GROWTH AND PREFERENCES FOR AMMONIUM OR NITRATE UPTAKE BY BARLEY IN RELATION TO ROOT TEMPERATURE

Barley plants (Hordeum vulgare L. cv. Atem) were grown from seed for 28 d in flowing solution culture, during which time root temperature was lowered decrementally to 5-degrees-C. Plants were then subjected to root temperatures of 3, 5, 7, 9, 11, 13, 17 or 25-degrees-C, with common air temperature of 25/15-degrees-C (day/night). Changes in growth, plant total N, and NO3- levels, and net uptake of NH4+ and NO3- from a maintained concentration of 10 mmol m-3 NH4NO3 were measured over 14 d. Dry matter production increased 6-fold with increasing root temperature between 3-25-degrees-C. The growth response was biphasic following an increase in root temperature. Phase 1, lasting about 5 d, was characterized by high root specific growth rates relative to those of the shoot, particularly on a fresh weight basis. During Phase 1 the shoot dry weight specific growth rates were inversely related to root temperature between 3-13-degrees-C. Phase 2, from 5-14 d, was characterized by the approach towards, and/or attainment of, balanced exponential growth between shoots and roots. Concentrations of total N in plant dry matter increased with root temperature between 3-25-degrees-C, more so in the shoots than roots and most acutely in the youngest fully expanded leaf (2.1-6.9% N). When N contents were expressed on a tissue fresh weight basis the variation with temperature lessened and the highest concentration in the shoot was at 11-degrees-C. Uptake of N increased with root temperature, and at all temperatures uptake of NH4+ exceeded that of NO3-, irrespective of time. The proportions of total N uptake over 14 d absorbed in the form of NH4+ were (%): 86, 91, 75, 77, 76, 73, 77, and 80, respectively, at 3, 5, 7, 9, 11, 13, 17, and 25-degrees-C. At all temperatures the preference for NH4+ over NO3- uptake increased with time. An inverse relationship between root temperature (3-11-degrees-C) and the uptake of NH4+ as a proportion of total N uptake was apparent during Phase 1. The possible mechanisms by which root temperature limits growth and influences N uptake are discussed.