Relative contribution of seed phosphorus reserves and exogenous phosphorus uptake to maize (Zea mays L.) nutrition during early growth stages

Abstract

Adequate phosphorus (P) nutrition during early stages is critical for maize growth. Our objective was to evaluate the relative contribution of seed P reserves and exogenous P to maize nutrition during early growth stages. Seedlings were grown with labeled nutrient solution (³²P). Seedlings were harvested periodically over the course of the three-week study. Initially, 87% and 77% of the total C and N in seeds were located in the endosperm, whereas 86% of seed P was located in the scutellum as phytate. Up to the 7th day after sowing, 96% of phytate was hydrolyzed. Hydrolyzed forms of P were temporarily stored in the seed before being translocated to growing organs, suggesting that the hydrolysis of phytate was not a limiting step for P supply to seedlings. Significant P uptake by roots was observed from the 5th day after sowing on. Both sources of P supplied roots and leaves, with a slightly higher proportion of P from seed reserves going to leaves rather than to roots. Of total seed P, 60% and 92% was exported towards newly growing seedlings till 7th and 17th days after sowing and ceased to be a significant source of P for growth thereafter. We conclude that although both P supply processes overlap in time, seed P was the main P source during early growth stages.

Appendix

Phytate extracted from endosperm and scutellum was assay by ion chromatography (IC) using a hydroxide eluent prepared by an eluent generator with a column designed to determine common inorganic anions. High performance liquid chromatography was carried out on a Dionex (Sunnyvale, CA, USA) Model ICS 2000 Ion Chromatography System with eluent generation (RFIC-EG system) equipped with an automatic gradient pump, a conductivity detector (DS6 heated conductivity cell) and an automated sampler (AS50). The eluent was automatically generated by the eluent generator module using purified water and an eluent generator cartridge (EGC II KOH P/N 058900) to produce a gradient of 12–90 mM potassium hydroxide. An eluent generator was used in conjunction with internal suppression (ASRS 300 4 mm at 179 mA). The concentration of the eluent was controlled by Chromeleon® Chromatography Data System software version 6.8. Separation was performed using an anion exchange column equipped with a guard column (Ion Pac AG11, 4 mm (P/N 044078)) to prevent sample contaminants from eluting onto the profiling analytical column (IonPac AS11, 4 mm P/N 044076). The detector was preceded by an ASRS® 2 anion self-regenerating suppressor, to suppress the background conductivity of the eluent. A standard solution containing a phytate salt (phytic acid sodium salt hydrated from rice, SIGMA product n° P8810-10 G) was prepared for the identification of solute peaks. For the AS11 column, the initial mobile phase was 12 mM potassium hydroxide. The gradient elution of KOH was generated automatically up to 90 mM. An injection volume of 20 μL was used and eluted with the KOH mobile phase at a flow rate of 0.80 ml min⁻¹. Phytate was eluted after 23.2 min for a procedure that lasted 47 min per sample including 5 min for equilibration of the AS11 with 12 mM KOH prior to injection of the next sample. This method has a detection limit of 0.12 mg P L⁻¹ for phytate. The instruments was calibrated with blanks and six standards prepared by dissolving phytic acid sodium salt in purified water (0, 200, 400, 800, 1000 and 2000 mg L⁻¹ of phytic acid). The linearity of the standards was excellent (r² ≥ 0.999) with peak area evaluation over this range of concentrations. Phytate content was converted into phytate-P by dividing phytate content by 3.548 (Raboy and Dickinson 1984).