Retention of shoot regeneration capacity of tobacco callus by Na2SO 4.

Callus of tobacco (Nicotiana tabacum L. cv. Wisconsin 38) was grown in the light on shoot-forming medium in the presence of Na2SO4 for over a year. An increase in Na2SO4 concentration resulted in decreasing callus growth, decreasing percentage of calli producing shoots and number of shoots per callus, and increasing callus percent dry weight. Regeneration of shoots from callus grown in the absence of Na2SO4 began to decline after 14 months in culture, and shoot regeneration capacity was completely lost after 18 months. In contrast, 18-month old callus continuously grown in the presence of Na2SO4 retained the ability to form shoots. The highest percent of callus pieces that formed shoots and the maximum number of shoots per callus occurred at 70 mM (1%) Na2SO4. All plants arising from the 18-month old callus were polyploid. Both 9- and 18-month old callus exhibited more negative water and osmotic potentials in the presence of increasing Na2SO4 concentration.