Water Supply, Evaporation, and Vapour Diffusion in Leaves

On the basis of experimental results published during the last25 years, but more particularly during the last 5 years andincluding some results presented here, the hypothesis is proposedthat an important portion of the water supply from major veinsin leaves travels within the epidermal tissue to sites of evaporationclose to the stomatal pores. These evaporation sites are innerepidermal walls especially subsidiary and guard cell walls becausethese are closest to air spaces with the highest water vapourdeficits. Less water than is traditionally supposed evaporatesfrom mesophyll cell walls. Low osmotic potentials of guard cells(large negative) are not required in building up high turgorpressures. However, they are required in competing for wateragainst the process of evaporation which causes low matric potentialsto develop in subsidiary and guard cell walls so that guardcolls can maintain the comparatively low turgor pressures whichhave been shown to operate the stomatal apparatus. Traditionalviews about leaf water relations and methods of estimating mesophyllresistances for carbon dioxide diffusion into leaves must bemodified.