Abstract:

The response of net photosynthetic rate (Pn) in new growth shoots of 18-year-old China fir (Cunninghamia lanceolata (Lamb.)Hook. ) to sudden exposure to high-light and lightflecks was measured and is discussed in this paper. Pn showed a S-shape response to sudden exposure following a period of low-light or darkness adaptation, and 10-15 minutes of photo induction was required for Pn to attain its maximum level. With increasing intensity of the high-light exposure a longer induction period was required. Longer exposure and/or lower levels of low-light also increased the induction period. Following the passage of the initial light fleck shoots showed a hysteretic response by maintaining an induction state during non-consecutive light-flecks unless lightflecks were separated by a long period of low light. The response of Pn to instantaneous light-flecks depended on induction status and light-fleck duration; fully inducted shoots could respond, more rapidly than low or non-inducted shoots; the raised photo induction associated with light-flecks of longer length could thus contribute to increased Ph. The slow decrease of Pn after light-flecks suggested that there existed postillumination CO2 fixation, and that in canopies with frequent instantaneous light-flecks, a great proportion of assimilation might be the result of postillumination CO2 fixation.