A I4C And ISN Study Of The Effects Of Ammonium Or Nitrate Nutrition On Carbon Allocation In Triticum Aestivum L. And • Zea Mays L.

ABSTRACT

The poor response of some plant species, e.g. wheat, to ammonium nutrition has been

attributed to a diversion of carbon allocation from structural material for root extension to functions

associated with the assimilation and translocation of ammonium in the root. The aim of this research

was to investigate carbon allocation in response to ammonium or nitrate nutrition in wheat, an

ammonium intolerant species, and maize, which exhibits ammonium tolerance. Experiments were ·

carried out at 4mM and 12mM nitrogen feeding levels in sand and hydroponic culture respectively. pH

of growth media was maintained at 6.0 to 6.5. Measurements made included shoot : root ratios,

photosynthetic and root re·spiratory rates, plant water content, xylem sap analysis, and 14c and 15N

allocation to soluble and bound nitrogen compounds, and soluble, storage and structural

carbohydrates.

Stunted root growth occurred in ammonium-fed wheat, which was exacerbated by increasing

the NH4 concentration. No difference in growth response was evident between ammonium- and

nitrate-fed maize. Photosynthetic rates of ammonium- and nitrate-fed plants within both species were

similar but maize showed a 3-fold higher photosynthetic rate than wheat. Root respiration of

ammonium- and nitrate-fed wheat was similar, while nitrate-fed maize appeared to have a higher root

respiratory rate than ammonium-fed maize. Xylem sap analysis showed that for both species,

· ammonium-fed plants translocated more amino compounds and more carbon to the shoots than

nitrate-fed plants, although maize appeared to have a more rapid translocation-rate than wheat.

14c allocation to nitrogenous compounds in roots of ammonium-fed plants was greater than

that in nitrate-fed counterparts for both species. In wheat this increase appeared to be accommodated

by a larger initial diversion of 14c to the root. In maize, reserve carbon in the root appeared to

accommodate this increase. A reduction in 14c allocation to structural material in ammonium-fed

plants compared to nitrate-fed counterparts was not evident in either species.

15N tracing in maize showed that signifi~tly more nitrogen was taken up by ammonium-fed

plants in comparison to nitrate-fed plants. The difference in total N between plants fed ammonium or

nitrate was, however, not nearly as pronounced, suggesting that ammonium may be cycled out of the

plant again.

The response of wheat and maize to ammonium or nitrate nutrition is discussed

independently, and sugge~tions for further research are made.