Quantifying the Effects of Chelated Calcium and Salicylic Acid on the Leaf Mechanical Strength of Poinsettia

Vegetatively-propagated unrooted cuttings for the U.S. market are typically imported from off-shore production facilities. Cutting quality, defined as the resistance to external forces such as physical damage and pathogen infection, impacts postharvest durability during shipping and propagation. During previous studies, foliar application of calcium chloride (CaCl₂) increased mechanical strength of leaf tissue of unrooted cuttings. Concentrations at or above 800 mg∙L^-1 were associated with phytotoxicity symptoms in poinsettia (Euphorbia pulcherrima).  Therefore, the potential of alternate calcium sources, such as chelated calcium CaEDTA (40, 80, or 160 mg∙L^-1), and salicylic acid (150 or 300 mg∙L^-1) to increase the mechanical strength of the leaves of poinsettia was investigated. Mechanical strength of leaves was assessed using a force-displacement graph generated from a texture analyzer using a ball probe to penetrate a clamped leaf.  The peak force to fracture the leaf (g) and the work of penetration or area under the force-displacement curve (g∙mm) were utilized as indicators of mechanical strength. Calcium content in the leaves increased by 27% with increased application of CaEDTA from 0 to 160 mg∙L^-1. Peak force was 26% greater in treatments with CaEDTA at 80 or 160 mg∙L^-1 compared to the control. Work of penetration was also 24% and 29% greater for treatments with CaEDTA at 80 and 160 mg∙L^-1 compared to control. Following application of salicylic acid, peak force and work of penetration were 18% and 19% greater, respectively, at 150 or 300 mg∙L^-1 compared to control.  Provision of foliar CaEDTA at 40 or 80 mg∙L^-1 or salicylic acid at 150 mg∙L^-1 to stock plants can improve mechanical strength of cuttings in poinsettia without phytotoxicity symptoms.