Arabidopsis thaliana grown in a light regime that included ultraviolet-B (UV-B) radiation (6 kJ m(-2) d(-1)) had similar light-saturated photosynthetic rates but up to 50% lower stomatal conductance rates, as compared to plants grown without UV-B radiation. Growth responses of Arabidopsis to UV-B radiation included lower leaf area (25%) and biomass (10%) and higher UV-B absorbing compounds (30%) and chlorophyll content (52%). Lower stomatal conductance rates for plants grown with UV-B radiation were, in part, due to lower stomatal density on the adaxial surface. Plants grown with UV-B radiation had more capacity to down regulate photochemical efficiency of photosystem II (PSII) as shown by up to 25% lower phi(PSII) and 30% higher non-photochemical quenching of chlorophyll fluorescence under saturating light. These contributed to a smaller reduction in the maximum photochemical efficiency of PSII (F (v)/F (m)), greater dark-recovery of F (v)/F (m), and higher light-saturated carbon assimilation and stomatal conductance and transpiration rates after a four-hour high light treatment for plants grown with UV-B radiation. Plants grown with UV-B were more tolerant to a 12 day drought treatment than plants grown without UV-B as indicated by two times higher photosynthetic rates and 12% higher relative water content. UV-B-grown plants also had three times higher proline content. Higher tolerance to drought stress for Arabidopsis plants grown under UV-B radiation may be attributed to both increased proline content and decreased stomatal conductance. Growth of Arabidopsis in a UV-B-enhanced light regime increased tolerance to high light exposure and drought stress.

We investigated the effects of an ambient dose of ultraviolet-B (UV-B) radiation on chamber-grown Pseudotsuga menziesii var. glauca (Beissn.) Franco (Douglas-fir) seedlings, to determine if the presence of UV-B radiation in the growth light regime induces tolerance to environmental stresses such as high light and drought. Douglas-fir seedlings were grown without UV-B radiation or with 6 kJ m-2 day-1 of biologically effective UV-B, which is ambient for the intermountain regions of Idaho. Non-stressed seedlings grown with UV-B radiation had 35% lower seedling dry mass, 36% higher concentrations of UV-B absorbing compounds per unit leaf area, 30% lower stomatal frequencies, 25% lower light-saturated photochemical efficiencies of Photosystem II and 45% lower light-saturated stomatal conductance than non-stressed seedlings grown without UV-B radiation. After 4 days of high-light stress, seedlings grown with UV-B radiation had 32% higher light-saturated carbon assimilation rates (A(CO2)) than seedlings grown without UV-B radiation. After water was withheld from the seedlings for up to 15 days, seedlings grown with UV-B radiation had 50% higher A(CO2) and 40% higher seedling water potentials than seedlings grown without UV-B radiation. The results support the hypothesis that UV-B radiation can act as an environmental signal to induce tolerance to high-light and drought stress in Douglas-fir seedlings. Possible mechanisms for the enhanced stress tolerance are discussed.

Unifoliolate leaves of seedlings of soybean [Glycine max (L.) Merr. cv. Northrup King S1346] exhibit diaheliotropic movements when well watered and paraheliotropic movements when water stressed. The movements originate at the pulvinus at the base of each leaf lamina. The receptor site for the light stimulus was located by covering portions of the leaf surface with India ink and observing the effect on leaf movement; the leaf lamina, leaf veins, and pulvinus were tested. The pulvinus was the receptor site for light that stimulates both diaheliotropic and paraheliotropic leaf movement. The role of different wavelengths of light in diaheliotropism and paraheliotropism was determined using filters that remove broad bands of light wavelengths. Blue light was necessary for both diaheliotropism and paraheliotropism. Interference filters were used to expose soybean leaves to selected blue‐light wavelengths. The amount of diaheliotropic leaf movement at each wavelength was used to obtain an action spectrum. There was one peak between 410 nd 440 nm and one between 470 and 490 nm. A nonphytochrome photoreceptor located in the pulvinus used blue light, in conjunction with other factors, to determine leaf orientation and thereby regulate light interception of leaves of soybean seedlings.

The induction of NADP-malic enzyme (NADP-ME) in maize (Zea mays) etiolated seedlings as a function of intensity and duration of exposure to UV-B radiation was investigated by measuring changes in activity and protein, as well as by levels of NADP-ME gene transcripts. Exposures to UV radiation from 2 to 60 min including photon flux densities of UV-B of 0.5–2 mmol quanta m-2 s-1 which are well below levels which occur naturally under full sunlight, caused an increase in Me gene mRNA and NADP-ME protein, as well as in NADP-ME activity. A similar dosage of UV-A radiation was ineffective. A low level of red light (10 µmol quanta m-2 s-1) for only 5 min was also effective in inducing increases in NADP-ME activity equivalent to that with UV-B radiation. A 5 min exposure to far red light (100 µmol quanta m-2 s-1) following UV-B or red light treatment largely reversed the induction of NADP-ME, and this effect of far-red could be eliminated by further treatment with UV-B or red light. These results indicate that physiological levels of UV-B radiation have a positive effect on induction of the synthesis of a photosynthetic enzyme which is involved in C4 photosynthesis.

A simulation of the quantitative influence of altitude on photosynthetic CO2 uptake capability (A'') included the effects of predicted changes (1) in air temperature (lapse rate) and (2) leaf temperature, (3) ambient pressure and CO2 concentration, and (4) the diffusion eoefficient for CO2 in air. When a dry lapse rate (0.01 °C m"') in air temperature was simulated, significant declines (up to 14%) in A'' were predicted from sea level to 4km altitude. A moist lapse rate of 0.003°C m~^ resulted in less than a 4% decrease in A*" over the same altitude range. When natural leaf temperatures (predicted from heat balance analyses) were simulated. A'' was significantly greater (-20%) than when leaf temperatures were considered equal to air temperature for all lapse conditions. There was virtually no change in A'" with altitude when predicted leaf temperatures and moist lapse conditions were simulated. There was a significant (-10%) increase in A*" with altitude when leaf temperature was held eonstant at 30°C (regardless of altitude) under moist lapse conditions. Future studies evaluating the effects of elevation on photosynthesis could benefit from the above considerations of the effects of natural leaf temperature regimes and prevailing lapse conditions on CO2 uptake potential.Key-words: photosynthesis; altitude; lapse rate; leaf temperature; carbon dioxide uptake. Abhreyiatiotis:A. photosynthetic uptake of CO; (moles m~' s"'); g. leaf conductance to CO2 (moles m''s"'); AC. leaf-to-air differenee in CO2 eoneentration (moles); C,,. ambient CO2 concentration (moles); Ci, CO2 eoneentration inside the leaf (moles); D^.. diffusion coefficient forCO2in air (m's"'); T. diffusion temperature (°C); P. atmospheric pressure at a given altitude (P,,); D,,, diffusion coeflkient for CO2 at standard temperatures (25°C) and pressure (t.55 x 10 "' m" s^-); Po, standard atmospherie pressure at sea level (t atmosphere, 0.1013 MPa); To. stahdard temperature (25°C); T,. leaf temperature (°C); T,,, air temperature (°C).