Yield and breadmaking quality responses of winter wheat to sulphur fertiliser

Abstract

Sulphur deficiency has become more common in wheat as a result of decreased S inputs from atmospheric deposition. This project was initiated to evaluate the responses of grain yield and breadmaking quality to the additions of S fertilisers under field conditions (Part I.), and to investigate the physiological basis of the S nutrition of wheat in terms of the critical phases of S supply and re-distribution of S to wheat grain (Part II.).

In Part I., twelve field experiments were carried out at Bridgets (Hampshire), Raynham/Barsham (Norfolk), Woburn (Bedfordshire) and in the Scottish Borders over the three growing seasons from 1994 to 1997. The winter wheat variety Hereward was used in the first two seasons, and three breadmaking varieties in the third season, Hereward, Rialto and Spark, were compared. The experiments in the first two seasons also compared applications of S as gypsum in early spring versus foliar applications of ammonium sulphate at the milky ripe stage. Complete data sets for yield and S uptake were obtained in 11 experiments, and for breadmaking quality parameters in 10 experiments. The main findings can be summarised as follows:

Significant yield increases in response to S additions in early spring were obtained in 3 out of 11 field experiments over the three seasons from 1994 to 1997. In addition, one experiment showed S deficiency symptoms during stem elongation, although the grain yield response to S did not reach a significant level. The responsive sites were a shallow calcareous soil at Bridgets and a sandy soil at Woburn. In the responsive experiments, yield increases due to S varied between 0.43 and 1.34 t ha-1, or between 8.7 and 26.5% on the relative basis. Most of the yield increase was obtained from the application of the first 20 kg S ha-1.

Applications of S in early spring increased loaf volume significantly in six out of the ten experiments that produced suitable grain samples for breadmaking tests. All 4 sites showed responses in one or two seasons, suggesting that breadmaking quality response to S was more common than yield response. Increases in loaf volume typically varied between 40 and 100 ml. In addition, S also improved crumb score in two experiments. Three breadmaking varieties, Hereward, Rialto and Spark, appeared to respond similarly to S. In comparison, increasing the amount of N applied from either 180 to 230 kg ha-1 in nine experiments, or from 230 to 280 kg ha-1 in one experiment, increased loaf volume significantly only in one case, even though this increased grain protein significantly in most experiments.
Loaf volume correlated more closely with grain S concentration than with grain N (grain protein). These results indicate that, within the range of grain protein concentration obtained in this series of experiments (8.5-14.3%), the concentration of crude protein was not as limiting a factor as the concentration of S in grain to breadmaking performance. Because grain S concentration correlated with loaf volume in a linear pattern, it was difficult to derive a critical value of grain S for breadmaking quality. In many cases, a low loaf volume was associated with a grain N:S ratio of greater than 16:1. These results confirm that grain S status is important for breadmaking quality of wheat.

There were significant effects of S on dough rheology, and the amount and elastic modulus of gel protein. Sulphur addition in general increased gel protein content, but decreased its elastic strength. Sulphur also decreased dough resistance, and increased dough extensibility. The effects of S on dough rheology and the elastic strength of gel protein could be explained by the positive influence on the ratio of LMW/HMW subunits of glutenin. Despite their different rheological properties, Hereward, Rialto and Spark responded similarly to S.

Compared to the spring applications of gypsum, foliar applications of ammonium sulphate at the milky ripe stage were not effective in correcting S deficiency for grain yield. In some cases, foliar applications resulted in scorching and yield losses. In terms of the effects on grain S concentration and breadmaking quality parameters, foliar applications of S produced inconsistent results. It was concluded that the best practice at present was to apply S, in a sulphate form, in spring.
It was established that winter wheat crops generally require >15 kg S ha-1 to ensure S sufficiency. The harvest index for S was much lower than that for N, even under S deficient conditions, indicating that the re-utilisation of S within plants was less efficient than of N. Analysis of plant samples at early stem elongation (GS 31-32) was useful in predicting S deficiency, with a critical value of 2 mg g-1 of total S in the whole plant shoots.

An extractable sulphate-S concentration in the soil profile of greater than 3 mg kg-1 in early spring appeared to indicate a sufficient S supply for grain yield. However, S deficient sites could not be predicted reliably even when soil extractable sulphate-S was less than 3mg kg-1. In this series of field experiments, breadmaking quality responses were not related to soil extractable S in early spring.

In Part II, pot experiments were conducted to investigate the effects of S deficiency and the timing of S addition on yield and yield components, and to quantify the re-distribution to grain of the S accumulated in wheat plants at different growth stages. A method was developed to use different S sources varying in their natural abundance of the stable isotope 34S as a tracer system for the quantification of S re-distribution under hydroponic conditions. The breadmaking variety Hereward was used in both experiments. Main findings are summarised as follows:

Severe S deficiency decreased grain yield markedly by affecting the number of ears and the number of grain per ear, whereas single grain weight was little affected. Compared to the S deficient control, ear number was increased significantly by the additional S given to the S-deficient plants at pre-stem elongation and stem elongation stages, but not by the additional S given after stem elongation. This indicates that S supply before and during stem elongation is important for the initiation and survival of tillers. In contrast, the critical phases for the number of grains per ear appeared to be the stem elongation and pre-anthesis ear development stages. Additional S given to the S deficient plants after anthesis did not correct the deficiency significantly.

Grain S concentration appeared to be influenced more by the S supply after stem elongation. Additional S given to the S deficient plants at the pre- and post-anthesis ear development stages restored the concentration of S in grain to levels similar to or above that found in the S sufficient control. Increasing proportion of low molecular weight gluten polymer was found to be associated with increasing grain S concentration.

At maturity, wheat grain derived 14, 30, 6 and 50% of its S from the accumulation during the following successive growth stages: between emergence and early stem elongation, between stem elongation and flag leaf emergence, between flag leaf emergence and anthesis, and after anthesis, respectively. It was estimated that 39, 32 and 52% of the S present in the flag leaves, older leaves and stems respectively, at anthesis, was exported during the post-anthesis period. These results demonstrate considerable cycling of S within wheat plants, and highlight the importance of S uptake after anthesis to the accumulation of S in grain under the experimental conditions employed.

Overall, the results suggest that the stem elongation stage is the most critical phase of S supply for grain yield, whereas S supply after anthesis is important for achieving a high concentration of S in grain to give a quality benefit.