Walter A. Pursley

Elevated Carbon Dioxide and Ozone Effects on Peanut: I. Gas-Exchange, Biomass, and Leaf Chemistry

The effects of elevated CO2 and ozone (O3) on net photosynthetic rate (A) and growth are generally antagonistic although plant responses are highly dependent on crop sensitivity to the individual gases and their concentrations. In this experiment, we evaluated the effects of various CO2 and O3 mixtures on leaf gas-exchange, harvest biomass, and leaf chemistry in peanut (Arachis hypogaea L.), an O3-sensitive species, using open-top field chambers. Treatments included ambient CO2 (about 375 micromol mol-1) and CO2 enrichment of approximately 173 and 355 micromol mol-1 in combination with charcoal-filtered air (22 nmol O3 mol-1), nonfiltered air (46 nmol O3 mol-1), and nonfiltered air plus O3 (75 nmol O3 mol-1). Twice-ambient CO2 in charcoal-filtered air increased A by 23% while decreasing seasonal stomatal conductance (gs) by 42%. Harvest biomass was increased 12 to 15% by elevated CO2. In ambient CO2, nonfiltered air and added O3 lowered A by 21% and 48%, respectively, while added O3 reduced gs by 18%. Biomass was not significantly affected by nonfiltered air, but was 40% lower in the added O3 treatment. Elevated CO2 generally suppressed inhibitory effects of O3 on A and harvest biomass. Leaf starch concentration was increased by elevated CO2 and decreased by O3. Treatment effects on foliar N and total phenolic concentrations were minor. Increasing atmospheric CO2 concentrations should attenuate detrimental effects of ambient O3 and promote growth in peanut but its effectiveness declines with increasing O3 concentrations.

Response of soluble sugars and starch in field-grown cotton to ozone, water stress, and their combination

Abstract Ozone (O3) stress is known to reduce the growth and yield of a number of crops, and water stress can modify the extent of these effects. Both O3 and water stress alter the carbohydrate status of plants. Little is known, however, concerning O3 effects on carbohydrate pools of field-grown plants and whether water stress will modify the carbohydrate response to O3. Cotton (Gossypium hirsutum L. “McNair-235”) plants were exposed to five O3 concentrations in open-top field chambers for 12 hr/day throughout the growing season at two levels of soil water (well-watered or periodically water-stressed). The O3 concentrations ranged from 0.021 to 0.073 μl/l (seasonal mean 12 hr/day concentration). Plants were sampled from each plot on four occasions encompassing the early- to late-reproductive stages of growth. Soluble sugars (glucose, fructose and sucrose) and starch were measured in leaves, stems and roots at each sampling date. Analysis of variance was performed for main effects and interactions of O3 and water treatments at each sampling date (O3 effects were partitioned in linear and quadratic components). Effects of O3 and water stress on soluble carbohydrates and starch were most common in stems and roots. Ozone suppressed carbohydrate concentrations in all cases where significant O3 effects were detected in the absence of O3 × water interactions. On the other hand, soluble carbohydrate concentrations were greater in water-stressed plant tissues when effects were significant and in the absence of interactions. Water-stress effects on starch were variable. Interactions of O3 and water stress were not consistent but often included interaction with the quadratic O3 component.

INFLUENCE OF A MYCORRHIZAL FUNGUS AND/OR RHIZOBIUM ON GROWTH AND BIOMASS PARTITIONING OF SUBTERRANEAN CLOVER EXPOSED TO OZONE

The influence of soilborne symbionts such as rhizobia or mycorrhizal fungi on plant response to ozone (O3) has not been well defined. Leguminous plants in the field are infected by both types of organisms, which influence plant nutrition and growth. We studied the effects of infection withRhizobium leguminosarum biovartrifolii and/orGigaspora margarita on response of subterranean clover (Trifolium subterraneum L. cv Mt. Barker) to O3. Exposures were conducted in greenhouse CSTR chambers using four O3 concentrations [charcoal-filtered (CF), 50, 100, or 150 ppb; 6 h day−1, 5 day wk−1 for 12 weeks] as main plots (replicated). Four inoculum types were subplot treatments, i.e., inoculated with one, both, or neither microorganisms. At 2-wk intervals, plants were exposed to14CO2 and harvested 24 h later for determination of biomass and14C content of shoots and roots. Ozone at 100 or 150 ppb suppressed clover growth during the experiment. Inoculation withG. margarita alone suppressed clover growth by the last two harvests, whereasR. leguminosarum alone enhanced growth during this time period. When both symbionts were present, the plants grew similarly to the noninoculated controls. Shoot/root ratios were increased by 100 or 150 ppb O3 compared to that for CF-treated plants. Shoot/root ratios were greater for all inoculated plants compared to noninoculated controls. Under low O3 stress (CF or 50 ppb), plants inoculated with bothR. leguminosarum andG. margarita transported a greater proportion of recent photosynthate (14C) to roots than did noninoculated plants; we attribute this to metabolic requirements of the microorganisms. At the highest level of O3 stress (150 ppb), this did not occur, probably because little photosynthate was available and the shoots retained most of it for repair of injury. Statistically significant interactions occurred between O3 and inoculum types for shoot and total biomass. When averaged across harvests, 50 ppb O3 suppressed biomass in the plants inoculated withG. margarita alone. Apparently, the mycorrhizal fungus is such a significant C drain that even a small amount of O3 stress suppresses plant growth under these conditions.

Effects of ozone and water stress on net photosynthetic rate of field-grown soybean leaves

Detailed physiological measurements were taken in controlled field experiments to better elucidate how environmental stresses interact to affect a soybean plants physiology and growth. Nondestructive techniques were used to measure weekly and diurnal effects of ozone and water stress on net photosynthesis, stomatal resistance, and water status of field-grown soybean leaves. Open-top field chambers were used to expose soybeans to seasonal 12-h/day mean concentrations of 0.018 (CF), 0.059 (1.3NF) and 0.085 (1.9NF) ppm ozone. Soil water levels were well watered (WW) and water stressed (WS). Net carbon exchange-rate measurements (CER) were taken between 1000 and 1230h EST on center trifoliates with uniform exposure to sunlight. Average CER rates expressed as percent of CF over the growing season were 75% and 55% for WW 1.3NF, and 1.9NF and 90% and 61% for WS 1.3NF and 1.9NF, respectively. Measured mean seasonal photosynthesis rates of the WS CF treatments were 68% of those for the WW CF treatments.