Abstract
The effects of low soil water potential (ψs= -1.7--1.6 MPa) on photosynthesis of coffee leaves grown under two light levels (shaded and unshaded) were examined. Water stress reduced the saturated photosynthetic rate by about 60 and 75% in unshaded and shaded leaves respectively. In both shaded and unshaded leaves, saturating irradiance and initial slope values of photosynthesis decreased by water stress. Stomatal resistance (Rs) and mesophyll resistance (Rm) increased about 2.7 and 2-fold in unshaded leaves and both resistances increased about 5-fold in shaded leaves. Rs/ (Rs+Rm) was 0.66 in unshaded and 0.40 in shaded leaves developed under stressed conditions. Photosynthetic O2 evolution rates and RuBPCase contents showed more pronounced reduction by water stress in shaded leaves. For these reasons, the decline in photosynthesis due to water stress can be mainly attributed to an increase in Rs for unshaded leaves and in Rm for shaded leaves.
