In winter, low temperature leads to the situation where the absorption of light energy occasionally exceeds the energy consumption of leaf photosynthesis. This imbalance often causes photoinhibition of photosystem I in cold-sensitive species. In evergreen needles in winter, photoprotection mechanisms of photosystem II are induced and excess light energy can be safely dissipated.

Evergreen herbaceous species in the deciduous forest understory maintain their photosystems in long-lived leaves under dynamic seasonal changes in light and temperature. However, in evergreen understory herbs, it is unknown how photosynthetic electron transport acclimates to seasonal changes in forest understory environments, and what photoprotection systems function in excess energy dissipation under high-light and low-temperature environments in winter. We used Asarum tamaense, an evergreen herbaceous species in the deciduous forest understory with a single-flush and long-lived leaves, and investigated seasonal dynamics of leaf photosynthesis in this species.

Both the rates of CO₂ assimilation and electron transport under saturated light were kept low in summer, but increased in autumn and winter in A. tamaense leaves. Although the contents of photosynthetic proteins including Rubisco did not increase in autumn and winter, the proton motive force and ΔpH across the thylakoid membrane were high in summer and decreased from summer to winter to a great extent. These decreases alleviated the suppression by lumen acidification and increased the electron transport rate in winter. The content and composition of carotenoids changed seasonally, which may affect changes in non-photochemical quenching from summer to winter.

In A. tamaense leaves, the increase in photosynthetic electron transport rates in winter was mainly due to the activation of photosynthetic enzymes and/or release of limitation of photosynthetic electron flow. These seasonal changes in the regulation of electron transport and the changes in several photoprotection systems should support the acclimation of photosynthetic C gain under dynamic environmental changes throughout the year.

In this talk, we also show recent studies on comparison of photosynthetic regulations between understory species with different phenological strategies.