In container substrates, moisture level and substrate porosity affect oxygen supply to roots. Oxygen can be supplied to roots either as a gas present in air-filled pores or as DO in water. In atmospheric air, oxygen is at 20.9%, equivalent to 274 mg·L−1 at 1 atm and 25 °C. Within the root zone, Van Iersel and Dove (2014) pointed out that in a well-irrigated substrate, the relative humidity is likely to be close to 100%, resulting in reduced partial pressure of oxygen because of water vapor (19.1–20.6 kPa), and root and microbial respiration could further lower O2 concentration. Oxygen saturation concentration in water decreases as temperature increases, with 8.3 mg·L−1 saturated DO at 25 °C, compared with 11.3 mg·L−1 at 10 °C. The diffusion rate of oxygen in water is ≈10,000 times slower than the diffusion in air (Colmer, 2003; Wegner, 2010). Consequently, transport of O2 in a substrate and availability for root uptake are dependent on factors that affect diffusion and advection (bulk flow) of oxygen, including the surface area of substrate particles and water that are in contact with air, the continuum of gas-filled pores, and substrate moisture level (Currie, 1970). Peat-based substrates are designed to avoid complete saturation and provide air-filled porosity for oxygen supply to roots, even when irrigated to CC (Argo et al., 1996; DeBoodt and Verdonck, 1971). In a typical peat-based substrate, air porosity can be up to 32% by volume at CC in a 1-L container, but this percentage can decrease to 5% in small cells used for propagation of seedlings and unrooted cuttings (Argo et al., 1996; Gislerod, 1982; Handreck and Black, 1994). Lower substrate oxygen conditions are likely to occur with fine substrate particles, small container size, and high moisture. These conditions may also occur during mist propagation of plant cuttings because high substrate moisture is needed to prevent plant wilting, there are frequent irrigation events, and plants are grown in small cells (Santos et al., 2011).
Oxygen supply to and within roots involves several processes and is vital for root physiological function. Diffusion and bulk flow of oxygen into container substrate often occurs rapidly, replacing oxygen used in respiration by roots and microbes (Van Iersel and Dove, 2014). Oxygen from air-filled pores can diffuse passively into the root tip (Lemon, 1962; Luxmoore et al., 1970) or DO can enter through water pathways (Luxmoore et al., 1970). Apoplastic porosity through contiguous intercellular spaces allows for the movement of oxygen within the root (Armstrong and Drew, 2002; Colmer, 2003). Channels in membranes or aquaporins also facilitate cell-to-cell symplastic transport of oxygen (Herrera and Garvin, 2011; Hub et al., 2009). These oxygen transport processes in the root allow the vital functions related to respiration and nutrient uptake to be carried out (Drew, 1988). Respiration rates have been estimated at 300–500 ng·cm−3·s−1 0.3–0.5 ng·L−1·s−1 for apical regions of the root at 23 °C (Armstrong et al., 2000). Over-irrigation fills substrate pores with water rather than air, and oxygen demand from respiring roots and microbes can further result in low (hypoxic) to no oxygen (anoxic) levels (Armstrong and Drew, 2002; Drew, 1983; Morard and Silvestre, 1996; Naasz et al., 2009). Plant stress has been observed in hydroponic production conditions under low DO concentrations from 0.1 to 2 mg·L−1 (Goto et al., 1996; Zheng et al., 2007). Stress responses include decreased root metabolism and nutrient uptake, root death, and wilting (Armstrong and Drew, 2002; Drew, 1983; Ehret et al., 2010; Handreck and Black, 1994; Morard and Silvestre, 1996). Low oxygen at the root zone also increases the risk of diseases from microbial pathogens such as Pythium (Chérif et al., 1997).
In some studies, oxygenation of irrigation water has increased plant biomass. Dissolved oxygen in water can be increased with turbulence, bubbling of air or purified oxygen, and the addition of chemicals such as hydrogen peroxide (Schröder and Lieth, 2002). Oxygen injecting technology can increase DO above oxygen-saturated levels in irrigation water (Schröder and Lieth, 2002; Zheng et al., 2007). Lei et al. (2016) grew corn in vermiculite substrate under completely saturated conditions. Irrigating plants with oxygenated water (two aeration systems) increased the DO from 3.5 to 6.5 mg·L−1, resulting in higher corn yield and biomass compared with those grown with ambient irrigation water with 0.3–4.5 mg·L−1 DO. In a hydroponic system, when DO ranged from ambient of ≈8.5 mg·L−1 to supersaturated 30 mg·L−1, there were no effects on tomato dry mass (Zheng et al., 2007). In the Zheng et al. (2007) study, plant growth decreased and roots appeared stunted and thick when DO was further increased to ≈40 mg·L−1. Although reports on oxygen injecting technology of irrigation water in propagation or container production of bedding plants are lacking, potential positive effects could include increased root or total growth and root health.
Our objective was to evaluate whether oxygenation of irrigation water affected plant growth and substrate DO levels during 1) mist propagation of unrooted cuttings and 2) subsequent growth in containers after transplant. Greenhouse experiments were run with mist propagation of vegetative cuttings of Calibrachoa ×hybrida ‘Aloha Kona Dark Red’ and Lobelia erinus ‘Bella Aqua’ in plug trays. Plants of these two species and Pelargonium ×hortorum ‘Patriot Red’ were subsequently grown to flowering in 10.2-cm-diameter containers. Supplemental experiments were conducted without plants to provide additional details on DO under the experimental growing conditions.
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