# Functional Differences of Glutamine Synthetase Isoenzymes in Wheat Canopy Ammonia Exchange

## TL;DR

This study explores how different forms of the enzyme glutamine synthetase in wheat affect ammonia exchange between the plant canopy and atmosphere, with implications for nitrogen efficiency and reducing emissions.

## Contribution

The study identifies distinct functional roles of TaGS isoenzymes in wheat canopy ammonia exchange dynamics.

## Key findings

- The leaf NH3 compensation point, influenced by apoplastic NH4+ concentration, is a key factor in canopy NH3 exchange.
- TaGS2 supports NH3 assimilation from photorespiration and nitrate reduction early in grain filling.
- TaGS1;1 aligns with scavenging NH3 from organic nitrogen degradation later in development.

## Abstract

Canopy ammonia (NH3) exchange is a major contributor to agricultural NH3 emissions and is closely linked to nitrogen-use efficiency. Glutamine synthetase (GS) mediates plant NH3 assimilation, yet the specific roles of different GS isoenzymes in regulating wheat canopy NH3 exchange remain unclear. This study aimed to clarify the functional differences of wheat TaGS isoenzymes in modulating canopy–atmosphere NH3 exchange dynamics using two wheat cultivars (Yumai 49-198 and Xinong 509) under two nitrogen application levels (120 and 225 kg N ha−1). Field experiments combined with FTIR-based NH3 flux measurement, biochemical assays, and molecular analyses were conducted at anthesis and 16, 24, and 30 days after anthesis (DAA). Results showed that the leaf NH3 compensation point, determined by apoplastic NH4+ concentration, is a key factor influencing canopy NH3 exchange. Leaf NH3 sources exhibited distinct temporal specificity: photorespiration and nitrate reduction dominated at anthesis to 16 DAA, whereas nitrogenous compound degradation prevailed at 24–30 DAA. This temporal partitioning was highly coordinated with TaGS isoenzyme expression: TaGS2 was highest in early grain filling, potentially supporting assimilate NH3 from photorespiration/nitrate reduction, while TaGS1;1 expression increased progressively, aligning with the scavenging of NH3 from organic nitrogen degradation. These coordinated patterns suggest that the TaGS isoenzymes play differentiated roles in influencing wheat canopy NH3 exchange. This study thus provides correlative insights that point to potential molecular targets for breeding nitrogen-efficient wheat cultivars and mitigating agricultural NH3 emissions sustainably.