Differences in ultrastructure and activity of rice roots under different irrigation and nitrogen supply levels

ObjectivesThis study observed root ultrastructure and measured metabolism of japonica rice under different irrigation and nitrogen supply levels to help understanding the mechanism of suitable irrigation and nitrogen fertilization coupling in yield increase of rice.

MethodsA pot experiment was conducted using a mid-season japonica rice cultivar of Xindao 20 as tested material in 2014 and 2015. Three nitrogen levels of 0, 240 and 360 kg/hm² and three irrigation regimes, including submerged irrigation, alternative wetting and moderate drying (–20 kPa) and alternative wetting and severe drying (–40 kPa) were designed. The root activity, root bleeding and total organic acid content in roots were measured, and root tip ultrastructure was scanned.

ResultsWith the elongation of rice growth, the root oxidation was increased firstly and then decreased with the peak at the panicle initial (PI) stage, the same trend was observed with the index of root exudates, and total organic acids in root exudates with the peak appeared at the heading stage. There was a significant interaction between irrigation regimes and nitrogen levels. At the same nitrogen level, the boundaries of nuclear membrane in root cells at the mid-tillering and PI stages were clear, and typical structure was found under the condition of moderate water stress. Compared with the submerged irrigation, the root activity was also increased at the main stages, the total contents of organic acid were enhanced significantly, such as root oxidative ability and the root exudates were increased by 25.6%–32.0% and 9.1%–18.8% at the mid-tillering stage, respectively, and the total organic acid production was improved by 16.4% at the pre-heading stage. The opposite results were observed under the condition of severe water stress, the number of organelles was reduced, osmiophilic bodies and amyloplast were increased, cells twisted and wrinkled at the later stages, organelles were broken and degraded, cell debris was remained in the cell matrix. Compared with the submerged irrigation, the root activity and root secretion of total organic acid were significantly decreased, such as the root oxidative ability and the root exudates were decreased by 8.8% and 25.6% at the PI stage respectively, and the total organic acids secretion was reduced by 22.8% at the whole stage. In the same irrigation regime, nuclear membrane at the main growth stages was clear and typical structure could be found under the condition of moderate N treatment, when compared with no nitrogen applied, meanwhile root activity and the total content of organic acid were enhanced significantly. The opposite result was observed under the condition of high N treatment, cells twisted and wrinkled in the later stage, organelles was broken and degraded, cell wall and membrane degradation was accelerated and cell debris was destroyed in the cell matrix, which indicated that heavy nitrogen application was not conducive to the maintenance of function of organelles, and root metabolic function declined. Compared with the moderate N treatment, the root oxidative ability and root exudates were reduced by 6.6%–9.8% and 7.7%–15.4% from the mid-tillering to PI stage respectively, meanwhile the content of organic acid in root exudates was reduced by an average of 11.6% under the condition of high N treatment before the heading. Overall, the root activity and the amount of organic acid secretion were significantly reduced.

ConclusionsThe nuclear envelope was most clear and root metabolism ability was strongest under the condition of moderate water stress and moderate N interaction. These results suggested increasing physiological function of root tip cells, enhancing root activity and improving root secretion of organic acids through the appropriate regulation of water and nitrogen will create a good rhizosphere environment for the growth of rice.