The experiment was conducted in a glass-house in order to determine the heterosis for photosynthetic characters in F1 hybrids produced by crossing a thermosensitive male sterile (TGMS) line, 103 s (female parent) and two drought-tolerant upland rice male-parent cultivars (Beodien and IR 71525). Three levels of soil-water potential, well watered (WW), moderate water deficit (MD), and severe water deficit (SD) were imposed at tillering stages. Heterosis for photosynthesis in terms of a carbondioxide exchange rate (CER) in single leaf as well as other related characters such as stomatal conductance, transpiration rate, specific leaf area, chlorophyll content and water use efficiency were measured in rice plants under drought and recovering conditions using a portable type of CO2/H2O gas analyzer (LI-6400, LI-COR, USA). The results showed that while suffering drought, the heterosis value for CER over the best parent (Hb) in two F1 decreased from 1.02 to 0.68. However, after recovering from drought this value increased in both F1 hybrids from 0.79 to 1.13. Heterosis for WUE increased in F1 103S/Beodien in both drought treatments (Hb from 0.73 to 3.07). This value increased in the 103S/IR 71525 (Hb = 0.66-0.55) in moderate drought conditions (Hb from 0.77 to 1.14), whereas it decreased (Hb from 1.14 to 0.23) under severe drought conditions. An insignificant heterosis for SPAD (an indicator of leaf chlorophyll content) was observed in F1 hybrid rice under all conditions. The interesting CER characteristic in F1 hybrid rice under drought conditions may be due to both greater stomatal conductance and water-use efficiency. Thus, the results indicate that it is possible to produce an F1 hybrid with high yield potential and drought tolerance by crossing a TGMS line with upland male-parent cultivars. Key Words: Photosynthesis, heterosis, F1 hybrid rice, drought tolerance, water use efficiency. ??
Author(s): Duong Thi Thu Hang, Pham Van Cuong, Hoang Viet Cuong, Toshihiro Mochizuki