Response of 'Hass' avocado (Persea americana Mill.) grafted on Mexican and West Indian clonal rootstocks to saline conditions

‘Hass’ scions on commercially important ‘Duke 7’ or putative salt-tolerant ‘VC 239’ or ‘VC 241’ rootstocks were irrigated with isotonic (3 dS•m−1) solutions containing 2 to 16 meq•L−1 Cl. After 12 months, total biomass of ‘Hass’ on ‘Duke 7’ was lowest for trees with the greatest leaf Cl concentrations, whereas growth of ‘Hass’ on the VC rootstocks was unaffected by significantly greater leaf Cl concentrations. Root Cl concentration of ‘VC 241’ was greater than the other rootstocks. Thus, the growth response of ‘Hass’ scions to Cl was mediated by a factor(s) within the rootstock and Cl tolerance of ‘Hass’ on the two VC rootstocks was not due to root Cl exclusion. Specific ion effects were contrasted with salinity by irrigating ‘Hass’ trees on Mexican race ‘Duke 7’, ‘Thomas’ and ‘Toro Canyon’ or putative West Indian selections ‘VC 207’ and ‘VC 256’ with 2 to 8 dS•m−1 (NaCl:CaCl₂, 2:1 molar ratio). After 100 days, Na and Cl concentrations were greater in meristems and leaves, but not roots, of trees irrigated with 8 dS•m−1 compared to 6•dS·m−1. Cl concentration of the treatment solution was 3-fold higher than Na concentration, but Na ion toxicity appeared to be the major factor influencing tree growth. Meristem (r = −0.53, P < 0.0001) and leaf Na concentrations (r = −0.37, P = 0.0048) were negatively correlated with final shoot biomass. ‘VC 207’ had the lowest root and shoot Cl concentrations in addition to restricted transport of Na from root to shoot, making it a candidate for further evaluation. Expression of the gene encoding 9-cis-epoxycarotenoid dioxygenase (NCED), which regulates abscisic acid biosynthesis, was investigated in roots of ‘Thomas’ and ‘Toro Canyon’, which was significantly more salinity tolerant than ‘Thomas’. For ‘Thomas’, PaNCED1 transcript was greater in salinity-treated trees than control trees 12 and 24 hours after the irrigation water reached 8 dS•m−1. In contrast, PaNCED1 transcript was detected later in ‘Toro Canyon’ roots, 24 and 48 hours after 8 dS•m−1 was reached. Taken together, results of the dissertation research established that there is significant genetic diversity related to salinity tolerance within both the Mexican and West Indian race rootstocks that should be investigated further.