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We compared the plant growth, stomatal conductance (gs), leaf water content (LWC), and root length density (RLD) of Oryza sativa L. ssp. japonica cv. Sensho (traditional upland), ssp. indica cv. Beodi...en (traditional upland), and ssp. japonica cv. Koshihikari (improved lowland) under two aerobic rice systems [well-irrigated (WI) and water-saving (WS) treatments]. Irrigation water was applied every 2 days from 21 to 68 days after sowing (DAS) and everyday thereafter in WI treatment and it was applied when soil water potential at 15 cm depth reached −15 kPa from 21 to 68 DAS and every 2 days thereafter in WS treatment to impose repetitive water stress. WS treatment used 35% less water than WI. Leaf area index (LAI) and shoot dry weight (SDW) were the lowest for Koshihikari in both treatments and the ratio of LAI and SDW in WS treatment to that in WI treatment was the lowest in Koshihikari. This indicates that aerobic cultivation was not suitable for Koshihikari even under well-irrigated conditions and that the effect of repetitive water stress was the most serious in Koshihikari. Midday gs of Sensho and Beodien in WS treatment were affected by irrigation, whereas that of Koshihikari was low and stable. LWC of Koshihikari was smaller than those of upland genotypes in both treatments. LWC of upland genotypes in WI and WS reached maximum and minimum values at predawn and evening, respectively, and recovered at night, but LWC of Koshihikari in WS treatment did not recover at night. RLD of upland genotypes was higher than that of Koshihikari, but no significant differences were observed among treatments. These results indicate that genotypic difference of physiological traits under aerobic conditions (both WI and WS) was caused by the genotypic difference of water uptake capacity, which can be partly caused by the RLD. In Koshihikari, however, the LWC difference between treatments can not be explained only by the RLD. Further studies will be needed to clarify physiological mechanism responsible for the water uptake capacity of roots in aerobic rice systems.show more
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