Effect Of Organic Acids On Zinc Accumulation In Apple Tree Roots Under Zinc Deficiency

As an important micronutrient for all plants, zinc plays a key role in the structural or functional of some enzymes and proteins. Zinc homeostasis in plants is the material basis to ensure the normal growth of plants. The proper growth of apple tree is based on the effective distribution of mineral elements and carbon nutrition in organs. Several studies demonstrated that zinc accumulation in different organs and different stages can be influenced by zinc deficiency. Organic acids(OAs) have chelate ability, and they can increase zinc bioavailability to promote zinc absorption and the translocation of roots. This research can provide a theoretical support for the further reuse of accumulated zinc in roots as well as a better guide to prevent the occurence of zinc deficiency and clarify the zinc deficiency tolerance mechanism in field.To explore the effects of OAs metabolism on zinc distribution, we carried out the experiment with the two-year-old apple trees and rootstock seedlings under different zinc levels with quartz sand and hydroponics respectivly. The main results are as following:(1) In this subject, the materials were grown in the glasshouse under different zinc levels, including zinc-adequate(15μmol·L-1), zinc-low(5μmol·L-1) and zinc-free(0μmol·L-1) treatments. With the time of different zinc levels was prolonged, apple trees showed obvious zinc-deficiency symptoms under zinc-low and zinc-free, for example the young developing leaves chlorisis and rosetted, while those symptoms were not obvious in zinc-adequate. The plant height and the quantity of fine roots were inhibited under zinc deficiency. These results suggest that the growth of apple trees was affected by zinc deficiency stress, especially under moderate zinc deficiency. At the initial stage of zinc deficiency treatment, compared with the normal treatment, the surface area of all roots and the area of new roots are both iecreased under zinc deficiency stress. It indicated that zinc deficiency stress promoted the growth of roots. But the growth of roots were inhibited by zinc deficiency appeared on the 7th day after treatment. The trend of root activity was consistent with it. These results show that zinc deficiency stress promoted the zinc absorption of root at the initial stage, however it decreased the zinc absorption with the extension of stress time by inhibited the growth of roots.(2) Under the zinc deficiency stress in July, the zinc concentration in shoots was the highest followed by other organs, and it was two times higher than zinc-adequate; however in other organs, the zinc concentration were lower than normal treanment. This finding may be attributable to the promotion of the zinc from the roots to the aboveground parts, especially to the organs which are metabolically active. While the zinc concentration reduced in shoots and stems under zinc-low treanment, it raised in fine roots with prolongation of treanment time. Meanwhile, the zinc concentration in different organs showed no significant changes under zinc-adequate. These data indicated that zinc accumulated in roots under zinc deficiency.(3) The total secretion rate of four organic acids under zinc deficiency stress was significantly higher than zinc-adequate; meanwhile the pH of rhizosphere was significant declined. Differents zinc levels vary dramatically in the rates of four types OAs which were secreted from roots. Oxalate was quantitatively the most important among the four types of organic acids under zinc deficiency stress, and citrate has been recognized as the most effective one. Significant positive correlation was existenced between citrate secretion and the zinc concentration in the roots of apple trees. Citrate acid secretion was raised with the prolonging of stress time and was the highest in zinc-free, and it was two times higher than that under adequate zinc. Overall, OAs secretion increased under zinc deficiency, which promoted zinc activation in rhizosphere of apple trees.(4) The total concentrations of four types of organic acids in roots were significant changed under different zinc levels, and they were decreased to 30%-50% of zinc-adequate in October. What is more, the citric acid concentration was decreased to 1/3-1/2 of zinc-adequate in October. Significant positive correlation was existenced between citrate concentration and the zinc concentration in the roots, while significant negative correlation was existenced between citrate concentration and the zinc concentration in the shoots of apple trees. It indicted that OAs concentration decreased, which attributed to the zinc transportation from roots to aboveground organs was inhibited in the form of metal composite through xylem.A possible physiological mechanism for zinc accumulation is that the zinc activation in the rhizosphere of apple trees was promoted by increased organic acids secretion, while the transportation of zinc from roots to aboveground organs was inhibited by the decreased concentration of organic acids in roots.