Auxin regulates leaf senescence and nitrogen assimilation to promote grain filling in maize (Zea mays L.)

Abstract

Context

Leaf senescence is the final stage of leaf development, which is often accompanied by nitrogen remobilization. And also, leaf senescence can be modulated by auxin.

Objective

The objective was to advance in the knowledge of the auxin-mediated mechanisms of leaf senescence and nitrogen assimilation that regulated grain yield.

Methods

Using maize variety Zhengdan 958 (ZD958) as material, a two-year field experiment was conducted to assess the leaf senescence and nitrogen assimilation-related physiological process regulated by Naphthalene acetic acid (NAA) (H₂O, 0.1 mmol/L NAA) and nitrogen levels (N0: No N application, N240: Normal N).

Results

The results showed that NAA significantly increased grain yield under low N conditions, which increased by 55.0 % in 2022 and 29.7 % in 2023, respectively. Auxin promotes grain filling mainly by extending the duration of filling days and increasing the average filling rate. In addition, auxin reduced nitrogen output from leaves, and significantly increased nitrogen accumulation in other organs. The total N accumulation of auxin treatment was increased by 34.7 % under low N condition, and increased by 17.5 % under normal N condition. Furthermore, NAA significantly increased the activities of NR (Nitrate Reductase), GS (Glutamine Synthetase) and GOGAT (Glutamate synthase), thus promoting the nitrogen assimilation ability of leaves. Conversely, auxin application reduced the content of abscisic acid and ethylene, increased the activity of antioxidant enzymes, and decreased the expression of senescence-related genes. This delayed leaf senescence, which was conducive to extending the grain-filling duration and the accumulation of grain starch and protein, subsequently improved the maize yield.

Conclusions

Auxin increased nitrogen assimilation ability and delayed leaf senescence, increase protein and carbohydrate accumulation in the grain, and thus increased the maize yield by 9.3 % and 42.4 %, under normal and nitrogen-deficiency condition.

Implications

This study highlights the impact of externally auxin application on N assimilation and senescence and its regulatory function in grain development.