Water potential and gas exchange did not reflect performance of Pinus radiata D. Don in an agroforestry system under conditions of soil-water deficit in a temperate environment.

In order to understand how radiata pines respond to declining supply of soil-water in agroforestry systems, we monitored water potential in xylem (ψ x ), osmotic potential (ψ) and relative water content ( q) for fascicles at pre-dawn and at mid-day for 3-year-old trees that were raised from either seedlings (Seedling) or from tissue culture (TC3 and TC4), and grown either alone (Control) or over lucerne ( Medicago sativa) pasture (Lucerne). Water relations at dawn were mostly similar for all the pines, except late in the season when ψ was lower, bulk turgor pressure ( P), deduced as the difference between ψ x and ψ, was higher, for TC3 than for the other two pines. At mid-day, Seedling often had higher ψ x and ψ, but because of its poor osmotic adjustment ( OA) had lower P, than either TC3 or TC4. The cell walls were more elastic in Seedling with modulus of elasticity ( e) of 6.5 MPa compared with 8.1 MPa for both TC3 and TC4, while loss of turgor was estimated to occur at ψ x of −1.45 MPa for Seedling, −1.38 MPa for TC3 and −1.35 MPa for TC4. All trees irrespective of their origin had higher ψ x , P, CO2 assimilation ( A), and stomatal conductance ( g s ), but lower ψ, in Control than in Lucerne in which the soil profile was consistently drier. The trends in ψ x , ψ, q and A did not reflect the known differences in dry weight of trees, P was in the order TC3 > TC4 > Seedling, consistent with previously reported tree weights. Both TC3 and TC4 had higher P, due to their larger OA, than Seedling, although the latter had higher A. Thus ψ x and A that are routinely measured may not always adequately explain differences in growth amongst pines; it is advisable that ψ be determined to allow deductions of P be made when using water relations to analyse plant growth. [ABSTRACT FROM AUTHOR]