Coordinated Activity of Photosystem I and Photosystem II in Selaginella martensii (Lycopodiophyta) Across Light Gradients

Selaginella martensii is a living member of lycophytes, an early divergent group of vascular plants. This shade-adapted species can acclimate to various light regimes, from deep shade to high light, with a gain in carboxylation capacity accompanied by modulations in thylakoid composition, including increased relative amount of photosystem I (PSI) compared to photosystem II (PSII). It was hypothesized that, as in angiosperms, the photosynthetic acclimation of S. martensii to high light may lead to increased cyclic electron flow around PSI (CEF), with changes in excitation distribution between PSI and PSII and with modulations of electron carrier pools. After long-term acclimation to deep shade (LL), intermediate shade (ML) and high light (HL) regimes, plants were compared using chlorophyll a fluorescence, P700 oxidation, electrochromic bandshift, and transmission electron microscopy of the thylakoid system. As expected, a 75% increase in CEF occurred from LL to HL plants, facilitated by a preferential energy distribution to PSI. However, some unique characteristics distinguish S. martensii from common models of long-term thylakoid photoregulation. The pools of electron transporters did not increase linearly with the electron fluxes from LL to HL plants; rather, the enhancement of electron transport depended on unique combinations of capacity and mobility of the electron carriers, the latter determined by the extent of thylakoid shrinkage. Moreover, especially in shade-acclimated plants, the fraction of oxidized PSI exceeded that of reduced QA, therefore keeping PSI under a persistent shortage of electrons already at low light intensities.