Carbon Partitioning and Export in Mature Leaves of Pepper (Capsicum annuum)

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Abstract
The partitioning of recently fixed carbon by mature pepper leaves has been examined over a 10 h photoperiod using a constant specific radioactivity 14CO2 labelling technique. Changes in the rates of carbon partitioning into export, starch, sucrose and hexoses were examined following changes in irradiance during the photoperiod. Leaves grown under 80 W m−2PAR were exposed to this irradiance for the first 4 h of the photoperiod then the iiradiance was decreased. Leaves accumulated sufficient reserves in the first 4 h to maintain export at the initial rate (approximately 20 μg carbon cm−2 leaf h−1) over the following 6 h of the photoperiod when the net photosynthesis rate (Pn) was decreased to 10% of the initial rate by the decreased irradiance. Export was initially maintained by the depletion of sucrose and hexose and then by carbon from the degradation of starch in the light. If leaves were exposed to low irradiance at the beginning of the photoperiod, then the export rate was linearly related to the Pn during that period. When Pn exceeded that required to maintain an export rate of approximately 20 μg carbon cm−2 h−1, then more carbon was partitioned into starch. At low initial irradiance, a greater proportion of photosynthate was partitioned into export rather than starch and at high initial irradiancc the reverse occurred. There was a linear relationship between starch accumulation rate and Pn for all leaves but the relationship between Pn and export rate was only significant for leaves with low levels of reserve carbon. The results show that mature pepper leaves subjected to different irradiances maintain constant export rates through alterations of carbon partitioning. Export at low Pn is maintained at the expense of sugar and starch reserves, with partitioning in high irradiance being predominantly to starch.