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© 1987 Oxford University Press

RESEARCH-ARTICLE

Comparison of the Effects of Root Temperature on Nitrate and Ammonium Nutrition of Oilseed Rape (Brassica napusL.) in Flowing Solution Culture

II. CATION-ANION BALANCE

J. H. MACDUFF1,, M. J. HOPPER2, A. WILD1 and F. E. TRIM2

1Department of Soil Science, The University of Reading London Road, Reading RG15AQ, U.K.
2Institute of Grassland and Animal Production, Hurley, Maidenhead Berks. SL6 5LR, U.K.

To whom correspondence should be addressed.

Macduff, J. H., Hopper, M. J., Wild, A. and Trim, F. E. 1987. Comparison of the effects of root temperature on nitrate and ammonium nutrition of oilseed rape (Brassica napusL.) in flowing solution culture. II. Cation-anion balance.—J. exp. Bot. 38: 1589-1602.

The effects of root temperature and form of N nutrition (NH4 or NOJ) on the mineral composition of the plant, the balance of inorganic cation-anion uptake and on the apparent net efflux of H +/OH–ions from the roots were studied with 49-d-old oilseed rape (Brassica napusL. cv. Bien venu) in flowing solution culture. Plants were pre-treated for 14 d at a root temperature of 5 °C prior to constant root temperatures of 3, 7, 11 or 17°C for 14 d, with a common shoot temperature of 20/15°C day/night. Nitrogen was supplied as NH+44 or NO–3 at 10 mmol m3. Values of Q10 (7-17°C) for mean unit absorption rates of all the major nutrient ions (K+ , Mg++ , NH+4, SO4, H2PO4, NO3), except Ca++, were > 2.0 over the first 5 d of treatment but thereafter were < 1.5; the apparent effect of temperature on uptake rates diminished with time. Under NH+4 nutrition, inorganic cation uptake (Mg+ + + K++Ca+ + +NH+4) exceeded inorganic anion uptake (SO4 –4+ H2PO4) over 14 d at all temperatures, with the proportion of cation uptake as NH4 remaining constant (0.67-0-68) irrespective of root temperature. The net efflux of H + from the roots approximately balanced NH+4 uptake (1:1) over 14 d at each temperature and also balanced the difference between the total uptake of inorganic cations and inorganic anions. Under NO3 nutrition, the sum of the net efflux of OH and the change in the carboxylate contents of plants over 14 d approximately balanced the sum of NO3 and SO –4 reduced in the plant. The majority of the negative charge associated with the reduction of NO3 and SO –4 was apparently effluxed as OH, but this fraction was lower at low root temperatures. The results are discussed in terms of mechanisms that have been proposed to regulate the internal pH of plants.

Key words: Brassica napus, oilseed rape, root temperature, cation-anion balance, H+/OH efflux.


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