Leaf injury characteristics of grassland species exposed to ozone in relation to soil moisture condition and vapour pressure deficit

Abstract

A range of plant species typical of semi-natural grasslands were tested for their sensitivity to short-term ozone injury under normal and reduced irrigation, and in relationship to air vapour pressure deficit. Potted specimens of 24 herbs, legumes and grasses were exposed during two seasons to four O$ treatments in open-top chambers. The ozone treatments were : (a) charcoal-filtered air ; (b) charcoal-filtered air plus ozone to match ambient levels ; (c) charcoal-filtered air plus O $ to ambient levels 1n5 and (d ) charcoal-filtered air with ozone added to twice ambient levels during selected episodes of 7-13 d. During these ozone episodes, half of the plants in each ozone treatment received reduced irrigation (dry treatment) while the rest was kept under full irrigation (wet treatment). Type and date of first occurrence of leaf injury were noted during individual growth periods. Plants were harvested three times per year, and the percentage of injured leaves was recorded. Depending on species, injury symptoms were expressed as flecking (O $ -specific injury), leaf yellowing or anthocyanin formation. Carum carvi and most species of the Fabaceae family (Onobrychis sativa, Trifolium repens, Trifolium pratense) were found to be most responsive to O $ , injury occurring after only a few days of exposure in treatment (b). An episodic reduction in irrigation tended to reduce the expression of O $ -specific symptoms, but only in species for which a reduction in soil moisture potential and an associated reduction in stomatal conductance during the dry episodes were observed. In other species, the protection from O $ injury seemed to be of little importance. Using artificial neural networks the injury response of nine species was analysed in relation to Species, stomatal conductance, ozone as AOT40 (accumulated exposure above a threshold of 0n04 ppm for periods with global radiation 50 W m −# (Fuhrer et al., 1997)), mean relative growth rate, air vapour pressure deficit and global radiation. In the model with all factors, Species was most important, and when Species was omitted, stomatal conductance was the most important determinant for leaf injury to occur, whereas mean relative growth rate was less important. With no plant-related factors included, air vapour pressure deficit and AOT40 were of highest importance. Only in eight species was a positive relationship found between these two factors during the five days before the onset of injury, indicating increasing protection from ozone with increasing air vapour pressure deficit in some but not all species. These data show that across a range of grassland species, leaf injury caused by elevated levels of ozone is most likely to occur in species with high stomatal conductance and that protection from ozone during dry periods is species-specific and depends on a reduction in stomatal conductance due to a decrease in soil moisture potential. Protection under increased vapour pressure deficit can occur in some but not all species, depending on the relationship between stom...

Plant–plant interactions and soil moisture might be important in determining ozone impacts on grasslands

“…Notably, the highly significant ozone effects in the wet treatment were strongly reduced in the dry treatment, probably due to a reduction in ozone uptake by stomatal closure caused by the low SWC (Table 2). Fabaceae species have been shown repeatedly to be relatively sensitive to ozone (Bennett & Runeckles, 1977 ;Blum et al, 1983 ;Fuhrer et al, 1994 ;Nebel & Fuhrer, 1994), and significant interactions of the ozone effect with reduced SWC were reported for individuals (Bungener et al, 1999a) and in mixtures (Heagle et al, 1989). In addition, we found the ozone effect in Experiment B to be dependent on the mixing ratio.…”

“…Ozone concentrations were measured in ppbv (nl l −" ) and ozone exposure calculated as AOT40 (sum of positive residuals of all hourly mean concentrations minus 40 ppb when global radiation exceeded 50 W m −# ). Watering occurred every night by spraying tap water via a diffuser (Cetonor, Ceton, France) placed 1.2 m above the plants (Bungener et al, 1999a). Except during episodes of reduced water supply, soil water content reached field capacity every night in all chambers.…”

“…The frustums of the OTCs were covered with a highly transparent polytetrafluoroethylene (PTFE) film, thickness 0.05 mm (Angst j Pfister, Switzerland) to exclude natural rainfall. Each OTC was mounted on four bricks and fixed at 10 cm above the ground, and the frustum was lifted leaving a 5 cm gap above the upper rim of the OTC to improve ventilation and to minimize chamber effects (Bungener et al, 1999a). The chamber floor was covered with pebbles.…”

Growth response of grassland species to ozone in relation to soil moisture condition and plant strategy

“…Soil moisture deficit (SMD) can reduce the impact of O $ on plants by decreasing stomatal conductance via stomatal closure (Tingey & Hogsett, 1985), and the combined effect of O $ stress and drought on growth could differ from effects caused by O $ stress alone. As shown on the basis of visible O $ injury, the interaction between O $ and SMD is not straightforward and depends on the species (Bungener et al, 1999). For crops, vegetables, and trees, it has been documented that SMD might reduce O $ effects on growth in some cases (Amundson et al, 1986 ;Heagle et al, 1988 ;Pearson & Mansfield, 1994 ;Reiner et al, 1996 ;Soja et al, 1996) but not in others Miller et al, 1989 ;Fangmeier et al, 1994).…”

Physiological disturbances promoted by ozone in five cultivars of Phaseolus vulgaris L.

“…Among the few articles that concern sensitivity to O 3 for native and naturalized species, most are based, for the short term, on observing the development of visible symptoms, and for the long term, on the effects on growth and reproductive effort (Bungener et al, 1999). However, these parameters are not always easy to interpret, since they are influenced by several other environmental factors or take a long time to evaluate.…”