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

The NH3 compensation point (chi(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 degrees 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 NH, exchange irrespective of the NH3 level. For both cultivars a marked decrease in chi NH3 was observed in the period from tillering to anthesis, In cv. Golf, chi NH3 decreased from 6.4 +/- 1.1 to 3.0 +/- 0.4 nmol NH3 mol(-1) air, while chi 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 H-2 during leaf senescence.