Free Amino Acids as Affected by Light Intensity and the Relation of Responses to the Shade Tolerance of White Spruce and Shade Intolerance of Jack Pine

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Abstract
Saplings of white spruce and jack pine, grown under field conditions, were exposed for 2 years to natural light, and shade at 45, 25, and 13% of the daily natural light intensity. Continuous shade affected the growth and shape of saplings and influenced metabolism by reducing levels of most nitrogenous compounds in all plant parts (leaves, stems and buds, and roots). Compared with the spruce, significant responses by amino acids to the range of shade treatment in the very intolerant jack pine were greater and covered a much greater proportion of the amino acid N. As saplings were entering winter dormancy, thresholds for significant responses by individual amino acids and intercalation of their metabolic paths with those not responsive reflected a metabolic basis for shade tolerance that represented not only past growth rates of sapling parts, particularly roots, but also the extent to which nitrogenous compounds accumulated and were compartmentalized amongst parts. The main effects of shade on nitrogen metabolism implicated the balance of four- and five-carbon α-keto acids (photosynthate) which evoked significant changes in levels of certain amino acids and the amides, glutamine, and asparagine. At low light intensities, and particularly in stems and buds of both species, soluble nitrogen was diverted into the storage compound arginine.

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