Discussion

- Relationship between malt qualities and beta-amylase activity and protein content as affected by timing of nitrogen fertilizer application

Previous studies showed that increasing N application rate at late growth stage of barley generally increased grain protein content and decreased grain weight (Xie and Ding, 1996; Zhao et al., 1988). The present results of two-year experiments indicated that N top-dressing at early stage (2-leaf stage) benefited improvement of malt quality due to increasing grain weight and decreasing grain protein concentration, in comparison with the treatment in which more N top-dressed at late growth stage (booting stage).

β-amylase activity in grains was significantly increased when barley plants were top-dressed with more N fertilizer at booting stage. β-amylase is predominantly synthesized and accumulates during grain maturation up to 1%~2% of total barley protein (Hejgaard and Boisen, 1980). As such, it is one of the most common protein components in barley grain. Thus the amount of β-amylase in barley grains is largely dependent on the conditions under which the grain develops, in particular on nitrogen metabolism and protein synthesis. It was found that there was a close association between β-amylase activity and grain nitrogen/protein content (Rutger et al., 1967; Hayter and Riggs, 1973; Swanston, 1980). Therefore it is expected that more N top-dressing at late stage will increase β-amylase activity.

This investigation showed that timing of N fertilizer application affected significantly most malt quality parameters. More application of N fertilizer at late growth stage significantly decreases malt extract due to more increase of grain protein concentration. Eagles et al.(1995) reported that malt extract would decrease with increased N application rate. It is implied that higher N level in soil and plants leads to more protein synthesis and accumulation in barley grains, as a result causing a reduced ability of grain components to be decomposed during malting and mashing, which may be demonstrated by the response of Kolbach index to timing of N application. Kolbach index is a value indicating protein solubility and is decreased with increased N application at late growth stage. It is interesting to note that the effect of timing of N fertilizer application on malt quality is also cultivar-dependent. For instance there was a significant difference in Kolbach index among three N treatments for Xiumai 3, while there was no difference for 92-11.

It is well documented that grain protein concentration is negatively correlated with malt extract and positively correlated with diastatic power (Arends et al., 1995; Bishop and Day, 1993; Howard et al., 1996; Smith, 1990). Molina-Cano et al.(1995) reported that grain protein concentration was significantly and negatively correlated with malt viscosity and Kolbach index, while malt extract significantly and positively correlated with Kolbach index. This research revealed that β-amylase activity was positively correlated with grain protein concentration, but its association with diastatic power was not close. It was found that β-amylase activity was negatively correlated with malt extract and Kolbach index. The results indicated that the influence of increasing protein concentration in barley grains on malt quality is dual: a positive effect by increasing β-amylase activity as well as diastatic power and a negative effect by reducing malt extract. So it is important to optimize rate and timing of N fertilizer application in order to obtain high grain yield and malt quality. According to the current results, it seems better to top-dress N fertilizer in equally at the 2-leaf and booting stage, respectively.

An effective approach to balance the conflict between high yield and malt quality on N application is to develop cultivars with grain protein concentration that are less sensitive, in terms of grain protein accumulation, to variable N levels in soil (Arends et al., 1995). Bertholdsson (1999) described an ideotype for low and stable grain protein concentration, which was characterized by late heading, many tillers and many seeds per ear. Thus it may be possible to increase β-amylase activity and diastatic power while keeping grain protein concentration below the critical level of 11.5%. In this study, the two barley cultivars had distinct difference in the response of grain quality and malt quality to N application timing. It is suggested that it is also possible to alleviate the contradiction between low protein level and high diastatic power through developing cultivars with high β-amylase activity and low/stable protein concentration.