Responses Of Root-related Parameters To N Applications And Its Relations To Photosynthates Accumulation In The Early Stage Of Growth In Rice (Oryza Sativa L.)

In 2005, eight rice varieties (Liangyoupeijiu, Pei'ai64s, Yangdao6, Shanyou63,Zhenshan97, Minghui63, 88s/0293, and Teqing) were employed to investigate thedifferences in responses of main root traits of plant grown in soil-pot culture at fournitrogen levels, respectively. In 2006, under three light intensities, the two varieties,Liangyoupeijiu and Shanyou63, which have significant differences root characters, weregrown hydroponically for investigating relationships between parameters of rootcharacters (maximum root length, total root length, root surface area, root volume androot number) and photosynthates accumulation at three nitrogen levels (10 mg N L^-1, 40mg N L^-1 and 80 mg N L^-1), respectively. The results showed that:1. Under low nitrogen supply, maximum root length, total root length, root surfacearea, root volume, and root number were higher compared with those under middle andhigh nitrogen supplies. The plants grown in low N treatment were higher in root dryweight, root/shoot ratio and concentration of soluble sugar in root compared with thoseunder middle and high nitrogen supplies. Root characters showed differences among theeight cultivars at three nitrogen levels. The root growth of hybrid rice had superiority toits parents and indica inbred, especially in low nitrogen treatment.2. The plants grown hydroponically at low nitrogen level had higher rootmorphological parameters compared with those undermiddle and high nitrogen. Withincrease of N supply, root dry weight and root/shoot ratio decreased while netphotosynthesis, leaf area and whole-plant photosynthesis increased. Concentration,accumulation and distribution of soluble sugar and starch in root were higher at lownitrogen level than those at the other two nitrogen levels, indicating that morephotosynthates were distributed into the root system and therefore enhanced root growth.3. Activities of enzymes involved in sucrose metabolism were changed under various nitrogen supplies. Activity of sucrose phosphate synthase (SPS) in leaf decreasedwith increasing nitrogen supply. Compared with those under high nitrogen treatment,activity of sucrose synthase (SS) in leaf was higher under low nitrogen supply, butactivity of invertase in leaf was lower. Activity of invertase (INV) in root decreased withincreasing nitrogen supply.4. There were significant and positive correlation among root characters andconcentration, accumulation and distribution of soluble sugar and starch in root,suggesting that root growth was closely related with assimilate supply to roots. It wasobserved that significantly positive correlations among soluble sugar and starch in rootand activity of SPS in leaf, activity of INV in root. Significantly positive correlationswere observed among concentration of soluble sugar in leaf and activity of SS in leaf andactivity of INV in root, regardless of significantly negative correlations betweenaccumulation of soluble sugar and SPS in leaf.5. There were significant effects of light intensity on rice growth. Under the samenitrogen supply, whole-plant photosynthesis decreased with decreasing light intensity.Higher ratio of chlorophyll a/b and lower content of total chlorophyll were found inleaves grown under high light intensity. Under high light intensity and low nitrogensupply, there was a higher accumulation of soluble sugar and starch in leaf and root,higher activities of SPS and SS in leaf, lower activity of INV in leaf, and higher activityof invertase in root. Under high light intensity, growth of the roots was observed toenhance.