Does transpiration limit the growth of vegetation or vice versa?

For hydrological or agronomic purposes, the potential rate of transpiration from vegetation is often calculated as a function of climatological variables, sometimes with the inclusion of a canopy resistance to water vapour diffusion. If, within leaves, the intercellular concentration of CO 2 is conservative, the canopy resistance must depend on the photosynthesis rate implying that potential transpiration depends on potential growth. The relevant form of the Penman-Monteith equation is developed to link water use efficiency with the conversion coefficient for solar radiation. When water is limiting, the maximum rate at which transpiration can occur depends mainly on the rate of extension of the root system and on the water “available” per unit soil volume. It follows that both potential and subpotential rates of transpiration are consequences of the assimilation of carbon by vegetation and its subsequent redistribution to form shoots and roots.