Species with contrasting shade tolerance were grown under three light by two nutrient treatments. Gas exchange by intact leaves, leaf disk O2 evolution and chlorophyll fluorescence were measured. In shade-tolerant evergreen species (Argyrodendron sp., A. trifoliolatum and Flindersia brayleyana) photosynthetic activity of seedlings in air at light saturation (A) was lower under weak (30 pmol quanta m-2 day-1 ), compared with medium (130) or strong light (535). In Toona australis, a shade-intolerant and deciduous tree, A was reduced 44% from strong to weak light treatment on high nutrients (71 mg N L-1 nutrient solution). Nevertheless, nitrogen-use efficiency for leaf photosynthesis was highest in Toona under all growing conditions and, with higher specific leaf area, probably contributes towards fast occupancy of sites which underlies early succession in this species. All species made photosynthetic and respiratory adjustments from strong to medium to weak light, which resulted in a lower light compensation point (Q0). Such adjustments were accentuated by low nutrient supply (1.0 mg N L-1 nutrient solution) and were especially pronounced for shade-intolerant Toona. Reduced Q0 in Toona was accompanied by lower A and light saturation point (QA). Both species of Argyrodendron showed no decrease in QA despite reduction in Q0 under weak light.Contrary to expectation, photosynthetic responses to light × nutrient treatments did not correlate with degree of shade tolerance accorded each species by rainforest ecologists.

To compare the effects of supplementary ultraviolet-B (UV-B) radiation on a tropical/subtropical and a temperate plant, two indica rice cultivars (Er Bai Ai and Lemont) and peas were exposed to supplementary UV-B radiation for 8 days (biologically effective irradiance of 0.68 W m-2, 12 h per day). Marked decreases occurred in the ratios of variable to maximum chlorophyll fluorescence yield and in the quantum yield of photosynthetic oxygen evolution over the 8 day treatment period. The greatest decline always occurred in pea leaves, while in rice, cv. Er Bai Ai was more susceptible to UV-B radiation than cv. Lemont. Compared with control plants, the content of soluble protein and ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) protein decreased significantly after the UV-B treatment; the greatest decrease occurred in pea leaves, while the rate of decrease in rice was greater in cv. Er Bai Ai than cv. Lemont. Over the 8 day UV-B treatment period, the increase of UV-B-absorbing compounds was greater in rice leaves than in pea leaves, and greater in cv. Er Bai Ai than in cv. Lemont, although cv. Lemont was more UV-B tolerant. We suggest that the increase in these compounds is not the only indicator of resistance to UV-B damage; other factors, yet to be identified, may also confer UV-B tolerance. While leaf orientation may be related to the severity of UV-B damage, naturally near-horizontal pea leaves were still much more susceptible to UV-B damage compared with rice leaves which were held horizontally over the 8 day treatment period. Brown spots appeared on the upper surface of leaves of cv. Er Bai Ai after 2 days of UV-B treatment and showed a cumulative increase with the duration of exposure. Our study confirms the multiplicity of photosynthetic responses and of different protective strategies that may account for the differential sensitivity of plants to supplementary UV-B radiation.

Canopy temperature and incident photosynthetically active radiation were measured in experimental rice plots in Guangzhou, South China, during conditions which approximated those of the 'dry cold dew wind', a chilling phenomenon which is responsible for substantial yield reduction in rice. These field conditions were then reproduced in controlled environment photosynthetic measurement chambers. Mature plants of five varieties of rice were arranged to simulate a canopy in these chambers which was then exposed to a 3-day chilling treatment (21°C day/lO°C night) 14 days after anthesis. Canopy photosynthesis was measured before, during and after the chilling period. A Japonica-like cultivar, Hungarian-1 from Central Europe, and an Indica-like cultivar, Lemont from Texas, were markedly less sensitive to a chilling-dependent reduction in photosynthesis than were cultivars Gui Chao-2, Er Bai Ai and IR-8 from South-East China and The Philippines. The extent of chilling-dependent reduction in photosynthesis was greater if roots of plants in the simulated canopy were allowed to track air temperature at night. This led to a 5-7°C differential between root and air temperature in the morning and was associated with a midday depression of photosynthesis in all varieties. If root temperature was kept at 20°C during the day and night and air temperature controlled at 21°C day/ 10°C night, the effects of chilling on photosynthesis were much less pronounced.These changes in canopy photosynthesis following chilling treatments were correlated with changes in leaf level parameters. The level of soluble sugars and starch in leaves was found to remain high during the first cool night and to increase further during the next day. In cultivars which showed pronounced inhibition of canopy photosynthesis throughout the chilling treatment (Gui Chao-2, IR-8), soluble sugars remained at high levels until temperatures were returned to control values. In the less sensitive cultivar, Hungarian-1, soluble sugars declined throughout the chilling treatment. These results suggest inhibition of photosynthesis may be associated with sugar retention in leaves of rice at low temperatures, and imply differences between cultivars in this response. Other photosynthetic parameters monitored included the maximum rate of light and COz saturated photosynthesis, the quantum yield of photosynthesis, and FO and F,IF, of chlorophyll fluorescence at 77K. Surprisingly, Hungarian-1, the cultivar which showed least impairment of canopy photosynthesis showed greater reduction in quantum yield and larger changes in fluorescence parameters during the chilling treatments. These changes were readily reversible and may be related to the more horizontal leaf habit of this cultivar compared with the more vertical orientation of leaves in the other cultivars. These observations imply that assimilate regulation of photosynthetic metabolism, rather than light dependent damage to primary photosynthetic processes, underlies low temperature effects on yield of rice during grain fill...

Characteristics of photosynthetic CO2 exchange and 77K chlorophyll fluorescence in attached and detached leaves of rice were used to examine differences between five varieties exposed to a combination of bright light and low temperature. Preliminary studies established that attached leaves of varieties did not differ in the light dependence of photosynthesis at CO2 saturation or fluorescence properties when grown in full sunlight and measured under comparable conditions. All varieties showed a depression in maximum photosynthetic rate, but not in quantum yield or chlorophyll fluorescence when examined in the late afternoon after a period of active photosynthesis in air at 25°C. Attached leaves of all varieties showed depression of the maximum rate of photosynthesis and of quantum yield as well as an increase in Fo and a decrease in Fv/Fm of chlorophyll fluorescence at 77K following horizontal exposure to bright light in air at 25°. Similar results were obtained with detached leaves exposed to bright light while floating on water at 25°C. The rate of photosynthetic CO2 fixation in air as a function of temperature was examined in attached leaves of two representative varieties, Japonica-like cv. Hungarian- 1, and Indica-derived cv. IR-8. The former was capable to rates of photosynthesis at 10°C which were twice those of the latter, although the rates at 25°C were similar in both varieties. When detached leaves of five varieties of rice were floated on water at 10°C while exposed to 700 �mol photons m-2 s-1 of white light, consistent differences in the response of quantum yield, light and CO2 saturated O2 evolion and 77K fluorescence properties were found. Exposure to bright light at low temperature exaggerated the effects of bright light on these parameters, and larger effects were found in the Indica-derived varieties than in Japonica-like cv. Hungarian-1. These treatments at 10°C did not influence the extent of increase in Fo, but led to a greater decline in Fv/Fm of 77K chlorophyll fluorescence compared with treatments at 25°C. Varieties in which there was a larger decline in Fv/Fm showed slower recovery of variable fluorescence in the dark at 25°C, and especially at 10°C. These responses to in vitro treatments seem capable of distinguishing varieties such as cvv. Hungarian-1 and Er Bai Ai, which are believed to be more chilling tolerant by other criteria, from chilling sensitive varieties such as Gui Chao-2 and IR-8. Even though there is potential for application of these methods in screening programs, there is little evidence that the lesions in primary photosynthetic processes indicated by these methods are significant determinants of photosynthesis under field conditions.