o reveal the dynamics of short-term photosynthetic acclimation to increased irradiance, the light response of photochemical (qp), non-photochemical (q^) and Fo (qo) quenchings of chlorophyll (Chl) fluorescence and Chl and carotenoids compositíon in Norway spruce needles were monitored within three days after transfer of saplings ffom low diffuse irradiance (maximum photosynthetic photon fluence density PPFD 50 pmol m'^ s'i) to direct sun radiation (maximum PPFD 2000 pmol m‘2 s'*). Irradiance responses of fluorescence quenching coefficients revealed the occurrence of substantíal changes in partítioning of excitation energy between photochemical reactions and radiatíonless dissipation within two days. The saturating irradiance for qj,j and the capacity of non-radiatíve dissipation processes was shifted from about 450 pmol m'^ s** to 1620 pmol m-2 s'L Whereas immediately after exposure to ťull sunlight was completely reduced at 1620 pmol m'^ s'*, two days later 40 % of was stíll present in oxidized form at this irradiance. A fast pigment photobleaching at noon prevented the over-reduction of and thus it was one of the possible short-term acclimation processes. No severe photoinhibition of photosystem 2 (PS 2) photochemistry occurred within the period of investigation as can be judged from the high F^/F^ value.
Morphological and ultrastructural changes, the chlorophyll (Chl) content and Chl a fluorescence induction were studied in primary leaves of runner beán plants (Phaseolus coccineus L. cv. Pi?kny Jaš) grown in Knop solution and treated with excess Cu [20 g(CuS04 x 5 H2O) m"^] at difíerent growth stages. The plants were exposed to the metal for 12 d. Cu added to the nutrient solution at the beginning growth stage induced significant leaf area reduction (31 %) as well as increase of Chl accumulation (148 %) and leaf density (122 %). No ultrastructural changes of chloroplasts were seen. Application of Cu at the advanced growth stage caused relatively smáli changes except local disturbances in stroma lamellae and leaf reduction to 70 % of control. Significant disorganization of chloroplast ultrastructure, smáli leaf area reduction (80 %) but specific leaf area increase (163 %) and leaf density decrease (67 %) were seen only in the primary leaves of plants treated with Cu at the finál stage of growth. These changes, similar to senescence response, were accompanied by Fy/Fo ratio decrease to 67 % in comparison with control.
Soybean [Glycine mctx (L.) Merr. cv. Jack] was grown in the field in rain-prolected plots to study effects of drought and atmospheric CO2 enrichnient on leaf gas exchange. Midday depressions in leaf photosynthetic CO2 exchange rates were found in drought-sfressed plants and the diumal changes were inostly stoinatal- regulated, although accumulated drought stress eventually resulted in some non- stomatal limitations. However, seasonal changes in were mostly limited by non- stomatal factors. Water use efficiency was always higher for drought stiessed plants and depended on the severity of stress and associated stomatal or nonstoinatal limitations. At enriched atmospheric CO2 levels, stomatal limitations to Pyj under drought stress were less important than at ambient atmospheric CO2 levels. Morning and aftemoon leaf starch levels were enhanced in both irrigated and nonirrigated plants in enriched CO2. Aftemoon starch levels were higher in stiessed leaves than in non-stressed leaves at normál CO2 levels.
Effects of electrical conductivity (EC) and substrate water content on photosynthetic response to irradiance were examined to understand the constraints in photosynthesis caused by these stresses. Tomato plants were grown under high (4.5 mS cm'1) or low (2.3 mS cm'1) EC and high (95 %) or low (55 % of capillary capacity) soil water content. Photosynthetic photon flux (PPF) was changed (I) from low to high and then in reverse from high to low, and (2) starting from high to low and then reversing from low to high. In both cases, photosynthetic rates (PN) at most levels of PPF were higher during the 2nd cycle than during the first one. The extent of this hysteresis was larger for high EC-treated and/or water-stressed plants. In addition, /’N was inhibited under veiy high PPF (1800 pmol nr2 s'1) in high EC and/or water-stressed plants (stress-induced photosynthetic depression under high PPF). After gradual declining for about 40 min under high PPF, /'N started to recover. When recovered to some extent, it went down again, showing oscillation cycles. Oscillation was clearly observed for the treatment of high EC combined with high soil water content, but not for the control. Thus the abnormal photosynthetic responses to irradiance, such as hysteresis, photosynthetic depression under high PPF and oscillations may be induced or promoted by some constraints in substrate water availability, which cause abnormal stomatal response and inactivity of mesophyll photosynthesis.
A decrease of F,„ followed with a ceitain delay by an increase of was detected in the cells of Synechococcus elongatus in the first 120 min of the photoinhibitory treatment at 56 <>0 (growth temperature). Then F,n started to rise in parallel with F,, and this process proceeded widi the same rate both in the light and in the dark at 56 °C after light treatment. On tíie contrary, an increase of F^ observed during the light treatment at 15 °C was largely reversed after subsequent transfer of the ceUs to the dark at 56 <>€ but F,q remained nearly unchanged during the dark incubation.