Genotypic Differences In The Responses Of Rice Root Growth And Development To Elevated CO₂Concentration-a FACE Study

The concentration of atmospheric CO2has risen from approximately280μmol mol-1in preindustrial times to381μmol·mol-1at present. The latest model predicts that the CO2concentration [CO2] will reach550μmol·mol-1around2050, and by the end of the21st century, it will reach730-1020μmol·mol-1. Rice (Oryza Sativa L) is one of the most important food crops. Root system of rice is a major organ responsible for water and nutrient acquisition, which also synthesizes physiological active compounds, therefore, rice roots have great impact on grain yield formation. To date, only a few number of studies conducted under flooded paddy soils aimed at examining root system response to rising CO2concentration. The following aspects are unclear:Will elevated atmospheric CO2concentration influence morphology characters and biological activities of rice roots? Are there genotypic differences among rice cultivars? What is the relationship between root characters and grain yield? In order to answer these questions, A FACE experiment was carried out in a field located in Jiangdu county, Jiangsu province, using12different rice cultivars as tested cultivars. The CO2concentration in FACE treatment was580μmol·mol-1(200μmol·mol-1above Ambient [CO2]). The main objective of this experiment was to study the effects of elevated CO2 concentration on rice root system and the relationship between root characters and grain yield, and to provide the experimental evidence for improving future rice production under high CO2concentration. Results showed as follows:1. There were significant differences among all tested cultivars (P<0.01) on average length, diameter and volume per adventitious root. Compared to ambient[CO2], the average weight, length, diameter and volume per adventitious root at tillering stage increased by6%(P>0.1),1%(P>0.1),6%(P<0.01) and16%(P<0.05) under FACE, respectively. The average weight, length, diameter and volume per adventitious root at heading stage were enhanced under FACE by13%(P<0.01),8%(P>0.1),5%(P>0.1) and6%(P<0.01), respectively. Similar trends were observed for all cultivars.2There were significant differences among all tested cultivars (P<0.01) on the dry weight, total length, volume and number of adventitious roots per hill. Elevated CO2concentration significantly increased root dry weight, total root length, root volume and number of adventitious roots per hill at tillering stage by26%,22%,37%and21%, respectively. Root dry weight, total root length, root volume and number of adventitious roots per hill at heading stage was enhanced under FACE by31%,22%,24%and15%, respectively. Similar trends were observed for all cultivars. The significant enhancement in root dry weight, root length and root volume of adventitious roots per hill under high CO2concentration were mainly due to higher number of adventitious root, secondly related to better growth of each adventitious root, such as the increment of length, diameter and volume per adventitious root).3There were significant differences among all tested cultivars (P<0.01) on the total absorption area per unit root dry weight, as well as the active absorption area per unit root dry weight. Elevated CO2concentration significantly decreased the total absorption area per unit root dry weight and the active absorption area per unit root dry weight by16.4%and16.1%, respectively. The reduction of rice root activity under high CO2probably related to the greater biomass accumulation and lower plant nitrogen content in rice plant.4There were significant differences among all tested cultivars (P<0.01) on the total absorption area per hill root dry weight, the active absorption area per hill root dry weight. Elevated CO2concentration significantly increased root dry weight per hill, but decreased root activity per unit root dry weight, hence little effect of elevated [CO2] on root activity per hill was observed. FACE treatments significantly decreased the total absorption area per hill root dry weight and the active absorption area per hill root dry weight at heading stage by3.0%and2.6%, respectively. Similar trends were observed for all cultivars.5Elevated CO2concentration increased grain yield of tested cultivars by15%in average, but such increases varied with cultivars. The result of correlation analysis showed that grain yield was positively correlated with root dry weight per hill, total root length per hill, root volume per hill, as well as average diameter per adventitious root. Among all investigated root characters, root dry weight per hill (r=0.497**, n=72) and average diameter per adventitious root (r=0.430**, n=72) showed most close relationship with grain yield. By contrast, negative correlation was found between grain yield and the total absorption area per hill root dry weight (r=-0.433**, n=72) or the active absorption area per hill root dry weight (1=-0.428**, n=72). These results indicated that higher grain yield of rice under FACE might attribute to the significant changes in root weight and root diameter.