Roles of Cultivar, Light and Carbohydrates in Rooting of Cuttings of Hydrangea macrophylla

The roles of light and carbohydrates in adventitious root formation of Hydrangea macrophylla cuttings of the cultivars ‘Caipirinha’ and ‘Clarissa’ were investigated. Cuttings were planted immediately or dark-stored for seven days prior to cultivation under light. The leaf and rooting phenotype, relative chlorophyll content, carbohydrate levels in different cutting sections and rooting response to hexose were analyzed. Surprisingly, pronounced leaf yellowing and reddening and a strong hexose accumulation in the cutting leaves indicated that the hydrangea cuttings experienced light stress under a photosynthetic photon flux density (PPFD) of 100 µmol m−2 s−1. Reduction in PPFD to 50 µmol m−2 s−1 decreased these symptoms and increased chlorophyll content, but impaired rooting. The effects of dark storage depended on cultivar, PPFD, and hydration of cuttings. ‘Clarissa’ exhibited lower rooting success, particularly after dark storage and low light, and showed lower hexose-to-sucrose ratios and hexose concentrations in the stem base than ‘Caipirinha’. Rooting of ‘Clarissa’ could not be rescued by sugar supplementation, whereas application of 27 mM glucose plus 30 mM fructose for 24 h before planting enhanced rooting of ‘Caipirinha’. The lower hexose level in the stem base of ‘Clarissa’ does not appear to be the critical factor underlying its low rooting capacity.