Apoplastic and bulk tissue concentrations of NH4+ and H+ were measured during senescence of intact (attached) and excised ryegrass (Lolium perenne) leaves differing in nitrogen and carbon status. The potential for NH3 emission from the senescing leaves was estimated on the basis of the ratio between [NH4+] and [H+], designated the Γ-value, in apoplastic solution and bulk tissue.

Attached leaves with visual symptoms of senescence showed two to three times higher [NH4+] and 0.5–1 unit lower pH in both apoplastic solution and bulk tissue extracts compared with green leaves. The Γ-values were, in all cases, low in attached leaves, ranging from 20 to 300 in the apoplastic solution and 500–900 in the bulk tissue.

In excised leaves with high nitrogen status and low C : N ratio (≈ 10), apoplastic [NH4+] increased from around 40 µm to 2 mm after senescence in darkness for 4–9 d. Bulk tissue water [NH4+] increased in the same period to > 30 mm. Apoplastic Γ-values were in all cases < 1000, while bulk tissue Γ-values increased dramatically and reached more than 60 000 in high-nitrogen leaves.

Ammonia compensation points predicted on the basis of apoplastic [NH4+] and pH in senescing leaves with high-nitrogen status reached 6–8 nmol mol-1 air. Consequently, senescing leaves may constitute a significant source of atmospheric NH3.