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Shuyang Zhen and Stephanie E. Burnett

There is currently little information regarding the impact of soil moisture on morphology and physiology of English lavender (Lavandula angustifolia). Therefore, our goal was to determine the impact of substrate volumetric water content (θ = volume of water ÷ volume of substrate) on this plant. We grew ‘Munstead’ and ‘Hidcote’ lavender at one of four θ: 0.1, 0.2, 0.3, or 0.4 L·L−1 for 54 days using a capacitance sensor-automated irrigation system. Plant height, greatest width, inflorescence number, and total leaf number and area of both cultivars increased with increasing θ. Shoot fresh and dry weight of lavender irrigated at θ 0.3 L·L−1 was generally twice that of those grown at the lowest θ (0.1 L·L−1). Leaf-level instantaneous net photosynthetic rate (AN) and transpiration (E) of ‘Munstead’ decreased with decreasing θ. This reduction in AN was likely due to the concurrent reduction in stomatal conductance (g S) at lower θ. Similar reductions in AN, E, and g S of ‘Hidcote’ were observed at lower θ (0.2 and 0.3 L·L−1) 5 weeks after the initiation of the study, but not at the end of the study probably due to acclimation of ‘Hidcote’ to mild drought.

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Shuyang Zhen and Marc W. van Iersel

Photosynthetic responses to light are dependent on light intensity, vary among species, and can be affected by acclimation to different light environments (e.g., light intensity, spectrum, and photoperiod). Understanding how these factors affect photochemistry is important for improving supplemental lighting efficiency in controlled-environment agriculture. We used chlorophyll fluorescence to determine photochemical light response curves of three horticultural crops with contrasting light requirements [sweetpotato (Ipomea batatas), lettuce (Lactuca sativa), and pothos (Epipremnum aureum)]. We also quantified how these responses were affected by acclimation to three shading treatments-full sun, 44% shade, and 75% shade. The quantum yield of photosystem II (ΦPSII), a measure of photochemical efficiency, decreased exponentially with increasing photosynthetic photon flux (PPF) in all three species. By contrast, linear electron transport rate (ETR) increased asymptotically with increasing PPF. Within each shading level, the high-light-adapted species sweetpotato used high light more efficiently for electron transport than light-intermediate lettuce and shade-tolerant pothos. Within a species, plants acclimated to high light (full sun) tended to have higher ΦPSII and ETR than those acclimated to low light (44% or 75% shade). Nonphotochemical quenching (NPQ) (an indicator of the amount of absorbed light energy that is dissipated as heat) was upregulated with increasing PPF; faster upregulation was observed in pothos as well as in plants grown under 75% shade. Our results have implications for supplemental lighting: supplemental light is used more efficiently and results in a greater increase in ETR when provided at low ambient PPF. In addition, high-light-adapted crops and crops grown under relatively high ambient light can use supplemental light more efficiently than low-light-adapted crops or those grown under low ambient light.

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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.