RESPONSES OF 2 HYBRID POPULUS CLONES TO FLOODING, DROUGHT, AND NITROGEN AVAILABILITY .2. GAS-EXCHANGE AND WATER RELATIONS

Repeated progressive drought and flooding stress were imposed on two h ybrid poplar clones (Populus x euramericana 'Eugenei' and P. tristis x P. balsamifera 'Tristis') grown under two levels of nitrogen. Diurnal responses of gas exchange variables were measured periodically. Over a period of 18 days of flooding, plants displayed no reduction in phot osynthesis during the initial days, followed by a midday depression, a nd finally whole-day declines as flooding was prolonged. Supplemental N enabled plants to resist the negative effects of flooding on photosy nthesis and conductance. Under well-watered conditions, additions of N enhanced photosynthesis; when soil water was restricted, however, pho tosynthesis of high-N plants drastically decreased. The experience of one cycle of progressive drought substantially improved the capacity o f plants to maintain a stable rate of photosynthesis during another dr ought cycle, whereas a 10-day period of stress interruption appeared t o increase the physiological vulnerability of high-N plants to another drought cycle. Drought-stressed plants gained full and quick recovery of photosynthesis upon relief from stress. In contrast, flooded plant s did not recover until 9 days after the removal of stress. Both flood ing and two cycles of progressive soil drying of up to 10 days did not invoke midday leaf water deficits. After a period of water stress int erruption followed by another drought cycle, stomata of 'Eugenei' tend ed to lose their sensitivity to declining soil moisture, resulting in moderate leaf water deficits. 'Tristis', on the other hand, appeared b etter able to maintain the adaptation produced by drought, even after stress was interrupted.