ORIGINAL ARTICLE Amarjit S. Basra ® Ashok K. Dhawan ® Sham S. Goyal DCMU inhibits in vivo nitrate reduction in illuminated barley (C 3 ) leaves but not in maize (C 4 ): A new mechanism for the role of light? Received: 14 February 2002 /Accepted: 8 April 2002 / Published online: 15 June 2002 Ó Springer-Verlag 2002 Abstract The leaves of C 4 plants possess a superior metabolic efficiency not only in terms of photosynthetic carbon assimilation, but also in terms of inorganic ni- trogen assimilation, when compared to C 3 plants. In vivo nitrate assimilation efficiency of leaves is dependent on light, but the obligatory presence of light has been debated and its role remains confounded. This problem has not been addressed from the standpoint of the C 3 vs. C 4 nature of the species investigated, which may actually hold the key to resolve the controversy. Here, we present the first report providing evidence for differential photo- regulation of leaf nitrate reduction in barley (Hordeum vulgare L.) vs. maize (Zea mays L.) plants, which may help explain the superior nitrogen-use efficiency (and hence superior productivity) of maize plants. The novel finding that carbohydrate-depleted maize leaves were able to reduce nitrate when photosynthesis was inhibited by 3-(3¢,4¢-dichlorophenyl)-1,1¢-dimethylurea (DCMU) in the presence of light, raises a very important question about the possibilities of a new photo-regulatory mechanism for supporting nitrate reduction in maize leaves operating independently of photosynthetic carbon dioxide fixation. On the other hand, leaves of barley could not carry out any in vivo nitrate assimilation, whatsoever, under these conditions. We find another fundamental difference between the two species in terms of differential regulation of nitrate reductase (NR; EC 1.6.6.1). In barley leaves, NR activity and activation state remained unaffected due to DCMU, but in sharp contrast, both were appreciably upregulated in maize. Collectively, the results indicate that enzyme capacity is not limiting for nitrate reduction in leaves, as the NR activity was higher in barley than in maize. The maize leaves may have had a selective advantage due to C 4 morphology/metabolism in terms of maintaining a better reductant/carbon skeleton supply for nitrate re- duction. Keywords Carbon dioxide assimilation ® Hordeum (nitrate reduction) ® Light ® Nitrate reduction in vivo ® Zea (nitrate reduction) ® DCMU Abbreviations DCMU: 3-(3¢,4¢-dichlorophenyl)-1, 1¢- dimethylurea; ® NR: nitrate reductase Introduction Nitrate reduction in most plant species occurs predom- inantly in the leaves, and is dependent on light not only for the generation of reducing power and ATP but also for the supply of carbon via photosynthetic CO 2 as- similation (Aslam et al. 1979; Aslam and Huffaker 1984; Li and Oaks 1994; Oaks 1994; Provan and Lillo 1999). Likewise, nitrate reductase (NR; EC 1.6.6.1), the first rate-limiting enzyme in the overall process of nitrate assimilation, is reportedly regulated by light (Campbell 1999; Kaiser et al. 1999; Appenroth et al. 2000). Central to the role of light in nitrate reduction is the issue of whether light per se is obligatory or its effect exerted indirectly via photosynthetically derived carbohydrates. Evidence for no direct dependence on light is based on observations that the light effect is mimicked by exter- nally supplied sugars (Aslam et al. 1979; Aslam and Huffaker 1984; Cheng et al. 1992; De Cires et al. 1992). On the other hand, Li and Oaks (1994) observed that on transfer of maize plants to darkness the decay of NR activity and NR mRNA in shoots commenced immedi- ately. Rapid inactivation of NR also occurs in the light when CO 2 fixation is prevented by CO 2 deprivation (Kaiser and Brendle-Behnisch 1991; Lejay et al. 1997), Planta (2002) 215: 855–861 DOI 10.1007/s00425-002-0802-9 A.S. Basra ® A.K. Dhawan ® S.S. Goyal (&) Department of Agronomy and Range Science, University of California, Davis, CA 95616, USA E-mail: ssgoyal@ucdavis.edu Fax: +1-530-7524361 Current address: A.K. Dhawan CCS Haryana Agricultural University, Regional Research Station, Karnal-132001, India