Discussion
Recently, PDMA, an effective Fe chelator, was synthesized based on the structure of the phytosiderophore DMA (Suzuki et al., 2021). PDMA, with low synthesis costs and high stability, was proven to improve Fe nutrition in rice and cucumber under conditions of hydroponic culture and calcareous substrates (Suzuki et al., 2021, Ueno et al., 2021). In this study, positive effects of PDMA on Fe nutrition and peanut yields were demonstrated in calcareous soil under field conditions.
The present study revealed that the application of PDMA significantly corrected Fe deficiency in pot and field trials (Fig. 2, Fig. 5). Consistent with the accumulation of Fe, because Fe is a vital element involved in chlorophyll synthesis (Kobayashi et al., 2019), the SPAD values were enhanced substantially by PDMA application (Fig. 1, Fig. 5). The growth of the PDMA-treated peanut plants was therefore promoted (Fig. 1). Similar increases were found in rice and cucumber (Suzuki et al., 2021, Ueno et al., 2021). Importantly, our work produced the novel finding that PDMA can increase the yield and kernel micronutrition of peanuts (Table 1, Fig. 6), implying the great ability of PDMA to improve the production and quality of plants under Fe-limited field conditions.
PDMA resulted in higher levels of dissolved Fe in the rhizosphere than did EDTA–Fe, but the improvement in peanut Fe nutrition between these treatments was not significantly different (Fig. 2, Fig. 5). These results suggest that PDMA can chelate insoluble Fe in soil to improve Fe nutrition in comparison to traditional Fe fertilizers without any exogenous Fe supplementation, which realizes sustainable utilization of the Fe in soil (Colombo et al., 2014). This type of metal-free fertilizer effectively utilizes the residual nutrients in native soil and is conducive to the development of sustainable agriculture (Bouis and Welch, 2010).
Another mechanism by which PDMA may increase the Fe nutrition in peanut is that PDMA–Fe complexes may be taken up directly by roots via AhYSL1. A previous study indicated that PDMA can trigger YS family gene expression in Strategy Ⅱ plants (Suzuki et al., 2021). In the present study, expression of the YS homolog AhYSL1 in peanut root was up-regulated by PDMA (Fig. 4), suggesting that PDMA–Fe complexes might be directly assimilated by roots. Similar characteristics were found for DMA–Fe, which was proven to be taken up via the YSL transporter in both Strategy I and Strategy Ⅱ plants (Xiong et al., 2013, Schenkeveld et al., 2014). Therefore, the absorption of PDMA–Fe, instead of regulation via AhNRAMP1 and AhFIT , might be an important pathway for improving the Fe nutrition of peanut (Fig. 4). Additionally, higher Fe levels in Strategy I plants down-regulate the capacity of ferric reductase and the expression of AhIRT1 (Martín-Barranco et al., 2020, Riaz and Guerinot, 2021). Similarly, the Fe uptake process in peanut root was decreased significantly (Fig. 3). Overall, PDMA improved peanut Fe nutrition by increasing Fe availability and up-regulating the absorption of PDMA–Fe. As an effective biostimulant, PDMA has great potential for the biofortification of plant growth, yields, and kernel micronutrition.
Chemosynthetic chelate–Fe, as represented by EDTA–Fe, is commonly used to improve the Fe nutrition of plants in the field (Abadía et al., 2011, He et al., 2013). However, chemosynthetic chelate–Fe is expensive and thus generally applied only to cash crops (Briat et al., 2015). Moreover, the residual chemosynthetic chelator does not degrade easily in the soil. Biological Fe fertilizers, such as humic acid–Fe and amino acid–Fe, have therefore been applied widely in the field in recent years to protect the soil environment (Saini et al., 2016). Nevertheless, because of the instability of biological Fe fertilizers in soil, the effect on plant Fe nutrition is limited, especially in calcareous soil (Briat et al., 2015). The neotype Fe chelator PDMA has been proven more stable than biological Fe fertilizers. Additionally, the biodegradability of PDMA can prevent the problem of environmental pollution caused by chemosynthetic chelate–Fe (Suzuki et al., 2021).
In conclusion, the novel Fe fertilizer PDMA can dissolve insoluble Fe in the rhizosphere and up-regulate AhYSL1 expression to improve Fe nutrition in peanut and thus plant growth. Importantly, under field conditions, PDMA enhanced the yield and kernel micronutrition of peanuts significantly (Fig. 7). Our results demonstrate the excellent potential of PDMA for wide use as an Fe fertilizer and biostimulant in agriculture.