Senescence-induced changes in apoplastic and bulk tissue ammonia concentrations of ryegrass leaves

•  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 .