ABSTRACT
Water loss is the price plants pay for obtaining CO2 from a dry atmosphere. The exchange rate is poor. To fix one molecule of CO2 the plant will typically tran-spire 200-400 molecules of water, depending on humidity and on stomatal conductance (Raven, 1984). That plants are generally more productive when grown in high humidity indicates that the cost of transpiration outweighs any direct benefits in terms of leaf cooling or nutrient transport (Salisbury and Ross, 1992). To obtain an adequate supply of water, plants invest a considerable portion of their resources in building and maintaining uptake and transport tis-sues that fix practically no CO2. The size of this invest-ment depends on plant and habitat: In cool desert shrubs, root biomass can be as much as three to nine times the shoot biomass (Dobrowolski et al., 1990); on the other extreme, in submerged aquatic plants the root biomass may be as little as 6% of the shoot (Waisel and Agami, 1996). Within each habitat, a suc-cessful competitor will achieve maximum water supply for a given investment.
