• Pinus radiata (D. Don);
  • Pinaceae;
  • radiata pine;
  • photosynthesis;
  • water potential;
  • tracheids

Abstract. Experiments were carried out on Pinus radiata (D. Don) trees grown as cuttings from clonal parent stock. Some of these trees were about 0.4 m high while others were about 5 m high; all were grown in containers. The stem diameters at the tops and at the bottoms of the large trees, rates of photosynthesis, and needle water potentials were measured both when the trees were well watered and as they dehydrated after water was withheld. The water potentials of well-watered plants was highest in the small trees and lowest at the top of the large trees. When water was withheld, photosynthesis was in most cases unaffected by a small reduction in water potential, but the rate of photosynthesis fell as water potentials declined further. The stems of the large trees expanded at a constant rate when the trees were well watered and for part of the dehydration period, while subsequent stem shrinkage and the fall in photosynthesis both occurred at approximately the same time.

Water potentials increased little in the 24 h after rewatering, and significant rates of photosynthesis were not measured until 2 or 3 d later while renewed stem expansion was not measured until 2 d after rewatering.

Water deficits reduced the lumen diameter of newly matured stem tracheids, but increased the thickness of their walls. After 1 month of water potentials of about −2.4 MPa, tracheid lumen diameter and wall thickness were both much reduced, and this reduction continued in tracheids maturing shortly after rewatering.