Interstock of ‘Valencia’ Orange Affects the Flooding Tolerance in ‘Verna’ Lemon Trees

Abstract

Previous work on citrus trees has shown that an interstock, grafted between the rootstock and scion combination, not only can improve tree growth, longevity, fruit production, and quality, but also can increase salinity tolerance. This research was designed to evaluate flooding responses of 2-year-old ‘Verna’ lemon trees [Citrus limon (L.) Burm.; VL] either grafted on ‘Sour’ orange (C. aurantium L.; SO) rootstock without an interstock (VL/SO) or interstocked with ‘Valencia’ orange (C. sinensis Osbeck;VL/V/SO) or with ‘Castellano’ orange (C. sinensis Osbeck; VL/C/SO). Well-watered and fertilized trees were grown under greenhouse conditions and half were flooded for 9 days. At the end of the flooded period, leaf water relations, leaf gas exchange, chlorophyll fluorescence parameters, mineral nutrition, organic solutes, and carbohydrate concentrations were measured. Leaf water potential (Ψw), relative water content (RWC), net CO2 assimilation rate (ACO2), and stomatal conductance (gS) were decreased by flooding in all the trees but the greatest decreases occurred in VL/V/SO. The Ci/Ca (leaf internal CO2 to ambient CO2 ratio), Fv/Fo (potential activity of PSII) and Fv/Fm (maximum quantum efficiency) ratios were similar in flooded and non-flooded VL/SO and VL/C/SO trees but were decreased in VL/V/SO trees by flooding. Regardless of interstock, flooding increased root calcium (Ca), iron (Fe), copper (Cu), and manganese (Mn) concentration but decreased nitrogen (N) and potassium (K) concentration. Based on the leaf water relations, gas exchange, and chlorophyll parameters, ‘Verna’ lemon trees interstocked with ‘Valencia’ orange had the least flooding tolerance. Regardless of interstock, the detrimental effect of flooding in ‘Verna’ lemon trees was the leaf dehydration which decreased ACO2 as a result of non-stomatal factors. Lowered ACO2 did not decrease the leaf carbohydrate concentration. Flooding decreased root starch in all trees but more so in VL/V/SO trees. Sugars were decreased by flooding in roots of interstocked trees but were increased by flooding in VL/SO roots suggesting that the translocation of carbohydrates from shoots to roots under flooded condition was impaired in interstocked trees.