Cost-benefit analysis of mesophyll conductance: diversities of anatomical, biochemical and environmental determinants

Background Plants invest photosynthates in construction and maintenance of their structures and functions. Such investments are considered costs. These costs are recovered by the CO2 assimilation rate (A) in the leaves, and thus A is regarded as the immediate, short-term benefit. In photosynthesizing leaves, CO2 diffusion from the air to the carboxylation site is hindered by several structural and biochemical barriers. CO2 diffusion from the intercellular air space to the chloroplast stroma is obstructed by the mesophyll resistance. The inverses is the mesophyll conductance (g(m)). Whether various plants realize an optimal g(m), and how much investment is needed for a relevant g(m), remain unsolved. Scope This review examines relationships among leaf construction costs (CC), leaf maintenance costs (MC) and g(m) in various plants under diverse growth conditions. Through a literature survey, we demonstrate a strong linear relationship between leaf mass per area (LMA) and leaf CC. The overall correlation of CC vs. g(m) across plant phylogenetic groups is weak, but significant trends are evident within specific groups and/or environments. Investment in CC is necessary for an increase in LMA and mesophyll cell surface area (S-mes). This allows the leaf to accommodate more chloroplasts, thus increasing A. However, increases in LMA and/or S-mes often accompany other changes, such as cell wall thickening, which diminishes g(m). Such factors that make the correlations of CC and g(m) elusive are identified. Conclusions For evaluation of the contribution of g(m) to recover CC, leaf life span is the key factor. The estimation of MC in relation to g(m), especially in terms of costs required to regulate aquaporins, could be essential for efficient control of g(m) over the short term. Over the long term, costs are mainly reflected in CC, while benefits also include ultimate fitness attributes in terms of integrated carbon gain over the life of a leaf, plant survival and reproductive output.