Diurnal changes in water conduction in loblolly pine (Pinus taeda) and Virginia pine (P-virginiana) during soil dehydration

We studied diurnal changes in water conduction during soil dehydration in 3 7-month-old seedlings of one Virginia pine (Pinus virginiana Mill.) and two loblolly pine (P. taeda L.) sources, one from North Carolina (NC) and the other from the "Lost Pines" areas of Texas (TX), in an environmentally cont rolled growth chamber. For seedlings of similar biomass, the TX source had higher values of transpiration, needle conductance, and plant hydraulic con ductivity under well-watered conditions than the NC source. Under dry soil conditions, the TX source had lower values of water conduction than the NC source. The Virginia pine source responded similarly to the TX source under both well-watered and dry soil conditions. For all three pine sources, gra dients between soil and needle water potentials were greatest when the seed lings were moderately stressed. The TX and Virginia pine sources had higher gradients and lower daytime needle water potentials under moderate stress conditions than the NC source. Predawn needle water potentials did not diff er among the pine sources. We conclude that the TX and Virginia pine source s have decreased daytime needle water potentials and increased water potent ial gradients during the daytime under moderate stress conditions, but with no disruption of recovery at predawn. The greater rates of transpiration a nd water conduction by the TX source compared with the NC source under well -watered conditions suggest a means by which growth can be maximized prior to the onset of drought, thereby enhancing survival of loblolly pines in dr ought-prone environments.