Diel trends in stomatal response to ozone and water deficit: a unique relationship of midday values to growth and allometry in Pima cotton?

Plant responses to ozone (O3 ) and water deficit (WD) are commonly observed, although less is known about their interaction. Stomatal conductance (gs ) is both an impact of these stressors and a protective response to them. Stomatal closure reduces inward flux of O3 and outward flux of water. Stomatal measurements are generally obtained at midday when gas exchange is maximal, but these may not be adequate surrogates for stomatal responses observed at other times of day, nor for non-stomatal responses. Here, we find in Pima cotton that stomatal responses to O3 observed at midday do not reflect responses at other times. Stomata were more responsive to O3 and WD near midday, despite being at quasi-steady state, than during periods of active opening or closing in morning or evening. Stomatal responsivity to O3 was not coincident with maximum gas exchange or with periods of active regulation, but coincident with plant sensitivity to O3 previously determined in this cultivar. Responses of pigmentation and shoot productivity were more closely related to stomatal responses at midday than to responses at other times of day under well-watered (WW) conditions, reflecting higher stomatal responsivity, sensitivity to O3 , and magnitude of midday gs . Under WD conditions, shoot responses were more closely related to early morning gs. Root responses were more closely related to early morning gs under both WW and WD. Responses of stomata to O3 at midday were not good surrogates for stomatal responses early or late in the day, and may not adequately predicting O3 flux under WD or when maximum ambient concentrations do not occur near midday.

Perchlorate content of plant foliage reflects a wide range of species-dependent accumulation but not ozone-induced biosynthesis.

Perchlorate (ClO4(-)) interferes with uptake of iodide in humans. Emission inventories do not explain observed distributions. Ozone (O3) is implicated in the natural origin of ClO4(-), and has increased since pre-industrial times. O3 produces ClO4(-)in vitro from Cl(-), and plant tissues contain Cl(-) and redox reactions. We hypothesize that O3 exposure may induce plant synthesis of ClO4(-). We exposed contrasting crop species to environmentally relevant O3 concentrations. In the absence of O3 exposure, species exhibited a large range of ClO4(-) accumulation but there was no relationship between leaf ClO4(-) and O3, whether expressed as exposure or cumulative flux (dose). Older, senescing leaves accumulated more ClO4(-) than younger leaves. O3 exposed vegetation is not a source of environmental ClO4(-). There was evidence of enhanced ClO4(-) content in the soil surface at the highest O3 exposure, which could be a significant contributor to environmental ClO4(-).

High ozone increases soil perchlorate but does not affect foliar perchlorate content.

Ozone (O) is implicated in the natural source inventory of ClO, a hydrophilic salt that migrates to groundwater and interferes with the uptake of iodide in mammals, including humans. Tropospheric O is elevated in many urban and some rural areas in the United States and globally. We previously showed that controlled O exposure at near-ambient concentrations (up to 114 nL L, 12-h mean) did not increase foliar ClO. Under laboratory conditions, O has been shown to oxidize Cl to ClO. Plant tissues contain Cl and exhibit responses to O invoking redox reactions. As higher levels of O are associated with stratospheric incursion and with developing megacities, we have hypothesized that exposure of vegetation to such elevated O may increase foliar ClO. This would contribute to ClO in environments without obvious point sources. At these high O concentrations (up to 204 nL L, 12-h mean; 320 nL L maximum), we demonstrated an increase in the ClO concentration in surface soil that was linearly related to the O concentration. There was no relationship of foliar ClO with O exposure or dose (stomatal uptake). Accumulation of ClO varied among species at low O, but this was not related to soil surface ClO or to foliar ClO concentrations following exposure to O. These data extend our previous conclusions to the highest levels of plausible O exposure, that tropospheric O contributes to environmental ClO through interaction with the soil but not through increased foliar ClO.

Demonstration of a diel trend in sensitivity of Gossypium to ozone: a step toward relating O3 injury to exposure or flux.

Plant injury by ozone (O3) occurs in three stages, O3 entrance through stomata, overcoming defences, and attack on bioreceptors. Concentration, deposition, and uptake of O3 are accessible by observation and modelling, while injury can be assessed visually or through remote sensing. However, the relationship between O3 metrics and injury is confounded by variation in sensitivity to O3. Sensitivity weighting parameters have previously been assigned to different plant functional types and growth stages, or by differentially weighting O3 concentrations, but diel and seasonal variability have not been addressed. Here a plant sensitivity parameter (S) is introduced, relating injury to O3 dose (uptake) using three independent injury endpoints in the crop species, Pima cotton (Gossypium barbadense). The diel variability of S was determined by assessment at 2h intervals. Pulses of O3 (15 min) were used to assess passive (constitutive) defence mechanisms and dose was used rather than concentration to avoid genetic or environmental effects on stomatal regulation. A clear diel trend in S was apparent, with maximal sensitivity in mid-afternoon, not closely related to gas exchange, whole leaf ascorbate, or total antioxidant capacity. This physiologically based sensitivity parameter provides a novel weighting factor to improve modelled relationships between either flux or exposure to O3, and O3 impacts. This represents a substantial improvement over concentration- or phenology-based weighting factors currently in use. Future research will be required to characterize the variability and metabolic drivers of diel changes in S, and the performance of this parameter in prediction of O3 injury.

Root and shoot gas exchange respond additively to moderate ozone and methyl jasmonate without induction of ethylene: ethylene is induced at higher O3 concentrations.

The available literature is conflicting on the potential protection of plants against ozone (O(3)) injury by exogenous jasmonates, including methyl jasmonate (MeJA). Protective antagonistic interactions of O(3) and MeJA have been observed in some systems and purely additive effects in others. Here it is shown that chronic exposure to low to moderate O(3) concentrations (4-114 ppb; 12 h mean) and to MeJA induced additive reductions in carbon assimilation (A (n)) and root respiration (R (r)), and in calculated whole plant carbon balance. Neither this chronic O(3) regime nor MeJA induced emission of ethylene (ET) from the youngest fully expanded leaves. ET emission was induced by acute 3 h pulse exposure to much higher O(3) concentrations (685 ppb). ET emission was further enhanced in plants treated with MeJA. Responses of growth, allocation, photosynthesis, and respiration to moderate O(3) concentrations and to MeJA appear to be independent and additive, and not associated with emission of ET. These results suggest that responses of Pima cotton to environmentally relevant O(3) are not mediated by signalling pathways associated with ET and MeJA, though these pathways are inducible in this species and exhibit a synergistic O(3)×MeJA interaction at very high O(3) concentrations.

No interaction between methyl jasmonate and ozone in Pima cotton: growth and allocation respond independently to both.

Ozone (O3) is damaging to plants, inducing signalling pathways involving antagonism between jasmonates and ethylene. These pathways mediate O3 responses, particularly to acute exposure, and their manipulation protected several species against acute and chronic O3. We use chronic daily exposure of up to 163 ppb O3, and twice weekly application of up to 320 microg plant(-1) methyl jasmonate (MeJA) to test two hypothesizes: 1) a low rate of MeJA does not affect growth but increases O3 sensitivity; 2) a high rate inhibits growth but reduces O3 sensitivity. Both hypotheses were rejected. Growth declined with increases in both MeJA and O3. MeJA at 40 microg plant(-1) caused no direct effect, and at 160 microg plant(-1) reduced growth similarly at all O3. Neither rate altered O3 sensitivity. These additive responses are not consistent with protection by MeJA in this system. They may reflect inter-specific differences in signalling, since O3 concentrations used here exceeded some reported acute exposures. Alternatively, parallel responses to O3 and MeJA may suggest that O3-induced jasmonates play a developmental role in chronic response but no protective role in the absence of lesions characteristic of acute exposure. MeJA appears useful as a probe of these mechanisms.

O3 impacts on plant development: a meta-analysis of root/shoot allocation and growth.

The mechanism of O3 action on plants remains poorly characterized. Symptoms include visible lesions on the leaf surface, reduced growth and a hypothesized reduction in allocation of carbohydrate to roots. The generality of this latter phenomenon has not been demonstrated. Here, a meta-analysis is performed of all available experimental data, to test the hypotheses that O3 exposure of the shoot inhibits biomass allocation below ground (the root/shoot allometric coefficient, k) and inhibits whole-plant growth rate [relative growth rate (RGR)]. Both k and RGR were significantly reduced by O3 (5.6 and 8.2%, respectively). Variability in k was greater than in RGR, and both exhibited some positive as well as mostly negative responses. The effects on k were distinct from the effects on RGR. In some cases, k was reduced while RGR was unaffected. Slow-growing plants (small RGR) exhibited the largest declines in k. These observations may have mechanistic implications regarding O3 phytotoxicity. There were no effects of type of exposure chamber on sensitivity to O3. The analyses indicate that the O3 inhibition of allocation to roots is real and general, but variable. Further experiments are needed for under-represented plant groups, to characterize exceptions to this generalization and to evaluate O3--environment interactions.