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NIR spectra of wheat grains with contrasting phytate concentrations

GRAEME BATTEN 1 ; Richard Trethowan 2 ; ROBERT CALDWELL 2 ; LINDSAY CAMPBELL 2 ; ROGER MEDER 3
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INTRODUCTION

NIR calibrations developed to predict phosphorus (P) in plant tissues are less precise than calibrations for nitrogen in the tissues. The r2 is typically ~ 0.7–0.9 for validation set samples. The basis of a P calibration in rice grains has been reported as due to absorbances at 25 wavelengths in the 1100 - 2500 nm region with a dominant peak due to P at 5240 cm-1 (1908 nm). The aim of this project is to test the hypothesis that NIR calibrations for phosphorus utilize predominantly the organically-bound P in the samples and thus calibrations for total-P rely on a strong organic : inorganic P ratio.

MATERIALS AND METHODS
Wheat (Triticum aestivum L.) genotypes, comprising locally adapted genotypes and progeny of crosses carrying the lpa-1 gene for low phytic acid, were grown in replicated experimental plots at the I.A. Watson Grains Research Institute at Narrabri in northern NSW Australia in 2011. At maturity, grain samples were taken for physical and chemical assessments including mineral concentrations, by ICPAE spectroscopy, and phytate-bound P by HPLC. Selected samples of whole grain were scanned using a FT-NIR spectrometer (MPA, Bruker Optik GmbH, Ettlingen, Germany) between 10,000 and 4000 cm-1 at a resolution of 8 cm-1 averaging 64 scans per spectrum Spectra were acquired using Opus software (Bruker Optik) and then processed using The Unscrambler (v 10.1, Camo Software, Oslo, Norway).

Here we report spectra of grains with low, medium and high total-P concentrations and with low and high proportions of this total-P present in the organic (phytate) form.

RESULTS AND DISCUSSION
The total-P in the 295 available grain samples ranged from 0.25 to 0.47%P. The phytate-P ranged from 0.11 to 0.31%P and the phytate-P as a proportion of the total-P (PP%TP) ranged from 33 to 94%.

From the available samples, we scanned grains with 0.30%, 0.35% and 0.40% total-P and for each of these total-P levels we scanned samples which contained low (less than 40%) and high (greater than 67%) PP%TP.

The dominant peak in each spectrum, for grains with low and high PP%TP, was observed at 5241 to 5249 cm-1 when the spectra were pre-treated with Savitzky-Golay, second derivative, (11 point smoothing, second order polynomial). This peak may be difficult to identify as it is close to the water peak at 5200 cm-1. However, it was similar in size, but not direction, to the O-H absorption peak at about 5300 cm-1. Small changes in the Savitzky-Golay pre-treatment smoothing settings had large effects on the appearance of the spectra and the identification of the P-peak.

Grains with a higher PP%TP ratio had stronger peaks when the total-P was 0.30 and 0.35% but not when the total-P was 0.40% TP. These initial results provide confidence to examine all 295 samples and to test the hypothesis that P-calibrations using NIR spectra are based on the organic (phytate) P concentration.