Effects of long-term soil drought on photosynthesis and carbohydrate metabolism in mature robusta coffee (Coffea canephora Pierre var. kouillou) leaves

Four clones of robusta coffee representing drought-tolerant (14 and 120) and drought-sensitive (46 and 109A) genotypes were submitted to a slowly imposed water deficit. Sampling and measurements were performed when predawn leaf water potential (Ψpd) approximately −2.0 and −3.0 MPa was reached. Regardless of the clone evaluated, drought led to sharper decreases in stomatal conductance than in photosynthesis, which was accompanied by significant declines in internal to ambient CO2 concentration ratio. Little or no effect of drought on chlorophyll a fluorescence parameters was observed. Regardless of the stress intensity, starch decreased remarkably. This was not accompanied by significant changes in concentration of soluble sugars, with the exception of clone 120 in which a rise in sucrose and hexose concentrations was found when Ψpd reached −3.0 MPa. At Ψpd = −2.0 MPa, activity of acid invertase increased only in clone 120; at Ψpd = −3.0 MPa, it increased in clones 14, 46 and 120, while activity of sucrose synthase declined, but only in clone 109A. Drought-induced decrease in ADP-glucose pyrophosphorylase activity was found only in clones 14 and 46, irrespective of stress intensity. At Ψpd = −3.0 MPa, maximal extractable and activation state of sucrose-phosphate synthase (SPS) decreased in all clones with the exception of clone 120, in which SPS activity was maintained in parallel to a rising activity of fructose-1,6-bisphosphatase. Changes in SPS activity could neither be explained by the CO2 decrease linked to stomatal closure nor by differences in leaf water status.