Temporal pattern of photosynthesis under continuous illumination of radish plants

Changes in photosynthetic activity, redox state of photosystem I (PSI) and photosystem 11 (PSII), as well as starch and sucrose content were studied on the source leaves of 18- to 20-day-old radish (Raphanus sativus L.) plants that were dark-adapted for 12 h and then exposed to continuous white light ( 170 mumol quanta/(m(2) s)). The kinetic pattern of photosynthetic activity comprised three phases. Within the first 6 h of light adaptation (first phase), the maximum photosynthetic rate and the quantum yield of photosynthesis increased 1.6 times in the illuminated leaves compared to the leaves of plants placed in darkness. Further illumination led to the decrease of both photosynthetic indices by about 20% (12 h after the onset of light exposure, second phase) and finally increased them to the level observed after 6-h light exposure (72 h, third phase). The stationary photooxidation level of PSI primary donor was relatively low within the first 6 h of light adaptation, and then it steeply increased. The linear relationship between the amounts of photoreduced PSII primary acceptor and photooxidized PSI primary donor did not change during prolonged light adaptation, showing a highly coordinated functioning of both photosystems. The amount of sucrose in leaves attained its peak after 12 h of light adaptation and did not change further on. The starch content increased to its peak within 24 h of illumination and decreased cyradually upon longer exposures. It is concluded that, despite active export of assimilates to the developing storL age organ, the source leaves exhibit a nonmonotonic temporal course of endogenously regulated photosynthetic activity, which was related to changes in the effectiveness and, possibly, the number ofthe components of photosynthetic apparatus.