Citrus rootstock responses to water stress

Tolerance to drought-stress (DS) of the citrus rootstock Forner–Alcaide no. 5 (FA-5) was tested and compared with that of its parents, Cleopatra mandarin (CM) and Poncirus trifoliata (PT). Nine-month-old seedlings of CM, PT and FA-5 and 15-month-old grafted trees of ‘Valencia’ orange scions on these three rootstocks were cultivated in sand under glasshouse conditions and irrigated with a nutrient solution. Plants were drought-stressed by withholding irrigation until leaves were fully wilted. Survival time of both seedlings and grafted trees under DS was linked to the water extraction rate from the soil, which depended mainly on leaf biomass and on transpiration rate. Seedling responses to DS affecting leaf water relationships and gas exchange parameters varied among genotypes. FA-5 seedlings survived longer than the other seedlings, maintaining the highest levels of water potential, stomatal conductance, transpiration rate and net CO2 assimilation towards the end of the experiment, when water stress was most severe. Thus, FA-5 was more resistant to DS than its parents (CM and PT). Moreover, rootstock affected the performance of grafted trees under water stress conditions. The higher drought tolerance induced by FA-5 rootstock could be related to the greater osmotic adjustment (OA), which was reflected by smaller reductions in leaf relative water content (RWC) and in higher turgor potentials and leaf gas exchange than the other rootstocks.

Research highlights▶ We are part of a relatively large network of scientists working in citrus agronomy. Citrus are not only the major fruit tree crop in the world, but they have very interesting features that makes them almost a model plant for woody and especially for fruit tree species. ▶ In 1974, a program was begun to breed citrus rootstocks by hybridisations at the Valencian Institute for Agricultural Research (IVIA), in which more than 500 hybrids were evaluated to determine their agronomic performance. Between them, two hybrids of Cleopatra mandarin (Citrus reshni Hort ex Tan) × Poncirus trifoliata (L) Raf: Forner–Alcaide no. 5 (FA-5) and Forner–Alcaide no. 13 (FA-13), were selected since both induce high productivity and good fruit quality on the scion cultivar. Moreover, these rootstocks were resistant to citrus tristeza virus (CTV) and displays good resistance to Phytophthora spp. In addition, FA-5 is resistant to citrus nematode (Tylenchulus semipenetrans Cobb.) and it appears to be tolerant to calcareous soils. FA-5 is a commercial rootstock in European Union since 2005 and FA-13 since 2008.