In both photoheterotrophic and heterotrophic tobacco cells a reduced supply of sulfate in the medium did not alter the rate but the duration of exponential growth. The higher the sulfate supply in the medium the longer exponential growth proceeded. However, the ion composition of photoheterotrophic and heterotrophic cells was affected by sulfur deficiency in completely different ways. The dynamics in the K+-, Na+-, Mg2*-, nitrate-, phosphate-, and malate-con-tents of photoheterotrophic cells during growth were not at all, or only slightly changed, when the sulfate supply in the medium was reduced from 1.8mM to 1.2 mM, 0.6 mM, or 0.3mM. In heterotrophic tobacco suspensions, however, severe sulfur deficiency caused K+, Na+, Mg2+, and malate to accumulate and nitrate to begin to accumulate earlier inside the cells. Addition of sulfate after 4 days to heterotrophic suspensions grown under sulfur-limiting conditions prevented the accumulation of these cations and anions. During the initial period of growth also phosphate accumulated inside heterotrophic tobacco cells to amounts found to be the higher the smaller the sulfate-content of the media. Apparently, in photoheterotrophic tobacco cells the ion composition can homeostatically be regulated independent from the cells' sulfate supply, whereas the ion composition of heterotrophic tobacco cells appears to be highly dependent on the sulfate supply of the cells.
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