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Recent carbon dioxide (CO2) concentrations promoted higher parthenin concentrations in an invasive Parthenium hysterophorusbiotype. Mean concentrations of parthenin, an allelopathic and defensive sesquiterpene lactone, were 49% higher at recent (~400 ppm) than at mid-twentieth-century (~300 ppm) CO2concentrations, but did not vary in a non-invasive biotype, suggesting that recent increases in atmospheric CO2may have already altered the chemistry of this destructive weed, potentially contributing to its invasive success.

Changes in temperature, CO2, and precipitation under the scenarios of climate change for the next 30 yr present a challenge to crop production. This review focuses on the impact of temperature, CO2,and ozone on agronomic crops and the implications for crop production. Understanding these implications for agricultural crops is critical for developing cropping systems resilient to stresses induced by climate change. There is variation among crops in their response to CO2, temperature, and precipitation changes and, with the regional differences in predicted climate, a situation is created in which the responses will be further complicated. For example, the temperature effects on soybean [Glycine max(L.) Merr.] could potentially cause yield reductions of 2.4% in the South but an increase of 1.7% in the Midwest. The frequency of years when temperatures exceed thresholds for damage during critical growth stages is likely to increase for some crops and regions. The increase in CO2contributes significantly to enhanced plant growth and improved water use efficiency (WUE); however, there may be a downscaling of these positive impacts due to higher temperatures plants will experience during their growth cycle. A challenge is to understand the interactions of the changing climatic parameters because of the interactions among temperature, CO2, and precipitation on plant growth and development and also on the biotic stresses of weeds, insects, and diseases. Agronomists will have to consider the variations in temperature and precipitation as part of the production system if they are to ensure the food security required by an ever increasing population.

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 sativaL.) from seven geographical locations to CO2concentration ([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 CO2levels than older lines; and (ii) phenotypic homogenization among modern lines could hamper efforts to identify desirable characteristics associated with CO2responsiveness.

Measurements were made over four growing seasons of the Na(+) and Cl(-) content of leaves and woody tissues (twigs, branches, trunk and roots) of mature, fruit-bearing Prunus salicina Lindl. (on Marianna 2624 rootstock) trees irrigated during the growing season with water containing 3, 14 or 28 mM salt (2/1 molar ratio of NaCl and CaCl(2)). At the beginning of the study, the trees were 19 years old. Woody tissues of trees irrigated with water containing 14 or 28 mM salt accumulated Na(+) and Cl(-). Leaves of trees irrigated with water containing 14 or 28 mM salt accumulated Cl(-), but not Na(+), unless they had visible symptoms of salt injury. X-Ray microanalysis of leaf mesophyll cells indicated some ability of the cells to sequester Cl(-) in the vacuole. The data demonstrate a capacity for ion compartmentation among tissues and cell organelles in mature Prunus salicina, which may explain the ability of the species to survive low levels of salinity for several years in the field.

In a concurrent study we reported that cowpea (Vigna unguiculata [L.] Walp.) had the ability to maintain seed yields when subjected to drought during the vegetative stage provided the subsequent irrigation interval was not too great. A 4 day irrigation interval produced maximum vegetative and seed yields, while an 8 day interval following a vegetative stage drought produced maximum seed yields and water-use-efficiency.

Cowpeas (Vigna unguiculata [L.] Walp.) were subjected to six irrigation treatments and two levels of soil nitrogen during two seasons under field conditions. Withholding irrigation during the vegetative stage in a rain-free environment following pre-irrigation resulted in lower water use (-11 to -20%) but negligible effects on seed yields (-2 to +3%) providing the irrigation interval during flowering and pod filling was not too great. Lengthening the irrigation interval resulted in less vine elongation and less shoot biomass production. Significant reductions in seed yield were observed only with the longest irrigation interval. The higher level of soil nitrogen resulted in small or negligible increases in shoot biomass production and seed yield.

Seeds from four plant pairs collected from contrasting elevations in Hawaii were grown in greenhouses at the University of Maryland at UV‐B radiation levels that approximated a 20% and 40% stratospheric ozone depletion anticipated at sea level in Maui. In general, increases in UV‐B radiation resulted in earlier reproductive effort, increased dark respiration and maintenance of relative water content (RWC), photosynthesis, and apparent quantum efficiency (AQE) in plants from higher elevations where natural UV‐B radiation is already high. In contrast, plants collected from low elevational ranges showed a significant decline in average plant and floral dry biomass, a decline in AQE and RWC, and a reduction in light‐saturated photosynthetic capacity. Increases in UV‐B‐absorbing compounds (e.g., flavonoids), were noted for low elevation but not high elevation plants. However, plants from high elevations produced a consistently larger amount of these compounds even in the absence of UV‐B radiation. This study suggests that plants growing in a naturally high UV‐B environment may have developed or maintained mechanisms related to reproductive phenology and carbon uptake which may maintain productivity in a high UV‐B environment. This would also suggest that ecotypic differentiation may have occurred in response to increasing UV‐B radiation over an elevational gradient. The range of adaptability to increased UV‐B also implies changes in species and community dynamics that might be anticipated in natural plant populations if stratospheric ozone depletion continues.

Sugarbeets (Beta vulgaris L.) on a Panoche clay loam soil were subjected to 3 different irrigation frequencies and 3 irrigation cutoff dates prior to harvest to determine the effects on evapotranspiration, growth, and sucrose yield. Lengthening the irrigation interval from 1 to 3 weeks reduced evapotranspiration without a significant decline in sucrose production. Increased irrigation cutoff from 3 to 7 weeks prior to harvest significantly increased sucrose percentage within the root and resulted in similar total sucrose yields. Lengthening the irrigation interval only slightly reduced both fresh vegetative biomass and leaf area index (significant differences occurred only at one plant sampling date). The combination of less frequent irrigation and an early cutoff date increased the amount of soil water extracted by sugarbeets. The water use of sugarbeets can be reduced without a significant decline in sucrose production through optimizing irrigation frequency to about 14 to 20 days on this soil and cutting off irrigations about 40 to 45 days before harvest, provided irrigations replenish soil water depletions.

In a previous study conducted at the University of California at Riverside, it was shown that water use of cowpea could be reduced while maintaining seed yields by withholding irrigation during the vegetative stage in a rain-free environment, and then irrigating when estimates based on potential evapotranspiration, indicated 40% depletion of available moisture in 90-cm depth of soil. The general applicability of this efficient irrigation management method was tested by experiments conducted at the West Side Field Station in the San Joaquin Valley of California with six irrigation treatments, three different row spacings (single rows on 76- and 102-cm beds, and double rows on a 102-cm bed), a semi-erect cultivar of cowpea (Vigna unguiculata [L.] Walp.), and a prostrate cultivar of lima bean (Phaseolus lunatus L.).

Seedlings of nine tropical species varying in growth and carbon metabolism were exposed to twice the current atmospheric level of CO2 for a 3 month period on Barro Colorado Island, Panama. A doubling of the CO2 concentration resulted in increases in photosynthesis and greater water use efficiency (WUE) for all species possessing C3 metabolism, when compared to the ambient condition. No desensitization of photosynthesis to increased CO2 was observed during the 3 month period. Significant increases in total plant dry weight were also noted for 4 out of the 5 C3 species tested and in one CAM species, Aechmea magdalenae at high CO2. In contrast, no significant increases in either photosynthesis or total plant dry weight were noted for the C4 grass, Paspallum conjugatum. Increases in the apparent quantum efficiency (AQE) for all C3 species suggest that elevated CO2 may increase photosynthetic rate relative to ambient CO2 over a wide range of light conditions. The response of CO2 assimilation to internal Ci suggested a reduction in either the RuBP and/or Pi regeneration limitation with long term exposure to elevated CO2. This experiment suggests that: (1) a global rise in CO2 may have significant effects on photosynthesis and productivity in a wide variety of tropical species, and (2) increases in productivity and photosynthesis may be related to physiological adaptation(s) to increased CO2.