Thermoperiod affects the diurnal cycle of nitrate reductase expression and activity in pineapple plants by modulating the endogenous levels of cytokinins.

Nitrate reductase (NR, EC 1.6.6.1) activity in higher plants is regulated by a variety of environmental factors and oscillates with a characteristic diurnal rhythm. In this study, we have demonstrated that the diurnal cycle of NR expression and activity in pineapple (Ananas comosus, cv. Smooth Cayenne) can be strongly modified by changes in the day/night temperature regime. Plants grown under constant temperature (28 degrees C light/dark) showed a marked increase in the shoot NR activity (NRA) during the first half of the light period, whereas under thermoperiodic conditions (28 degrees C light/15 degrees C dark) significant elevations in the NRA were detected only in the root tissues at night. Under both conditions, increases in NR transcript levels occurred synchronically about 4 h prior to the corresponding elevation of the NRA. Diurnal analysis of endogenous cytokinins indicated that transitory increases in the levels of zeatin, zeatin riboside and isopentenyladenine riboside coincided with the accumulation of NR transcripts and preceded the rise of NRA in the shoot during the day and in the root at night, suggesting these hormones as mediators of the temperature-induced modifications of the NR cycle. Moreover, these cytokinins also induced NRA in pineapple when applied exogenously. Altogether, these results provide evidence that thermoperiodism can modify the diurnal cycle of NR expression and activity in pineapple both temporally and spatially, possibly by modulating the day/night changes in the cytokinin levels. A potential relationship between the day/night NR cycle and the photosynthetic pathway performed by the pineapple plants (C(3) or CAM) is also discussed.