PHOTOSYNTHETIC CARBON METABOLISM AND TRANSLOCATION IN WILD-TYPE AND STARCH-DEFICIENT MUTANT NICOTIANA-SYLVESTRIS L

A high rate of daytime export of assimilated carbon from leaves of a s tarch-deficient mutant tobacco (Nicotiana sylvestris L.) was found to be a key factor that enabled shoots to grow at rates comparable to tho se in wild-type plants under a 14-h light period. Much of the newly fi xed carbon that would be used for starch synthesis in leaves of wild-t ype plants was used instead for sucrose synthesis in the mutant. As a result, export doubled and accumulation of sucrose and hexoses increas ed markedly during the day in leaves of the mutant plants. The increas ed rate of export to sink leaves appeared to be responsible for the in crease in the proportion of their growth that occurred during the day compared to wild-type plants. Daytime growth of source leaves also inc reased, presumably as a result of the increased accumulation of recent ly assimilated soluble carbon in the leaves. Even though starch accumu lation did not occur in the leaves of mutant plants, nearly all the su gar that accumulated during the day was exported in the period of decr easing irradiance at the end of the diurnal light period. Changes in c arbon allocation that occurred in leaves of wild-type and mutant plant s near the end of the light period appeared to result from endogenous diurnal regulation associated with the day-night transition.