ORANGE VARIETIES AS INTERSTOCK IN 'VERNA' LEMON TREES INCREASE THE SALT TOLERANCE BUT NOT THE DROUGHT OR FLOODING TOLERANCE

A research on the ability of interstocks to increase the abiotic stress tolerance in lemon trees was conducted with three independent experiments. A comparison of 2-year-old ‘Verna’ lemon trees [Citrus limon (L.) Burm.; VL] grafted on Sour Orange (C. aurantium L.; SO) rootstock either 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) in salinity, drought and flood conditions was carried out. In the salinity experiment, trees were supplied with nutrient solutions containing 0, 30, or 60 mM NaCl for two months. In the drought experiment, half the trees were reduced by 25% watering every 5 days for 20 days. In flood experiment, half the trees were flooded for 9 days. The experiments were conducted under greenhouse conditions. Leaf water relations, leaf gas exchange and mineral nutrition were measured. Interstocked Citrus trees are more salt-tolerant than non-interstocked trees. Leaf concentrations of Cl^- and Na⁺ were higher in VL/SO trees than in VL/C/SO or VL/V/SO trees. The mechanism involved in the reduction of leaf Cl^– ions by an interstock could be linked to its low E_leaf values during the day and/or to their high S:R DW ratios. ‘Valencia’ variety reported greater salt-tolerance to VL trees than ‘Castellano’, even though leaf Cl^– and Na⁺ levels were similar in the two interstocked trees. ‘Verna’ lemon trees interstocked with orange varieties did not improve the flooding and drought tolerance. When ‘Valencia’ orange was used as the interstock, the flooding and drought tolerance was drastically reduced.