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Thomas Graham and Michael A. Dixon

Marchantia polymorpha L. (a thalloid liverwort) is a common plant pest in nursery and greenhouse production systems. The rapid growth and dissemination of this pest can result in heavy mats of thallus tissue on the surface of pots, which restrict water penetration, compete for nutrients, and provide a habitat for other pests and disease vectors. The sensitivity of liverwort to aqueous ozone was examined to determine if routine use of ozone, as a component of an irrigation water remediation strategy, could provide ancillary services in the form of liverwort management. Three experiments were performed to evaluate contact time (CT) thresholds and application frequencies suitable for liverwort management applications. The first two experiments confirmed that CT is a suitable process control parameter with a base liverwort management threshold occurring between CT 0.84 and 1.68 mg·L−1·min under the conditions used. The third experiment examined the effect of application frequency at a CT of 3.75 mg·L−1·min, which was previously determined to be compatible with select woody perennial species. Three and five applications per week resulted in reduced liverwort growth and fecundity.

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Thomas Graham, Ping Zhang, Youbin Zheng, and Michael A. Dixon

The phytotoxic threshold of five woody perennial nursery crops to applications of aqueous ozone was investigated to determine if aqueous ozone could be used for remediation of recycled nursery irrigation water and for pathogen control. The perennial nursery crops [Salix integra Thunb. ‘Hakura Nishiki’; Weigela florida Thunb. ‘Alexandra’; Spiraea japonica L.f. ‘Goldmound’; Hydrangea paniculata Seib. ‘Grandiflora’; Physocarpus opulifolius L. Maxim. ‘Summer Wine’] were evaluated for aqueous ozone phytotoxicity after 6 weeks of overhead spray irrigation in which five aqueous ozone treatments (0, 10.4, 31.2, 62.5, 125.0 μmol·L−1) were applied on a daily basis. The concentrations applied represent levels useful for irrigation system maintenance (pathogen and biofilm control) with the highest levels selected to clearly demonstrate phytotoxicity. Aqueous ozone solutions were prepared and injected in-line during irrigation for 7.5 min every day for 6 weeks, after which growth parameters (leaf area, shoot dry weight, root dry weight, height, flower number) were measured and leaf injury was evaluated. High residual aqueous ozone (62.5 μmol·L−1 or greater at emitter discharge; 0.3 m from canopy) in the irrigation water was shown to negatively affect the growth parameters measured; however, low residual ozone concentrations (31.2 μmol·L−1 or less at emitter discharge; 0.3 m from canopy) did not present any measurable risk to plant growth. Furthermore, even at higher dose levels, leaves produced during the treatment period showed reduced damage levels. It is concluded that ozone residuals of 31.2 μmol·L−1 (at emitter discharge) can remain in overhead irrigation water without negatively affecting the crop species examined under the application protocols used. At the ozone concentrations demonstrated to be tolerable by the crop species examined, it is reasonable to surmise that control of pathogens at all points within the irrigation system will be achievable using aqueous ozone as part of an irrigation management strategy. The use of aqueous ozone in this fashion could also aid in dramatically reducing chemical residuals on crops by reducing the input requirements of traditional chemical controls.

Free access

Youbin Zheng*, Thomas Graham, Stefan Richard, and Mike Dixon

Pot gerbera (Gerbera jamesonii Var. `Shogun') plants were subirrigated with one of four nutrient solutions (10, 25, 50, and 100% of full strength) in order to determine whether currently used commercial nutrient solution concentrations can be reduced without negative impact on crop production. Nutrient concentration levels did not affect leaf area, flower number and appearance, and plant total dry weight. There were no significant differences in leaf chlorophyll content between the plants that received the 50 and 100% strength nutrient solutions. It is concluded that nutrient solution concentrations typically used in commercial greenhouse, for pot gerbera production, can be safely reduced by at least 50% without adversely affecting crop production. Nutrients accumulated in the top section of the growth substrate under all treatment levels; however, no phytotoxic effect was observed. Fertilizer inputs were reduced in the 50%, 25%, and 10% treatments by 54%, 75%, and 90% respectively. After 4 weeks recirculating, the quality of the nutrient solutions was still within acceptable limits.

Free access

Youbin Zheng, Thomas Graham, Stefan Richard, and Mike Dixon

To determine whether currently used commercial nutrient solution concentrations can be reduced during the final stage (last 4 to 5 weeks) of production of potted gerbera (Gerbera jamesonii `Shogun') under recirculating subirrigation conditions, plants were grown under one of four nutrient levels (10%, 25%, 50%, and 100% of full strength). Nutrient concentration levels did not affect leaf area, flower number and appearance, and plant total dry weight. There were no significant differences in the greenness (as measured by SPAD meter) of leaves from plants that received the 50% and 100% strength nutrient solutions. However, leaves from plants that received the 10% and 25% strength solution showed significantly less greenness than that of the plants that received 50% and 100% strength nutrient solutions. There were interveinal chlorosis symptoms on the younger leaves of some plants in the 10% and 25% strength nutrient treatments. It is suspected that this interveinal chlorosis was due to iron (Fe) deficiency caused by the increased substrate pH. It is concluded that the nutrient solution concentrations typically used for potted gerbera production in commercial greenhouses at the final stage (4 to 5 weeks) under recirculating subirrigation conditions, can be safely reduced by at least 50% without adversely affecting crop production. Nutrient salts accumulated in the top section of the growth substrate under all treatments levels; however, no phytotoxic effects were observed. No differences in water use (141 mL per plant per day) were observed amid the various nutrient levels. Fertilizer inputs were reduced in the 50%, 25%, and 10% treatments by 54%, 75%, and 90% respectively, relative to the 100% treatment. After 4 weeks under recirculating conditions, the qualities of the nutrient solutions were still within acceptable limits.

Open access

Dave Hawley, Thomas Graham, Michael Stasiak, and Mike Dixon

The influence of light spectral quality on cannabis (Cannabis sativa L.) development is not well defined. It stands to reason that tailoring light quality to the specific needs of cannabis may increase bud quality, consistency, and yield. In this study, C. sativa L. ‘WP:Med (Wappa)’ plants were grown with either no supplemental subcanopy lighting (SCL) (control), or with red/blue (“Red-Blue”) or red-green-blue (“RGB”) supplemental SCL. Both Red-Blue and RGB SCL significantly increased yield and concentration of total Δ9-tetrahydrocannabinol (Δ9-THC) in bud tissue from the lower plant canopy. In the lower canopy, RGB SCL significantly increased concentrations of α-pinine and borneol, whereas both Red-Blue and RGB SCL increased concentrations of cis-nerolidol compared with the control treatment. In the upper canopy, concentrations of α-pinine, limonene, myrcene, and linalool were significantly greater with RGB SCL than the control, and cis-nerolidol concentration was significantly greater in both Red-Blue and RGB SCL treated plants relative to the control. Red-Blue SCL yielded a consistently more stable metabolome profile between the upper and lower canopy than RGB or control treated plants, which had significant variation in cannabigerolic acid (CBGA) concentrations between the upper and lower canopies. Overall, both Red-Blue and RGB SCL treatments significantly increased yield more than the control treatment, RGB SCL had the greatest impact on modifying terpene content, and Red-Blue produced a more homogenous bud cannabinoid and terpene profile throughout the canopy. These findings will help to inform growers in selecting a production light quality to best help them meet their specific production goals.

Free access

Gioia Massa, Thomas Graham, Tim Haire, Cedric Flemming II, Gerard Newsham, and Raymond Wheeler

Significant advances in controlled-environment (CE) plant production lighting have been made in recent years, driven by rapid improvements in light-emitting diode (LED) technologies. Aside from energy efficiency gains, LEDs offer the ability to customize the spectrum delivered to a crop, which may have untold benefits for growers and researchers alike. Understanding how these specific wavebands are attenuated by plant tissue is important if lighting engineers are to fully optimize systems for CE plant production. In this study, seven different greenhouse and field crops (radish, Raphanus sativus ‘Cherry Bomb II’; red romaine lettuce, Lactuca sativa ‘Outredgeous’, green leaf lettuce, Lactuca sativa ‘Waldmann’s Green’; pepper, Capsicum annuum ‘Fruit Basket’; soybean, Glycine max ’Hoyt’; cucumber, Cucumis sativus ‘Spacemaster’; canola, Brassica napus ‘Westar’) were grown in CE chambers under two different light intensities (225 and 420 μmol·m−2·s−1). Intact, fully expanded upper canopy leaves were used to determine the level of light transmission, at two to three different plant ages, across seven different wavebands with peaks at 400, 450, 530, 595, 630, 655, and 735 nm. The photosynthetic photon flux (PPF) environment that plants were grown in affected light transmission across the different LED wavelengths in a crop-dependent manner. Plant age had no effect on light transmission at the time intervals examined. Specific waveband transmission from the seven LED sources varied similarly across plant types with low transmission of blue and red wavelengths, intermediate transmission of green and amber wavelengths, and the highest transmission at the far-red wavelengths. Higher native PPF increased anthocyanin levels in red romaine lettuce compared with the lower native PPF treatment. Understanding the differences in light transmission will inform the development of novel, energy-saving lighting architectures for CE plant growth.

Free access

Unaroj Boonprakob, David H. Byrne, Charles J. Graham, W.R. Okie, Thomas Beckman, and Brian R. Smith

Diploid plums (Prunus L. sp.) and their progenitor species were characterized for randomly amplified polymorphic DNA polymorphisms. Bootstrap analysis indicated the variance of genetic similarities differed little when the sample size was >80 markers. Two species from China (Prunus salicina Lindl. and P. simonii Carr.) and one species from Europe (P. cerasifera Ehrh.) contributed the bulk (72% to 90%) of the genetic background to the cultivated diploid plum. The southeastern plum gene pool was more diverse than those from California, Florida, or South Africa because of the greater contribution of P. cerasifera and P. angustifolia Marsh. to its genetic background.