Acidic mist and nitrogen fertilization effects on growth, nitrate reductase activity, gas exchange, and frost hardiness of red spruce seedlings

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Abstract
Summary: Acidic deposition may contribute to the decline of red spruce (Picea rubensSarg.) in eastern North America; response to acidic mist may depend on mist content and plant nutrient status. We grew red spruce seedlings with high or low nitrogen fertilization to roots (other elements in constant proportion to N), exposed only shoots to pH 3.0, high sulphate (10:1, S:N) or high nitrate (1:10, S:N) mist for 5 wk, then moved seedlings outdoors for cold acclimation. Effects of mist content were found until the third week of cold acclimation. Foliar necrosis was higher after exposure to sulphate than to nitrate mist and greater in older than new needles. Sulphate‐misted seedlings had lower new‐shoot biomass, shoot water content, and nitrate reductase activity, and higher foliar sulphur levels than nitrate‐misted seedlings. They also had greater needle frost hardiness (estimated by electrolyte leakage) at the start of cold acclimation. Effects of fertilization were found throughout the experiment. High‐N fertilization increased biomass, tissue N level, and shoot water content, and decreased foliar necrosis from mist. During cold acclimation, high‐N seedlings had lower transpiration and photosynthetic rates and higher water use efficiency than low‐N seedlings. High‐N fertilization increased needle frost hardiness after 5 wk of acclimation and decreased visible needle injury after an outdoor frost. However, high‐N seedlings had fewer needle primordia in terminal buds and more bud frost‐damage than low‐N seedlings. Fertilization, mist content, and their interaction strongly affected seedlings, showing that nutrient status can influence the response of red spruce to acidic deposition and frost.