TEMPORAL COMPARTMENTATION IN LEMNA PAUCICOSTATA: PHOTOPERIODISM, RESPIRATION, NITROGEN NUTRITION AND HETEROTROPHIC GROWTH OF DIFFERENT STRAINS

In previous work with strain 6746 of Lemna paucicostata Hegelm. in heterotrophic culture, changes in the light schedule affected certain features of the daily respiratory pattern, on some but not all nitrogen sources, in a manner parallelling their effects on timing in the photoperiodic flowering response. Seeking further guidance on which metabolic processes should be investigated to understand this relationship, twelve additional strains were compared with 6746 in regard to 1) heterotrophic flowering under short-day skeleton photoperiods, and 2) daily respiratory patterns under 0.25 hr daily of dim red light. Heterotrophic flowering occurred in eight strains; among these, several differed sharply from 6746 in the character of their respiratory patterns and the probable relation of those patterns to photoperiodic timing. For example, in 6746, the second daily peak on NO3, NH4 and probably on glutamine reflects photoperiodic timing; the patterns on N-deficient, aspartate and glutamate media do not. In strains 6609 and 381, in contrast, glutamate also elicits a second peak probably reflective of photoperiodic timing; in strain 421, neither the NO3 nor NH4 patterns resemble those reflecting photoperiodic timing in 6746. Other strain differences in flowering, respiratory patterns and heterotrophic growth provide useful material for studying phytochrome action, N assimilation and respiration, and they confirm the view that strain identity may be crucial in biochemical investigations on Lemnaceae. These results also reinforce the concept that the "temporal compartmentation" of systems entrained to light/dark cycles can provide important insights, and should be more widely used, in work on regulation of plant metabolism. FLOWERING in strain 6746 of Lemna paucicostata Hegelm., extensively studied in this laboratory and elsewhere, is controlled by photoperiod and other environmental factors. The respiration rate of heterotrophic cultures changes in regular daily patterns under entrainment by repeated brief light exposures acting through phytochrome; depending on the nitrogen source supplied, certain features of these daily patterns change with different light schedules in a manner suggesting that they reflect processes also involved in photoperiodic timing (Hillman, 1975a, 1976a,b, 1977). Thus, identification of the biochemical reactions controlling such patterns would advance an understanding of the photoperiodic timing mechanism as well as of the regulation of nitrogen metabolism and the respiratory role of phytochrome. However, present data are not adequate to suggest which among many possible enzymes or small molecules it would be worth assaying to establish the origin I Received for publication 12 January 1979; revision accepted 28 March 1979. I thank Susan S. Lamm for expert technical assistance and Neal Tempel for continued attention to the CO2 monitoring systems. This research was carried out at Brookhaven National Laboratory under the auspices of the U.S. Department of Energy. of the respiratory patterns. Such assays would have to supply values numerous and reliable enough to correlate with patterns based on points recorded at 0.5-hr intervals; without additional guidance, any single series would combine a large demand for resources with a small probability of usefulness. Hence as many approaches as possible are needed to help discriminate between aspects of respiration and N metabolism that are closely related to photoperiodic timing and those that are not. One approach, pattern modification, involves seeking conditions or substances, besides light schedules and N sources, that affect particular features of the daily patterns; differential temperature effects have been studied (Hillman, 1979) and work with metabolic and respiratory inhibitors is in progress. Another approach, which can loosely be called genetic, is suggested by the fact that different strains (clones) of L. paucicostata differ not only in general appearance and growth habit under standard conditions but also in flowering responses (Landolt, 1957; Yukawa and Takimoto, 1976) and in the distribution of various flavonoids (McClure, 1967). Such variations occur even among clones of the first generation from selfed strain 6746 (Hillman, 1975b; McClure, 1967). Differences in chromo-