RESULTS
Nitrate inhibits iron
accumulation in the newly growing leaves of apple
Two-year-old trees of the apple cultivar ‘Fuji’ were treated with 10,
30, 60, or 120 g nitrate fertilizer per plant. We found that increased
nitrate application resulted in more severe iron deficiency symptoms in
new leaves (Fig. S1), while chlorophyll content gradually decreased
(Fig. S2). To further explore the effects of nitrate on iron uptake in
apple, apple seedlings were treated with +Fe and -Fe nutrient solutions
containing high concentration of nitrate (HN, 15 mM) and low
concentration of nitrate (LN, 0.5 mM). To create an ideal experimental
environment, we chose vermiculite rather than soil to cultivate the
seedlings, which has no soluble Fe but contains
Fe2O3. The application of ferrozine
(FRZ) in -Fe treatments enabled chelation of Fe2+which limits the uptake of Fe2+. After 15 days, the
newly growing leaves showed severe chlorosis and reduced chlorophyll
content under HN-Fe treatment (Fig. 1a, b, c). By contrast, slight
chlorosis was observed under LN-Fe treatment. Additionally, when there
was no iron shortage, LN treatment seedlings had a lower chlorophyll
content than HN treatment seedlings (Fig. 1a, c). These results
suggested that LN treatment alleviates iron deficiency in seedlings.
Meanwhile, the soluble Fe content of both newly growing leaves (Fig. 1d)
and roots (Fig. 1e) significantly increased under LN-Fe treatment, even
though vermiculite has very little soluble Fe. Additionally, soluble Fe
content in young leaves of seedlings which were treated with LN solution
with sufficient iron supplementation was higher than seedlings which
were treated with HN solution with sufficient iron supplement treatment
(Fig. 1d). These results implied that LN-triggered alleviation of Fe
deficiency may be due to an increase in iron reutilization.