Photoinhibition of photosynthesis in intact kiwifruit (Actinidia deliciosa) leaves: Changes in susceptibility to photoinhibition and recovery during the growth season.

Kiwifruit (Actinidia deliciosa (A. Chev.) C.F. Liang et A.R. Ferguson) plants grown in an outdoor enclosure were exposed to the natural conditions of temperature and photon flux density (PFD) over the growing season (October to May). Temperatures ranged from 14 to 21° C while the mean monthly maximum PFD varied from 1000 to 1700 μmol · m(-2) · s(-1), although the peak PFDs exceeded 2100 μmol · m(-2) · s(-1). At intervals, the daily variation in chlorophyll fluorescence at 692 nm and 77K and the photon yield of O2 evolution in attached leaves was monitored. Similarly, the susceptibility of intact leaves to a standard photoinhibitory treatment of 20° C and a PFD of 2000 μmol · m(-2) · s(-1) and the ability to recover at 25° C and 20 μmol · m(-2) · s(-2) was followed through the season. On a few occasions, plants were transferred either to or from a shade enclosure to assess the suceptibility to natural photoinhibition and the capacity for recovery. There were minor though significant changes in early-morning fluorescence emission and photon yield throughout the growing season. The initial fluorescence, Fo, and the maximum fluorescence, Fm, were, however, significantly and persistently different from that in shade-grown kiwifruit leaves, indicative of chronic photoinhibition occurring in the sun leaves. In spring and autumn, kiwifruit leaves were photoinhibited through the day whereas in summer, when the PFDs were highest, no photoinhibition occurred. However, there was apparently no non-radiative energy dissipation occurring then also, indicating that the kiwifruit leaves appeared to fully utilize the available excitation energy. Nevertheless, the propensity for kiwifruit leaves to be susceptible to photoinhibition remained high throughout the season. The cause of a discrepancy between the severe photoinhibition under controlled conditions and the lack of photoinhibition under comparable, natural conditions remains uncertain. Recovery from photoinhibition, by contrast, varied over the season and was maximal in summer and declined markedly in autumn. Transfer of shade-grown plants to full sun had a catastrophic effect on the fluorescence characteristics of the leaf and photon yield. Within 3 d the variable fluorescence, Fv, and the photon yield were reduced by 80 and 40%, respectively, and this effect persisted for at least 20 d. The restoration of fluorescence characteristics on transfer of sun leaves to shade, however, was very slow and not complete within 15 d.