Effect of Zn and P supply on grain Zn bioavailability in wheat

Phytic acid is not only the richest form of P, but also a main element reducing zinc bioavailability in wheat grains. A field experiment was conducted in a split-split design with 4 wheat genotypes (“Zhongyu 6”, “Xiaoyan 22”, “Xiza1” and “Zhengmai 9023”) of varied zinc efficiency. The experiment investigated the interactions of P, Zn and phytic acid in wheat, and the effects of P and Zn fertilization on Zn bioavailability in wheat grains. Two rates of zinc 0 kg(Zn)·hm^-2 and 7 kg(Zn)·hm^-2 and five rates of P 0 kg(P₂O₅)·hm^-2 (deficient), 50 kg(P₂O₅)·hm^-2 (low), 100 kg(P₂O₅)·hm^-2 (suitable), 150 kg(P₂O₅)·hm^-2 (high) and 200 kg(P₂O₅)·hm^-2 (excess) were designed. The results showed that Zn application obviously increased soil available Zn content. Even after wheat cultivation, soil available Zn content under Zn treatment was higher than that under no Zn treatment. Meanwhile, P concentration, P/Zn ratio, phytic acid concentration and phytic acid/Zn²⁺ molecular ratio in wheat grains slightly dropped by 0.5%, 1.85%, 2.7% and 6.2%, respectively. Zinc bioavailability increased slightly because of Zn application. Also phytic acid concentration and phytic acid/Zn²⁺ molecular ratio dramatically increased with P treatment. For instance, increasing P treatment from 0 to 100 kg·hm^-2 increased phytic acid concentration and phytic acid/Zn²⁺ molecular ratio by 13.4% and 25.94% respectively, while decreasing zinc bioavailability in wheat grain. Similarly, wheat grain phytic acid concentration and phytic acid/Zn²⁺ molecular ratio increased at low P treatment and decreased at high P treatment, and peaked at 100 kg·hm^-2 and 150 kg·hm^-2 P treatment with values of 5.02 mg·g^-1 and 11.56 respectively. Excessive P treatment decreased phytic acid concentration in wheat grain. Furthermore, wheat grain phytic acid/Zn²⁺ molecular ratio under suitable (100 kg·hm^-2) P treatment was similar to that under low (50 kg·hm^-2) P treatment, but lower than that under high (150 kg·hm^-2) P treatment. Hence suitable P treatment was considered a better measure for zinc nutrition in wheat grain. It was concluded that zinc bioavailability in wheat grain could increase under suitable combinations of Zn and P treatment. P treatment was noted to be more important than Zn treatment.