Total phenolic, flavonoid contents and antioxidant capacity
Wide range of variations was observed in total phenolic, flavonoid contents and antioxidant capacity in mini core (Table 1). Among 164 rice accessions, TPC ranged from 91.8 (Plant Identification: SORNAVARI; Ancestry: AUS) to 1329.8 (Karayal, AUS) mg GAE/100g. The mean value was 272.4 mg GAE/100g with a CV of 97.0%. TPC ranged from 130.2 (Tauli, AUS) to 1329.8 (Karayal, AUS) mg GAE/100g in red rice with a mean value of 755.2 mg GAE/100g and 34.1% for CV. In white rice, the range was from 91.8 (SORNAVARI, AUS) to 250.8 (LUSITANO, TEJ) mg GAE/100g with mean value of 155.5 mg GAE/100g and CV of 20.6%. The mean value in red rice was significantly higher than that of white rice (755.2 versus 155.5), however, some red rice still showed lower total phenolic content than white rice.
Table 1 Means and ranges of antioxidant traits in 164 sampled accessions from the mini-core collection1
1Different letters in the same column indicate significant difference at P < 0.05 level.
TFC in 164 accessions ranged from 43.4 (SORNAVARI, AUS) to 411.9 (Romeno, TEJ) mg CAE/100g with mean value of 103.3 mg CAE/100g and CV of 79.6%. The range in white rice was from 43.4 (SORNAVARI, AUS) to 153.3 (4595, IND) mg CAE/100g with an average of 67.6 mg CAE/100g and CV of 28.0%; and from 54.0 (Dhan, IND) to 411.9 (Romeno, TEJ) mg CAE/100g with a mean value of 254.1 mg CAE/100g and CV of 30.5% in red rice. Not all red rice had higher TFC although the mean value in red rice was significantly higher than that of white rice (254.1 versus 67.6).
The total antioxidant capacity varied greatly in 164 rice accessions. The range was from 187 (BHIM DHAN, Admix) to 8286.8 (Nang Bang Bentre, AUS) μM TEAC with an averaged at 1118.8 μM TEAC and CV of 138.2%. Among the white rice, the ABTS.+ ranged from 187 (BHIM DHAN, Admix) to 621.2 μM (GPNO 1106, TRJ) TEAC and averaged at 415.9 μM TEAC with CV of 18.6%, the red rice had a significantly higher mean value at 4038.2 μM TEAC with a range from 1707.0 (Tauli, AUS) to 8286.8 μM (Nang Bang Bentre, AUS) TEAC and CV of 33.3%.
Antioxidant properties in different ancestry groups
The 164 accessions were divided into six ancestry groups in present study according to Li et al. (2010) and the total phenolic, flavonoid and ABTS.+ antioxidant capacity were compared among groups (Table 2). The AUS group had the highest mean TPC, TFC and ABTS.+ antioxidant capacity, followed by TEJ group. It showed be pointed out that the AUS and TEJ rice had the highest and the second highest percentage of pigmented rice respectively (20 out of 35 accessions in AUS group and 5 out of 22 accessions in TEJ group are red rice, respectively), indicating that the significant higher values in parameters observed in AUS and TEJ rice may be attributed to its relatively higher percentage of red rice.
Table 2 Means and ranges of antioxidant traits in rice accessions of different ancestry groups1, 2
1Different letters in the same column indicate significant difference at P < 0.05 level.
2IND indica, AUS aus, TEJ temperate japonica, TRJ tropical japonica, ARO aromatic, Admix accessions with mixed ancestry.
Compare to other groups, the ARO group showed lower mean values in antioxidant traits and smaller extent of variation (CV) which was probably because of its small number of accessions. However, the Admix group still showed wide variations in TPC and ABTS although its number of accessions was not high neither (n=12).
When red rice accessions were excluded, there was no significant difference in TFC among ancestry groups; TEJ rice showed significant high value in TPC compared to all the other groups and the highest antioxidant capacity among groups.
Correlations among total phenolic, flavonoid content, antioxidant capacity and apparent amylose content
Pairwise correlation analysis showed that the three parameters of antioxidant property were positively correlated with each other (r = 0.923 − 0.953, P < 0.0001) in 164 rice accessions, which was also observed in 31 red rice (Table 3). However, the correlation between TPC and ABTS, TFC and ABTS were higher in all accessions (r = 0.944, 0.923 respectively) than that of red rice (r = 0.684,0.656 respectively). Also, the correlation between parameters in white rice was pretty weak. This difference may be due to the smaller variation in values of parameters in red rice and white rice, compared to that in the total rice accessions.
Table 3 Correlation analysis of antioxidant traits and amylose content
**** indicate significance at P < 0.0001 level.
The relationship between red pericarp and AAC, AAC and antioxidant traits
25 out of 31 red rice had AAC > 23%, In 164 rice accessions, apparent amylose content positively correlated with TPC, TFC (P < 0.05) and antioxidant (P < 0.05) (Table 3).
Association mapping in 164 rice accessions
Association mapping between three antioxidant traits and 155 SSR markers (Li et al., 2010) in 164 rice accessions was performed based on Q + K model. The results were shown in Table 4 and Figure 1. Overall, a total of 12 different markers from 7 chromosomes were detected in 23 significant marker-trait associations. Four markers, Rid12, RM484, RM162, RM5371 were commonly detected for phenolic content, flavonoids content and antioxidant capacity. The Rid12 marker which targeted an InDel at the Rc locus showed the strongest association with all three traits, this confirmed the Rc gene acting as one main loci responsible for antioxidant traits in rice grain.
Table 4 Marker-trait associations detected in MLM analysis (Q + K model)
Most markers detected in association with antioxidant traits were on chromosome 6, including two of the four common markers detected for TPC, TFC and ABTS. Furthermore, three markers on chromosome 6 were identified for ABTS antioxidant capacity.