Performance of Navel orange trees grafted onto two new dwarfing rootstocks (Forner-Alcaide 517 and Forner-Alcaide 418)

The aim of the present work was to study the performance of two new dwarfing citrus rootstocks (Forner-Alcaide 517 and Forner-Alcaide 418) in comparison with Carrizo citrange. To carry out these experiments, rootstocks were budded with Navelina Navel orange scions. In young plants, rootstocks’ size-controlling capacity was associated to their hydraulic conductance. In the case of Forner-Alcaide 418, lower hydraulic conductance was related to a smaller lumen area of root xylem vessels with respect to the other rootstocks. Also, hydraulic resistance of bud union segments of the stem was found to decrease with increasing vigour of plants grafted onto the different rootstocks. Furthermore, the graft union may provide resistance to sucrose transport in dwarfing rootstocks, as indicated by the reduced translocation of 13C-labelled photoassimilates from leaves to roots and reduced soluble sugar and starch concentrations under the graft union. This may result in a deficient carbohydrate supply to roots in dwarfing rootstocks, thereby stunting their growth. Accordingly, small root size and low hydraulic conductivity may cause water deficit in leaves during periods of high evaporative demand, inducing stomatal closure. This leads to a decrease in photosynthetic assimilation rate, which may affect plant growth negatively. These effects occur to a different extent in scions on both dwarfing rootstocks, appearing to be the cause of their reduced growth.Moreover, plants on both dwarfing rootstocks showed decreased translocation of 13C-photoassimilates to roots, possibly contributing to higher availability of these compounds in the scion, and resulting in increased 13C transport towards the fruits. This effect could explain the higher yield efficiencies found in dwarfing rootstocks.A ten-year long field experiment, with trees grafted onto the three rootstocks, showed that Forner-Alcaide 517 and Forner-Alcaide 418 induced lower canopy volumes and higher yield efficiencies than Carrizo citrange. In addition, our results show both dwarfing rootstocks produce good fruit quality, are suitable for high density plantings and can be cultured in alkaline soils.