PHOTOSYNTHETIC RESPONSES TO DYNAMIC LIGHT UNDER FIELD CONDITIONS IN 6TROPICAL RAIN-FOREST SHRUBS OCCURRING ALONG A LIGHT GRADIENT

We examined in the field the photosynthetic utilization of fluctuating light by six neotropical rainforest shrubs of the family Rubiaceae. T hey were growing in three different light environments: forest underst ory, small gaps, and clearings. Gas exchange techniques were used to a nalyse photosynthetic induction response, induction maintenance during low-light periods, and light-fleck (simulated sunfleck) use efficienc y (LUE). Total daily photon flux. density (PFD) reaching the plants du ring the wet season was 37 times higher in clearings than in the under story, with small gaps exhibiting intermediate values. Sunflecks were more frequent, but shorter and of lower intensity in the understory th an in clearings. However, sunflecks contributed one-third of the daily PFD in the understory. Maximum rates of net photosynthesis, carboxyla tion capacity, electron transport, and maximum stomatal conductance we re lower in understory species than in species growing in small gaps o r clearings, while the reverse was true for the curvature factor of th e light response of photosynthesis. No significant differences were fo und in the apparent quantum yield. The rise of net photosynthesis duri ng induction after transfer from low to high light varied from a hyper bolic shape to a sigmoidal increase. Rates of photosynthetic induction exhibited a negative exponential relationship with stomatal conductan ce in the shade prior to the increase in PFD. Leaves of understory spe cies showed the most rapid induction and remained induced longer once transferred to the shade than did leaves of medium-or high-light speci es. LUE decreased rapidly with increasing lightfleck duration and was affected by the induction state of the leaf. Fully induced leaves exhi bited LUEs up to 300% for 1-s lightflecks, while LUE was below 100% fo r 1-80 s lightflecks in uninduced leaves. Both induced and uninduced l eaves of understory species exhibited higher LUE than those of species growing in small gaps or clearings. However, most differences disappe ared for lightflecks 10 s long or longer. Thus, understory species, wh ich grew in a highly dynamic light environment, had better capacities for utilization of rapidly fluctuating light than species from habitat s with higher light availability.