Interactive Effects of Elevated [CO2] and Drought on the Maize Phytochemical Defense Response against Mycotoxigenic Fusarium verticillioides.

Changes in climate due to rising atmospheric carbon dioxide concentration ([CO2]) are predicted to intensify episodes of drought, but our understanding of how these combined conditions will influence crop-pathogen interactions is limited. We recently demonstrated that elevated [CO2] alone enhances maize susceptibility to the mycotoxigenic pathogen, Fusarium verticillioides (Fv) but fumonisin levels remain unaffected. In this study we show that maize simultaneously exposed to elevated [CO2] and drought are even more susceptible to Fv proliferation and also prone to higher levels of fumonisin contamination. Despite the increase in fumonisin levels, the amount of fumonisin produced in relation to pathogen biomass remained lower than corresponding plants grown at ambient [CO2]. Therefore, the increase in fumonisin contamination was likely due to even greater pathogen biomass rather than an increase in host-derived stimulants. Drought did not negate the compromising effects of elevated [CO2] on the accumulation of maize phytohormones and metabolites. However, since elevated [CO2] does not influence the drought-induced accumulation of abscisic acid (ABA) or root terpenoid phytoalexins, the effects elevated [CO2] are negated belowground, but the stifled defense response aboveground may be a consequence of resource redirection to the roots.

Accumulation of terpenoid phytoalexins in maize roots is associated with drought tolerance.

Maize (Zea mays) production, which is of global agro-economic importance, is largely limited by herbivore pests, pathogens and environmental conditions, such as drought. Zealexins and kauralexins belong to two recently identified families of acidic terpenoid phytoalexins in maize that mediate defence against both pathogen and insect attacks in aboveground tissues. However, little is known about their function in belowground organs and their potential to counter abiotic stress. In this study, we show that zealexins and kauralexins accumulate in roots in response to both biotic and abiotic stress including, Diabrotica balteata herbivory, Fusarium verticillioides infection, drought and high salinity. We find that the quantity of drought-induced phytoalexins is positively correlated with the root-to-shoot ratio of different maize varieties, and further demonstrate that mutant an2 plants deficient in kauralexin production are more sensitive to drought. The induction of phytoalexins in response to drought is root specific and does not influence phytoalexin levels aboveground; however, the accumulation of phytoalexins in one tissue may influence the induction capacity of other tissues.

Effects of elevated [CO2 ] on maize defence against mycotoxigenic Fusarium verticillioides.

Maize is by quantity the most important C4 cereal crop; however, future climate changes are expected to increase maize susceptibility to mycotoxigenic fungal pathogens and reduce productivity. While rising atmospheric [CO2 ] is a driving force behind the warmer temperatures and drought, which aggravate fungal disease and mycotoxin accumulation, our understanding of how elevated [CO2 ] will effect maize defences against such pathogens is limited. Here we report that elevated [CO2 ] increases maize susceptibility to Fusarium verticillioides proliferation, while mycotoxin levels are unaltered. Fumonisin production is not proportional to the increase in F. verticillioides biomass, and the amount of fumonisin produced per unit pathogen is reduced at elevated [CO2 ]. Following F. verticillioides stalk inoculation, the accumulation of sugars, free fatty acids, lipoxygenase (LOX) transcripts, phytohormones and downstream phytoalexins is dampened in maize grown at elevated [CO2 ]. The attenuation of maize 13-LOXs and jasmonic acid production correlates with reduced terpenoid phytoalexins and increased susceptibility. Furthermore, the attenuated induction of 9-LOXs, which have been suggested to stimulate mycotoxin biosynthesis, is consistent with reduced fumonisin per unit fungal biomass at elevated [CO2 ]. Our findings suggest that elevated [CO2 ] will compromise maize LOX-dependent signalling, which will influence the interactions between maize and mycotoxigenic fungi.

Leaf photosynthesis and carbohydrates of CO2-enriched maize and grain sorghum exposed to a short period of soil water deficit during vegetative development.

Among C4 species, sorghum is known to be more drought tolerant than maize. The objective was to evaluate differences in leaf gas exchanges, carbohydrates, and two enzyme activities of these nicotinamide adenine dinucleotide phosphate-malic enzyme (NADP-ME) C4 subtype monocots in response to water deficit and CO2 concentration ([CO2]). Maize and sorghum were grown in pots in sunlit environmental-controlled chambers. Treatments included well watered (WW) and water stressed (WS) (water withheld at 26 days) and daytime [CO2] of 360 (ambient) and 720 (elevated) µmol mol⁻¹. Midday gas exchange rates, concentrations of nonstructural carbohydrates, and activities of sucrose-phosphate synthase (SPS) and adenosine 5'-diphosphoglucose pyrophosphorylase (ADGP) were determined for fully expanded leaf sections. There was no difference in leaf CO2 exchange rates (CER) between ambient and elevated [CO2] control plants for both maize and sorghum. After withholding water, leaf CER declined to zero after 8 days in maize and 10 days for sorghum. Sorghum had lower stomatal conductance and transpiration rates than maize, which resulted in a longer period of CER under drought. Nonstructural carbohydrates of both control maize and sorghum were hardly affected by elevated [CO2]. Under drought, however, increases in soluble sugars and decreases in starch were generally observed for maize and sorghum at both [CO2] levels. For stressed maize and sorghum, decreases in starch occurred earlier and were greater at ambient [CO2] than at elevated [CO2]. For maize, drought did not meaningfully affect SPS activity. However, a decline in SPS activity was observed for drought-stressed sorghum under both [CO2] treatments. There was an increase in ADGP activity in maize under drought for both [CO2] treatments. Such a response in ADGP to drought, however, did not occur for sorghum. The generally more rapid response of maize than sorghum to drought might be related to the more rapid growth of leaf area of maize.

Elevated CO2 increases water use efficiency by sustaining photosynthesis of water-limited maize and sorghum.

Maize and grain sorghum seeds were sown in pots and grown for 39 days in sunlit controlled-environment chambers at 360 (ambient) and 720 (double-ambient, elevated)µmol mol(-1) carbon dioxide concentrations [CO(2)]. Canopy net photosynthesis (PS) and evapotranspiration (TR) was measured throughout and summarized daily from 08:00 to 17:00h Eastern Standard Time. Irrigation was withheld from matched pairs of treatments starting on 26 days after sowing (DAS). By 35 DAS, cumulative PS of drought-stress maize, compared to well-watered plants, was 41% lower under ambient [CO(2)] but only 13% lower under elevated [CO(2)]. In contrast, by 35 DAS, cumulative PS of drought-stress grain sorghum, compared to well-watered plants, was only 9% lower under ambient [CO(2)] and 7% lower under elevated [CO(2)]. During the 27-35 DAS drought period, water use efficiency (WUE, mol CO(2)Kmol(-1)H(2)O), was 3.99, 3.88, 5.50, and 8.65 for maize and 3.75, 4.43, 5.26, and 9.94 for grain sorghum, for ambient-[CO(2)] well-watered, ambient-[CO(2)] stressed, elevated-[CO(2)] well-watered and elevated-[CO(2)] stressed plants, respectively. Young plants of maize and sorghum used water more efficiently at elevated [CO(2)] than at ambient [CO(2)], especially under drought. Reductions in biomass by drought for young maize and grain sorghum plants were 42 and 36% at ambient [CO(2)], compared to 18 and 14% at elevated [CO(2)], respectively. Results of our water stress experiment demonstrated that maintenance of relatively high canopy photosynthetic rates in the face of decreased transpiration rates enhanced WUE in plants grown at elevated [CO(2)]. This confirms experimental evidence and conceptual models that suggest that an increase of intercellular [CO(2)] (or a sustained intercellular [CO(2)]) in the face of decreased stomatal conductance results in relative increases of growth of C(4) plants. In short, drought stress in C(4) crop plants can be ameliorated at elevated [CO(2)] as a result of lower stomatal conductance and sustaining intercellular [CO(2)]. Furthermore, less water might be required for C(4) crops in future higher CO(2) atmospheres, assuming weather and climate similar to present conditions.

Soil organic carbon and nitrogen accumulation in plots of rhizoma perennial peanut and bahiagrass grown in elevated carbon dioxide and temperature.

Carbon sequestration in soils might mitigate the increase of carbon dioxide (CO2) in the atmosphere. Two contrasting subtropical perennial forage species, bahiagrass (BG; Paspalum notatum Flügge; C4), and rhizoma perennial peanut (PP; Arachis glabrata Benth.; C3 legume), were grown at Gainesville, Florida, in field soil plots in four temperature zones of four temperature-gradient greenhouses, two each at CO2 concentrations of 360 and 700 micromol mol(-1). The site had been cultivated with annual crops for more than 20 yr. Herbage was harvested three to four times each year. Soil samples from the top 20 cm were collected in February 1995, before plant establishment, and in December 2000 at the end of the project. Overall mean soil organic carbon (SOC) gains across 6 yr were 1.396 and 0.746 g kg(-1) in BG and PP, respectively, indicating that BG plots accumulated more SOC than PP. Mean SOC gains in BG plots at 700 and 360 micromol mol(-1) CO2 were 1.450 and 1.343 g kg(-1), respectively (not statistically different). Mean SOC gains in PP plots at 700 and 360 micromol mol(-1) CO2 were 0.949 and 0.544 g kg(-1), respectively, an increase caused by elevated CO2. Relative SON accumulations were similar to SOC increases. Overall mean annual SOC accumulation, pooled for forages and CO2 treatments, was 540 kg ha(-1) yr(-1). Eliminating elevated CO2 effects, overall mean SOC accumulation was 475 kg ha(-1) yr(-1). Conversion from cropland to forages was a greater factor in SOC accumulation than the CO2 fertilization effect.