A chamber fumigation study to evaluate the performance of the snap bean biomonitoring system under simulated Mediterranean climatic conditions.

The O3 biomonitoring system based on sensitive (S156) and resistant (R123) genotypes of snap bean (Phaseolus vulgaris L.) (Burkey et al. 2005), is being field-tested under the ICP Vegetation since 2008. Although OTCs studies have found clear relationships between O3 exposure, physiological parameters, leaf injury, and yield of the S156/R123 system (Flowers et al., 2007), some biased results have emerged during field campaigns carried out in southern Europe. In a chamber fumigation experiment, we have tested the performance of the snap bean system under simulated Mediterranean microclimatic conditions. Sixteen seeds per genotype were grown in 12 l pots inside a walk-in chamber (mean day/night T: 28.2/24.0 °C; RH: 60%; leaf-level PAR: 350 E m-2s-1; Photoperiod: 8:00-19:00 h; daily irrigation at field capacity); after the development of the 2nd trifoliate leaf, eight plants per genotype were transferred in a twin chamber under the same conditions, and fumigated 7 hours per day with 80 ppb of O3 for 20 days, for a POD6 of 4.7 mmol m-2 and 3.0 mmol m-2 for S and R, respectively. In the fumigated chamber, both genotypes showed visual O3 symptoms (ICP Vegetation Experimental Protocol 2011). However, symptoms appeared earlier on the S plants, and were more severe at the end of the experiment than in the R plants (Fig. 1). Net photosynthesis (Pn) and stomatal conductance (gs) were significantly reduced in both cultivars (Pn: -43.4% and - 54.1%; gs: -42.8% and -31.6% than the control for S and R, respectively), with no difference between S and R at the end of the fumigation. Interestingly, despite the Pn reduction, the total photosynthetic Performance Index (PItot, Strasser et al., 2010) increased significantly in the fumigated S plants since the onset of flowering, suggesting the triggering of detoxification processes (Bussotti et al., 2011; Mereu et al., 2011). The final pod harvest showed that, although a significant O3-induced reduction of pod yield was evident in both genotypes, the S/R total pod weight ratio was lower than 1 both in the control and in the fumigated sets (0.63 and 0.57, respectively). In conclusion, our data suggests that, although prudence is needed in generalizing the results obtained in closed chambers, further studies are required to clarify the O3 response of the S156/R123 system before its application in the field, particularly under Mediterranean climatic conditions