EFFECT OF TRANSPIRATION RATE ON WATER TRANSPORT RESISTANCE IN APPLE TREE ROOTS

Water relationships and microclimate in apple trees in the field were measured and analysed in order to determine the effect of transpiration rate on transport resistance of water in the roots and in order to evaluate different ways to estimate transpiration rate in the field.
The young apple trees, cv.
Jonagold, were growing in 25 l containers in soil kept near field capacity.
The course of transpiration rate was measured by weighing; calculated from the microclimate, energy balance and stomatal diffusion conductance and measured by automated potometers containing cut branches.
The course of twig water potential (_twig) was measured as well.

Throughout a range of soil to twig water potential gradients of -2 to -11 bar the apparent root transport resistance was constant at approx. 1.9 x 10⁵ bar kg^-1 m² s.
Transpiration rates were controlled by the stomates, effectively reducing water potential decreases at high evaporative conditions.
No hysteresis was found in the course of _twig throughout increasing and decreasing transpiration rates.

Limitations are made clear of transpiration estimation of intact trees by calculation from the microclimate and by potometer measurements.

I International Symposium on Water Relations in Fruit Crops

P.A.M. Hopmans, M.P.K. Stallen, H.E. Schouwink

https://doi.org/10.17660/ActaHortic.1985.171.4