Evaluation of critical phases of sulphur supply for optimum yield and quality of wheat

Abstract

This project combined an agronomic study of optimum timing for S-fertilizer application with physiological and biochemical approaches to investigate sulphur metabolism in wheat. The major applied aim was to determine the optimum window for spring S-application and to provide this information as a guideline for levy payers.

 In field trials undertaken in two successive years, S was applied to a winter wheat crop on a known deficient site at 4 different times throughout the spring/early summer seasons. S-application (30 kg/ha) as sulphate (gypsum) to the soil had a remedial effect of increasing yield by 30% in 2000 and by 40% in 2001. Application was effective when applied between March and May (GS 13 to 31). Application after GS 39-59 in June failed to increase grain yield.

 Application of S-fertilizer also influenced grain S-concentrations, irrespective of application date (March to June, GS 13-59). Analysis of the grain from all plots receiving S-application indicated that grain S concentration was increased from 0.83 to around 1.2 or from 0.99 to 1.57 mg g-1 DW in 2000 and 2001, respectively. Grain nitrogen was not affected by S-fertilizer application, however N:S ratios were decreased from around 25 to below or equal to 17 by S-application. A direct consequence of the S-concentration was a modification of the protein profiles and the amino acid composition.

In glasshouse experiments in which wheat was grown on nutrient solution and continuously supplied with S at five rates, grain yield was maximal at supply rates of 0.1 mM ('adequate') to 5 mM ('substantially well-fertilized'), but decreased at rates above this. Grain sulphur ranged from 2.1 to 2.9 mg g-1 over a range of S-supply from adequate (0.1 mM in solution) to a substantial excess (20 mM).

These high S-concentrations most likely result from the continuous supply of S throughout all growth stages which is beneficial compared to the single dose usually applied in the field. S is required for optimal vegetative development (hence a requirement for early season application) and for grain S accumulation (which may require a subsequent application). In all cases unassimilated inorganic S was present in the grain, and this greatly increased with increasing levels of supply. This represents an un-exploited resource, potentially available to improve quality parameters.