Increasing the size of mesophyll cells as a mechanism for adaptation to climate aridity of two steppe plant species in Kazakhstan

We studied the leaf traits of steppe species - Artemisia austriaca Jacq. (Asteraceae) and Stipa lessingiana Trin. et Rupr. (Poaceae) in two communities of true steppe and desert steppe in Kazakhstan. The whole-leaf traits changed differently within species, mesophyll traits had similar changes with aridity. The leaf thickness and the leaf mass area increased in desert steppe in Artemisia, while in grass species these leaf traits did not change. However, mesophyll cell sizes increased in desert steppe in both species. An increase in cell sizes, as well as in the chloroplast number per cell in A. austriaca and in the cell number per leaf area in S. lessingiana, led in both species to a rise of the total chloroplasts’ surface per leaf area unit being important to carbon dioxide diffusion. We concluded that despite the generally accepted stereotype about «small-cell mesophyll» in steppe xerophytes, namely in some species an increase in cell size, aimed at increasing the internal assimilation surface of the leaf, can be considered as a mechanism for adapting the photosynthetic apparatus of steppe plants to climate aridity.