Quantifying the Effects of Preharvest Calcium Nutrition on the Toning of Unrooted Cuttings

Vegetatively-propagated unrooted cuttings for the U.S. market are typically imported from off-shore production facilities. Growers empirically evaluate the cutting quality that impacts post-harvest durability during shipping and propagation and refer to the desired characteristics as ‘toning’. Toning can be defined as the resistance to external forces such as physical damage or pathogen infection. While growers employ strategies to create well-toned cuttings, these strategies have not been quantified in controlled studies. During the current study, degree of toning was quantified via analysis of mechanical strength of leaves following pre-harvest foliar calcium treatments, using poinsettia and geranium as model plants. Calcium chloride (CaCl₂) was applied weekly as a foliar spray to stock plants at the rate of 0, 400 or 800 mg∙L^-1 Ca. Mechanical strength of leaves was assessed using a force-displacement graph generated from a texture analyzer using a ball probe to penetrate a unit area of a clamped leaf.  The peak force to fracture the leaf (g) and work of penetration or area under the force-displacement curve (g∙mm) was utilized as indicators of mechanical strength. For geranium, work of penetration increased 15% with the application of 800 mg∙L^-1 Ca compared to the control (P = 0.02). For poinsettia, work of penetration was greater for treatments with CaCl₂ with an average of 277 g∙mm as compared to 251 g∙mm without CaCl₂ (P = 0.04). Peak force was also greater (P = 0.01) with CaCl₂ (198.5 g) as compared to treatments without CaCl₂ (182.5 g). No differences were evident between 400 and 800 mg∙L^-1 Ca for poinsettia, indicating 400 mg∙L^-1 of CaCl₂ is sufficient to significantly increase the mechanical strength of leaves as compared to geranium, which required 800 mg∙L^-1 Ca. The increased leaf strength due to foliar calcium application was retained in both species following a 3-day simulated post-harvest treatment. Provision of foliar calcium applications to stock plants can improve mechanical strength/toning of cuttings both in geraniums and poinsettia. Leaf mechanical strength could be used effectively to quantify the degree of toning of unrooted cuttings.