# Fine Root Length of Maize Decreases in Response to Elevated CO2 Levels in Soil
## 4 Discussion  
This study examined the morphology of maize root systems exposed to simulated CO\({}_{2}\) leakage from CCS. The significant decrease of the total root length induced by elevated soil CO\({}_{2}\) indicated that leaked CO\({}_{2}\) could inhibit root growth. More specifically, the shortening of the total root length was mainly because of the reduction of fine root length (Table 2). [PERSON] et al. demonstrated that root length was significantly less (by 47%) in CO\({}_{2}\) gas injection treatments (with an introduced flow rate of 250 mL min\({}^{-1}\)) relative to the control [27]. We confirmed that the mean root lengths for the elevated soil CO\({}_{2}\) (G1000 and G2000) treatments were reduced by 7.59% and 44.73%, respectively, compared to the CK treatment. Our findings suggest that the knowledge of the decrease in fine root length is essential for explaining how plants respond to CO\({}_{2}\) leakage from CCS. Apart from the discussion of the morphological characteristics of the total root system for the CK and elevated soil CO\({}_{2}\) treatments, it is important to highlight the root diameter, as this would demonstrate a noteworthy and interesting response to leaked CO\({}_{2}\) in soil. The median basal diameter of nodal-root whorls was substantially lower in CK than those in the elevated soil CO\({}_{2}\) treatments (Figure 3, \(p\)\(<\) 0.05). The diameter results for each nodal-root whorl was equivalent to the mean diameter changes of total root systems (Figure 3), indicating that there were fewer lateral roots in the CO\({}_{2}\) treatments than in CK. Additionally, root diameter and length distribution are important features of root systems [28]. Many studies have examined how the diameter class lengths of plants are affected by different fertilization measures [29; 30; 31], however, more information is needed to understand the effects of elevated CO\({}_{2}\) levels on plant roots. Figure 4 shows the exponential relationship between root length and basal diameter classes, and the observed significant root length and diameter change between CK and CO\({}_{2}\) treatments. Injected CO\({}_{2}\) resulted in reduced root lengths and increased root diameters. Other studies of maize roots have shown that CO\({}_{2}\) leakage produced adventitious roots [20], the prop roots of maize arising from the lower stem, which would increase the mean root diameter. However, the model in our study was different from that of [PERSON] et al. who used the non-linear regression extreme value model to fit the diameter class length (DCL; mm cm\({}^{-3}\)) [29]. Possibly due to different scanner resolutions, the minimum class of the maize root diameter was 0.1 mm in our study and 0.01 mm in the [PERSON] et al. study. Furthermore, it should be noted that the present study scanned all the roots of maize without distinguishing between above-ground nodal roots (adventitious roots) and below-ground nodal roots. This may be a possible explanation for the fact that the root lengths with a diameter \(>\)2.9 mm did not show a consistent trend regarding the root diameter-class distribution (Figure 4a, Table 3). \begin{table}
\begin{tabular}{c c c c c} \hline \hline \multirow{2}{*}{**Treatments**} & \multicolumn{4}{c}{**Root Length (cm)**} \\ \cline{2-5}  & **0 \(<\) d \(\leq\) 1.00** & **1.0 \(<\)d \(\leq\) 2.00** & **2.0 \(<\) d \(\leq\) 2.90** & **d \(>\) 2.90** \\ \hline CK & 34763.6 \(\pm\) 3769.6a & 2107.4 \(\pm\) 74.0a & 519.4 \(\pm\) 34.3 ab & 532.6 \(\pm\) 41.3a \\ G1000 & 24529.3 \(\pm\) 1325.5b & 1341 \(\pm\) 85.7b & 544.8 \(\pm\) 22.2a & 625.5 \(\pm\) 58.5a \\ G2000 & 18814.3 \(\pm\) 2735.3b & 1202.8 \(\pm\) 83.9b & 419.3 \(\pm\) 34.8b & 533.0 \(\pm\) 47.3a \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Impact of different CO\({}_{2}\) treatments on root length (mean \(\pm\) SE) of different diameter classes. d represents root diameter (mm). The numbers with the same letters are not significantly different at a 5% significance level.