SOURCE STRENGTH REGULATES AN EARLY PHASE-TRANSITION OF TOBACCO SHOOT MORPHOGENESIS

We have taken advantage of specific reductions in the ribulose-1,5-bis phosphate carboxylase/oxygenase concentration in rbcS antisense mutant s of tobacco (Nicotiana tabacum L.) to assess the contribution of sour ce strength (carbohydrate production) to the control of shoot developm ent. Wild-type and antisense plants undergo distinct phases of shoot d evelopment that can be distinguished from one another on the basis of differences in stem elongation rates, internode distances, plastochron indices, leaf sizes, and leaf morphologies. An early phase of shoot m orphogenesis is markedly prolonged in the antisense plants, and an inc reased number of leaves emerge during this phase in the mutants. This delay is specific, inasmuch as the duration and expression of traits c haracteristic of later phases of shoot development proceed normally. I n addition to altered shoot developmental patterns, the antisense muta nts have enhanced shoot/root ratios and markedly increased leaf longev ities. It is likely that these are adaptations that enhance photosynth etic rates. Consistent with this proposal, the total leaf areas and dr y weights of the mutant and wild type are similar at flowering. Collec tively, our results indicate that source strength regulates the durati on of an early phase of tobacco shoot development and the transition t o a later phase. We suggest that this phase change may occur in respon se to the attainment of a threshold source strength, which is delayed in the mutant plants.