Physiological impacts of Mg deficiency in Pinus radiata: growth and photosynthesis

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 deficie ncy and in their needle Mg concentrations. Plants were grown in flowing nut rient solutions with 2 mg l(-1) (control) and 0.8 mg l(-1) (low) Mg. Plant growth, needle Mg concentration, photosynthesis, chlorophyll fluorescence a nd carotenoid pigment content were measured. At low Mg, needle Mg concentra tion was about half that of control plants, height growth was reduced 15-25 %, and the needles showed strong visual characteristics of Mg deficiency. P hotosynthesis was also halved, and was associated with closure of the stoma ta 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 l ight and in the dark, and was strongly dependent on needle Mg concentration s below a threshold concentration of 0.02-0.025% (d. wt basis). The electro n transport rate (ETR) at saturating photon flux density in low-Mg-grown ne edles was reduced to about half that of their Mg controls, bur the photon e fficiency of ETR was unaffected by the Mg concentration the plants were gro wn in. Photosynthetic quenching was markedly reduced and non-photosynthetic quenching was increased following growth in low Mg. Growth under low Mg al so increased levels of zeaxanthin. Although family differences in growth an d photosynthetic physiology were present, few family x Mg interactions were significant. We conclude that Mg deficiency probably affects growth throug h severe reductions in photosynthesis.