PLANT-GROWTH IN RELATION TO THE SUPPLY AND UPTAKE OF NO3- - A COMPARISON BETWEEN RELATIVE ADDITION RATE AND EXTERNAL CONCENTRATION AS DRIVING VARIABLES

Two approaches to quantifying relationships between nutrient supply an d plant growth were compared with respect to growth, partitioning, upt ake and assimilation of NO3- by non-nodulated pea (Pisum sativum L. cv . Marma). Plants grown in flowing solution culture were supplied with NO3- at relative addition rates (RAR) of 0.03, 0.06, 0.12, and 0.18 d- 1, or constant external concentrations ([NO3-]) of 3, 10, 20, and 100 mmol M-3 over 19 d. Following acclimation, relative growth rates (RGR) approached the Corresponding RAR between 0.03-0.12 d-1, although grow th was not limited by N supply at RAR=0.18 d-1. Growth rates showed li ttle change with [NO3-] between 10-100 mmol M-3 (RGR=0.15-0.16 d-1). T he absence of growth limitation over this range was suggested by high unit absorption rates of NO3-, accumulation of NO3- in tissues and pro gressive increases in shoot: root ratio. Rates of net uptake of NO3- f rom 1 mol m-3 solutions were assessed relative to the growth-related r equirement for NO3-, showing that the relative uptake capacity increas ed with RGR between 0.03-0.06 d-1, but decreased thereafter to a theor etical minimum value at RGR congruent-to 0.15 d-1. Balancing concentra tions of NO3-, defined as the steady-state external concentration requ ired to sustain a given RAR, were calculated to be 0.3, 1.0 and 12.1 m mol M-3, respectively, for RAR of 0.03, 0.06, and 0.12 d-1. These show ed reasonable agreement with concentrations measured in the culture so lutions. The results are discussed in the context of the supposed theo retical and technical advantages of the relative addition approach com pared with the use of concentration in quantifying limiting nutrient s upply. Comparison of a number of parameters related to growth and nitr ate assimilation suggests that plant response to N supply is intrinsic ally independent of the method employed.