Agronomic response to saline irrigation of two citrus species grafted on one salinity tolerant rootstock

Citrus crop are very sensitive to salinity, but their actual response may depend on several factors, among them the species used as scion. When growers have to choose a species for planting they need to know how it will behave in salt stress situations. The aim of this experiment was to evaluate the behaviour of two citrus species 'Clemenules' mandarin (CLM) and 'Star Ruby' grapefruit (SR), both grafted on 'Cleopatra' mandarin rootstock, to establish their relative salt tolerance. The experiment was carried out over a 3 year period (2008-2010) in the IMIDA experimental farm located in Torre Pacheco (Murcia) using 15-year-old trees irrigated with 2 treatments: 0 mM NaCl and 40 mM NaCl. After the application of saline water for 3 consecutive years, the species revealed a different behaviour in response to salinity. Higher leaf Cl-and Na+ concentrations were observed in SR, which led to an important nutritional imbalance in the trees. The high leaf Na+ concentration in SR salinized-trees produced an antagonistic effect on leaf K+ concentrations. The salinity applied also produced a different response in the vegetative growth parameters of each species. SR was more affected by salinity in this respect, showing a substantial degree of defoliation during the first year. However, the leaf biomass of the trees gradually recovered from the second year onwards, pointing to signs of salinity acclimation. In CLM, on the other hand, leaf density was hardly affected by salinity during the three years of the experiment. The total accumulated yield ( 20082010) was only reduced by salinity in SR trees, mainly as a result of reduced fruit weight, although a lower fruit load was also observed in 2009. As regards fruit quality, salinity increased total soluble solids and acidity in both species, without affecting the maturity index. The external peel colour of SR fruit was greener as a result of salinity.