This paper reports the effects of nutrient magnesium (Mg) concentrations on the growth and photosynthetic
physiology of clonal Pinus radiata from four female parents (families) known to differ in their tolerance to Mg
deficiency and in their needle Mg concentrations. Plants were grown in flowing nutrient solutions with 2 mg l−1
(control) and 0.8 mg l−1 (low) Mg. Plant growth, needle Mg concentration, photosynthesis, chlorophyll
fluorescence and carotenoid pigment content were measured. At low Mg, needle Mg concentration was about half
that of control plants, height growth was reduced 15–25%, and the needles showed strong visual characteristics
of Mg deficiency. Photosynthesis was also halved, and was associated with closure of the stomata under low Mg
and with reductions in the residual conductance. In needles from plants grown at low Mg, photochemical yield
was reduced both in the light and in the dark, and was strongly dependent on needle Mg concentrations below a
threshold concentration of 0.02–0.025% (d. wt basis). The electron transport rate (ETR) at saturating photon flux
density in low-Mg-grown needles was reduced to about half that of their Mg controls, but the photon efficiency
of ETR was unaffected by the Mg concentration the plants were grown in. Photosynthetic quenching was
markedly reduced and non-photosynthetic quenching was increased following growth in low Mg. Growth under
low Mg also increased levels of zeaxanthin. Although family differences in growth and photosynthetic physiology
were present, few family × Mg interactions were significant. We conclude that Mg deficiency probably affects
growth through severe reductions in photosynthesis.