Involvement of rooting factors and free IAA in the rootability of citrus species stem cuttings

Abstract

Two-year-old trees of cultivar ‘Rangpur’ lime (Citrus limonia Osb.) and of cultivar ‘Oroblanco’, a triploid pummelo-grapefruit hybrid (Citrus grandis Osb. × Citrus paradisi Macf.), which had not reached flowering stage, were grown under greenhouse conditions. Cuttings from the last vigorous vegetative flush were taken from each species for rooting experiments. Callus formation and percentage of rooting were determined after 19 and 36 days. Endogenous indole-3-acetic acid (IAA) content and avocado rooting promoter (ARP)-like activity were determined in leaves and bark (cortex) of the lower end of the cutting on day of excision and 19 days later. Rooting of the easy-to-root ‘Rangpur’ lime reached 77% after 19 days and 100% after 36 days. At those times the difficult-to-root ‘Oroblanco’ did not root at all or reached 12% rooting, respectively. At Day 19 the level of free IAA in the bark of ‘Rangpur’ lime was 3.5 times higher than that at Day 0, and 3.6 times higher than in the ‘Oroblanco’ bark on the same day. ARP was found in both species at excision day and after 19 days using gas-chromatography analysis. The ARP activity on the day of excision was only slightly higher in the leaves and bark of ‘Rangpur’ lime than in ‘Oroblanco’, but after 19 days ARP-like activity rose approximately 45% in the basal bark of ‘Rangpur’ lime with no such increase in its leaves or in the leaves and bark of ‘Oroblanco’. The differences in IAA level and ARP-like activity in the two citrus cultivars appear to be correlated with their ease of rooting, but it is not possible to tell whether the increase in IAA and ARP over the rooting period is the cause or result of root initiation.