The significance of plant energy status for the uptake and incorporation of NH4-nitrogen by young rice plants

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Abstract
Uptake and assimilation of inorganic N in young rice plants has been studied with labelled N (N-15). Depletion of the plants' carbohydrate content, obtained by a preceding dark period, resulted in a drastic reduction of NH4 +-N uptake. Plants exposed to low light intensity showed diminishing NH4 +-N uptake rates as compared with plants exposed to full light intensity, the latter showing constant NH4 +-N uptake rates during the whole experimental period. The percentage of labelled insoluble N in total labelled N was not significantly affected by a preceding dark period, whereas the low light intensity resulted in a lower proportion of insoluble N in roots and shoots. The incorporation of labelled N into the insoluble fraction (proteins, nucleic acids) was higher in plants fed with NH4 +-N than in those fed with NO3 -. The uptake of NH4 +-N was not significantly affected by NO3 -, whereas the NO3- uptake rate was considerably reduced in the presence of NH4 +-N. Low energy status of plants affected the nitrate uptake more than the uptake of NH4 +-N. The results show that uptake and assimilation of inorganic N depend much on the energetic status of plants. Nitrate uptake and assimilation is more sensitive to low energy conditions than NH4 +-N.