Boron Alleviates Aluminum Toxicity By Regulation Of PH Variationin Different Root Zones Of Pea(Pisum Sativum)

Aluminum toxicity is an important limiting factor for plant growth in acid soil, and many studies have demonstrated that boron can alleviate aluminum toxicity, because of the complexity of interactaction between boron and aluminum, the mechanism of boron alleviate aluminum toxicity is still not clear. Root apices is the most sensitive targets for aluminum toxicity and B deficiency. We hypothesized that root surface p H differed in root zones which was regualted by B, thus reduced Al accumulation in and toxicity to the Al-sensitive root zone. We studied realtionship between boron and aluminumin different root zones of pea(Pisum sativum) in aspects of root performance, oxidative stress, p H of rhizospere, surface and apoplast in different zones, rhizosphere acidification, H+ flux and the involvement of plasma membrane(PM)H+-ATPase. The main results of this study are as follows:1. The differences of boron in alleviating of aluminum toxicity to different root zones. The effect of boron in alleviating of aluminum toxicity to meristem zone and transition zone is more significant than that of elongation zone and mature zone. Boron supply significantly reduced the accumulation of aluminum in the transition zone, thus boron alleviated the inhibition of root elongation by aluminum toxicity, improved the root activity of meristem zone and transition zone, and reduced the production of callose.The effects of boron in alleviating of aluminum toxicity to elongation zone and mature zone was not obvious.2. The relationship between boron alleviate aluminum toxicity and oxidative stress. Reactive oxygen species(ROS) increased while malondialdehyde(MDA) decreased with the increase of aluminum concentration, suggesting that there was no positive correlation between the generation of ROS and MDA. Boron and aluminum had different effects on ROS and MDA production in different root zones. Under 15 μmol/L Al stress, boron inhibited ROS production induced by aluminum stress in meristem zones, and alleviating aluminum toxicity. But the reason of boron in aggravating oxidation stress to transition zone needs further research.3. Regulation of p H variation in different root zones by B and Al The p H differences of different root zone was obvious, the p H of meristem zone and transition zone was significantly higher than that in elongation zone and mature zone. Fluorescence indication of apoplast p H in different root zone showed that apoplast p H of meristem zone and transition zone was significantly higher than that of elongation zone and mature zone, and boron promoted apoplast alkalzation of transition zone. Regulation of root surface p H in the transition zone and the elongation zone by boron and aluminum was different, boron promoted protons influx in transition zone, maintained the alkalization of transition zone, improved the root surface p H. Aluminum inhibited proton influx, mainly controlled the p H of elongation zone. Boron can maintain transition zone alkalization and alleviate aluminum toxicity under aluminum stress in a short period of time duration.4. The effect of boron and aluminum on rhizosphere acidification and the relationship with the PM-H+-ATPase. Boron deficiency promote rhizosphere acidification, supply boron inhibit rhizosphere acidification, and boron regualte rhizosphere acidification with spatial-temporal difference, because the main acidification root zone is different with different time of boron treatment; aluminum can also inhibit acidification, and inhibitory effect of alumnium on rhizosphere acidification is more significant with the exist of boron. In agar medium, rhizosphere acidification were inhibited after sodium vanadate(VA, a specific inhibitor of PM-H+-ATPase) was added; in the presence of boron and aluminum, inhibition of sodium vanadate on rhizosphere acidification are more significant; suggested that the rhizosphere acidification was caused by PM-H+-ATPase, boron and aluminum act on the PM-H+-ATPase. Microelectrode measurement results show that acidification of elongation zone is directly regulated by PM-H+-ATPase, boron deficiency enhance activity of PM-H+-ATPase by FC, boron supply enhance the inhibition of VA on PMH+-ATPase activity, aluminum inhibit rhizosphere acidification, under the condition of adding boron aluminum inhibit PM-H+-ATPase stronger under the condition of boron supplied; PM-H+-ATPase was also involved in alkalization adjustment of transition zone, and the regulation of PM-H+-ATPase is more complex, boron plays an important role in the regulation, under the condition of aluminum or without aluminum, adding boron promote rhizosphere p H, negative feedback to adjust PM-H+-ATPase activity by FC.In sum, boron regulate PM-H+-ATPase activity and inhibit rhizosphere acidification, improve rhizosphere p H, promote apoplast alkalization of transition zone, reduce accumulation of aluminum and callose formation in transition zone, alleviate inhibition of root elongation which induced by aluminum, improve the root activity, and consequently alleviate aluminum toxicity.