Assessment of combined effects of mild water stress, elevated tropospheric O_3, and CO_2 on crops using fluorescence imaging

摘要

As a part of the environmental air quality assessment studies, we have conducted spectral characterizations of steady state fluorescence and canopy level reflectance measurements in responses to the air quality treatments. These studies have been carried out under well watered and restricted soil moisture regimes. Our primary objective was to evaluate fluorescence technique for the nondestructive assessment of plants grown in elevated CO_2, elevated O_3, and both elevated CO_2 and O_3 environmentsunder normal and restricted soil moisture conditions. The experimental field site is located at the South Farm, USDA/BARC, Beltsville, Maryland. We used field open-top chamber systems to simulate four altered combinations of tropospheric CO_2 and O_3 environments under well watered and restricted soil moisture conditions. The simulated gaseous environments included: 1) charcoal-filtered (CF) ambient air as the control chamber; 2) elevated CO_2 consisted of CF ambient air plus 150 +- 10 mu l/l CO_2; 3) elevated O_3 that consisted of Non-Filtered (NF) ambient air and addition of 35 +- 5 nl/l O_3; and 4) high CO_2 and O_3 chamber which consisted of NF ambient and additions of 150 +- 10 mu l/l CO_2 and 35 +- 5 nl/l O_3. The CO_2 was supplied for 18 hours (3:00 to 21:00 h EST) per day from a CO_2 bulk tank. In the elevated O_3 treatment chambers, 35 +- 5 nl/l O_3 was supplied during seven hour period from 9:00 to 16:00 h EST per day. Both gases were supplied for the entire growing season from early July tolate-October. Two soybean cultivars 'Essex' and 'Forrest' were planted in the OTC. Fluorescence images were acquired at F450, F550, F680, and F740. The results showed that fluorescence responses of 'Essex' and 'Forrest' were significantly different in all four bands. Restricted soil moisture treatment had significant effects in that means for F450 and F550 bands significantly increased in both cultivars. The increases in fluorescence intensities at F450 and F550 for 'Essex' due to restricted soil moisture were much greater than those of 'Forrest'. At F680 and F740 bands, only 'Essex' was significantly affected by soil moisture treatment. The most significant effects of elevated CO_2 and O_3 were observed at F450 and F550 bands. Means for elevated O_3in ambient air in both cultivars were significantly higher in both soil moisture regimes. However, no significant differences between CF and NF+CO_2+O_3 within a soil moisture treatment were observed. Means for CF+CO_2 was significantly lower for both cultivars under well watered soil condition at F450. In general, none of the treatment means were significantly affected by elevated O_3 treatments at F680 and F740. Although visible stress symptoms such as chlorosis, discoloration, or necrosis were not evident in the leaves used in this study, F450 and F550 fluorescence images depicted the effects of elevated O_3, partial compensation of elevated O_3 effects by the elevated CO_2, and positive physiological effects of elevated CO_2. Plant stresses that cause localized damage may not be easily detected with small field of view point measurements. Imaging a whole leaf may provide better means to study the effects of non-systemic stresses. We are also currently investigating reflectance techniques for a possible complementary use with fluorescence measurements in the remote assessment of above treatment effects.