Abstract
Responses to salinity stress of four Japonica type rice cultivars were studied. Rice seedlings were grown in growth pouch by hydroponic culture with or without 50mM NaCl. The length and root surface area for each root diameter were measured by image analysis. Norin 18 was the most tolerant cultivar according to the least reduction rate of total dry weight and relative growth rate (RGR). The RGR was related to the net assimilation rate (NAR), and not to leaf area ratio. NAR was positively correlated with root length and root surface area per plant, and negatively with Na content of shoot and the ratio of leaf area to root surface area (LA/RA). The Na content of shoot decreased exponentially with increasing root length and root surface area per plant. There was a positive correlation between Na content of shoot and LA/RA. The relatively small reduction of secondary and tertiary lateral root under 0.169mm in diameter, and crown root about 0.5mm in diameter in Norin 18 under salinity stress resulted in the largest root length and largest root surface area per plant. A small reduction in the root system and a low LA/RA under salinity stress may serve to maintain the high water absorption ability of this cultivar. These results suggested that a high transpiration rate of Norin 18 efficiently excluded Na from transpiration stream, which resulted in low Na content of shoot under salinity stress, and that a high water content of the cultivar prevented a reduction of NAR, which attributed to the highest RGR under salinity stress.
