Shoot—root Plasticity and Episodic Growth in Red Pine Seedlings

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Abstract
Red pine seedlings of a half-sib seed source were grown in growth chambers under thermoperiodic regimes of 30/20 °C, 25/15 °C and 20/10 °C day/night temperatures. Classical growth analyses based on weekly harvests of leaves, stem and roots were employed to study the first 3 to 15 weeks of seedling development. Leaf and root growth were inversely related and episodic. Significant short term surges in growth of either organ were effective in reversing periodic imbalances that occurred, thus maintaining a long term dry weight equilibrium between above and below ground seedling parts. Adaptive plasticity in the leaf-root balance at different temperatures gave plants grown at 25/15 °C a larger proportion of leaves relative to roots and a greater size compared to seedlings grown under other regimes. Episodic fluctuations in leaf and root growth occurred simultaneously with depressions in net assimilation rate. Apparently, balanced growth is maintained at an assimilatory cost to the plant, periodic ‘corrections’ of shoot—root imbalances requiring carbohydrate conversion and energy expenditure.