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Darren J. Hayes and Bryan J. Peterson

We assessed adventitious root formation on stem cuttings of mountain fly honeysuckle [Lonicera villosa (Michx.) Schult.] in separate experiments using overhead mist and subirrigation systems. The concentration of applied potassium salt of indole-3-butyric acid (K-IBA) and the proportions of coarse perlite and milled peatmoss in the propagation medium were varied within both systems. Across treatments, 98% of cuttings in the overhead mist system and 85% of cuttings in the subirrigation system produced roots. In the overhead mist system, root volume, root dry weight, and number of root tips were greatest among cuttings treated with 4000 to 12,000 mg·L−1 K-IBA and stuck into 100% perlite. In the subirrigation system, root dry weight was not significantly affected by K-IBA concentration, but the greatest root volume and number of root tips were produced by cuttings treated with 8000 or 12,000 mg·L−1 K-IBA and stuck into 100% perlite. Despite the natural affinity of mountain fly honeysuckle for moist, organic soils, all of the 18 rooted cuttings we planted in a landscape trial survived and grew appreciably with minimal care over 2 years in a mineral field soil. We conclude that cuttings of mountain fly honeysuckle can be propagated readily by overhead mist or subirrigation, that root system quality is improved substantially by increasing K-IBA concentration and using coarse perlite without peatmoss, and that mountain fly honeysuckle can be grown in typical horticultural landscapes.

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Bryan J. Peterson and William R. Graves

Numerous genera of plants are distributed in both eastern North America and in portions of California with the dry summers of Mediterranean climates. We compared effects of flooding and drought on relative growth rate (RGR), photosynthesis, and biomass of seedlings of two genera, Sambucus L. and Ptelea L., with congeners in both regions. Ptelea crenulata Greene and Sambucus nigra ssp. cerulea (Raf.) R. Bolli from the San Francisco Bay area and Ptelea trifoliata L. and Sambucus nigra ssp. canadensis (L.) R. Bolli from mesic, deciduous forests in eastern North America were studied. Potted seedlings were subjected to six treatments, three extents of root-zone inundation and three severities of drought (irrigation when soil moisture by volume decreased to 5%, 10%, and 20%). After 5 weeks, deleterious effects of inundation were more pronounced among plants from the West than among their eastern congeners. For example, RGR of western and eastern Sambucus with completely inundated root zones was reduced 116% and 25%, respectively. All western and eastern Ptelea died when root zones were completely inundated, but inundating the lower half of the root zone killed all western plants but only reduced RGR among eastern plants. Photosynthesis of Sambucus from the West was lowest with complete inundation and was similar across the remaining treatments. In contrast, photosynthesis of eastern Sambucus was lowest during severe drought but otherwise similar. Photosynthesis of Ptelea was sensitive to both drought and flooding, and moderate root-zone water content led to the highest rates for both western and eastern plants. For both genera, maximal photosynthesis per unit leaf area was greater among western than eastern plants, but eastern plants had greater total leaf area and biomass. Root-to-shoot ratios of western Sambucus were greater than ratios of plants from the East after all treatments, whereas western Ptelea had greater root-to-shoot ratios than eastern Ptelea only under severe drought. Although comparative sensitivity to drought of plants from California and eastern North America varied in these genera, Mediterranean Sambucus and Ptelea both showed greater sensitivity to root-zone inundation than did their eastern congeners.

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Bryan J. Peterson and William R. Graves

Horticulturists have not promoted use of Dirca palustris L. (eastern leatherwood) despite its suite of traits valued by gardeners and landscapers. Horticultural production of D. palustris may be hindered by slow shoot growth and sensitivity of plants to edaphic conditions. Because of discrepancies in reported tolerances of D. palustris to root-zone pH, we assessed whether pH of soils supporting indigenous populations in Florida, Maine, and North Dakota corresponded to responses of seedlings from the three provenances to root-zone pH of 4.5 to 7.3 in soilless media. Regression showed that root zones at pH 5.8 promoted maximum stem length of seedlings from Florida and North Dakota, whereas root zones at pH 4.5 led to maximum stem length of seedlings from Maine. Root-zone pH 5.6 and 5.5 fostered maximum root and shoot dry weight, respectively, for seedlings from Florida, whereas root zones at pH 4.5 promoted maximum root and shoot dry weights of seedlings from Maine and North Dakota. Averaged over provenance, relative leaf greenness decreased by 62%, and foliar nitrogen, iron, manganese, and zinc decreased by 49%, 70%, 95%, and 48%, respectively, as root-zone pH increased from 4.5 to 7.3. Foliar phosphorus decreased at both low and high pH. The pH of soils where seeds were collected did not predict optimal root-zone pH for stem length or biomass accrual in soilless media; genotypes from soils with a pH of 7.4 in North Dakota did not exhibit greater tolerance to high pH than genotypes from Maine or Florida, where pH of indigenous soil was 6.1 and 5.2, respectively. Averaged over pH treatments, seedlings from Florida showed the greatest stem length and formed the most shoot biomass, whereas seedlings from North Dakota had stouter stems, greater root biomass, and greater root-to-shoot ratios than did seedlings from Florida and Maine. Our results illustrate that acidic media facilitate horticultural production of D. palustris, that further evaluation of provenance differences could facilitate selection of genotypes for horticulture, and that tolerances of genotypes to root-zone pH do not strictly correspond to the pH of soils on which they were indigenous.

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Bryan J. Peterson, Stephanie E. Burnett and Olivia Sanchez

Although overhead mist revolutionized the propagation industry, it does suffer from potential drawbacks that include the application of large volumes of water, potentially unsanitary conditions, irregular misting coverage, and leaching of foliar nutrients. We explored the feasibility of submist as an alternative as it might avoid these problems by applying water exclusively from below the cutting, which is inserted basally into an enclosed rooting chamber. We propagated cuttings of korean lilac (Syringa pubescens ssp. patula) and inkberry (Ilex glabra) using both overhead mist and submist to compare effectiveness of the systems. Cuttings of korean lilac were wounded and dipped basally into 8000 mg·L−1 of the potassium salt of indole-3-butyric acid (K-IBA), and those in the overhead mist systems were inserted into coarse perlite. Cuttings of inkberry were wounded and treated with 5000 mg·L−1 K-IBA, and those in the overhead mist systems were inserted into 50:50 peat:perlite (by vol). Cuttings of korean lilac in the submist systems produced more than twice as many roots as cuttings in the overhead mist systems, with roots more than 2.6 times the length. Similarly, cuttings of inkberry in the submist systems produced more than three times the root counts and root lengths as cuttings in the overhead mist systems. For korean lilac, root dry weights averaged 58 mg for cuttings in the submist system, compared with only 18 mg among cuttings receiving overhead mist. Likewise, root dry weights averaged 70 and 7 mg for cuttings of inkberry propagated by submist and overhead mist, respectively. Rooted cuttings of korean lilac transplanted well into a soilless substrate, where they more than tripled their root biomass to 218 mg (vs. 59 mg for cuttings transplanted from overhead mist). We did not evaluate transplant performance of inkberry. Our results show that submist systems might merit consideration for the propagation of woody plants by leafy stem cuttings.

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Renae E. Moran, Bryan J. Peterson, Gennaro Fazio and John Cline

To identify genotypes of apple (Malus ×domestica) rootstock with vulnerability to low temperature, we measured the low temperature tolerance of xylem, phloem and cambium in 2-year-old shoot pieces from cultivars Budagovsky 9 (B.9), M.7 EMLA (M.7), M.9 EMLA (M.9), Geneva® 41 (G.41), Geneva 30 (G.30), Geneva 214 (G.214), Geneva 814 (G.814), and Geneva 935 (G.935), as well as six advanced selections in the Geneva (G.) series and three in the Vineland (V.) series. From Oct. 2013 to Apr. 2014, injury was measured as a 0–10 rating based on percentage of discolored cross-sectional xylem and phloem, and cambial length and circumference with brown discoloration, with 0 indicating no browning and 10 indicating browning in the entire tissue. From Oct. 2014 to Apr. 2015, injury was measured as xylem, phloem and cambium browning using a similar rating scale that accounted for both the percentage of browned tissues and the intensity of browning. Following exposure to −35 to −40 °C, many genotypes, including ‘M.7’, ‘M.9’, ‘G.935’, G.4011, G.4292, G.5087, and V.5, had only partial xylem injury in the fall, whereas others, ‘M.7’, ‘G.41’, ‘G.214’, and G.4011, had more extensive xylem browning at −30 °C and colder. ‘G.30’ had moderate to severe xylem browning at −15 to −19 °C. In late October of both years, G.4013 exhibited severe phloem browning at relatively high temperatures, but accrued additional hardiness by Nov. 2014, whereas genotypes ‘B.9’, ‘M.9’, ‘G.30’, and ‘G.41’ developed considerable phloem hardiness by late October with no additional increase in hardiness in November. Geneva and Vineland genotypes exhibited a low degree of susceptibility to injury at −35 to −40 °C in Jan. 2014 and Mar. 2015. Shoot hardiness in Apr. 2014 and 2015 was highly variable between the 2 years, with severe browning of xylem and cambium at −40 °C in every genotype sampled in Apr. 2014, but not in Apr. 2015. ‘M.9’ and G.3902 appeared to be the least vulnerable to injury in April, whereas ‘G.30’, ‘G.41’, ‘G.814’, G.4292, and G.5257 seem more likely to suffer injury in spring. ‘G.30’ had tender xylem in both fall and spring, G.4013 had the least hardy cambium and phloem in fall, and G.5257 the least hardy cambium in the spring. These genotypes are vulnerable to damaging temperatures during fall acclimation and spring deacclimation.

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Bryan J. Peterson, Olivia Sanchez, Stephanie E. Burnett and Darren J. Hayes

Overhead mist (OM) facilitates the propagation of stem cuttings by preventing transpirational water loss. However, drawbacks to OM include the application of large volumes of water, potentially unsanitary conditions, irregular misting coverage, and leaching of foliar nutrients. We explored three alternatives to OM that might avoid these problems by applying moisture below, rather than overhead. These included 1) a submist (SM) aeroponic system configured to provide intermittent mist only to the rooting zone, 2) a subirrigation (SI) system that provided water via capillary action through perlite from a reservoir maintained below the base of each cutting, and 3) a subfog (SF) aeroponic system that was configured to provide constant fog only to the rooting zone. To initiate each system, we wetted perlite or filled reservoirs using either water or quarter-strength Hoagland solution. Stem cuttings of ‘Wizard Mix’ coleus (Plectranthus scutellarioides) were propagated in the systems for 21 days. Cuttings in the SM system produced more than three times as many roots as cuttings in the OM system, with roots more than six times the length. Root dry weights averaged 28 mg for cuttings in the SM system, compared with only 3.5 mg among cuttings receiving OM. The SF and SI systems produced results broadly comparable to the OM. Fertilizer did not consistently improve rooting measures across the systems. Although we observed few fine roots on cuttings rooted using SM, they transplanted well into a soilless substrate and quickly produced new root growth. The SM system used less than 1/5 the water used by the SI system, and less than 1/50 the water used by the SF system. In comparison, a single OM nozzle operating for 10 seconds released about one-third of the total water lost through transpiration from each SM system over the entire experiment. Our results show that SM systems merit further evaluation for propagation of plants by stem cuttings.