Seasonal structural and functional changes in the photosynthetic apparatus of evergreen conifers

We conducted a detail study of the photosynthetic apparatus in assimilating organs of three introduced evergreen conifer species: Taxus cuspidate S. et Z. ex E. (Far-Eastern yew), Thuja occidentalis L. (arbovitae “green”), and Th. occidentalis f. “Reingold” (arbovitae “yellow”) at various times in their life cycle. We studied the potential photosynthesis rate; composition and ratios of pigments, including primary carotenoids; the violaxanthin cycle (VC) activity, the synthesis of a secondary carotenoid, rhodoxanthin; and chloroplast ultrastructure. In winter and spring, β-carotene and lutein (primary carotenoids) contents were relatively constant in yew and arbovitae “yellow”. In December, the VC in yew was balanced and in arbovitae “yellow” unbalanced. In arbovitae “yellow”, the zeaxanthin pool was heterogeneous, and only part of it took part in the VC. It can be assumed that the other part of the pool can be oxidized to form a secondary carotenoid, rhodoxanthin. This secondary carotenoid was also accumulated in arbovitae “green”; its synthesis took place during the season, when the photosynthesis rate of plants was the lowest, and a significant chloroplast reorganization occurred (the number of thylakoids in grana decreased and plastoglobules appeared). We suppose that rhodoxanthin forms a filter for the light under the conditions of high insolation in winter. Thus, the evergreen conifer plants studied, which are adapted to growing at high latitudes where temperature is low and insolation is high in winter and spring, have a system for protecting the photosynthetic apparatus against photodestruction. In the basis of this system, the primary and secondary carotenoids lie, whose content changes during the year.