Role of sugars in nitrate utilization by roots of dwarf bean

Abstract

Nitrate uptake and in vivo, nitrate reductase activity (NRA) in roots of Phaseolus vulgaris, L. cv. Witte Krombek were measured in nitrogen‐depleted plants of varying sugar status, Variation in sugar status was achieved at the start of nitrate nutrition by excision, ringing, darkness or administration of sugars to the root medium.The shape of the apparent induction pattern of nitrate uptake was not influenced by the sugar status of the absorbing tissue. When measured after 6 h of nitrate nutrition (0.1 mol m−3), steady state nitrate uptake and root NRA were in the order intact>dark>ringed>excised. Exogenous sucrose restored NRA in excised roots to the level of intact plants. The nitrate uptake rate of excised roots, however, was not fully restored by sucrose (0.03–300 mol m−3).When plants were decapitated after an 18 h NO3− pretreatment, the net uptake rate declined gradually to become negative after three hours. This decline was slowed down by exogenous fructose, whilst glucose rapidly (sometimes within 5 min) stimulated NG−3 uptake. Presumably due to a difference in NO3− due to a difference in NO3− uptake, the NRA of excised roots was also higher in the presence of glucose than in the presence of fructose after 6 h of nitrate nutrition. The sugar‐stimulation of, oxygen consumption as well as the release of 14CO2 from freshly absorbed (U‐14C) sugar was the same for glucose and fructose. Therefore, we propose a glucose‐specific effect on NO3− uptake that is due to the presence of glucose rather than to its utilization in root respiration. A differential glucose‐fructose effect on nitrate reductase activity independent of the effect on NO3− uptake was not indicated.A constant level of NRA occurred in roots of NO3− induced plants. Removal of nutrient nitrate from these plants caused an exponential NRA decay with an approximate half‐life of 12 h in intact plants and 5.5 h in excised roots. The latter value was also found in roots that were excised in the presence of nitrate, indicating that the sugar status primarily determines the apparent rate of nitrate reductase decay in excised roots.