Control of transpiration in three coffee cultivars: the role of hydraulic and crown architecture

Water use and hydraulic architecture were studied in the coffee (Coffea ara bica) cultivars San Ramon, Yellow Caturra and Typica growing in the field u nder similar environmental conditions. The cultivars differed in growth hab it, crown architecture, basal sapwood area and total leaf surface area. Tra nspiration per unit leaf area (E), stomatal conductance (g(s)), crown condu ctance (g(c)), total hydraulic conductance of the soil/leaf pathway (G(t)) and the stomatal decoupling coefficient, omega (Omega) (Jarvis and McNaught on 1986) were assessed over a range of soil moisture and during partial def oliation treatments. The relationship between sap flow and sapwood area was linear and appeared to be similar for the three cultivars. Variation in g( c), E, and G(t) of intact plants and leaf area-specific hydraulic conductiv ity (k(1)) of excised lateral branches was negatively correlated with varia tion in the ratio of leaf area to sapwood area. Transpiration, g(c), and g( s) were positively correlated with G(t). Transpiration and G(t) varied with total leaf area and were greatest at intermediate values (10 m(2)) of leaf area. Omega was greatest in Yellow Caturra, the cultivar with the greatest leaf area and a dense crown, and was smallest in Typica, the cultivar with an open crown. Differences in omega were attributable primarily to differe nces in leaf boundary layer conductance among the cultivars. Plants of each cultivar that were 40% defoliated maintained sap flows comparable to pretr eatment plants, but expected compensatory increases in g(s) were not consis tently observed. Despite their contrasting crown morphologies and hydraulic architecture, the three cultivars shared common relationships between wate r use and hydraulic architectural traits.