 ] on An were considerably larger than those shown for many other woody species, but similar to those in other studies on European beech. The absolute value of An for a Romanian provenance of beech was 5-18% lower than in the Danish provenance at low [CO2 ] and 14-26% lower at high [CO2 ]. There was no statistically significant interaction between the provenances and [CO2 ], or between provenance and temperature. A model of the response of An to [CO2 ] at different temperatures gave predictions close to the measured results, except at the lowest temperature treatment where the model over-predicted the effect of elevated [CO2 ]. This and measurements of An made at a common, low [CO2 ] indicated a down-regulation of photosynthesis in the lowest temperature treatment at high [CO2 ]. Root plus soil respiration on a whole-tree basis (Rtr ) was increased by elevated [CO2 ] at all but the lowest temperature, but no effect was seen of [CO2 ] on root respiration per unit root d. wt. Mean Rtr on any given date was significantly correlated with An , except at the lowest temperature treatment. It is hypothesized that low temperature limited the ability of the roots to use photosynthates resulting in a feedback inhibition of An when elevated [CO2 ] was combined with low temperature.

Changes in Photosystem II fluorescence in Chlamydomonas reinhardtii exposed to increasing levels of irradiance in relationship to the photosynthetic response to light.

The effects of a 60 min exposure to photosynthetic photon flux densities ranging from 300 to 2200 µmol m(-2)s(-1) on the photosynthetic light response curve and on PS II heterogeneity as reflected in chlorophyll a fluorescence were investigated using the unicellular green alga Chlamydomonas reinhardtii. It was established that exposure to high light acts at three different regulatory or inhibitory levels; 1) regulation occurs from 300 to 780 µmol m(-2)s(-1) where total amount of PS II centers and the shape of the light response curve is not significantly changed, 2) a first photoinhibitory range above 780 up to 1600 µmol m(-2)s(-1) where a progressive inhibition of the quantum yield and the rate of bending (convexity) of the light response curve can be related to the loss of QB-reducing centers and 3) a second photoinhibitory range above 1600 µmol m(-2)s(-1) where the rate of light saturated photosynthesis also decreases and convexity reaches zero. This was related to a particularly large decrease in PS IIα centers and a large increase in spill-over in energy to PS I.

Shoot structure, leaf area index and productivity of evergreen conifer stands.

For 12 conifer species, the maximum ratio of shoot to leaf silhouette area of shade-acclimated shoots was found to vary from 0.50 to 0.99. Maximum leaf area index (leaf area per unit ground area) of conifer stands varied from 3.5 to 20, and maximum mean annual increment varied by a factor of 2. Significant correlations were found between leaf silhouette area ratio of shade-acclimated shoots and the maximum leaf area index (R(2) = 0.84) and the maximum mean annual increment (R(2) = 0.93). These results support a hypothesis that species to species differences in the morphology of shade-adapted shoots strongly affect both the development of leaf area and the productivity of stands of evergreen conifers.

The effects of photoinhibition on the photosynthetic light-response curve of green plant cells (Chlamydomonas reinhardtii).

The photosynthetic response to light can be accurately defined in terms of (1) the initial slope (quantum yield); (2) the asymptote (light-saturated rate); (3) the convexity (rate of bending); and (4) the intercept (dark respiration). The effects of photoinhibition [which damages the reaction centre of photosystem II (PSII)] on these four parameters were measured in optically thin cultures of green plant cells (Chlamydomonas reinhardtii). The convexity of the light-response curve decreased steadily from a value of 0.98 (indicating a sharply bending response) to zero (indicating Michaelis-Menten kinetics) in response to increasing photoinhibition. Photoinhibition was quantified from the quantum yield of inhibited cells relative to that of control cells. The quantum yield was estimated by applying linear regression to low-light data or by fitting a non-rectangular hyperbola. Assuming the initial slope is linear allowed comparison with earlier work. However, as the convexity was lowered this assumption resulted in a significant underestimate of the true quantum yield. Thus, the apparent level of photoinhibition required for a zero convexity and the initial decrease in light-saturated photosynthesis depended upon how the quantum yield was estimated. If the initial slope of the light response was assumed to be linear the critical level of inhibition was 60%. If the linear assumption was not made, the critical level was 40%. At the level of inhibition where the convexity reached zero, the light-saturated rate of photosynthesis also began to decrease, indicating that this level of inhibition caused photosynthesis to be limited at all light intensities by the rate of PSII electron transport. At this level of inhibition the Fm-Fi signal (where Fm is maximal chlorophyll fluorescence and Fi is intermediate chlorophyll fluorescence of dark adapted cells; Briantais et al. 1988) from the fluorescence induction curve was zero and the Fi-Fo signal (where Fo is initial chlorophyll fluorescence of dark adapted cells) was 30% of the control, indicating dramatic reduction or complete elimination of one type of PSII. These data do not contradict published mathematical models showing that the ratio of the maximum speed of electron transport in PSII relative to the maximum speed of plastoquinone electron transport can determine the convexity of the photosynthetic response to light.