Photosynthesis adaptation of the desert-steppe shrub Caragana bungei to larch forest conditions at mountainous slopes in Mongolian Khangai

Leaf parameters, gas exchange parameters, photosynthetic pigments content, mesophyll structure, and sizes of shrubs were studied for Caragana bungei Ledeb. in contrasting habitats of the mountain slope—herb-grass-shrub steppe and larch forest. It was shown that the adaptation of photosynthetic apparatus of C. bungei to forest conditions does not lie in biochemical and primary photosynthetic processes but consists of quantitative changes in the leaf tissue-cellular structure. Steppe and forest individuals differed neither in photosynthetic capacity nor in kinetic parameters of photosynthesis. It is assumed that the development of Caragana’s leaves at high light in the spring larch forest leads to the formation of a strong photosynthetic apparatus as a preadaptation to fluctuating light under the forest canopy. At the same time, forest shrubs had thinner and less dense leaves due to the decreased size and number of mesophyll cells. The decrease in the structural complexity of the mesophyll was compensated for by enhancement of the photosynthetic capacity of cells and chloroplasts. The low rate of real photosynthesis under the forest canopy shadow was observed, but it was compensated by a high relative mass and area of the leaves. As a result there were no differences in shrub sizes between steppe and forest shrubs of Caragana. We concluded that photosynthesis adaptation at the levels of leaf mesophyll and of the ratio of photosynthetic organs allow this species to maintain growth and to gain an advantage in an open larch forest.