The gaseous exchange of NH3 between plants and the atmosphere is important in the N budgets of agricultural cropping systems, yet the physiological bases underlying plant NH3 fluxes remain obscure. The objective of this study is to describe NH3 fluxes from shoots of well fertilized growth‐chamber grown wheat (Triticum aestivum L.) plants from early vegetative growth through maturity. Effects of developmental stage and photorespiration on shoot NH3 and CO2 exchange were investigated with a steady‐state, whole‐plant gas exchange cuvette. Ammonia compensation points were also determined during the grain‐filling period. Volatilization (loss) of NH3 from wheat shoots at ambient [NH3] was consistantly observed throughout the study. Plant CO3 exchange rates and NH3 volatilization rates were strongly influenced by leaf area, although trends for exchange of the two gases were dissimilar when expressed on a leaf area basis. Photorespiration was only modestly associated with level of NH3 volatilization. The highest rates of NH3 volatilization occurred during grain filling, with two bursts of high activity after anthesis and prior to maturity. Ammonia compensation points rose as plants approached maturity. Increased potential for NH3 volatilization during later developmental stages is attributed to ontogenetic changes in plant N metabolism that elevate tissue [NH+4] from concentrations occurring in younger, vegetative tissue.
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Morgan et al. (1989) studied this question.