Photosynthetic traits of upper three leaves in the vietnamese F₁ hybrid rice vietlai 45 and its parents during the ripening period

Vietlai 45 (VL45) is F₁ hybrid rice that has been recently introduced in Vietnam. Despite its high yield potential, the grown period is about two weeks shorter than the parents. To elucidate the factor resulting in this superior trait, we investigated photosynthetic traits in upper three leaves including the flag leaf (FL) during the ripening period in VL45 in comparison with its female and male parents, 103S and R45. At 3 days after heading (DAH), shoot dry weight was higher in VL45 than in the parent cultivare, whereas the leaf morphological traits of VL45 were similar to those of R45. The nitrogen content in leaves tended to be higher in VL45 than in R45 at 14 DAH. At 24 DAH, the FL of VL45 showed higher net photosynthetic rate (PJ and stomatal conductance (GJ than those of the parents, with a symptom of senescence in other leaves. Values of chlorophyll fluorescence parameters did not greatly differ among the three cultivare. P_n, per unit soluble protein content (P_n/SP) in the FL was much higher in VL45 than in the parents at 24 DAH. Thus, it seems that higher Pn in the FL of VL45 is because of high G _a, and P_n/SP. These data demonstrate that VL45 shows the heterosis effect on the dry matter production until heading and the maintenance of high P_n in the FL at late ripening stage, which would contribute to the superior agronomic trait of VL45.