Effects of supplied nitrogen form on growth and water uptake of French bean (Phaseolus vulgaris L.) plants -: Nitrogen form and water uptake

In order to investigate the effect of N form on dry matter (DM) formation and water uptake rate, French bean (Phaseolus vulgaris L. `Sotaxa') plants were grown with a split-root system. Three treatments were compared: sole nitrate (NO3-) supply (NN), sole ammonium (NH4+) supply (AA) and spatially separated supply of NO3- and NH4+ (NA). The pH of the nutrient solutions was kept constant at 6.3 using a pH-stat system. 9 days after onset of the treatments, NN plants had higher root (36%) and shoot dry matter (11%) than AA plants. N form drastically influenced partitioning of assimilates: in the NA treatment, the root half exposed to NO3- revealed a 170% higher DM than the root half exposed to NH4+. N form affected stable carbon-isotope discrimination (delta) of leaf tissue. In leaves of plants which were supplied with NH4+ (AA; NA) delta was significantly more negative (-29.4parts per thousand, -29.6parts per thousand) than in NN treatment (-28.2parts per thousand). We explain this effect by differences in stomatal conductance. We suppose that the significantly less negative delta of root tissue under NH4+ supply is most probably related to higher PEP-case activity. The water uptake rate was higher in NN than in AA grown plants. This effect was found in both, short- and long-term experiments. In case of NA plants, the water uptake in the root part being exposed to NO3- was 104% higher than in those receiving NH4+. At least in the case of the NA treatment we can exclude shoot growth effects as being responsible for differences in water uptake. We therefore assume that differences in root hydraulic conductivity are responsible for the observed effects.