The characteristics of seasonal and ontogenetic changes in the tissue – water relations of Acer, Populus, Tsuga, and Picea

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Abstract
The Scholander-Hammel pressure bomb was used to measure ontogenetic and seasonal changes in .pi.o (the osmotic pressure of the symplasm at zero water potential), .pi.p (the osmotic pressure of the symplasm at incipient plasmolysis), .epsilon.max (the bulk elastic modulus near maximum turgor) and a number of other water relations parameters in single leaves of A. saccharum and several species of Populus [served clones of P. alba, P. nigra, P. deltoides and P. balsamifera] and in shoots of T. canadensis and Picea abies. In newly emerged leaves of Acer, Populus and Picea .pi.o, .pi.p and .epsilon.max are all small but rise rapidly with leaf development. These parameters stabilize at a maximum value or slowly increase with progress in season. In Acer, .epsilon.max declines shortly before senescence. In developing leaves, the water content reaches a maximum before the soluble solutes. This accounts for the low values of .pi.o and .pi.p. In Tsuga .pi.o cycles through an annual maximum in winter and a minimum in summer. These changes may correlate with frost hardiness.
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