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Fang Geng, Renae Moran, Michael Day, William Halteman, and Donglin Zhang

The influence of red and blue light wavelengths was tested to improve the initial in vitro multiplication of apple (Malus × domestica) rootstock cultivars Budagovsky 9 (B.9), Geneva 30 (G.30), and Geneva 41 (G.41). Single-node segments were established in semisolid Murashige and Skoog media and then transferred to proliferation media and cultured 40 days under white, red, or blue light irradiance. In a second experiment, G.30 was cultured under red, blue, or white light with and without gibberellic acid (GA3). The three rootstocks responded similarly under white light in terms of shoot number, length of the longest shoot, and the number of elongated shoots. Red light increased the number of shoots, length of the longest shoot, and the number of elongated shoots of B.9 and G.30 when compared with white or blue light. Red light increased the number of elongated B.9 and G.30 shoots to five per explant compared with one per explant under white light. In contrast, shoot growth of G.41 showed no difference under the three light quality treatments, and the number of elongated shoots per explant was less than one. When compared with an absence of GA3, a concentration of GA3 at 0.5 mg·L−1 promoted in vitro shoot growth of G.30 under red and blue light.

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Fang Geng, Renae Moran, Michael Day, William Halteman, and Donglin Zhang

These studies were conducted to determine the most effective methods for increasing shoot elongation during the initial proliferation stage of micropropagation in two dwarfing apple, Malus ×domestica (Borkh.), rootstock cultivars. Several experiments were conducted to compare explant collection date, exposure to chilling (5 ± 1 °C) temperatures, and varying concentrations of plant growth regulators in Murashige and Skoog (MS) media. Microshoot growth of ‘Geneva 41’ (‘G.41’) was very low and unaffected by chilling duration from 0 to 8 weeks or by gibberellic acid (GA3) concentration from 0 to 1.0 mg·L−1, but was improved by an additional subculture which increased shoot length from 1 to 15 mm. In ‘Geneva 30’ (‘G.30’), shoot elongation was most affected by date, chilling explants, and by optimizing cytokinin concentration and type. Explant collection date in April increased shoot growth compared with August or November. Microshoot growth of ‘G.30’ was increased by chilling nodal explants for 4 and 6 weeks when explants were collected in August and November, but not in April. Eight weeks chilling was detrimental for explants collected in April, and generally had little or no effect with August and November. The cytokinin 6-benzylaminopurine (BA) increased shoot number to a greater extent than thidiazuron (TDZ) or zeatin (ZT), and was also more effective for increasing shoot elongation with concentrations of 0 to 2.0 mg·L−1. In ‘G.30’, GA3 increased shoot growth at the optimum concentration of BA, but not with lower concentrations. ‘G.30’ microshoots were fewer and shorter with 24-epi-brassinolide (EBR) at concentrations of 0.1 and 1.0 mg·L−1. Chemical names: N-phenyl-N’-(1,2,3-thiadiazol-5-yl)urea (TDZ), 6-(4-hydroxy-3-methylbut-2-enylamino)purine (ZT).

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Shuyang Zhen, Stephanie E. Burnett, Michael E. Day, and Marc W. van Iersel

Two experiments were conducted to determine how different substrate volumetric water contents (θ equals volume of water per volume of substrate) affected morphology and physiology of three popular perennials using a capacitance sensor-automated irrigation system. In the first study, rosemary (Rosmarinus officinalis) was grown at one of eight θ set points ranging from 0.05 to 0.40 L·L−1. In the second study, Canadian columbine (Aquilegia canadensis ‘Pink Lanterns’) and cheddar pink (Dianthus gratianopolitanus ‘Bath’s Pink’) were grown at one of nine θ set points ranging from 0.05 to 0.45 L·L−1. Total leaf number and area as well as shoot fresh and dry weight of rosemary plants grown at θ of 0.20 L·L−1 or greater were approximately twice that of those grown at lower θ. Canadian columbine height increased as θ increased. Leaf area of cheddar pink grown at θ of 0.35 L·L−1 or higher was twice that of plants grown at the lowest θ. Shoot dry weight of Canadian columbine was not significantly affected by θ. Shoot dry weight of cheddar pink responded quadratically to increasing θ and peaked at θ of 0.35 L·L−1. θ also significantly influenced photosynthetic activities; net photosynthetic rate (AN) and stomatal conductance (g s) of Canadian columbine increased with increasing θ. AN of cheddar pink also increased as θ increased. Greater water volumes were applied to maintain higher θ set points. Irrigation water use efficiency (IWUE = shoot dry weight ÷ total amount of water applied per plant) of Canadian columbine and cheddar pink was not influenced by θ. Growth of all three plants was reduced when grown at lower θ; in the case of cheddar pink and Canadian columbine, this was attributable at least in part to reduced AN.

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Katherine F. Garland, Stephanie E. Burnett, Michael E. Day, and Marc W. van Iersel

Two investigations were conducted to determine the morphological and physiological impacts of varying light and substrate water levels on Heuchera americana ‘Dale's Strain’ (american alumroot). Both investigations used a capacitance sensor automated irrigation system to maintain constant substrate volumetric water contents (θ = volume of water/volume of substrate). In the first study, the substrate was maintained at one of eight θ ranging from 0.15 to 0.50 L·L−1. Leaf area of plants grown at the highest θ was more than twice that of plants grown at the lowest θ. Shoot dry weight also responded positively to θ increasing from 0.15 to 0.35 L·L−1, but plants did not have greater dry weights when maintained at θ higher than 0.35 L·L−1. The second experiment assessed american alumroot's performance under four daily light integrals (DLIs) (7.5, 10.8, 14.9, and 21.8 mol·m−2·d−1) with θ maintained at 0.35 L·L−1. Increasing DLI from 7.5 to 21.8 mol·m−2·d−1 caused shoot dry weight, leaf area, maximum width, and leaf count to change quadratically. Dry weight and leaf area reached their maximum at 10.8 mol·m−2·d−1, whereas leaf count was greatest at 14.9 mol·m−2·d−1. Increasing DLI to 21.8 mol·m−2·d−1 negatively impacted leaf area and leaf count but did not lower shoot dry weight. Leaf area ratio and petiole length of the uppermost fully expanded leaf decreased with increasing DLI. Measures of leaf-level net photosynthesis, light response curves, and CO2 response curves indicated no physiological differences among plants grown under different water or light levels. In both studies, long-term, whole crop measures of water use efficiency based on shoot dry weight and water applied (WUEc) did not reflect the same water use trends as instantaneous, leaf-level measures of WUE based on leaf gas exchange (WUEl). WUEc decreased with increasing θ and DLI, whereas WUEl was not influenced by θ and increased with increasing DLI. WUEl is often used to provide insight as to how various abiotic and biotic factors influence how efficiently water is used to produce biomass. However, these findings demonstrate that there are limitations associated with making such extrapolations.