GROWTH OF TOBACCO IN SHORT-DAY CONDITIONS LEADS TO HIGH STARCH, LOW SUGARS, ALTERED DIURNAL CHANGES IN THE NIA TRANSCRIPT AND LOW NITRATE REDUCTASE-ACTIVITY, AND INHIBITION OF AMINO-ACID SYNTHESIS

Diurnal changes in carbohydrates and nitrate reductase (NR) activity w ere compared in tobacco (Nicotiana tabacum. L.cv. Gatersleben) plants growing in a long (18 h light/6 h dark) and a short (6 h light/18 h da rk) day growth regime, or after short-term changes in the light regime . In long-day-grown plants, source leaves contained high levels of sug ars throughout the light and dark periods. In short-day-grown plants, levels of sucrose and reducing sugars were very low at the end of the night and, although they rose during the light period, remained much l ower than in long days and declined to very low levels again by the mi ddle of the night. Starch accumulated more rapidly in short-day-than l ong-day-grown plants. Starch was completely re mobilised during the ni ght in short days, but not in long days. A single short day/long night cycle sufficed to stimulate starch accumulation during the following light period. In long-day-grown plants, the Nia transcript revel was h igh at the end of the night, decreased during the day, and recovered g radually during the night. In short-day-grown plants, the Nia transcri pt level was relatively low at the end of the night, decreased to very low levels at the end of the light period, increased to a marked maxi mum in the middle of the night, and decreased during the last 5 h of t he dark period. In long-day-grown plants, NR activity in source leaves rose by 2- to 3-fold in the first part of the light period and decrea sed in the second part of the light period. In short-day-grown plants, NR activity was low at the end of the night, and only increased sligh tly after illumination. Dark inactivation of source-leaf NR was partia lly reversed in long-day-grown plants, but not in short day-grown plan ts. In both growth regimes, mutants with one instead of four functiona l copies of the Nia gene had a 60% reduction in maximum NR activity in the source leaves, compared to wild-type plants. The diurnal changes in NR activity were almost completely suppressed in the mutants in lon g days, whereas the mutants showed similar or slightly larger diurnal changes than wild-type plants in short days. When short-day-grown plan ts were transferred to long-day conditions for 3 d, NR activity and th e diurnal changes in NR activity resembled those in long-day-grown pla nts. Phloem export from source leaves of short-day-grown plants was pa rtially inhibited by applying a cold-girdle for one light and dark cyc le. The resulting increase in leaf sugar was accompanied by an marked increase in the Nia transcript level and a 2-fold increase in NR activ ity at the end of the dark period. When wild-type plants were subjecte d to a single short day/long night cycle of increasing severity, NR ac tivity in source leaves at the end of the night decreased when the end ogenous sugars declined below about 3 mu mol hexose (g FW)(-1). In sin k leaves in short-day conditions, sugars were higher and the light-ind uced rise in NR activity was much larger than in source leaves on the same plants. The source leaves of wild-type plants in short-day condit ions contained very high levels of nitrate, very low levels of glutami ne, low levels of total amino acids, and lower protein and chlorophyll , compared to long-day-grown plants. Plants grown in short days had re latively high levels of glutamate and aspartate, and extremely low lev els of most of the minor amino acids in their source leaves at the end of the night. Illumination led to a decrease in glutamate and an incr ease in the minor amino acids. A single short day/long night cycle led to an increase in glutamate, and a large decrease in the minor acids at the end of the dark period, and reillumination led to a decrease in glutamate and an increase in the minor amino acids. It is proposed th at sugar-mediated control of Nia expression and NR activity overrides regulation by nitrogenous compounds when sugars are in short supply, r esulting in a severe inhibition of nitrate assimilation. It is also pr oposed that sugars exert a global control on amino acid metabolism The importance of sugars for the regulation of nitrogen metabolism is str ikingly illustrated by the finding that tobacco is carbon and nitrogen limited when it is grown in short-day conditions.