Carbon and Nitrogen Assimilation by Vicia faba L. at Low Temperature: the Importance of Concentration and Form of Applied-N

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Abstract
Low temperature (6 °C) growth was examined in two cultivars of Vicia faba L. supplied with 4 and 20 mol m−3 N as nitrate or urea. Both cultivars showed similar growth responses to increased applied-N concentration regardless of N-form. Total leaf area increased, as did root, stem and leaf dry weight, total carbon content and total nitrogen content. In contrast to findings at higher growth temperatures, 20 mol m−3 urea-N gave substantially greater growth (all parameters measured) than 20 mol m−3 nitrate-N. The increased carbon content per plant associated with increased applied nitrate or urea concentration, or with urea in comparison to nitrate, was due to a greater leaf area per plant for CO2 uptake and not an increased CO2, uptake per unit area, carbon, chlorophyll or dry weight, all of which either remained constant or decreased. Nitrate reductase activity was substantial in plants given nitrate but negligible in plants given urea. Neither free nitrate nor free urea contributed greatly to nitrogen levels in plant tissues. It is concluded that there is no evidence for a restriction in nitrate reduction at 6 °C, and it is likely that urea gives greater growth than nitrate because of greater rates of uptake.

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