The stock plant irradiance environment influences stem breakage of flowering poinsettias (Euphorbia pulcherrima)

Experiments were conducted to determine the effect of the stock plant environment on the stem breakage of poinsettia (Euphorbia pulcherrima Willd. 'Freedom Red') that occurs on the lateral stems that develop following the propagation and pinching of the cuttings removed from those stock plants. In the first experiment, stock plants were grown in greenhouses under ambient irradiance or 50% irradiance-reduction shade cloth. Within each irradiance treatment, stock plants were placed at three different spacings, 410, 730, or 1140 cm² plant^-1. Flowering plants were dropped onto a concrete pad from different heights to quantify stem breakage. The stock plants grown under the 50% shade and 410 cm² plant^-1 spacing treatment produced cuttings that experienced 29% stem breakage when dropped 30 cm, while the stock plants grown under ambient irradiance and 1140 cm² spacing produced cuttings that had only 3% stem breakage. In a second experiment, stock plants were grown outdoors with 0, 62 or 81% shade. Stock plants grown under 81% shade produced cuttings that were significantly more susceptible to stem breakage, e.g., 35% stem breakage at 30 cm drop height, than cuttings harvested from stock plants grown under ambient irradiance, which experienced only 9% stem breakage. These data support the hypothesis that the irradiance delivered to the stock plants can impact stem breakage of the plants grown from the cuttings harvested from those stock plants.