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 (ψx), osmotic potential (π) 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 π was lower, bulk turgor pressure (P), deduced as the difference between ψx and π, was higher, for TC3 than for the other two pines. At mid-day, Seedling often had higher ψx and π, 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 ψx 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 ψx, P, CO₂ assimilation (A), and stomatal conductance (gs), but lower π, in Control than in Lucerne in which the soil profile was consistently drier. The trends in ψx, π, 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 ψx and A that are routinely measured may not always adequately explain differences in growth amongst pines; it is advisable that π be determined to allow deductions of P be made when using water relations to analyse plant growth.