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Carbon dioxide is a principle resource for plant growth; as such, the ongoing increase in its concentration may allow breeders to begin selecting for optimal cultivars among current crop lines. Such a program may not be necessary, however, if breeders have, by empirical selection, already chosen the most responsive cultivars. To test this hypothesis, we examined the growth and vegetative characteristics of individual plants of cultivated oat (Avena sativa L.) from seven geographical locations to CO2 concentration ([CO2]) increases that corresponded roughly to the [CO2] from the 1920s, the current [CO2], and the [CO2] projected for the middle of this century (300, 400, and 500 μmol mol-1, respectively). In contrast to our predictions, newer lines were less responsive than older lines to rising [CO2] in terms of both leaf area and tiller number. A comparison of phenotypic variability indicated that for almost all measured parameters, older lines had a greater degree of intraspecific variability (i.e., newer lines were more uniform). As a consequence, a number of CO2-sensitive lines among older cultivars were identified with respect to parameters such as vegetative biomass or tiller production. Our results suggest that for oat: (i) newer lines are not intrinsically more responsive to rising CO2 levels than older lines; and (ii) phenotypic homogenization among modern lines could hamper efforts to identify desirable characteristics associated with CO2 responsiveness. [ABSTRACT FROM AUTHOR]

The effects of ultraviolet-B (UV-B between 290 and 320 nm) on photosynthesis and growth characteristics were investigated in field grown cassava (Manihot esculentum Crantz). Plants were grown at ambient and ambient plus a 5.5 kJ m-2 d-1 supplementation of UV-B radiation for 95 d. The supplemental UV-B fluence used in this experiment simulated a 15% depletion in stratospheric ozone at the equator (0°N). Carbon dioxide exchange, oxygen evolution, and the ratio of variable to maximum fluorescence (Fv/Fm) were determined for fully expanded leaves after 64-76d of UV-B exposure. All plants were harvested after 95d of UV-B exposure, assayed for chlorophyll and UV-B absorbing compounds, and separated into leaves, petioles, stems and roots. Exposure to UV-B radiation had no effect on in situ rates of photosynthesis or dark respiration. No difference in the concentration of UV-B absorbing compounds was observed between treatments. A 2-d daytime diurnal comparison of Fv to Fm ratios indicated a significant decline in Fv/Fm ratios and a subsequent increase in photoinhibition under enhanced UV-B radiation if temperature or PPF exceeded 35°C or 1800 μmol m-2 s-1, respectively. However, UV-B effects on fluorescence kinetics appeared to be temporal since maximal photosynthetic rates as determined by oxygen evolution at saturated CO2 and PPF remained unchanged. Although total biomass was unaltered with UV-B exposure, alterations in the growth characteristics of cassava grown with supplemental UV-B radiation are consistent with auxin destruction and reduced apical dominance. Changes in growth included an alteration of biomass partitioning with a significant increase in shoot/root ratio noted for plants receiving supplemental UV-B radiation, The increase in shoot/root ratio was due primarily to a significant decrease in root weight (-32%) with UV-B exposure. Because root production determines the harvestable portion of cassava, UV-B radiation may still influence. [ABSTRACT FROM AUTHOR]