Analysis of the causes of variability of the dry leaf mass-per-area ratio

The lamina dry mass-area ratio (LMA: Leaf Mass per Area) is a very variable characteristic. Leaf dry mass consists of symplast mass (the set of all leaf protoplasts) and apoplast mass (the set of all cell walls in a leaf). The ratio between symplast and apoplast masses is positively related to any functional leaf trait calculated per unit of dry mass. The value of this ratio is defined by cell size and the number of cells per unit of leaf area, by the number of mesophyll cell layers and their differentiation into palisade and spongy ones, and also by cell packing density. The LMA value is determined by leaf thickness and density. The extent and direction of variability in both leaf traits determine the extent and direction of variability in LMA. A negative correlation between leaf thickness and density reduces the variability of LMA. As a consequence of this correlation, the following pattern emerges: the thinner a leaf, the denser it is. Changes in the traits that determine the LMA value take place both within a species under the influence of environmental factors and between species that differ in leaf structure and functions. Light is the most powerful environmental factor that influences LMA, with an increase in illumination leading to an increase in LMA. This effect occurs during leaf growth at the expense of structural changes associated with the reduction of the symplast-apoplast mass ratio. Under intense illumination, LMA may increase due to accumulation of starch. With regard to the majority of leaf functions, the mass of the starch may be ascribed to apoplast. Starch accumulation in leaves is also observed under conditions of elevated CO2 concentration in the air. Under high illumination, however, LMA increases also due to increased apoplast contribution to leaf dry mass. A low level of mineral nutrition leads to LMA increase due to a lowering of growthzone demands for phothosynthates and, therefore, to an increase in starch content in leaves. A high level of mineral nutrition during leaf growth leads to LMA increase at the expense of mesophyll thickening where components of the photosynthesis system are located. When additional environmental factors are involved, starch accumulation may be partly responsible for the increase in LMA. The LMA increase at the expense of starch accumulation, unlike that which occurs at the expense of mesophyll thickening, is accompanied by an increase in leaf density. LMA increases under conditions of water deficiency, which in mature leaves may be caused by starch accumulation. LMA increase during the leaf growth period under conditions of water deficiency is associated with a decrease in the symplast-apoplast mass ratio.