e a psi c u m a n n u u m plants under summer field conditions were grown in 17 cm plastic pots with different substrates: (1) Peat-Polystyrene (1:1) (v/v), (2) Composted Pine Bark, (3) Composted Grape Refuse-Olot s Volcanic Ashes (1:1) (v/v), and (4) Peat-Composted Pine Bark-Perlite (1:1:1) (v/v/v). Transpiration rate and stomatal resistance daily variation depended on irradiance and on substrate water contento Plants with one morning irrigation, 2 l/pot, showed stomatic symptoms of water deficit during the morning, in substrate (3), and in the afternoon in the other media. E.A. \rJ. % vol: (1) 22.7; (2) 21.8; (3) 8.0; and (4) 19.1 • Two more irrigations, 1 l/pot each, overcame restrictions of gaseous exchange in substrate (3), increasing ~anspiration rates and decreasing stomatal resistance substantially. Furthermore in _all substrates this three irrigation treatment extended, at least for one hour in the afternoon, the potential gaseous exchange in relation to irradiance assuming photosynthesis and stomatal resistance dependence of photon fluence rateo