Rhizometer root tracing techniques and digital image analysis for assessing and quantifying seedling root growth in substrates

Abstract

The ability to observe and measure root growth of horticultural plants grown in pot culture is difficult to do without disturbing or damaging the roots of growing plants. The rhizometer was recently developed at North Carolina State University to observe plant root growth and measure the effect of root growth on undisturbed substrate physical properties over time. The clear cylinder design of the rhizometer allows for visible observations of the rhizosphere so that root data collection, such as root count, root length, and root hairs can be quantified without disturbance. The objective of this study was to measure several root system parameters on plants grown in the rhizometer using tracing techniques and digital image software. Root growth in three substrates were compared in this study; peat amended with 20% (v/v) of either perlite (PL), pine-wood-chips (PWC) or shredded-pine-wood (SPW). Both PWC and SW were produced by hammer-milling freshly chipped or shredded loblolly pine trees (Pinus taeda) through a 6.35 mm hammer mill screen. Twenty rhizometers were filled with each individual substrate and four species of seeds were individually planted directly into the rhizometers (one seed per rhizometer); bean (Phaseolus vugaris 'Gold Rush'), corn (Zea mays 'Jubilee'), tomato (Solanum lycopersicum L. 'Better Boy'), and marigold (Tagetes erecta 'Inca Orange') resulting in 60 rhizometers used. Three root measurements were taken once root tips were visible along the rhizometer cylinder; number of root tips (RT), number of roots with visible root hairs (RH), and cumulative root length (RL). Root length was measured by tracing the roots on a transparency sheet, taking a digital photograph, and using RootReader 2D software to select the traced roots and calculate total root length of the picture. Number of RT, RH, and RL measurements had linear responses over time for all four species. At 12 days after emergence (DE), corn had the largest total RL in the SPW substrate, and tomato had a higher total RL in PWC substrate at 12 DE compared to the other substrates. The higher total RL observed with these two species in SW and PWC substrates at 12 DE could possibly be attributed to structural particle differences (size and shape) of these three aggregates and the matrix of those particles in the container that facilitated faster or easier root growth through the substrate. As the roots of these plants grew into the substrates and likely altered the substrates' physical properties over time, there were no observable differences in RT, RH or RL among the substrates for each species at measurement dates after 12 DE. Root tracing and digital analysis were effective tools in further assessing root growth characteristics of seedlings in the rhizometer.