INFLUENCE OF NITROGEN SUPPLY AND GROWTH IRRADIANCE ON PHOTOINHIBITIONAND RECOVERY IN HEUCHERA-AMERICANA (SAXIFRAGACEAE)

Prior work demonstrated that Heuchera americana, an evergreen herb inh abiting the deciduous forest understory in the southeastern United Sta tes, has a 3-4-fold greater photosynthetic capacity under the low-temp erature, strong-light, open canopies of winter compared to the high-te mperature, weak-light, closed canopies of summer, Moreover, despite th e reductions in soil nitrogen, the chilling temperatures, and the incr eased quantum flux associated with winter, chronic photoinhibition was not observed in this species at this time of the year. We were intere sted in the photosynthetic acclimation and photoinhibition characteris tics of this species when grown under contrasting light and nitrogen r egimes. Newly expanded shade-acclimated leaves of forest-grown plants exposed to strong light varying in intensity and duration at 25 degree s C showed a reduction in F-v/F-m (the ratio of variable to maximum ro om temperature chlorophyll fluorescence measured after dark adaptation ), which was correlated with a decline in phi(a) (the intrinsic quantu m yield of CO2-saturated O-2 evolution on an absorbed light basis). Pl ants grown in the glasshouse under contrasting light (high and low lig ht; HL and LL, respectively) and nitrogen supply (high and low nitroge n; HN and LN, respectively) regimes showed that photosynthetic acclima tion to HL was impaired in LN regimes. The HL-LN plants also had the l owest values of F-v/F-m and of phi on both incident and absorbed light bases and had 50% less chlorophyll (per unit area) compared to plants from other growth regimes. Controlled exposure to bright light at low temperatures (2-3 degrees C) for 3 h resulted in a sharp decrease in F-v/F-m (and rise in F-o, the minimum fluorescence yield) in all plant s. Shade-grown plants from both N regimes were highly susceptible to c hronic photoinhibition, as indicated by a greater reduction in F-v/F-m and incomplete recovery after 18 h in weak light at 25 degrees C. The HL-HN plants were the least susceptible to chronic photoinhibition, h aving the smallest decrease in F-v/F-m with near full recovery within 6 h. The decline in F-v/F-m in HL-LN plants was comparable to that of shade-acclimated plants, but recovered fully within 6 h. Low-N plants from both light regimes displayed greater increases in F-o which did n ot return to pretreatment levels after IS h of recovery. These studies indicate that HL-LN plants were sensitive to chronic photoinhibition and. at the same time, had a high capacity for dynamic photoinhibition . Experimental garden studies showed that H. americana grown in an ope n field in summer were photoinhibited and did not fully recover overni ght or during extended periods of weak light. These results are discus sed in relation to the photosynthetic acclimation of H. americana unde r natural conditions.