POSITIVE CORRELATION BETWEEN LEVELS OF RETAINED ZEAXANTHIN PLUS ANTHERAXANTHIN AND DEGREE OF PHOTOINHIBITION IN SHADE LEAVES OF SCHEFFLERA-ARBORICOLA (HAYATA) MERRILL

Attached intact leaves of Schefflera arboricola grown at three differe nt photon flux densities (PFDs) were subjected to 24-h exposures to a high PFD and subsequent recovery at a low PFD. While sun leaves showed virtually no sustained effects on photosystem II (PSII), shade-grown leaves exhibited pronounced photoinhibition of PSII that required seve ral days at low PFD to recover. Upon transfer to high PFD, levels of n onphotochemical quenching in PSII as well as levels of zeaxanthin were initially low in shade leaves but continued to increase gradually dur ing the 24-h exposure. The xanthophyll cycle pool size rose gradually during and also subsequent to the photoinhibitory treatment in shade l eaves. Upon return to low PFD, a marked and extremely long-lasting ret ention of zeaxanthin and antheraxanthin was observed in shade but not sun leaves. During recovery, changes in the conversion state of the xa nthophyll cycle therefore closely mirrored the slow increases in PSII efficiency. This novel report of a close association between zeaxanthi n retention and lasting PSII depressions in these shade leaves clearly suggests a role for zeaxanthin in photoinhibition of shade leaves. In addition, there was a decrease in beta-carotene levels, some decrease in chlorophyll, but no change in lutein and neoxanthin (all per leaf area) in the shade leaves during and subsequent to the photoinhibitory treatment. These data may be consistent with a degradation of a porti on of core complexes but not of peripheral light-harvesting complexes. A possible conversion of beta-carotene to form additional zeaxanthin is discussed.