| Acid rain(AR) is a serious global environmental issue, resulting in incalculable economic loss on agriculture and forestry. Therefore, finding ways for enhancing adaptation of plants to AR are of great significance to alleviate the deleterious effects caused by AR, which has became a research focus. Abscisic acid(ABA) as a stress hormone, has been widely concerned enhancing the tolerance in plants to environmental stresses. So we studied effects of exogenous abscisic acid(ABA1/10 μM, ABA2/100 μM) on the growth, endogenous hormones and nutrient elements in rice seedlings, as well as the regulating mechanism on nutrient uptake in root exposed to acid rain(AR1/ pH 4.5, AR2/ pH 3.5) by advanced technologies such as leaf area analyzer, WinRHIZO root analyzer, high performance liquid chromatograph, Real-time PCR and atomic absorption spectrometer. The major results are shown as follows:(1) Effect of exogenous ABA on the growth in rice seedlings exposed to acid rain was studied. The results show that AR at pH 4.5 or 3.5 decreased growth indicators compared with those of the control. AR1 + ABA1 had no obvious effect on growth indicators, whereas AR1 + ABA2 decreased all of growth indicators. Moreover, growth indicators in rice seedlings treated with AR(pH 4.5 or 3.5) and 10 μM ABA was higher than that treated with the single AR, indicating that exogenous 10 μM ABA can alleviate the inhibition caused by acid rain on plant growth. In comparison with that of the seedlings exposed to pH 3.5 AR and 10 μM ABA, the alleviating effect of 10 μM ABA on the growth in rice seedlings exposed to pH 4.5 AR was better. However, growth indicators in rice seedlings treated with AR(pH 4.5 or 3.5 AR) and 100 μM ABA were lower than those of the control and those treated with the single AR. Hence, the application of 100 μM ABA could aggravate the inhibition induced by acid rain on plant growth. After a 5-day recovery, growth indicators in rice seedlings treated with pH 4.5 AR and 10 μM ABA were close to the control. And growth indicators treated with pH 3.5 AR and 10 μM ABA were still lower than those of the control, but higher than those measured during the exposure period. However, growth indicators of rice seedlings treated with AR and 100 μM ABA were lower than those treated with the single AR, even lower than those measured during the exposure period. Therefore, the application of 10μM ABA could promote the recovery of rice seedlings following the withdrawal of acid rain stress, while 100 μM ABA could hinder this process.(2) Effect of exogenous ABA on concentrations of endogenous hormones and nutrient elements in rice seedlings exposed to acid rain was studied. Compared with the control, AR at pH 4.5 or 3.5 increased the ABA concentration in leaves whereas decreased it in roots. Moreover, concentrations of IAA, GA and ZT in leaves and roots treated with pH 4.5 or 3.5 AR were decreased. The ABA concentration in leaves treated with AR and 10 μM ABA was higher than that of the control, but lower than that treated with the single AR. Contrarily, the ABA concentration in roots treated with AR and 10 μM ABA was lower than that of the control, but higher than that treated with the single AR. Moreover, concentrations of IAA, GA and ZT in rice treated with AR and 10 μM ABA were still lower than those of the control, but higher than those treated with the single AR. Concentrations of IAA, GA and ZT in rice treated with pH 4.5 AR and 10 μM ABA were even higher than those of the control. These results indicate that exogenous 10 μM ABA could promote the transport of ABA in roots to leaves, alleviate the decrease in IAA, GA and ZT concentrations, and then benefit to alleviate the inhibition caused by acid rain on plant growth. AR at pH 4.5 or 3.5 increased the NH4+-N concentration in leaves and concentrations of Ca and NH4+-N in roots whereas it decreased concentrations of K, Mg, NO3--N and P. pH 4.5 AR and 10 μM ABA decreased concentrations of K and Mg whereas they had no obvious effect on Ca, N and P. pH 3.5 AR and 10 μM ABA increased the NH4+-N concentration in leaves and concentrations of Ca and NH4+-N in roots, decreased concentrations of the other nutrients. These results indicate that exogenous 10 μM ABA promote the absorption of Ca and N in rice roots to transport to leaves, and alleviate the inhibition caused by acid rain on concentrations of K, Mg, N and P. However, the ABA concentration in seedlings treated with AR and 100 μM ABA was higher than that of the control, even higher than those treated with the single AR. In addition, concentrations of IAA, GA and ZT treated with AR and 100 μM ABA were still lower than those of the control, even lower than those treated with the single AR. Moreover, AR and 100 μM ABA increased concentrations of Ca and NH4+-N in leaves and the NH4+-N concentration in roots, decreased concentrations of K, Mg, NO3--N and P. Therefore, exogenous 100 μM ABA could induce the excess accumulation of endogenous ABA and the decrease in concentrations of IAA, GA and ZT, and aggravate the inhibition on K, Mg, N and P uptake due to hinder the transport of Ca and N to leaves. After a 5-day recovery, 10 μM ABA could alleviate the inhibition on concentrations of endogenous hormones and nutrients in rice treated with AR. However, exogenous 100 μM ABA could aggravate the inhibition caused by acid rain on plant growth because the combined treatment of AR and 100 μM ABA had heavier inhibition on concentrations of hormones and nutrients of rice than those measured during the exposure period.(3) Effect of exogenous ABA and AR on plasma membrane H+-ATPase acitivity and its expression level as well as root activity in rice roots were studied to clarify the mechanism on nutrient uptake in rice roots treated with pH 3.5 AR and 10 μM ABA. Compared to the control, AR at pH 4.5 or 3.5 caused a decrease in the root activity and an increase in the plasma membrane H+-ATPase activity. The root activity in roots treated with pH 4.5 AR and 10 μM ABA was close to the control. The root activity treated with pH 3.5 AR and 10 μM ABA was lower than that of the control, but higher than that treated with the single AR. Moreover, the combined treatment of pH 3.5 AR and 10 μM ABA decreased transcriptional level of genes including OSA1, OSA3, OSA6 and OSA8, increased the transcript level of genes OSA2, OSA4, OSA5, OSA7, OSA9 and OSA10. These results indicate that exogenous 10 μM ABA could enhance the root activity, the plasma membrane H+-ATPase activity and its expression of genes such as subfamilies II(OSA5, OSA7), III(OSA9) and IV(OSA4, OSA10), benefiting to maintain normal absorption and transfer function of roots exposed to acid rain. However, the root activity and plasma membrane H+-ATPase activity in roots treated with AR and 100 μM ABA were lower than those of the control, and even lower than those treated with the single AR. It means that exogenous 100 μM ABA could aggravate the inhibition caused by acid rain on the root activity and H+-ATPase activity, and then hinder nutrient elements across the membrane into cell by reducing the secondary transport capacity. After a 5-day recovery, the root activity treated with pH 4.5 AR and 10 μM ABA was close to the control. The root activity treated with pH 3.5 AR and 10 μM ABA was still lower than that of the control, but higher than that measured during the exposure period. The plasma membrane H+-ATPase activity in roots treated with AR and 10 μM ABA was higher than that of the control. Moreover, the combined treatment of AR and 10 μM ABA decreased the transcriptional level of OSA2, increased the other transcriptional level of genes. However, the root activity and the plasma membrane H+-ATPase activity in roots treated with AR and 100 μM ABA were still lower than that of the control, and even lower than that measured during the exposure period.In conclusion, exogenous 10 μM ABA could enhance the signaling molecule(ABA) and alleviate the decrease of IAA, GA and ZT concentrations. Moreover, ABA(10 μM) could promote the root activity and plasma membraneH+-ATPase activity to maintain the normal absorption and transferable function of roots exposed to acid rain, alleviating the inhibition on K, Mg, N and P uptake, and facilitating Ca and N transporting to the leaves. Meanwhile, ABA(10 μM) could promote the recovery of rice seedlings. Furthermore, the regulating effect of ABA(10 μM) on rice treated with pH 4.5 AR was better than those treated with pH 3.5 AR. Contrarily, the application of 100 μM ABA aggravated the inhibition of growth caused by acid rain, and the recovery degree were even worse than those treated with the single AR. |
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