Ammonia compensation points in two cultivars of Hordeum vulgare L. during vegetative and generative growth

The NH₃ compensation point (χ_NH3) in Hordeum vulgare cvs Golf and Laevigatum was determined at different growth stages under controlled environmental conditions. The plants were grown to maturity in hydroponics under N limitation. When plants were exposed to NH₃ at realistic ambient levels ranging from 0 to 25 nmol NH₃ mol^?1 air at an air temperature of 20°C, a significant (P < 0.001) linear correlation between the NH₃ flux and the atmospheric NH₃ mole fraction was observed, showing a constant conductivity to NH₃ exchange irrespective of the NH₃ level. For both cultivars a marked decrease in χ_NH3 was observed in the period from tillering to anthesis. In cv. Golf, χ_NH3 decreased from 6.4 ± 1.1 to 3.0 ± 0.4 nmol NH₃ mol^?1 air, while χ_NH3 in cv. Laevigatum dropped from 4.2 ± 0.3 nmol NH₃ mol^?1 air to below the detection limit (< 0.9 nmol NH₃ mol^?1 air). The NH₃ compensation points increased again during senescence, peaking at 5.3 ± 0.8 nmol NH₃ mol^?1 air for cv. Laevigatum. The modern and high-yielding cv. Golf had significantly higher (P < 0.01) NH₃ compensation points than the old and primitive cv. Laevigatum. Golf also had higher shoot NH₄⁺ and total nitrogen concentrations than Laevigatum. During generative growth the ratio between NH₃ and water vapour conductivities increased 10-fold, suggesting a shortening of the diffusive path length for NH₃ compared to H₂O during leaf senescence.