Nutrient requirements for plant growth are expected to rise in response to the predicted changes in CO(2) and temperature. In this context, little attention has been paid to the effects of soil temperature, which limits plant growth at early stages in temperate regions. A factorial growth-room experiment was conducted with winter wheat, varying soil temperature (10 degrees C and 15 degrees C), atmospheric CO(2) concentration (360 and 700 ppm), and N supply (low and high). The hypothesis was that soil temperature would modify root development, biomass allocation and nutrient uptake during vegetative growth and that its effects would interact with atmospheric CO(2) and N availability. Soil temperature effects were confirmed for most of the variables measured and 3-factor interactions were observed for root development, plant biomass components, N-use efficiency, and shoot P content. Importantly, the soil temperature effects were manifest in the absence of any change in air temperature. Changes in root development, nutrient uptake and nutrient-use efficiencies were interpreted as counterbalancing mechanisms for meeting nutrient requirements for plant growth in each situation. Most variables responded to an increase in resource availability in the order: N supply >soil temperature >CO(2).

The effect of ambient and elevated atmospheric CO(2) on biomass partitioning and nutrient uptake of mycorrhizal and non-mycorrhizal pea plants grown in pots in a controlled environment was studied. The hypothesis tested was that mycorrhizae would increase C assimilation by increasing photosynthetic rates and reduce below-ground biomass allocation by improving nutrient uptake. This effect was expected to be more pronounced at elevated CO(2) where plant C supply and nutrient demand would be increased. The results showed that mycorrhizae did not interact with atmospheric CO(2) concentration in the variables measured. Mycorrhizae did not affect photosynthetic rates, had no effect on root weight or root length density and almost no effect on nutrient uptake, but still significantly increased shoot weight and reduced root/shoot ratio at harvest. Elevated CO(2) increased photosynthetic rates with no evidence for down-regulation, increased shoot weight and nutrient uptake, had no effect on root weight, and actually reduced root/shoot ratio at harvest. Non-mycorrhizal plants growing at both CO(2) concentrations had lower shoot weight than mycorrhizal plants with similar nutritional status and photosynthetic rates. It is suggested that the positive effect of mycorrhizal inoculation was caused by an enhanced C supply and C use in mycorrhizal plants than in non-mycorrhizal plants. The results indicate that plant growth was not limited by mineral nutrients, but partially source and sink limited for carbon. Mycorrhizal inoculation and elevated CO(2) might have removed such limitations and their effects on above-ground biomass were independent, positive and additive.

The objective of this study was to investigate the direct effect of elevated atmospheric CO2 concentrations on leaf respiration in darkness (R) over a broad range of measurement temperatures. Our aim was to further elucidate the underlying mechanism(s) of the often-reported inhibition of leaf R by a doubling of the atmospheric CO2 concentration. Experiments were conducted using two species of Plantago that differed in maximum relative growth rate (fast-growing Plantago lanceolata L. and the slow-growing P. euryphylla Briggs, Carolin & Pulley). Rates of leaf respiration (R) were measured at atmospheric CO2 concentrations ranging from 75 to 2000 &mgr;mol mol-1 at temperatures from 12 to 42 degrees C. R was measured as CO2 release with a portable gas exchange system with infrared gas analysers. Our hypothesis was that the changes in temperature alter the flux coefficient (i.e. the extent to which changes in potential enzyme activity has an effect on the rate of a reaction) of enzymes potentially affected by CO2. Initial analysis of our results suggested that R was inhibited by elevated CO2 in both species, with the apparent degree of inhibition being greatest at low temperature. Moreover, the apparent degree of inhibition following a doubling of atmospheric CO2 concentration from 350 to 700 &mgr;mol mol-1 was similar to that reported by several previous studies (approximately 14% and 26% for P. lanceolata and P. euryphylla, respectively) at a temperature equal to the mean of the previous studies. However, subsequent correction for diffusion leaks of CO2 across the gas exchange's cuvette gaskets revealed that no significant inhibition had occurred in either species, at any temperature. The inhibitory effect of elevated CO2 on leaf respiratory CO2 release reported by previous studies may therefore have been overestimated.

As a consequence of the ongoing reduction of the stratospheric ozone layer, the vegetation is exposed to increasing levels of UV‐B radiation (280–320 nm). In addition ozone in the troposphere is a pollutant and also capable of affecting the photosynthetic machinery. In this study, 5‐year‐old European beech trees were exposed from 1 July to October 1993 to two levels of UV‐B radiation and two levels of ozone, alone and in combination, in open‐top chambers equipped with lamps. The simulated UV‐B levels corresponded to either clear sky ambient level or a 14% decrease in the stratospheric ozone column over eastern Denmark, resulting in a 23% difference in biologically effective UV‐B (UV‐BBE) irradiance. The maximum UV‐Bbe given was 8.61 kJ m−2 day−1. The ozone levels were either the ambient (average 32 nl l−1) or ambient with ozone addition (average resulting concentration 71 nl l−1). Compared to the control treatment (ambient UV‐B, ambient O3) the elevated levels of UV‐B and O3 affected the trees negatively, expressed as declines in net photosynthesis (Pn), stomatal conductance (gs), chlorophyll fluorescence (Fv/Fm) and acceleration of senescence, measured as yellowing of the leaves. The UV‐B treatment induced stomatal closure before the other treatments did. The magnitude of the decreases in Pn and Fv/Fm occurred in the order: control

No abstract