'Haying-off', the negative grain yield response of dryland wheat to nitrogen fertiliser II.Carbohydrate and protein dynamics

Abstract

Changes in carbohydrate and protein in stems, leaves, spikes, and grain between anthesis and maturity were measured in 3 dryland wheat crops whose responses to applied nitrogen (N) ranged from increases in grain yield through to decreases in grain yield. This decrease in grain yield, known as haying-off, was described in Paper I in this series. Measurements reported there showed that apparent retranslocation, defined as the decrease in weight of vegetative organs during grain filling, was generally greater for crops of high-N status than for those of low-N status. Retranslocation in this context is the process of moving compounds assimilated before anthesis to the grain.

The largest source of assimilates available for retranslocation in all crops at anthesis was water-soluble carbohydrates (WSC) contained in the stems and spikes, and represented a potential contribution of 34-50% to yield for the most severely hayed-off crops. The absolute amount of WSC present in high-N crops was less than that in low-N crops, despite a greater biomass. The lack of this form of assimilate available for retranslocation was the greatest single contributor to the yield reduction of the crops of high-N status. The quantity of protein retranslocated increased with crop N status, but the amounts involved were smaller than the quantity of WSC. Virtually all of the WSC reserves were utilised in all crops, in contrast to the protein reserves which were poorly retranslocated in the hayed-off crops. Most of the WSC was contained in the stems and most of the protein in the leaves.

The potential contribution of retranslocated WSC and protein from leaves was more difficult to estimate because of an apparent loss of 40-50% of leaf tissue after anthesis. The nature of the loss was estimated from the amounts of acid detergent fibre (ADF; fibre not solubilised by hot acid detergent) present at anthesis and maturity. Since ADF comprises cellulose and lignin which decompose slowly, the loss of 30-37% of ADF was applied as a correction factor in calculating potential retranslocation from leaves. There was no loss of stem ADF. Using the correction, the potential retranslocation of leaf protein and leaf WSC was equivalent to 6-15% of yield.

The export of all WSC and protein failed to account for the total decrease in leaf biomass, even after correction of leaf losses. We identified hemicellulose as an additional and previously unsuspected source of carbohydrate for retranslocation. Unlike WSC, the amount of leaf and stem hemicellulose at anthesis increased with crop N status, and the increase in hemicellulose between anthesis and maturity was equal to 10-17% of yield.