Influence of water deficit on durum wheat storage protein composition and technological quality − Rakyat Biologi

Environmental and genetic influence on grain technological quality

Our results confirm the considerable variation in grain protein content and technological quality parameters due to year-on-year variations in climate and especially in rainfall, typical of the Mediterranean area (Garrido-Lestache et al., 2004). The highest values in protein and gluten content were observed in the year showing the best rainfall distribution and the highest rainfall during grain filling. Similar results are reported by López-Bellido et al. (2001) and Garrido-Lestache et al. (2004) who found that high rainfall or irrigation during grain filling is often positively associated with protein content, increasing nitrogen uptake at a time when N availability has a large impact on grain nitrogen concentration (Gooding et al., 2003).

The positive response of gluten index to moderately high temperatures during grain filling, observed under our experimental conditions, has also been reported by Garrido-Lestache et al. (2005). On the contrary, the same authors (Garrido-Lestache et al., 2004) found high temperatures to reduce gluten quality by accelerating grain maturity in bread wheat. This difference between the two species may be due to differences in ripening, which generally takes place earlier in durum wheat (Garrido-Lestache et al., 2005). On the other hand, Borghi et al. (1995) found for both species an increase in alveograph index P/L ratio in the presence of a long period of temperature in the range 30-35 °C. This caused dough strengthening which is considered to be positive for pasta-making quality but detrimental to bread-making quality. Furthermore, also the duration of exposure to elevated temperature may influence grain quality response. In fact, Graybosch et al. (1995) observed optimal protein quality as determined by SDS sedimentation volumes with exposure to approximately 60 hours of temperature greater than 32 °C during grain filling. Instead, protein quality declined with exposure to more than 80 hours of elevated temperature.

With regard to protein composition we observed the influence of year both on gliadin content, which was related to changes in protein content as already reported by Daniel et al. (2000), Panozzo and Eagles (2000) and Saint Pierre et al. (2008), and on glutenin content, which increased on increasing gluten index, as also observed by Gupta et al. (1992). Furthermore, HMW-GS/LMW-GS ratio showed the same trend of protein content according to Pechanek et al. (1997) who found an increase of this ratio at higher protein content.

Relative to water deficit effect on grain quality, we found an increase in protein content that had already been previously observed by other authors in different environments (Rao et al. 1993; Rharrabti et al., 2003a; Garrido-Lestache et al., 2005; Guttieri et al., 2005). Only in 2002-2003 crop season, a slight but not significant decrease in protein and gluten content was observed under rain-fed condition. This might be due to the highest number of days of maximum temperature ranging from 30 to 35 °C recorded with respect to the other seasons (Table 1)….. In fact, in Mediterranean climates, high spring temperatures in combination with water stress could impede filling and translocation of nitrogen to the grain (Garrido-Lestache et al., 2005). Also technological indexes improved under water deficit, according to Garrido-Lestache et al. (2004) who recorded maximum values of alveograph index W when rainfall over the September-May period was lowest in Mediterranean distribution areas, and to Rharrabti et al. (2003b) who found a positive relationship between SDS volume and water deficit during grain filling.

Relative to protein composition, the increase in glutenin content we observed under water deficit appear in contrast with Panozzo et al. (2001) who found no change in the gliadin/glutenin ratio between irrigated and unirrigated environment subjected to high temperature at mid-grain filling. Hajheiadari et al. (2007) and Saint Pierre et al. (2008) found an increase in gliadin content under water deficit consistent with an increase in protein content. On the other hand, in agreement with our data, Panozzo et al. (2001) found an increase in the proportion of highly polymeric glutenins under water deficit. Daniel and Triboї (2002) reported that drought didn’t affect the rate of soluble and insoluble protein accumulation per degree day, but only the onset of polymer insolubilization.

With regard to cultivar effect, our results are consistent with Borghi et al. (1995), D’Egidio et al. (2000), Flagella et al. (2002) and De Vita et al. (2007) who reported that cultivar Simeto, tested in different locations, has a higher pasta-making quality than Ofanto, due to higher values of W and P/L. Moreover, the higher values of glutenins, HMW-GS, LMW-GS, HMW-GS/LMW-GS ratio and %UPP observed for Simeto (Table 3) confirm those reported by Ciaffi et al. (1995) in a study on four durum wheat cultivars grown in four locations in Italy. In this study, Simeto exhibited the highest relative amount of HMW-GS along with the highest amount of insoluble proteins, while Ofanto had the lowest HMW-GS/LMW-GS ratio and the lowest amount of insoluble polymeric proteins. According to De Vita et al. (2007), the presence of the favourable allele b at the Glu-B1 locus coding for glutenin subunits 7+8 allowed a better technological performance in Simeto; instead, the presence of the e allele coding for the HMW-GS 20 subunit in Ofanto, is related to low strength and also to low UPP content as reported by Edwards et al. (2007). Also Panozzo and Eagles (2000) found that the proportion of glutenins in flour protein was highly dependent on cultivar. Furthermore, Zhu and Khan (1999) found that the best bread-making wheat cultivars were characterized by higher proportions of glutenins and greater proportion of HMW-GS. On the other hand, Bénétrix et al. (1994) and Porceddu et al. (1998) showed that in durum wheat LMW-GS are the main protein subunits responsible for cultivar differences and also mainly involved in the largest complexes. According to Edwards et al. (2007), a well developed network for durum wheat would preferentially involve LMW-GS over HMW-GS.

Correlations among quality characters and PCA

The technological parameters SDS test and gluten index resulted to be correlated with each other and with many quality characters. In agreement with Brites and Carrillo (2001) and Clarke et al. (2004) we found no correlation between gluten index and protein and gluten content. On the contrary, we found a positive correlation between SDS sedimentation volume and protein content as already reported by Flagella et al., (2002), Ames et al. (2003) and Clarke et al., (2004). However, in contrast to our findings, Boggini et al. (1997) and Rharrabti et al. (2003b) found a negative correlation between protein content and SDS test. These contrasting results may be observed since SDS sedimentation volume may depend on both protein content and gluten strength. These two characters were found to respond differently to climatic conditions occurring during grain filling (Rharrabti et al., 2003b), which may influence their relationships. The highly positive correlation between glutenin fractions, %UPP and gluten index highlights the key role of these components in durum wheat technological quality. Also Gupta et al. (1992) found a consistent relationship between percentage of glutenins and different wheat quality parameters. Lafiandra and MacRitchie (1997) found a strong correlation between %UPP, bread-making quality and wheat flour dough strength. Also Don et al. (2003) found a clear relationship between glutenin particle size and dough mixing requirements. The relative amount of polymeric protein was found to increase with increasing gluten strength (Kuktaite et al., 2000; Johansson et al., 2001).

The absence of a high correlation between gliadin content and technological indexes was in agreement with Johansson et al. (2001) and Wieser and Kieffer (2001) who reported that the amount of gliadins is not correlated with any of the rheological dough and gluten properties.

The glutenins present in the residue, the HMW-GS, the LMW-GS and %UPP content were highly correlated with each other, but didn’t show significant correlations with protein, gluten and gliadin content. Also Gupta et al. (1992) found no correlation between the proportion of glutenins and protein content, while they observed an increase in the proportion of gliadins on increasing protein content as also observed by Panozzo and Eagles (2000) and Saint Pierre et al. (2008). However, in agreement with our findings, Peckanek et al. (1997) found no relationship between flour protein and gliadin/glutenin ratio, but a significant correlation between HMW-GS/LMW-GS ratio and protein content as observed under our experimental conditions.

Relative to the observed highly significant correlations between glutenin fractions, also Triboї et al. (2000) found an increase in HMW-GS and LMW-GS on increasing glutenin content, but no change in HMW-GS/LMW-GS ratio; however, the relative contribution of HMW-GS and LMW-GS subunits was also found to change. In fact, Jia et al. (1996b) reported a strong correlation between the accumulation of HMW-GS and glutenin content.

A high correlation was found between %UPP, glutenin content, HMW-GS and LMW-GS. This finding was to be expected because HMW-GS and LMW-GS are by far the main components of the glutenin fraction, where they are organized in a variety of polymers whose size distribution ranges from dimers to polymers with molecular weights up to millions (Weegels et al., 1996). These polymeric proteins are known to be the best determinants of technological quality (Dupont and Altenbach, 2003). Johansson et al. (2001) also found a close relationship between SDS insoluble protein polymers and total amount of HMW-GS. On the other hand, Jia et al. (1996b) observed that, while the composition in HMW-GS remained unchanged, the aggregation level of glutenin polymers developed differently depending on the maturation conditions.

Due to the high correlations found among the different quality traits, PCA was performed in order to give a general view of durum wheat response to water deficit in relation to qualitative characters.

The existence of both a “quantity” and a “quality” factor was in agreement with D’Egidio et al. (1990) and Flagella et al. (2002) and highlighted the absence of a correlation of protein and gluten content with gluten quality which is highly dependent on protein composition. By means of PCA analysis genetic differences between the two cultivars under study were more evident across the different environmental conditions. In all the crop seasons, Simeto showed the best technological response as previously observed by D’Egidio et al. (2000), Flagella et al. (2002, 2004) and De Vita et al (2007). The difference between the two cultivars was higher in the first year where Simeto showed also the higher protein and gluten content. Environmental influence on protein composition of different cultivars was already observed by Panozzo and Eagles (2000) and Yahata et al. (2005).

From PCA analysis the progressive improvement in technological quality from the first to the last crop season (factor 1) was clearly evident (Fig 3 a and b), suggesting that the different temperature trends occurring in the experimental years might have been responsible for changes in technological quality, as already discussed. Furthermore, the best performance observed in the second year relative to factor 2 was due to the better rainfall distribution which allowed a higher grain protein and gluten content. Finally, the effect of water stress was an increase in protein and gluten content in the first two cropping seasons, and an improvement in technological quality and protein composition (Fig. 3b) only in the last cropping season when water stress occurred during grain filling. Our results are in contrast with Triboi et al. (2003) and Saint Pierre et al. (2008) who found that changes in protein composition due to water stress were only due to differences in protein content. In fact, in the last cropping season, despite a change in protein composition due to an increase in glutenin content, no significant changes in protein content were observed in the rain-fed treatment. However, in agreement with our results, Jia et al. (1996b) reported that a severe decrease in water availability during grain filling induces a fall in gliadin accumulation, so increasing the glutenin/gliadin ratio. Furthemore, Panozzo et al. (2001) and Panozzo and Eagles (1999) reported that higher rates of synthesis of glutenins were observed in the dryland trials where higher spike temperatures were recorded.