Effects of high concentration of CO2 and/or Cd stress on endogenous hormones and organic acids contents in rice (Oryza sativaL.) seedling roots

Base on atmosphere CO₂ concentration increases and Cd pollution stress, the response mechanism of rice to environmental change was studied. To explore the changes of endogenous hormones and organic acids in rice roots under high CO₂ and Cd stress, which provide the theoretical basis for future rice production under the double environmental impacts of atmospheric CO₂ changes and Cd stress. Rice seedlings (Oryza sativa L., “Beijing No. 2”) were treated from two-leaf stage, with two CO₂ concentrations (400 ± 20 μmol/mol and 800 ± 20 μmol/mol, controlled by an artificial climate chamber) and/or three CdCl₂ concentrations (0, 50, 150 μmol/L) for 7 days. The growth parameters of rice seedlings were measured. The root endogenous hormones and organic acids contents were determined by high-performance liquid chromatography (HPLC). Results:(1) Increased CO₂ concentration promoted the accumulation of aboveground dry weight by 45.6%. The IAA (Indole-3-acetic acid), GA₃ (Gibberellins A3) and ZT (Zeatin) contents increased by 15.7%, 1.6% and 26.7%. Citric and fumaric acid contents in roots increased11.7 and 19.8 fold, malic acid secreted from roots decreased by 23.4%. (2) The growth was inhibited under Cd stress alone, including the fresh weight and dry weight of the aboveground part decreased by 48.5% and 15.4%, respectively. The IAA, GA₃, ZT, ABA (Abscisic acid), SA (Salicylic acid) and JA (Jasmonic acid) contents increased in roots. The large accumulation of malic acid, lactic acid and citric acid under Cd stress. Tartaric acid content increased 87.5% in roots. (3) Compared with Cd stress, under high CO₂ and Cd stress, IAA, ZT and GA₃ contents and endogenous hormones ratios significantly increased, and root length and biomass of rice increased (29.9%, 34.1% under high CO₂ concentration and heavy Cd stress). The total organic acids secretions decreased. In conclusion, Cd stress inhibited the rice growth, the more produced (such as SA, JA and ABA) and the secreted (as Cd chelation agents) by roots were involved in the defense mechanisms and produced a detoxification mechanism; High CO₂ promoted the root growth and resistance to Cd stress by changing hormones and organic acids contents.