Photosynthetic light harvesting was investigated under low-light stress conditions relevant to the problem of interior longevity of potted ornamental plants. Comparisons of leaf pigment levels and chlorophyll fluorescence excitation spectra were made for `Gutbier V-10 Amy' poinsettia (Euphorbia pulcherrima Willd.), which has poor interior longevity, and `Eckespoint Lilo' poinsettia, which has superior interior longevity. The results show that `Eckespoint Lilo' had higher total chlorophyll content per leaf area and lower chlorophyll a: chlorophyll b ratio than `Gutbier V-10 Amy'. In low-light stress, `Eckespoint Lilo' retained its chlorophyll or even accumulated higher levels than in high light, while `Gutbier V-10 Amy' did not exhibit higher chlorophyll retention in low light. Both cultivars acclimatized to low-light stress by decreasing the chlorophyll a: chlorophyll b ratio, and this acclimatization was evident sooner in younger, outer-canopy leaves above the pinch than in older leaves below the pinch. Both cultivars also increased the chlorophyll: carotenoid ratio in low light. These changes in pigment composition, which were essentially structural changes, were reflected in functional changes in light harvesting, as assessed by measurements of chlorophyll fluorescence excitation spectra.
J. Lorene Embry and Eugene A. Nothnagel
Kurt D. Nolte, Eugene A. Nothnagel and Charles W. Coggins Jr.
Studies were conducted to determine whether certain physiological effects of gibberellic acid (GA3) on the peel of citrus fruits may be attributed to GA3 interaction with cellular membranes. Excised mesocarp tissue from pummelo [Citrus maxima (Burm.) Merrill] fruits was analyzed for electrolyte and K+ release over time in varying concentrations of GA3. Electrolyte leakage and K+ efflux was significantly reduced (up to 30%) when tissue was incubated in the presence of GA3. GA3 improved the viability of mechanically isolated protoplasts during 72 hr of storage at 7C, as shown by the use of fluorescein diacetate. These results suggest that some of the GA3-elicited responses in citrus fruits may be membrane related.