Transplanting often causes root damage, and rapid root growth following transplanting may help to minimize the effects of transplant shock. The objective of this study was to determine the effects of NAA and IAA on posttransplant growth of vinca (Catharanthus roseus L.). Bare-root seedlings were germinated in a peat-based growing mix and transplanted into diatomaceous earth 10 days after seeding. Immediately after transplanting, seedlings were drenched with several concentrations of IAA or NAA (62.5 mL/plant). Both auxins increased posttransplant root and shoot growth, but the response was dose-dependent. The maximum growth occurred at concentrations of 10 mg·L-1 (IAA) or 0.1 mg·L-1 (NAA). The growth-stimulating effect of these auxins decreased at higher rates and NAA was highly toxic at 100 mg·L-1, killing most of the plants. Unlike the growth of bare-root seedlings, plug seedling growth was not stimulated by drenching with NAA solutions. These results show that auxins have the ability to stimulate posttransplant growth of vinca, but their effects may depend on the application method, rate, and timing, and transplanting method. Chemical names used: 1-naphthaleneacetic acid (NAA); 1-indole-3-acetic acid (IAA).
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