Commercial production of flowering poinsettias begins with the propagation of shoot tip cuttings that are harvested from stock plants having many stems in a dense canopy. The cuttings are removed from the stock plants on a weekly schedule. A typical unrooted poinsettia cutting for the North American market consists of a 5.0- to 6.5-cm stem, two to three mature leaves, and three to four immature leaves. The caliper at the base of the stem is typically 4 to 6 mm (Ecke et al., 2004). Individual shoots require 6 to 8 weeks to develop from an axillary bud to a shoot with commercial cutting specifications.
Previous studies on poinsettia growth and development have concentrated on the branching (Faust and Heins, 1996), plant height (Berghage and Heins, 1991; Clifford et al., 2004), bract size, and flowering dates (Liu and Heins, 2002). These studies typically describe the growth and development of the dominant shoots in a canopy with a relatively low canopy density (shoots/m2), i.e., low competition for sunlight. Thus, the results describe optimal rates of growth and development. In contrast, poinsettia stock plants consist of a relatively dense canopy of competing shoots, which appear to be developing at less than optimal rates. Stock plant crops are schedules so that the canopies reach the point of closure, i.e., when the leaves of neighboring plants overlap, before the cutting harvest season. If a stock plant canopy does not reach canopy closure, then production space is wasted. If a stock plant canopy reaches canopy closure far in advance of the harvest season, then labor is wasted removing cuttings until there is market demand while not increasing branching.
Previous studies on stock plant management techniques are relatively few and have primarily focused on the effect of pinching techniques on cutting yield (Grueber, 1985). The growth and development of individual shoots within the stock plant canopy have not been quantified. Therefore, the objective of this study was to measure the effects of temperature, CD, and DLI on shoot growth and development within a poinsettia stock plant canopy. These data can provide insight into the management of greenhouse environments and cultural practices for the production of commercial poinsettia cuttings.
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