Diurnal variation in the water balance of 2–3 year Pinus sylvestris in pots has been compared at four levels of soil moisture during 24 hours.
The physical component of the soil moisture suction was measured with tensiometers and gravimetric gypsum blocks and estimated as o.1, 0.5, 1–2 and >5 atmospheres for the four levels. Leaf water deficit was expressed as a percentage of the water at turgidity, transpiration as the loss in fresh weight of a leaf sample during 1–11 minutes after cutting, and stomatal opening was measured by infiltration with alcoholic gentian violet.
Increase in soil moisture tension caused a progressive increase in leaf water deficit, and a decrease in transpiration and stomatal opening, the latter being reduced both in number of open stomata and in the time for which they remained open during the day.
The relative value of these measurements as indices of plant water balance and of soil moisture conditions is discussed together with a brief consideration of the relation between transpiration rate and soil moisture.
It is concluded that leaf water deficit, transpiration and stomatal opening all reflect increase in soil moisture suction but that, during the day, they are affected also by other factors which obscure the direct effect of soil moisture. However, in the case of leaf water deficit an equilibrium is attained between plant and soil before sunrise. If measured in the morning, before the onset of stomatal transpiration, leaf water deficit appears to be closely related to the soil moisture conditions experienced by the plant. This measurement is suggested as the best plant index of the soil moisture suction in the vicinity of the roots.
The persistence of appreciable deficits in wet, drained soil indicates that factors other than the physical soil moisture tension oppose water uptake.