The alternation between PSII and PSI in ivy (Hedera nepalensis) demonstrated by in vivo chlorophyll a fluorescence and modulated 820 nm reflection.

To examine the coordination between photosystem II (PSII) and photosystem I (PSI) in response to varying environmental conditions, both diurnal fluctuations and seasonal variability of photosynthetic electron transport activity in ivy ( Hedera nepalensis , Araliaceae) were investigated: by measuring prompt fluorescence, delayed fluorescence (DF) and modulated reflection of 820 nm light (MR). During diurnal fluctuations, the PSII electron donor side was damaged, as evidenced by decreases of the fast amplitude of DF decay kinetics at I 1 , although there was no significant change in relative variable fluorescence at K-step to amplitude of F J − F o . Decreases in the maximum photochemical efficiency (i.e., PSII photoinactivation) were accompanied by an increased maximum decrease in the slope of MR/MR o (i.e., PSI photoactivation). Subsequently, PSII recovery and PSI relaxation occurred in the afternoon. Throughout the season, alternations between PSII and PSI were also suggested by the down-regulation of PSII and the up-regulation of PSI from summer to winter. Significant negative linear correlations between the activity of PSII and PSI across both diurnal fluctuations and seasonal variability were verified by correlation analyses. In addition, PSI was active throughout the year, suggesting PSI is independent from high temperatures. High PSI activity may maintain the functional integrity of the photosynthetic apparatus in overwintering ivy. The alternation between PSII and PSI activity may regulate the distribution of excitation energy between the two photosystems and balance the redox state of the electron transport change, thereby enabling ivy to respond to varying environmental conditions. [ABSTRACT FROM AUTHOR]