Stomatal regulation and xylem cavitation in Clementine (Citrus clementina Hort) under drought conditions

In many trees species, a strong relationship exists between stomatal closure and the onset of cavitation in xylem vessels. This relationship was investigated in a Citrus species. Young potted 'Clementine' (Citrus clementina Hort 'Clementine') grafted on 'Carrizo Citrange' (Citrus sinensis X Poncirus trifoliata) or 'Trifoliate Orange' (Poncirus trifoliata Raf), and grown under greenhouse conditions, were submitted to periods of drought by withholding irrigation. Pre-dawn water potential, leaf stomatal conductance, plant transpiration, the degree of xylem embolism, and xylem vulnerability curves were measured. Transpiration and stomatal conductance were reduced to a minimum value when the pre-dawn water potential reached -1 MPa.This value corresponded to the threshold pressure below which cavitation was induced. Higher intensities of water stress provoked more cavitation, but the degree of xylem embolism, as measured by percentage loss of conductivity, always remained below 50%.'Clementine' is therefore vulnerable to embolism, but early stomatal regulation prevents the occurrence of embolism. Under severe water stress, 'Clementine' is able to maintain functional xylem vessels, which probably enhances its survival during periods of intense drought, and favours its recovery. This suggests that xylem cavitation is a key process in understanding the response of Citrus to drought and, hence, is a promising criterion by which to screen for more drought-tolerant genotypes