Exogenous DCPTA ameliorates the soil drought effect on nitrogen metabolism in maize during the pre-female inflorescence emergence stage

Abstract 2-(3,4-Dichlorophenoxy) triethylamine (DCPTA) regulates many aspects of plant development; however, its effects on soil drought tolerance are unknown. We pre-treated maize (Zea mays L.) by foliar application of DCPTA and subsequently exposed the plants to soil drought and rewatering conditions during the pre-female inflorescence emergence stage. Exogenous DCPTA significantly alleviated drought-induced decreases in maize yield, shoot and root relative growth rate (RGR), leaf relative water content (RLWC), net photosynthetic rate (Pn), stomatal conductance (Gs) and transpiration rate (Tr), nitrate (NO3−), nitrite (NO2−), and soluble protein contents, and nitrate reductase (NR), nitrite reductase (NiR), isocitrate dehydrogenase (ICDH), alanine aminotransferase (AlaAT) and aspartate aminotransferase (AspAT) activities; increases in the intercellular CO2 concentration (Ci), the ammonium (NH4+) and free amino acid contents, and the glutamate dehydrogenase (GDH) and protease activities. Simultaneously, exogenous DCPTA improved the spatial and temporal distribution of roots and increased the root hydraulic conductivity (Lp), flow rate of root-bleeding sap and NO3− delivery rates. Moreover, Exogenous DCPTA protected the chloroplast structure from drought injury. Taken together, our results suggest that exogenous DCPTA mitigates the repressive effects of drought on N metabolism and subsequently enhances drought tolerance during the pre-female inflorescence emergence stage of maize. Highlights This is the first article that explores the effects of DCPTA on nitrogen metabolism and the first article that explores the effects of DCPTA on crops under soil drought conditions.

conditions. Compared with the well-watered treatment, in the DCPTA treatment, the 2 6 4 grain number increased by 5.97% in 2016 and by 6.50% in 2017, and the grain yield The shoot growth rate was inhibited during the drought period, the root growth 2 6 8 rate was promoted over days 0-10 and subsequently decreased, and the growth rates of shoots and roots recovered during rehydration ( Fig. 6 and 7). In the drought and days 11-30 in 2017, respectively. 30, there was a significant (P > 0.05) difference in RLD for the well-watered day 20, there was a significant difference in RLD between the drought treatment and the declines in RLD were partially reversed by rehydration. At day 30, there was a 3 0 0 significant (P > 0.05) difference in RLD between the drought treatment and there was a significant difference between RLD for the well-watered treatment and  The root hydraulic conductivity, flow rate of root-bleeding sap and NO 3 -3 0 7 concentrations in the root-bleeding sap declined continuously during the drought 3 0 8 period and recovered during rehydration (Fig. 9). In the drought treatment compared 3 0 9 with the control, the root hydraulic conductivity, flow rate of root-bleeding sap and Under well-watered conditions, DCPTA significantly increased root hydraulic  The RLWC declined continuously over the drought period and recovered 3 3 0 during rehydration (Fig. 10). In the drought treatment compared with the control, drought+DCPTA treatment compared with the control, the RLWC decreased by 11.75% had no significant effect on the RLWC. during rehydration (Fig. 11). In the drought treatment compared with the control, Pn, 25.10% and 12.39%, respectively, on day 20 and by 13.55% and 4.96%, respectively, The activities of foliar GS and GOGAT first increased and then decreased 4 3 5 during the drought period and recovered during rehydration (Fig. 16) caused an increase in the foliar GS and GOGAT activities. In contrast, drought led to faster recovery of the foliar NAD-GDH and NADH-GDH activities after rehydration.

5 6
In the drought+DCPTA treatment compared with the control, the activities of foliar The activities of foliar AlaAT and AspAT first increased and then continuously 4 6 8 decreased during the drought period and recovered during rehydration (Fig. 17). In the 4 6 9 drought treatment compared with the control, the activities of foliar AlaAT and AspAT The protease activity and free amino acid contents increased continuously 4 8 7 during the drought period and decreased during rehydration (Fig. 18). In the drought respectively, on day 20 and by 57.08% and 43.29%, respectively, on day 30 in 2017.

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However, the DCPTA application partially reversed the increases in the protease 4 9 3 activity and free amino acid contents caused by drought.

9 4
In the drought+DCPTA treatment compared with the control, the protease In contrast, drought led to a marked decrease in the foliar protein content. In the 5 0 0 drought treatment compared with the control, the foliar protein content decreased by Significant differences between the foliar protein contents of the well-watered by the stable soot and root RGR, GN and GY (Fig. 5, Fig. 6 and Fig. 7). Moreover, 5 1 6 exogenous DCPTA promoted growth and yield under well-watered conditions. emergence stage is the important stage determining maize yield (Talaat et al., 2015).
The inhibition of maize growth induced by drought could be partly attributed to al., 2011). Similar to previous reports for tomato (Sánchez-Rodríguez et al., 2011) and maize leaves in both the DCPTA-treated and non-treated leaves (Fig. 14). This 5 2 6 decrease may be explained by drought-induced inhibitions in nitrate uptake from the 5 was beneficial to NO 3 uptake under drought conditions.

5
Whether stomatal or non-stomatal factors are the main cause of a reduced Pn 5 5 6 2 1 may be determined by changes in Gs and Ci (Bethke and Drew, 1992). During the 5 5 7 early period of drought, the change of Ci were accompanied by continuously declined 5 5 8 Gs, then Gs decreases but Ci shows an increase (Fig. 11).

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Thus, the decrease of the Pn in drought-treated plants was mainly attributed to 5 6 0 stomatal limitations firstly, and then, non-stomatal limitations induced by the damage 5 6 1 of photochemical mechanism, partly reflected by damaged chloroplast (Fig. 12).

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However, DCPTA application maintains relatively high Gs, ensuring the availability DCPTA application also promoted photosynthesis under well-watered conditions. the leaf cell, is converted to NH 4 + by two successive steps catalysed by NR and NiR.

7 2
NR, the rate-limiting enzyme of nitrogen assimilation, is highly sensitive to stress The up-regulation of foliar NR activity in the drought+DCPTA treatment may result 5 7 8 from the increase in the foliar NO 3 − content (Fig. 14). Moreover, the reduction in the 5 7 9 foliar NiR activity under drought conditions was significantly reversed by the 5 8 0 application of DCPTA, which may be because the DCPTA-stabilized photosynthesis 5 8 1 resulted in a sufficient supply of Fd red (Fig. 11 and Fig. 15), thus promoting the could maintain a high NO 3 − assimilation ability in maize under drought conditions.

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Although the foliar NR and NiR activities declined during the drought period, the foliar NH 4 + content exhibited an increasing tendency in our experiment (Fig. 14). This the cellular spaces of the leaf, implying that photorespiration was partly alleviated 5 9 0 (Fig. 11).

9 1
In plants cells, excessive levels of NH 4 + are destructive, and the major NH 4 + 5 9 2 assimilation pathway is the GS/GOGAT cycle in higher plants. When the 5 9 3 GS/GOGAT cycle is suppressed and the NH 4 + content rises continuously under