Physiological Mechanisms Of Rapid Inhibition Of Rice Root Elongation Growth By Aluminum Toxicity

Aluminum toxicity in acid soil is one of the primary factors limiting the growth and yield of rice.Al toxicity causes the main symptom of the rapid inhibition of plant root growth.But there are few reports about the mechanisms of rapidly inhibiting the elongation growth of rice roots by toxic aluminum(Al³⁺).Here,an Al-sensitive rice variety IR24 was used to study the effects of Al toxicity on the seed germination, seedlings growth as well as the changes of some physiological indicators in the cell wall,cytoplasmic membrane,cytosol of the root apical cells in the course of the root elongation growth rapidly inhibited by Al toxicity.The mechanism of elongation growth of rice root rapidly inhibited by Al toxicity was then discussed.The main results are as follows:1.Aluminum stress retarded the germination speed of rice seeds.And the higher concentration of aluminum,the more obvious effect of the germination speed inhibited by Al toxicity.The germination rate of rice seeds were not different for 0,50, 200μM Al stress,showed that aluminum stress had little effect on the germination rate of rice seeds.After the rice seedlings were exposed respectively to 0,50,200μM Al for 3 d,the root elongation length,the root fresh weight,root/shoot ratio decreased significantly with the increase in the aluminum concentration(P＜0.05).No significant difference was found between the shoot fresh weight under 50,200μM Al stress(P＞0.05),but the shoot fresh weight for 50 and 200μM Al treatment showed significant difference(P＜0.05) from their control.There was no significant difference in the root elongation length of the seedlings under 200μM Al stress for 1 d,2 d and 3 d(P＞0.05),indicating that the root of rice seedlings was damaged severely by Al toxicity in 1 d.2.Dynamic analysis showed that the content of Al on the root tip of rice seedlings reached the maximum in its exposure to aluminum stress for 1 h(P＜0.05).While the elongating growth of rice seedling roots was inhibited obviously by aluminum toxicity within 3 h comparing to that of the control(P＜0.05).After the seedlings were exposed to Al stress for 3 h,the relative Evans blue uptake in root tip cells increased by 2.28 times than that of Al stress for 0 h(P＜0.05),demonstrating that Al stress could cause a rapid breakage of plasma membrane integrity of root tip cells. Microscopic observation showed that,the higher Al concentration,the more severe breakage of plasma membrane integrity(P＜0.05).3.After the rice seedlings were exposed respectively to Al stress for 24 h,it was found that H₂O₂ content and Malondialdehyde(MDA) content in root tip increased as the Al concentration increased(P＜0.05).Dynamic analysis showed that Al induced production of H₂O₂ within 3 h and there was a significant difference between H₂O₂ contents of root tip of rice seedlings exposed to Al for 0h,3h,6h(P＜0.05).MDA content started to increase significantly after Al stressed for 3 h(P＜0.05),indicating that Al toxicity induced the occurrence of oxidative stress in root tip of rice seedlings, but it occurred later than root elongation growth inhibition of seedlings was initiated. The dynamic changes of the surface pH in the root tip of rice seedlings were monitored using a modified technique of agarose gelling plates containing bromocresol purple as the pH indicator.It was shown that surface pH in the root tip of rice seedlings without Al-treatment was strongly alkalized from the starting pH 4.4 in a time-dependent manner.By contrast,the increase in the surface pH was inhibited in the Al-treated root tip of rice seedlings.4.Under 200μM Al stress,there were no significant changes for the oxalic acid content in the root tips(P＞0.05).Citric acid content in the root tips changed little(P＞0.05) within the initial 3 h of Al stress,afterwards increased rapidly and significantly,and reached the maximum at 12 h.After that,citric acid content turned to decrease,but was still higher than that of 6 h.Malic acid content decreased but had no significant difference at the beginning 3 h of Al stress(P＞0.05).Al stress for 3h later,malic acid content declined significantly but remained stable after 6 h exposed to Al stress(P＞0.05).5.Al stress induced rapidly the production and accumulation of lignin and thecontent of lignin in the root of seedlings was increased as the Al stress time extended(P＜0.05).Callose formation and accumulation were caused by Al stress.There was no significant change in the callose content between 0 h and 3 h(P＞0.05),but 3 h later increased significantly(P＜0.05).The activity of the ionically bound cell wall POD in the root tip cells increased rapidly under Al stress.It was significantly different between the POD activities of 0h,3h and 6h(P＜0.05)..The activity of covalently bound cell wall POD of root tip cell changed little during Al exposure(P＞0.05) and was also much lower than that of the ionically bound cell wall POD.