Physiological Response Mechanism Of Ginger To Lead, Cadmium Toxicity In Soil

To determine the effects of heavy metals such as Pb, Cd on growth and yield of ginger, and explore the time and dose effects of Pb or Cd toxicity in ginger, the effects of the different Pb or Cd levels on the growth, active oxygen metabolism, photosynthetic carbon assimilation and nitrogen metabolism of a ginger（Zingiber officinale Roscoe）cultivar ?Laiwu ginger? were studied with applying Pb（NO₃）₂ or Cd Cl₂.2.5H₂O solution by using the method of sewage irrigation, meanwhile, the physiological mechanism of exogenous H₂S alleviating Pb or Cd stress were also investigated. The aims of these studies are to provide a theoretical basis for the production detection and regulation in ginger under Pb or Cd stress, provide technical support for the efficient and safe production of ginger. The main results are as follows:1 Ginger had a certain adaptability to mild Pb stress. At the earlier stage, 150 mg.kg-1Pb stress could stimulate the ginger growth, which was correlated with the acc umulations of shoot and rhizome biomass. But at the later stage, especially the rhizome swelling stage, Pb exposure resulted in growth inhibition and biomass reduction in a dose-dependent manner, which was correlated with the significant decline of root activity. Furthermore, the toxicity effects of Pb stress in ginger had a significant difference in organs that was to say that Pb stress had the most serious toxicity to the root and significantly inhibited the stem growth which caused the stem to be thin and weak.2 Cd exposure significantly resulted in ginger growth and stem diameter inhibition, especially accompanied by the declines of the branches and leaves at the later stress. In addition, above-ground and under-ground biomasses were also reduced, espec ially at seedling stage that showed a significant dose effect of Cd stress in ginger roots and root activity was significantly inhibited.3 With the increase of Pb or Cd stress level and exposure duration, O₂·?productivity rate and H2O₂ content increased significantly and then decreased under excessive Pb stress, while MDA content and electrolyte leakage rate increased continuously. After 40 days of stress which was during the seedling stage, with the increase of Pb or Cd stress level, the activities of SOD, POD, CAT, GR, APX, MDHAR and DHAR were enhanced markedly, and the contents of As A, GSH, acid soluble SH and phytochelatins were increased remarkably and the ratios of As A/DHA, GSH/GSSG were also increased s ignificantly. Moreover, with ginger growth going on and the stress time increasing significantly, the antioxidant enzymes activities and the antioxidant substance contents were higher mostly under 150 mg·kg-1 mild Pb stress and inhibited under excessive Pb stress, which indicated that the As A-GSH cycle suffered serious damage. At the same time,With the increase of Cd stress level and exposure duration,related protective enzymes activities and antioxidant substances were decreased significantly. The results suggested that ginger has the adaptability to 40 days or mild Cd stress. But for 120 days and under excessive Cd stress, the antioxidant system in ginger leaves was damaged seriously and the balance of active oxygen metabolism was destroyed.4 While the low Pb concentration treatment stimulated the increase of leaf pigment content in the short term, high Pb or Cd concentration stress resulted in a significant decrease of the pigment content in the whole growth period. In addition, the utilization of the light energy in ginger leaves was reduced. Pb or Cd stress significantly inhibited the ginger leaf photosynthesis by reducing Pn, Gs, Fv/Fm and ΦPSⅡ, led to the increase of soluble sugar content. Moreover, at the seedling stage, the reason of the photosynthet ic rate decline caused by Pb or Cd pollution was mainly stomatal limitation, but, at the tillering and rhizome swelling stage, the reason were mainly due to non stomatal limitation.5 Nitrogen metabolism related enzyme activities in ginger leaves could be induced by short-term Pb or Cd stress, forming a self-protective properties, but with the stress sustaining, the activities of NR, GS and GOGAT were inhibited significiantly, and ammonium balance was broken, moreover, under late stress, excess endogenous a mmonium was cleared mainly through GDH way. Pb or Cd stress could directly inhibit the nitrate uptake and assimilation in ginger roots, cause a significant decrease of nitrate nitrogen in ginger leaves and roots, result in accumulation of ammonium nitrogen, induce the degradation of soluble protein and stimulate free amino acids especially proline content increasing significantly.6 Exogenous H₂S enhanced the stimulatory effect of Pb or Cd stress on the activities of antioxidant enzymes at the beginning of the stress, reduced the decline range of these enzyme activities at the later stage, inhibited the increase of AAO activity, thus improving the ability of scavenging O₂-. and H2O₂, maintaining a low ROS level, reducing the increase of MDA content and inducing significantly H₂S content.7 Under Pb or Cd stress, spraying Na HS improved the pigment content effectively, relieved the decline rate of Pn, Fv/Fm, ΦPSⅡ, delayed the Ci rising velocity and Ls decreased rate that meaned the non stomatal limitation could be delayed, alleviated effectively the increase of NPQ, protected the physiological functions of PSⅡ, induced the leaf stomatal opening in the light, maintained the transpiration rate. On the ten days of treatment, Pn of the control, Pb T1, Pb T3, Cd T1, Cd T3 at 13:00 decreased respectively by 27.25%, 34.58%, 58.48%, 27.38%, 36.75% than that at 11:00, while Pb T2, Cd T2 only decreased respectively by 15.24% and 21.96%, which showed that application of H₂S could alleviate effectively the “midday depression of photosynthesis ”in ginger leaves, help to improve Pn, decrease the effect of strong light on Pn in ginger leaves under Pb or Cd stress.