A large portion of the U.S. green industry is involved with growing plants in containers, including bedding plants, vegetable plants, foliage plants, potted flowering plants, potted nursery stock, and other assorted floriculture crops. Root growth of crops grown in containers is a central element in overall plant performance, whether it is during propagation, production, or post-production (e.g., transplant success) as a result of the combined functions of roots being anchorage, support, and water and nutrient uptake (Wraith and Wright, 1998). Considering the large portion of the industry involved with growing plants in containers and the importance of understanding the physiology and morphology of roots, the factors that influence root growth in container production need to be continually investigated. However, root growth and root architecture are frequently excluded in horticultural research (Wright and Wright, 2004), and the study of natural root development is a challenge as a result of the difficulty of root observations in containers during crop production (Silva and Beeson, 2011).
Strategies and techniques for observing, studying, and quantifying root growth have been reported over the past nine decades. Observing and measuring root growth of crops began in the field, and methods such as hand-drawing were time-consuming and difficult (McDougall, 1916; Weaver et al., 1922). Several of the field techniques, like photography, have since been modified to also measure root growth of plants grown in containers. Fortunately, many advances have been made over the decades in the study of root measurements, including techniques that can be easier, faster, and more descriptive of root growth, like the rhizotron or mini-rhizotron for field-grown plants (Taylor et al., 1990).
Currently, the most common root system evaluations of plants grown in containers are 1) subjective root ratings; and 2) root dry weight determination. Subjective root ratings can be a simple and easy way to qualitatively describe rootballs, washed roots, and propagated rooted cuttings. Ratings can evaluate root density, appearance, branching, and distribution. However, the person rating the root system must first understand how to accurately rate the quality of the root system (Walters and Wehner, 1994), and because the root rating is subjective, it often varies with each examiner. Root dry weight is a destructive method, which involves extracting and drying the plant root system (Aung, 1974). For dry weight measurements, and possibly for root ratings, substrate must be washed from the roots and many of the fine roots and root hairs are lost in this process as are the natural positions and arrangements of the roots. In standard methods of washing and storing root samples, losses of dry weight from 20% to 40% may occur (Oliveira et al., 2000; van Noordwijk and Floris, 1979).
The Horhizotron™ was developed at Auburn University and Virginia Tech as a non-destructive technique to measure horizontal root growth from rootballs of plants grown in nursery containers, allowing for post-transplant assessment (Wright and Wright, 2004). The Horhizotron™ is constructed of eight panels of glass attached to an aluminum base to form four wedge-shaped quadrants and is suitable for greenhouse or field use and fits a range of nursery stock rootballs. Previous studies with the Horhizotron™ have shown the design allows for each quadrant to be modified in different ways to examine the effects of different rhizosphere conditions (Jackson et al., 2005; Price et al., 2009; Wright and Wright, 2004). However, the Horhizotron™ works best with large-sized rootballs (3.8 to 11.4 L); the glass panels are not permanent and can move and crack; and the shade box does not restrict all light from the root system. Silva and Beeson (2011) developed a large-volume rhizotron to observe root growth in an environment closer to natural soil conditions and still have the apparatus above ground and therefore relatively easier to collect measurements. However, this design is even larger than the Horhizotron™ and intended for woody plants with large rootballs to imitate post-transplant/or field growing conditions.
Root growth in transparent containers/root boxes may be quantified using different methods. Digital imaging can be used by computer programs to evaluate root systems, as Silva and Beeson (2011) reported with a rhizotron. Digital imaging includes photographs or videos, scanned images of exposed roots, or scanned root tracings. Advantages to using image analysis include the advancing technology that allows computers to determine structure and segmentation of root growth, which will aid in describing rate of change in roots and their growth pattern (Spalding and Miller, 2013). However, images can often be unfocused or blurry, and when taking images of glass or reflective surfaces, the glare from the sun or artificial lighting is a problem. In some cases, the soil/substrate might not contrast with the plant roots enough to be completely visible. This causes the program user to have to manually pick out each root and trace the length of it themselves and, depending on the number of roots per image, could be time-consuming. Root growth rates can also be quantified by measuring the length of the longest roots against the transparent walls. Measuring the length of the five longest roots on each side of a quadrant is commonly used with the Horhizotron™ (Jackson et al., 2005; Price et al., 2009; Wright and Wright, 2004) with the roots of two sides of one quadrant averaged to obtain the experimental value for that quadrant (Wright and Wright, 2004).
The use of different wood products as substrate components have been widely researched in the past decade and have been proven to be an acceptable and sustainable alternative to greenhouse and nursery substrates as well as suitable substrate components when amended to peatmoss and pine bark. Gruda and Schnitzler (2004b) examined spruce (Picea sp.) wood fiber substrates (WFS) and noted particularly well-developed root systems of plants grown in the WFS compared with plants grown in peat and rockwool substrate. The physical properties of both a coarser and finer WFS resembled peat substrates with the amount of total pore space; the finer WFS had a high air volume compared with peat, and mixing both WFS and peat substantially improved the air volume (Gruda and Schnitzler, 2004a). Wright and Browder (2005) also noted an increase in root growth with plants grown in 75% pine bark:25% loblolly (Pinus taeda L.) pine chips compared with 100% pine bark. Jackson et al. (2010) observed the highest root rating for plants grown in loblolly pine tree substrates compared with plants grown in peat-lite or pine bark. However, these observations were quantified with root dry weights or subjective ratings, so the extent of the apparent root growth enhancement remains to be accurately quantified and explained.
Further investigations of plant root growth in containers and additional understanding of the factors that affect it are critical for improving overall growth and quality of container-grown plants. Developing new techniques to study, observe, and measure root growth of seedlings and small-sized plants during production (vegetable transplants, plugs for floricultural crops, and nursery liners) will be beneficial in future research of root development. The premise of this work was to develop a new apparatus to measure root growth in a greenhouse production setting using a system/technique similar to the Horhizotron™ but with a different purpose, design, and construction components (discussed below). The name of this apparatus is the mini-Horhizotron, based off of the name Horhizotron™ (horizontal root growth measurement instrument), which is used with permission of the Horhizotron™ designers (Wright and Wright, 2004). The objectives of this work were: 1) to design the mini-Horhizotron and test its suitability for observing and measuring container-grown plant roots non-destructively; 2) to compare the effect of two experimental designs (number of substrates/treatments per mini-Horhizotron) on plant root growth; and 3) use the mini-Horhizotrons to compare root growth of plants in different container substrates.
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