Quantification of Poinsettia Root Growth by Image Scans or X-ray Computed Tomography Scans in Three Propagation Substrates at Varied Moisture Levels

The objective was to refine protocols to quantify rooting of plant cuttings and the water/air microenvironment of substrates using x-ray computed tomography (CT). Poinsettia (Euphorbia pulcherrima) cuttings were propagated in three substrates (peat, rockwool, and phenolic foam) at varied moisture levels. In Experiment 1, adventitious rooting occurred in cuttings grown in rockwool or foam from 12% to 86% volumetric water content (VWC) and 12% to 80% volumetric air content (VAC). The highest root growth occurred in rockwool at 59% VWC and 33% VAC. There was an advantage to quantifying root growth by CT in rockwool and foam, in contrast to peat, because of clear differences in material density between root and substrate during image processing. In Experiment 2, root growth was quantified in peat by two-dimensional image scans under similar growth chamber environments, with rapid root growth from 52% to 63% VWC and 16% to 26% VAC. In Experiment 3, CT was used to quantify the substrate microenvironment at 0.5 cm slices to further describe the environment at the base of the plant cutting. Rapid rooting occurred in microenvironments above 56% VWC and 14% VAC, whereas the low 3% VAC in foam at high moisture may limit root growth.