|Sadok, Walid -|
Submitted to: Journal of Experimental Botany
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: December 7, 2011
Publication Date: May 1, 2012
Citation: Fiscus, E.L., Booker, F.L., Sadok, W., Burkey, K.O. 2012. Influence of atmospheric vapour pressure deficit on ozone responses of snap bean (Phaseolus vulgaris L.) genotypes. Journal of Experimental Botany. 63:2557-2564. Interpretive Summary: In an attempt to understand the relationships between the effects of pollutant ozone and other current and expected future environmental variables, an ozone bioindicator pair of genotypes were exposed to combinations of pollutant ozone (0 and 60 ppb daily mean) and two levels of atmospheric vapor pressure deficits (1.26 and 1.96 kPa) representing relative humidities of about 60% and 30%, respectively at a constant daytime temperature of 26C. The high VPD (low humidity) in clean air resulted in significant losses of seed yield in both genotypes. In polluted air high VPD increased seed yield in both genotypes. Additionally, ozone exposure significantly lowered seed yield at low VPD and at high VPD had no effect on seed yield. We speculate that the high VPD may have caused unsustainable rates of water loss from the leaves, resulting in partial stomatal closure with a resulting decrease in ozone entry into the leaves, thus preventing much of the damage the exposure might have caused.
Technical Abstract: Two genotypes of snapbean (Phaseolus vulgaris L.), one known to be sensitive to ozone and the other resistant, were examined to determine their response to atmospheric vapor pressure deficit (VPD) in the presence and absence of ozone. Plants were grown in Outdoor Plant Environment Chambers in combinations of high and low VPD (1.96 kPa and 1.26 kPa) and high and low levels of ozone (12 h mean ozone concentrations of 60 nmol mol-1 and <10 nmol mol-1). Plants were sampled at five weekly intervals during the season for determination of stem, root and leaf dry mass and leaf area. The remaining plants were grown to maturity and sampled for dry bean yield. Water use of the plants sampled during the season was measured for several days before each sampling so that water use per unit leaf area could be calculated after each destructive sampling. Yield was significantly higher in R123 across all treatments. In clean air seed production was significantly reduced by similar fractions in both genotypes by high VPD. At low VPD the ozone treatments reduced seed yield by ~55% in R123 and ~72% in S156 while at high VPD there was no significant ozone effect on seed production. Within the ozone treatments, high VPD increased yields in both genotypes compared with yields at low VPD, but a differential response between the two genotypes remained evident. In R123 root dry mass increased in response to high VPD in both clean and polluted air while the root dry mass was decreased by ozone in both VPDs. In S156 there was no effect of VPD in either clean or polluted air but ozone reduced root dry mass in both VPDs. Ancillary experiments suggest that a stomatal response to VPD per se under well watered conditions may be lacking in both genotypes and we hypothesize that the high VPD results in unsustainable transpiration rates which may induce mild water deficits resulting in partial stomatal closure and yield reductions. In the high ozone treatments the high VPD and water stress-induced stomatal closure may have reduced the ozone flux into the leaves and resulted in a yield increase compared to the low VPD treatment in both genotypes. In R123 the water deficits may also have stimulated root production in both clean and polluted air whereas S156 did not exhibit this response. High VPD did have significant effects on seed production in both genotypes in both clean and polluted air but the ozone-related yield reductions were proportionally higher in S156 thus preserving the usefulness of these genotypes as an ozone bioindicator pair. We suggest that more detailed work is necessary to better understand the interactions reported here as well as the influence of vpd on the growth and productivity of this crop.