Ontogenetic Variation of Nitrogenase, Nitrate Reductase, and Glutamine Synthetase Activities in Oryza sativa

Abstract

The relatship between the rates of nitrogenase, nitrate reductase, and gluta e synthetase acdviies, and plant ontogeny in rice (O'yza sativa L.), cultivar 'M9', grown in salt marsh sediment with and without nitrate treatment was studied. In both treatments, nitrogenase activity measured as the Immediate lnear rate of acetylene reduction by bacteria associated with the roots varied with plant age. In control plants, the nitrogenase activity devel During the vegetative and early reproductive stages of growth, the development of maximal rates of nitrogenase activity coincided with an increase of total nitrogen of the plants in both treatments.Grasses are considered to assimilate mainly inorganic nitrogen for growth, but they may stimulate nitrogen fixation by bacteria associated with their roots (6, 9, 10). The rate of nitrogen fixation in legumes has been shown to be dependent on plant age (13). The seasonal variations of leaf NRA3 and nitrogen fixation in soybeans have been reported, and each contributes nitrogen during plant development (14, 28 3 Abbreviations: NRA, nitrate reductase activity; GS, glutamine synthetase; GOGAT, glutamine:a-ketoglutarate aminotransferase.(20).The rate of nitrogenase activity associated with grass roots has been reported also to vary with plant development (2,7,8,17,23,24). The rates of leaf NRA and nitrogenase activity have been suggested to peak successively (2, 7, 24). However, the measurements of acetylene reduction were made with excised roots after an overnight incubation. This procedure was adopted because acetylene reduction by excised grass roots was not detected for 8 to 18 h (1). Acetylene reduction by excised grass roots after a long delay is caused by the induction of nitrogenase activity in a proliferating microbial population (3). In a recent review, van Berkum and Bohlool (6) concluded that acetylene reduction by excised roots after an extended delay does not indicate the ability of grasses to support and benefit from nitrogen fixation. In contrast, excised nodulated legume roots reduce acetylene immediately at linear rates (13), and the reported rates of nitrogenase activity in legumes are based on short term assays.We have reported immediate linear rates of acetylene reduction by several salt marsh grasses and rice grown in salt marsh sediment (5). The immediate linear rates of acetylene reduction by rice roots removed from the sediment are consistent with the kinetics of the detection of nitrogenase activity by other nitrogen-fixing systems. Therefore, rice roots have the ability to support nitrogen fixation by associated bacteria. Preliminary studies of 15N2 incorporation have demonstrated that rice may benefit from root-and leafassociated nitrogen fixation (15).The objectives of this work were: (a) to determine how the rate of root-associated nitrogenase activity in rice varies with plant ontogeny using techniques that measure immediate linear rates of acetylene reduction and (b) to study the relationship between the rates of nitrogenase, NRA, and GS a...

Physiology of Root-Associated Nitrogenase Activity in Oryza sativa

“…Rice (Oryza sativa L.) cultivar 'M9' was grown in saltmarsh sediment as previously described without the addition of fertilizer N (14). The water in which the plant growth containers were placed was heated to day/night temperatures of 35/26°C in the greenhouse.…”

“…The rates of nitrogenase activity were determined with a modified, intact-root assay method enabling linear rates of acetylene reduction to be recorded immediately after preparing the plants for the determinations and upon the addition of acetylene to the assay chambers (13). The assay chambers used by van Berkum and Sloger (14) were replaced by 473-ml Mason jars with Plexiglass lids designed to accomodate the plant stems, to allow the exchange of water in the jars for argon and acetylene, and to sample for acetylene reduction (Fig. 1).…”

“…1). The preparation of the plants to determine nitrogenase activity and the methods of gas analysis were as previously described (12)(13)(14).…”

“…The water in which the plant growth containers were placed was heated to day/night temperatures of 35/26°C in the greenhouse. The plants were grown until flowering before they were harvested for the experiments to ensure that maximal rates of root-associated nitrogenase activity had developed (14).…”

“…Immediately linear rates of acetylene reduction have been reported for these plants with intact methods for measurement of root-associated nitrogenase activity (13). This method has been used to interrelate plant age, fertilizer N application, NRA,2 and GS with root-associated nitrogenase activity in rice (14).…”