Influence of phosphorus deficiency on the growth response of four families of Pinus radiata seedlings to CO2-enriched atmospheres

Four half-sib families of Pinus radiata (D. Don) (20080, 20010, 20022 and 20062) were grown in pots under conditions where P was either deficient or adequate and water was supplied daily. Seedlings of all four families were exposed to 340 (ambient levels) or 660 μl CO₂l^?1 in controlled-environment chambers for 16 weeks. Families 20010 and 20062 were grown at the same CO₂ concentrations for 22 weeks. A subset of plants grown in the chambers were subject to drought and their water potentials and relative water contents measured. In the glasshouse experiment, photosynthesis, conductance and elongation of needles were measured. The fresh and dry-weights of the tops of the plants were measured in both experiments.

Elevated CO₂ concentrations increased shoot growth in all families, apparently through increases in photosynthesis. At 340 μl CO₂l^?1, family 20062 accumulated the most dry-weight and 20010 the least. However, family 20010 responded so strongly to 660 μl CO₂l^?1 that it produced as much dry-matter as 20062. Phosphorus deficiency generally diminished the response to high CO₂. Again, 20010 was outstanding in that its CO₂ response was least reduced by P deficiency. Carbon-dioxide enrichment did not increase the drought-tolerance of any of the four families, either by reducing conductance or by facilitating osmotic adjustment.

These results provide strong indications that the higher levels of atmospheric CO₂ expected in the next century will increase the growth of P. radiata where P availability is adequate, but may reduce growth in areas where rainfall is low. Finally, the capacity to respond to CO₂ enrichment appears to be widely distributed among superior P. radiata genotypes selected by the Forestry Commission of New South Wales. However the magnitude of response varies considerably, particularly where P is deficient.