Abstract
High yielding genotypes differing for high molecular weight glutenin subunits at Glu D1 locus in national wheat programme of India were examined for bread loaf volume, gluten and protein contents, gluten strength, gluten index and protein-gluten ratio. Number of superior bread quality genotypes in four agro-climatically diverse zones of Indian plains was comparable in both categories of wheat i.e., 5 + 10 and 2 + 12. There wasn’t any difference in average bread loaf volume and grain protein content either. 5 + 10 wheats showed better gluten strength and their gluten quality was also superior in the zones where protein content was high. 2 + 10 wheats exerted more gluten due to better protein-gluten ratio. Good bread making in 5 + 10 was derived by better gluten strength and also gluten quality in certain regions but bread quality in 2 + 12 wheats was channelized through higher gluten content as they were more efficient in extracting gluten from per unit protein. Difference in route to bread quality was apparent as gluten content and gluten strength were the key gluten attributes in 5 + 10 whereas protein content and gluten index were prominent in 2 + 12 types. Unlike 2 + 12, there was a ceiling in gluten harvest of 5 + 10 wheats as higher protein failed to deliver more gluten after some limit.
Keywords: Bread loaf volume, Grain protein, Indian wheats, Protein-gluten ratio, Wet gluten content
Introduction
High molecular weight glutenin subunits (HMWGS) are important for bread quality of common wheat (Triticum aestivum L.) as they articulate quality of gluten needed to make good dough (Payne et al. 1981; Pena 2008). Their relevance as selection parameter has been demonstrated to enhance bread quality in Indian wheats, too (Mohan and Gupta 2013a). Genes controlling HMWGS are located on the Glu-A1, Glu-B1 and Glu-D1 loci. There is differential quality effect linked to glutenin subunit composition as 1, 2* (Glu A1); 7 + 8, 17 + 18 (Glu B1); and 5 + 10 (Glu D1), generally contribute positively to high dough strength. Divergence for HMWGS in wheat has been highlighted in several studies (Horvat et al. 2002; Ram 2003). Significance of 5 + 10 in wheat dough quality, especially to bake high quality breads, has been strongly advocated whereas other allelic subunits at Glu D1 locus recognized as 2 + 12 are associated with poor bread making properties (Payne et al. 1987).
Relevance of Glu D1 locus in bread quality appears at variance in Indian wheat varieties. In All India Coordinated Wheat and Barley Improvement Project (AICW&BIP), 15 new high-yield varieties of bread loaf volume ≥ 580 cc were developed and released during the 10 year period 2005–14; seven belonged to 5 + 10 glutenin subunits and eight to 2 + 12. It shows that 2 + 12 glutenin subunits are also equally relevant in imparting good bread quality. In this scenario, it is imperative to understand what contributes to quality elevation in wheats of 2 + 12 glutenin subunits. The present study examines 10 year wheat quality data of genotypes evaluated by AICW&BIP in Indian plains to investigate the factors that balance bread quality in wheats of 5 + 10 and 2 + 12 glutenin subunits.
Material and methods
Environments and study material
Study material included released and pre-released bread wheat varieties tested under irrigated trials of AICW&BIP in four zones of Indian plains i.e., north-western plains zone (NWPZ), north-eastern plains zone (NEPZ), central zone (CZ) and peninsular zone (PZ). Hill region of this system i.e., northern hills zone (NHZ) was excluded as no bread superior genotype occur in this region. During the 10 year period 2005–14, 422 entries developed in the plains were evaluated in accordance to their adaptation in different zones. Environment for wheat growth is different in each zone. NWPZ and NEPZ represent the Indo-Gangetic plains (IGP) where NWPZ has the most soothing wheat growth environment whereas adjoined NEPZ has shorter winter and the climate is also humid. Two zones of central-peninsular India (CPI) are represented by CZ and PZ. Crop in CZ often faces soil moisture stress as climate is hot and dry. Peninsula in down south i.e., PZ has similar temperature and soil moisture conditions but climate is not that dry.
Observations and data analysis
Samples received from 3 to 5 locations of each zone were analyzed at ISO 9001–2008 certified laboratory located at Karnal. For HMWGS divergence, electrophoresis was performed on vertical gel according to SDS-PAGE protocol followed by Payne et al. (1981). Sedimentation value, gluten index and bread loaf volume was recorded as per international standards (AACC 2000). Protein content measured by infra-red transmittance based instrument Infra-tec 1125 is presented at 14 % grain moisture (GPC). Wet gluten per unit protein was described as protein-gluten ratio (PGR).
Statistical analysis
Difference in mean value was statistically analyzed by Student t-test. Comparison was made within and across the zones where data of four zones were pooled. Pearson correlations and regression analysis was done by data analysis tool provided in microsoft excel programme of computer.
Results
Comparison of grain quality
Analysis of quality characteristics in different zones revealed that average bread loaf volume in 5 + 10 wheats was significantly better than 2 + 12 in two zones of CPI only (Table 1). This advantage could not be traced in IGP where difference in bread quality was either non-significant as observed in NEPZ or the balance tilted towards 2 + 12 wheats as noticed in NWPZ. Sedimentation value in 5 + 10 group was significantly higher than 2 + 12 in every zone but gluten index emerged significantly better in NWPZ and PZ only. GPC matched in two categories of wheat at all places but wet gluten content was significantly better in 2 + 12 group in majority area except CZ where the advantage was non-significant. This pattern was visible in PGR also as 2 + 12 wheats had significant advantage in NWPZ, NEPZ and PZ and non-significant in CZ.
Table 1.
Quality parameters in wheats differing at Glu D1 locus
| Parameters | NWPZ | NEPZ | CZ | PZ | ||||
|---|---|---|---|---|---|---|---|---|
| 5 + 10 | 2 + 12 | 5 + 10 | 2 + 12 | 5 + 10 | 2 + 12 | 5 + 10 | 2 + 12 | |
| (103) | (21) | (61) | (68) | (15) | (73) | (33) | (48) | |
| Bread loaf volume (cc) | 559 | 567* | 567 | 564 | 568** | 553 | 586* | 576 |
| Grain protein content (%) | 12.1 | 12.1 | 11.5 | 11.5 | 11.8 | 11.7 | 12.1 | 12.4 |
| Wet gluten content (%) | 30.5 | 32.4** | 28.9 | 29.9* | 31.4 | 31.7 | 31.8 | 33.4** |
| Sedimentation value (ml) | 44** | 40 | 46** | 43 | 43** | 40 | 46* | 43 |
| Gluten index (%) | 62** | 55 | 63 | 61 | 55 | 53 | 63** | 57 |
| Protein-gluten ratio | 2.53 | 2.68** | 2.51 | 2.61** | 2.66 | 2.71 | 2.63 | 2.68* |
Since the pattern was almost similar in the zones, mean across the zones was also analyzed to get clear picture at the national level. Difference between 5 + 10 and 2 + 12 glutenin subunits continued to remain non-significant in bread loaf volume and protein content across the zones, too. 5 + 10 wheats registered highly significant advantage in sedimentation value and gluten index whereas advantage in 2 + 12 wheats was highly significant in gluten content and PGR (Table 2). Exercise was also done to compare entries of bread loaf volume ≥575 cc in these two classes. Again number of elite entries was almost similar. Differences in gluten characteristics were same as observed in the pooled analysis but divergence was noticeable in GPC as it was significantly higher in 2 + 12 wheats in comparison to 5 + 10.
Table 2.
Quality parameters across the Indian plains in wheats differing at Glu D1 locus
| Parameters | Mean – Indian plains | Elite entries – Indian plains | ||
|---|---|---|---|---|
| 5 + 10 | 2 + 12 | 5 + 10 | 5 + 10 | |
| (212) | (210) | (51) | (53) | |
| Bread loaf volume (cc) | 566 | 563 | 590 | 589 |
| Grain protein content (%) | 11.9 | 11.8 | 12.1 | 12.5* |
| Wet gluten content (%) | 30.3 | 31.6** | 31.7 | 33.4** |
| Sedimentation (ml) | 45** | 42 | 47** | 42 |
| Gluten index (%) | 62** | 57 | 66** | 58 |
| Protein-gluten ratio | 2.55 | 2.67** | 2.61 | 2.69** |
Contribution of gluten related attributes in bread quality
Regression statistics of different parameters suggest that coefficient of determination (R2) was highly significant in 5 + 10 group for three main parameters of gluten i.e., gluten content, sedimentation value and gluten quality (Table 3). The most important contributor in this category was sedimentation value followed by gluten index and gluten & protein contents. In case of 2 + 12 wheats, highly significant R2 was noted in gluten and protein contents only. In other parameters R2 value was either insignificant as in case of sedimentation value or carried little significance as observed in gluten index. The most important contributor among three was protein content.
Table 3.
Regression statistics for role of quality components in bread making
| Components | 5 + 10 group | 2 + 12 group | ||
|---|---|---|---|---|
| R2 | Significance (P value) | R2 | Significance (P value) | |
| Grain protein | 0.056 | <0.001 | 0.092 | <0.001 |
| Wet gluten | 0.055 | 0.001 | 0.054 | 0.001 |
| Sedimentation value | 0.145 | <0.001 | 0.017 | 0.059 |
| Gluten index | 0.090 | <0.001 | 0.025 | 0.021 |
| Protein-gluten ratio | 0.003 | 0.438 | 0.002 | 0.478 |
Multiple regression statistics further indicated that collective contribution of five gluten related traits in bread quality was highly significant in both wheat types and the R2 value was 0.245 in 5 + 10 and 0.146 in 2 + 12 class, However, with two key components also similar R2 was achievable (Table 4). Key component traits in this analysis were gluten content and sedimentation value in 5 + 10 (R2: 0.243); and GPC and gluten index in 2 + 12 wheats (R2: 0.144). It shows that route to bread making could entirely be different in 5 + 10 and 2 + 12 wheats.
Table 4.
Multiple regression statistics of key gluten properties in bread making
| Components | Coefficients | Significance (P value) | Coefficients | Significance (P value) |
|---|---|---|---|---|
| 5 + 10 glutenin subunit wheats R2: 0.243 | 2 + 12 glutenin subunit wheats R2: 0.144 | |||
| Grain protein | – | – | 7.50 | <0.001 |
| Wet gluten | 1.93 | <0.001 | – | 0.001 |
| Sedimentation value | 1.05 | <0.001 | – | 0.059 |
| Gluten index | – | – | 0.56 | <0.001 |
Pattern of protein-gluten relationship
Pooled data was plotted to study the pattern of association between gluten and protein contents. It is obvious that wet gluten increases with rise in protein content, the coefficient of determination (R2) therefore was high in both types of wheat (Fig. 1). However, the rate of increase in 5 + 10 wheats was sharp up-to GPC 12 % only. It slowed down thereafter and reached plateau when GPC touched 13 % level. However, wheats of 2 + 12 glutenin subunits expressed continuous increase in wet gluten under advancing protein levels. This pattern in 5 + 10 wheats had reasonably good coefficient of determination (R2:0.13). In 2 + 12 category however, R2 declined to 0.05. It was also observed that highest PGR (2.6) in 5 + 10 group was achieved when GPC and wet gluten content contents were 12 and 35 %, respectively. In comparison, highest PGR in 2 + 12 wheats was 2.7 achieved at GPC 11 % and wet gluten 37.5 %. Highest wet gluten level in 5 + 10 wheats was 32 % and it was achieved when GPC touched 13 % level. In case of 2 + 12 wheats however, this much gluten could be harvested even at 12 % GPC. In wheats of 2 + 12 glutenin subunits, highest wet gluten level was 36 %, achieved at 14.5 % GPC.
Fig. 1.
Polymer trend (Order 2) in wheats of 5 + 10 and 2 + 12 glutenin subunits
Discussion
Importance of HMWGS is rated high in wheat quality. Variations at Glu D1 locus are reported relevant in articulating dough quality for good product making (Pena 2008; Kolster et al. 1991). During the selection process, they advised to promote 5 + 10 glutenin subunits over 2 + 12 to bring improvement in bread quality. This analysis of high yielding Indian wheats clearly demonstrates that wheats of 2 + 12 glutenin subunits can also be equally efficient in bread making. Study demonstrates that 5 + 10 glutenin subunits are no guaranty of better bread quality under all type of environments. In India, 5 + 10 type wheats exert better bread quality only in the central-peninsular region. In some other region like IGP, 5 + 10 wheats fail to register higher bread loaf volume over the 2 + 12 type wheats.
Wheats of 5 + 10 glutenin subunits have better gluten strength (sedimentation value) and gluten quality (gluten index) which make the dough quality superior (Payne et al. 1981, 1987). This study put a condition to such relationship. Investigations amply demonstrated that 5 + 10 wheats do have better gluten strength under all environments but superiority in gluten quality is restricted only to the areas where protein content is high as observed in NWPZ and PZ. There was no major difference in gluten index under NEPZ and CZ conditions as GPC was low in these zones. In fact, gluten index in CZ is lowest in the country which reflects that dry-hot weather is hindrance to gluten quality (Mohan et al. 2011). Loss in gluten strength and gluten quality is supplemented by high gluten content in 2 + 12 wheats. In this study, sedimentation value was negatively associated with gluten content in 5 + 10 wheats (p < 0.05). High gluten strength in 5 + 10 wheats therefore could have ended in lower gluten content.
Study tried to explain the gluten properties which aid good bread making in 2 + 12 glutenin subunits. It demonstrated that despite lower gluten strength and gluten quality, it is possible to augment bread quality by elating gluten levels. In Indian wheats, gluten content is the major quality defining parameter (Mohan and Gupta 2013a). In this study, 2 + 12 wheats recorded wet gluten content higher than 5 + 10 wheats in three zones. It was CZ where gluten content remained at par in the two wheat groups. Higher gluten content in 2 + 12 wheats occurred due to better gluten harvest per unit protein. PGR in all zones except CZ was higher in 2 + 12 wheats which helped to attain higher gluten content. In CZ, PGR matched in both categories of wheat; therefore differences in gluten content were also diminished. PGR in CZ was already highest amongst all zones, further addition therefore was not happening. This analysis of high yielding Indian wheats clearly demonstrates that wheats of 2 + 12 glutenin subunits can also be equally efficient in bread making. Unlike 5 + 10 wheats where bread quality is derived from better gluten strength and gluten quality, 2 + 12 make it up through more gluten content as they derive more gluten from per unit protein.
It was interesting to observe that significant difference in gluten content even when GPC matched in two categories of wheat. It happened mainly protein-gluten ration was different. Genotypic differences in PGR have been highlighted in wheat quality (Simic et al. 2006) and at Glu D1 locus in Indian wheat programme (Mohan and Gupta 2013b). This investigation also illustrated that pattern of gluten-protein relationship differs in the two groups as gluten content keeps on increasing with advancing protein levels in 2 + 12 group whereas protein fails to add gluten after a certain level in 5 + 10 wheats. It shows that there is always a ceiling in 5 + 10 wheats for gluten content and high protein levels prove no good for gluten content after 13 % GPC. The 2 + 12 wheats, even though not rich in gluten strength and quality, make up bread loaf volume through higher gluten content. It was a clear demonstration to depict better efficiency of gluten harvest in 2 + 12 wheats through higher gluten from per unit protein. Besides gluten content, PGR was found relevant for gluten index and gluten strength as this association was highly negative (p < 0.01). Since 2 + 12 have high PGR in comparison to 5 + 12, therefore their gluten strength and gluten quality was also low.
With glaring difference in quality and quantity of gluten, the route to bread quality will certainly differ in these two classes of wheat. Protein and gluten contents, gluten strength and gluten quality have high relevance in bread quality of 5 + 10 wheats. In 2 + 12 wheats however, only protein and gluten contents and to certain extant gluten index express significant role in bread making. This study asserts that gluten content and gluten strength are the key gluten characteristics in 5 + 10 whereas protein content and gluten quality hold the key in 2 + 12 wheats. Such revelations are useful to the breeders in devising selection criteria for selection of bread superior genotypes in wheats of two different classes. In the elite material, 2 + 12 wheats can also register better nutritional quality because of higher protein content. In 2 + 12 wheats, gluten index correlated adversely with GPC (p < 0.01). Since gluten index is lower in 2 + 12 wheats, this could benefit protein content. These revelations concerning to gluten characteristics are important for the researchers to enhance bread quality in wheats differing at Glu D1 locus.
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