Developmental Stage of ‘Snow White’ Indian Hawthorn and ‘Sky Pencil’ Holly Affects Response to Cyclanilide

Abstract

A study was conducted to determine the role that plant developmental stage (DS) has on the efficacy of cyclanilide (CYC), a plant growth regulator (PGR) with cytokinin-like properties. In 2007, single foliar applications of 200 ppm CYC were applied to ‘Snow White’ Indian hawthorn (Rhaphiolepis indica (L.) Lindl. Ex Ker Gawl.) and ‘Sky Pencil’ holly (Ilex crenata Thunb.) at progressively later stages of lateral shoot development: bud break (DS 1), active shoot elongation (DS 2), or to recently matured shoots (DS 3). The experiment was repeated in 2008 and included an additional application to Sky Pencil holly before bud break (DS 0). In 2007 and 2008, Snow White Indian hawthorn treated with CYC at all DS formed more shoots than untreated plants, except in summer 2008 (DS 3 only) and fall 2008 (DS 1 only). New shoot counts of CYC-treated Indian hawthorn were 53 to 67% and 46 to 65% higher than those of untreated plants in summer and fall, respectively, in 2007 and 26 to 39% and 29 to 48% higher in summer and fall, respectively, in 2008. In the fall of 2008, the only effect of DS on shoot counts of Indian hawthorn was that plants treated at DS 3 formed 29% more new shoots than plants treated at DS 1; DS had no effect on shoot counts in 2007. Symptoms of foliar injury to Indian hawthorn included reddening, chlorosis, and cupping of new growth that appeared between 15 and 30 days after treatment (DAT) and, although not quantified in 2007, were generally less severe and more transitory as DS increased. In 2007, foliar injury lasted until about 90 to 120 DAT in DS 2 and DS 3 plants, but was more persistent in DS 1 plants. In 2008, foliar injury was also transient, regardless of DS, but was highest when CYC was applied at DS 1 or DS 2. CYC-treated Sky Pencil holly had formed more shoots than untreated plants by summer and fall of 2007. Although Sky Pencil holly treated at DS 2 in 2007 formed fewer shoots than DS 1 plants in fall, quality ratings were higher due to a larger canopy that was dense and compact. In the fall of 2008, shoot counts of CYC-treated Sky Pencil holly were greater than those of untreated plants, regardless of DS, and there were no effects of DS on shoot counts. Quality ratings of Sky Pencil holly treated at DS 2 or DS 3, but not at DS 0 and DS 1, were higher than those of untreated plants. In both years of the study, symptoms of foliar injury on Sky Pencil holly were minimal, suggesting a relatively high tolerance to foliar-applied CYC, and all plants were considered marketable.