TYREE, M. T., J. DAINTY, and M. BENIS. 1973. The water relations of hemlock (Tsugn carzaderlsis). I. Some equilibrium water relations as measured by the pressure-bomb technique. Can. J. Bot. 51 : 1171-1480. Theoretical and experimental as ects of the equilibrium water relations of excised hemlock (Tsrtga canadensis) shoots are examined. T f e equilibrium water relations of hemlock were determined by using the pressure-bomb technique on shoots 15 to 40 g in fresh weight. At or near full turgor the osmotic pressure of the cells averages 16.5 + 0.4 bars. The fraction of the total water content of hemlock shoots residing in the living cells is 0.77 + 0.04. When hemlock shoots are near full turgor the balancing pressure will change linearly with the volume expressed provided the volume changes represent less than $O/;, of the total shoot water content. TYREE, M. T., J. DAINTY et M. BENIS. 1973. The water relations of hemlock (Tsrrga carzadt.,z.sis). I. Some equilibrium water relations as measured by the pressure-bomb technique. Can. J. Bot. 51 : 1471-1480. Les auteurs ont CtudiC des aspects thCoriques et expkrimentaux des relations d'equilibre de l'eau dans des branches isolCes de pruche (Tsnga catzadensis). Ces relations ont CtC dCterminCes en appliquant la technique de la bombe ? i pression sur des branches dont le poids frais variait entre 15 et 40 g. Dans 1'Ctat de turgesceilce compkte ou presque compkte, la pression osmotique moyenne des cellules est de 16.5 + 0.4 bars. La fraction de contenu total en eau des rameaux situCe dans les cellules vivantes est de 0.77 + 0.04. Quand les rameaux sont presque compktement turgescents, la pression d'Cquilibre change lineairement avec le volume exprim6 pourvu que les changements de volume representent moins de ;% du contenu total en eau du rameau.[Traduit par le journal]

No abstract

The tempo of rehydration of sunflower (Helianlhus annuus L.) leaves was measured after dehydration in a pressure bomb down to water potentials of -0.5 to -1.6 MPa, When rehydrated f"rom small water deficits ( -0.5 to -0.8 MPa) the plot of log rehydration rate versus time is concave. When rehydration starts from large deficits (-1,2 to -1.6 MPa) the semilog plot has a characteristic shoulder, i.e. a rehydration phase of long half-time is followed by a phase of short half-time. The experimental curves were fitted with parallel and series models of rehydration. In the parallel ljiodel two compartments are connected by resistances in parallel with the water source and rehydrate independently. In the series model one compartment is connected with the water source via a resistance and the second compartment is connected in series with the first by another resistance so that water entering the seeond compartment must pass through the first. Amongst nineteen experiments, ten could be fitted very closely by both the parallel and series models and nine could not be fitted by either model.

The pressure-bomb technique has been used to measure the temperature dependence of water exchange between the apoplast and symplast of hemlock (Tsuga canadensis) shoots. By applying the Arrhenius rate theory, the activation energy for water exchange in the whole shoot equals 25.9 + 0.6 × 103 J per mole. A theory is proposed on the basis of the Arrhenius rate equations which allows us to predict the relative magnitudes of the membrane resistance, Rm, and the extracellular resistance, Rx, to water permeation. On the basis of our calculations, we believe Rm is at least one-quarter of Rx and may even exceed Rx, but more work needs to be done to be sure.