This study investigates effects of climate warming (+ 2-5 °C above ambient) and elevated CO2 concentration (600 jumo] mol"') on the stomatal functioning and the water relations of Lotiuin perenne, using Free Air Temperature Increase (FATI) and Free Air CO2 Enrichment (FACE). Compared to growth at ambient temperature, whole-season temperature increase reduced leaf stomatal conductance, but only at the top of the canopy (-14-6 and -8-8% at ambient and elevated CO2, respectively). However, because higher canopy temperature raised the leaf-to-air vapour pressure difference, leaf transpiration rate increased (+28% at ambient and +48% at elevated CO2) and instantaneous leaf water use efficiency, derived from short-term measurements of assimilation and transpiration rate, declined (-11% at ambient and -13% at elevated CO2). Nevertheless, at the stand level, growth at + 2-5 °C reduced transpiration due to fewer tillers per plant and a smaller leaf area per tiller. This sparser vegetation was also more closely coupled to the atmosphere and maintained a drier internal microclimate. To assess whether the stomatal behaviour observed in this experiment could be explained by prevailing concepts of stomatal functioning, three models were applied (Cowan 1977; Ball, Woodrow & Berry 1987; Leuning 1995). The latter model accounted for the highest proportion of variability in the data (58%) and was insensitive to CO2 and temperature regime, which suggests that tlie principles of stomatal regulation are not affected by changes in CO2 or climate.

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A new technique, called Free Air Temperature Increase (FATI), was developed to artificially induce increased canopy temperature in field conditions without the use of enclosures. This acronym was chosen in analogy with FACE (Free Air CO2 Enrichment), a technique which produces elevated CO2 concentrations [CO2] in open field conditions. The FATI system simulates global warming in small ecosystems of limited height, using infrared heaters from which all radiation below 800 nm is removed by selective cut‐off filters to avoid undesirable photomorpho‐genetic effects. An electronic control circuit tracks the ambient canopy temperature in an unheated reference plot with thermocouples, and modulates the radiant energy from the lamps to produce a 2.5°C increment in the canopy temperature of an associated heated plot (continuously day and night). This pre‐set target differential is relatively‐constant over time due to the fast response of the lamps and the use of a proportional action controller (the standard deviation of this increment was <1°C in a 3 week field study with 1007 measurements). Furthermore, the increase in leaf temperature does not depend on the vertical position within the canopy or on the height of the stand. Possible applications and alternative designs are discussed.

The relationship between leaf photosynthetic capacity (p n, max), net canopy CO2- and H2O-exchange rate (NCER and E t, respectively) and canopy dry-matter production was examined in Lollium perenne L. cv. Vigor in ambient (363±30 μl· l(-1)) and elevated (631±43 μl·l(-1)) CO2 concentrations. An open system for continuous and simultaneous regulation of atmospheric CO2 concentration and NCER and E t measurement was designed and used over an entire growth cycle to calculate a carbon and a water balance. While NCERmax of full-grown canopies was 49% higher at elevated CO2 level, stimulation of p n, max was only 46% (in spite of a 50% rise in one-sided stomatal resistance for water-vapour diffusion), clearly indicating the effect of a higher leaf-area index under high CO2 (approx. 10% in one growing period examined). A larger amount of CO2-deficient leaves resulted in higher canopy dark-respiration rates and higher canopy light compensation points. The structural component of the high-CO2 effect was therefore a disadvantage at low irradiance, but a far greater benefit at high irradiance. Higher canopy darkrespiration rates under elevated CO2 level and low irradiance during the growing period are the primary causes for the increase in dry-matter production (19%) being much lower than expected merely based on the NCERmax difference. While total water use was the same under high and low CO2 levels, water-use efficiency increased 25% on the canopy level and 87% on a leaf basis. In the course of canopy development, allocation towards the root system became greater, while stimulation of shoot dry-matter accumulation was inversely affected. Over an entire growing season the root/shoot production ratio was 22% higher under high CO2 concentration.

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To evaluate the effect of different naturally occurring irradiation conditions on the sensitivity of bean (Phaseolus vulgaris cv. Label) to increased UV‐B levels, plants were grown under six different light treatments. In the control series (at ambient levels of UV‐B), UV‐B and visible light were decreased in parallel, resulting in three different total irradiation treatments with the same UV‐B/PAR ratio. A second series with a 15% increase in UV‐B irradiation at each PAR level was used to investigate the effect of UV‐B under the varying total irradiance levels. The different total irradiance levels resulted in large differences in total dry weight, specific leaf weight, photosynthesis‐light response and pigment concentrations. Nevertheless, the 15% increase in UV‐B resulted in equal reductions in total dry weight (from 24.5 to 34.3%) and effective photosynthesis for all light levels. The accumulation of protective pigments in the primary bean leaves was strongly correlated to the total irradiance level (200% increase from the lowest to the highest light level), but was not influenced by increasing UV‐B levels. As the UV‐B/PAR ratio outside increases with decreasing total irradiance (when induced by cloud cover) this implies that low radiation levels are potentially dangerous to some plants, even though the UV‐B levels may seem negligible.

Responses of abaxial, adaxial, and total leaf conductance to incident photosynthetic photon flux density, solar irradiance, and changing leaf water potential, as well as diurnal and seasonal patterns of stomatal conductances, were examined under field conditions for six Populus clones. Clones belonged to the species P. trichocarpa and to different interspecific crossings including P. deltoides, P. nigra, and P. maximowiczii. Black Cottonwood clone Columbia River showed stomatal behavior different from other clones in many aspects: (i) it was the only clone with hypostomatous leaves; (ii) its stomata remained open for a longer period of time, both diurnally and seasonally; (iii) the hysteresis effect in stomatal response to solar irradiance during the day was less pronounced; and (iv) its stomata showed hardly any response to declining leaf water potential. Leaf area duration and seasonal stomatal activity showed considerable clonal differences, which are in agreement with girth growth increment patterns. Clone Columbia River showed a much longer leaf life-span with considerable stomatal activity near late autumn, which might explain the substantial late-autumn girth growth increment of this clone.