Growth and Preferences for Ammonium or Nitrate Uptake by Barley in Relation to Root Termperature

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Abstract
Barley plants (Hordewn 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°C. Plants were then subjected to root temperatures of 3, 5, 7, 9, 11, 13, 17 or 25 °C, with common air temperature of 25/15 °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 °C. The growth response was biphasic following an increase in root temperature. Phase I, 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 I the shoot dry weight specific growth rates were inversely related to root temperature between 3–13 °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 °C, more so in the shoots than roots and most acutely in the youngest fully expanded leaf (2·l–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 °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, Il, 13, 17, and 25 °C. At all temperatures the preference for NH4+ over NO3− uptake increased with time. An inverse relationship between root temperature (3–11 °C) and the uptake of NH4+ as a proportion of total N uptake was apparent during Phase I. The possible mechanisms by which root temperature limits growth and influences N uptake are discussed.

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