Interactive effects of phosphorus supply and light intensity on glucosinolates in pakchoi (Brassica campestris L. ssp. chinensis var. communis)

The influences of phosphorus (P) supply levels (1.00 and 0.01 mM) and different light intensities (normal and 60% shading) on plant growth and glucosinolate (GS) concentrations were investigated in pakchoi (Brassica campestris L. ssp. chinesis (L.) Makino var. communis Tsen et Lee) plants grown in nutrient solution. Low light intensity significantly decreased the shoot and root growth, particularly under P sufficiency. P deficiency significantly decreased the shoot growth under normal light intensity, but did not significantly affect the root growth irrespective of the light intensity. The root/shoot ratio was increased by P deficiency and high light intensity. P deficiency largely increased the concentration of total GSs under normal light intensity by 164%, but this effect was not significant under low light intensity. However, the effects of P supply level and light intensity on seven individual GSs and three classes of GSs (aliphatic, aromatic and indolyl GSs) were quite different. The concentrations of gluconapin, glucobrassicanapin, glucobrassicin, neoglucobrassicin, 4-methoxyglucobrassicin and gluconasturtiin were increased by P deficiency, especially under normal light intensity resulting in increases of 122%, 497%, 352%, 115% and 225%, respectively. Low light intensity decreased the concentrations of gluconapin, glucobrassicanapin, glucobrassicin and neoglucobrassicin under P deficiency condition, and increased the concentrations of glucobrassicin and 4-methoxyglucobrassicin and neoglucobrassicin under P sufficiency condition. The ratios of aliphatic, aromatic and indolyl GSs to total GSs were not significantly affected by P supply level under low light intensity. However, under normal light intensity, P deficiency increased the ratio of aliphatic GSs to total GSs from 58% to 67%, at the expense of aromatic ones, and did not affect the ratio of indolyl GSs to total GSs. Altered concentrations of free amino acids (GS precursors) could not be related to changes of individual GSs. The concentration of sulfur in pakchoi shoots was not significantly affected by different P levels and light intensities. The results indicate a role of P and light, and their interaction, on the concentrations and profiles of GSs in the shoots of pakchoi plants. However, the physiological and molecular mechanisms underlying this response await further investigation.