Water relations of loblolly pine seedlings from diverse geographic origins.

Leaf conductance at three absolute humidity deficits (AHDs) (7, 14 and 21 g m(-3)), hydraulic conductance and components of tissue water potential were measured in one-year-old loblolly pine seedlings from six origins representing the geographic range of the species. Measurements were made on seedlings grown (a) with ample water (moist regime) and (b) with recurring severe drought (dry regime). However, all seedlings were well-watered prior to and during measurements. Seedlings grown in the moist regime had greater mean leaf conductances (0.30 versus 0.13 cm s(-1)) and greater responses to AHD than seedlings grown in the dry regime. They also exhibited greater hydraulic conductances (0.53 versus 0.35 microg cm(-2) s(-1) MPa(-1), less negative osmotic potentials (-1.45 versus -1.57 MPa) and higher relative water contents at turgor loss (0.72 versus 0.65). Seed source differences in water relations characteristics were detected only in seedlings grown in the moist regime. In these, trees from the three interior origins had greater mean leaf conductances than those from the three coastal sources (0.32 versus 0.28 cm s(-1)), but no differences in response to changing AHD were observed. Seedlings from North Carolina had lower osmotic potentials at turgor loss than those from Florida, Georgia or Texas. These differences in water relations characteristics are not clearly related to the observed greater survival ability of trees from interior origins compared with those from coastal origins.