Abstract
In the present study, the characteristics of the CO2 exchange rate (CER) of flag leaves during the grain filling stage were compared between a Vietnamese hybrid rice cultivar Vietlai 20 (VL20) and its parent cultivars, the female line 103S and the male line R20. The CER and the related parameters were measured in the morning (8:00-10:00) and in the afternoon (14:00-16:00) at 3 days before heading (DBH), and at 7 and 17 days after heading (DAH). The results showed that there was no significant difference in the CER of flag leaves among the three cultivars at 3 DBH, while at 17 DAH, VL20 exhibited a higher CER than the parents both in the morning and afternoon. Stomatal conductance (Gs) decreased considerably in the afternoon, compared to that in the morning at 7 and 17 DAH for all the cultivars. In fact, VL20 showed lower Gs than the parents at 17 DAH, indicating that the CER was negatively affected by the stomatal behavior. The changes in the mesophyll conductance (Gm), as an indicator of CO2 fixation ability, were similar to those in CER in all the cultivars at the three stages. In particular, VL20 always exhibited higher Gm than the male parent R20 both in the morning and afternoon, which is consistent with the higher nitrogen content in the flag leaves. These results suggested that the high mesophyll function could contribute to the high photosynthetic potential of flag leaves in VL20 during the grain filling stage despite of the stomatal limitation.
