Effect of elevated C[O.sub.2] concentration and nitrate: ammonium ratios on gas exchange and growth of cassava

Aims This study evaluated how different nitrogen forms affect growth and photosynthetic responses of cassava to C[O.sub.2] concentration. Methods Cassava was grown in 14-L pots in a greenhouse at 390 or 750 ppm of C[O.sub.2]. Three nitrogen treatments were applied: (a) 12mM N[O.sub.3.sup.-],(b) 6mM N[O.sub.3.sup.-]+ 6mM N[H.sub.4.sup.+], and (c) 12mM N[H.sub.4.sup.+]. Results Thirty-six days after treatments began, plants grown under elevated C[O.sub.2] and fertilized only with N[O.sub.3.sup.-] (750_N[O.sub.3.sup.-]) had photosynthetic rates similar to plants grown under 390_N[O.sub.3.sup.-], indicating significant photosynthetic acclimation to C[O.sub.2]. In contrast, photosynthetic rates at elevated C[O.sub.2] increased as N[H.sub.4.sup.+] increased in the nutrient solution, such that photosynthetic acclimation was reduced for plants fertilized with only N[H.sub.4.sup.+]. However, this positive effect of N[H.sub.4.sup.+] on photosynthesis was not observed in more advanced growth stages, and the toxic effects of N[H.sub.4.sup.+] severely reduced total dry mass for these plants measured at the end of the experiment. Conclusions Our results indicate that cassava will respond with increased biomass accumulation in response to raising atmospheric C[O.sub.2] levels, and that N form can have an important impact on the photosynthetic response. However, the positive effect of N[H.sub.4.sup.+] fertilization on cassava photosynthetic C[O.sub.2] response eventually led to a toxicity problem that reduced biomass production. The challenge is to determine how to manage N[H.sub.4.sup.+] fertilization so that the photosynthetic benefit observed in the initial phase may persist throughout the crop cycle. Keywords Photosynthesis * Water use efficiency. Stomatal conductance * Transpiration * Nitrogen * Climate change