4.3. Analysis of Root Morphology

Root systems were scanned with an Epson Perfection V700 Photo scanner set at 600 dpi, and scanned images were measured using the root image analysis software WinRhizo (Regent Instruments, Inc., Québec, QC, Canada). The software separates roots into user-specified diameter classes and generates root length, surface area, volume, and other values for each diameter class. Based on results from previous experiments, we used different diameter class distributions when analyzing field and greenhouse root systems. For greenhouse-grown root systems, our analysis included roots between 0.0–2.0 mm diameter at 0.1 mm increments to allow adequate resolution in distinguishing among the fine roots of young plants. For field-grown roots in which many of the finest roots are not retained through the winter and during excavation, we measured roots at 1 mm diameter intervals up to 10 mm. Additionally, we measured roots of 0.0–0.5 mm and 0.5–1.0 mm diameters to account for differences in diameter of the finest roots previously observed in field-grown grapevines. Roots were dried at 65 °C and weighed to determine dry biomass.

High plasticity and the heterogeneous nature of soils and growing media result in considerable variation in root systems, even among genetically identical plants. For this reason, absolute values derived from image analysis were used to calculate the proportion of total root length represented by diameter classes, mean root diameter and specific root length (SRL; root length ⋅ dry biomass⁻¹), three parameters that are independent of the absolute size of root systems.