Leaf structure and specific leaf mass: the alpine desert plants of the Eastern Pamirs, Tadjikistan

This study examines interrelationships between eight leaf attributes (speci fic leaf mass, area, dry mass, lamina thickness, mesophyll cell number per cm(2), mesophyll cell volume, chloroplast volume, and number of chloroplast s per mesophyll cell) in field-grown plants of 94 species from the Eastern Pamir Mountains, at elevations between 3800 and 4750 m. Unlike most other m ountain areas, the Eastern Pamirs, Karakorum system, Tadjikistan provide lo calities where low temperatures and radiation combine with moisture stress at high altitudes. For all the attributes measured, significant differences were found between plants with different mesophyll types. Leaves with dors iventral palisade structure (dorsal palisade, ventral spongy mesophyll cell s) had thicker leaves with larger but fewer mesophyll cells, containing mor e and larger chloroplasts. These differences in mesophyll type are reflecte d in differences in the total surface of mesophyll cells per unit leaf area (A(mes)/A) or volume (A(mes)/V). Plants with isopalisade leaf structure (p alisade cells under both dorsal and ventral surfaces) are more commonly xer ophytes and their increased values of A(mes)/A and A(mes)/V decrease CO2 me sophyll resistance, which is an important adaptation to drought. Path analy sis shows the critical importance of mesophyll cell volume in leading to th e covariance between the different leaf attributes and hence to specific le af mass (SLM), even though mesophyll cell volume is not itself strongly cor related with SLM. This is because mesophyll cell volume increases SLM throu gh its effects on leaf thickness and chloroplast number per cell, but decre ases SLM through its negative effect on mesophyll cell density.