Photosynthetic responses to dynamic light under field conditions in six tropical rainforest shrubs occuring along a light gradient

Abstract

We examined in the field the photosynthetic utilization of fluctuating light by six neotropical rainforest shrubs of the family Rubiaceae. They were growing in three different light environments: forest understory, small gaps, and clearings. Gas exchange techniques were used to analyse photosynthetic induction response, induction maintenance during low-light periods, and lightfleck (simulated sunfleck) use efficiency (LUE). Total daily photon flux density (PFD) reaching the plants during the wet season was 37 times higher in clearings than in the understory, with small gaps exhibiting intermediate values. Sunflecks were more frequent, but shorter and of lower intensity in the understory than in clearings. However, sunflecks contributed one-third of the daily PFD in the understory. Maximum rates of net photosynthesis, carboxylation capacity, electron transport, and maximum stomatal conductance were lower in understory species than in species growing in small gaps or clearings, while the reverse was true for the curvature factor of the light response of photosynthesis. No significant differences were found in the apparent quantum yield. The rise of net photosynthesis during induction after transfer from low to high light varied from a hyperbolic shape to a sigmoidal increase. Rates of photosynthetic induction exhibited a negative exponential relationship with stomatal conductance in the shade prior to the increase in PFD. Leaves of understory species showed the most rapid induction and remained induced longer once transferred to the shade than did leaves of medium- or high-light species. LUE decreased rapidly with increasing lightfleck duration and was affected by the induction state of the leaf. Fully induced leaves exhibited LUEs up to 300% for 1-s lightflecks, while LUE was below 100% for 1-80 s lightflecks in uninduced leaves. Both induced and uninduced leaves of understory species exhibited higher LUE than those of species growing in small gaps or clearings. However, most differences disappeared for lightflecks 10 s long or longer. Thus, understory species, which grew in a highly dynamic light environment, had better capacities for utilization of rapidly fluctuating light than species from habitats with higher light availability.

Gas Exchange, Photochemical Efficiency, and Leaf Water Potential in Three Salix Species

“…Generally, S. matsudana showed much lower P N and WUE than others during the whole day, while high Ψ l was noted. This trend consisted with the previous reports that species responding to their habitats present different eco-physiological characteristics although they can grow in similar habitats (Landnauser et al 1997, Valladares et al 1997. These trends could be explained as follows:…”

Evidence of higher photosynthetic plasticity in the early successional Guazuma ulmifolia Lam. compared to the late successional Hymenaea courbaril L. grown in contrasting light environments

“…The apparent quantum efficiency (α) was significantly higher in G. ulmifolia than in H. courbaril in both environments (p < 0.05) ( Table 1). The α values did not show significant differences between environments in H. courbaril (p > 0.05), which is expected since quantum yield tends to remain insensitive to the light environment (Björkman, 1981;Sims and Pearcy, 1989;Ramos and Grace, 1990;Valladares et al, 1997). However, G. ulmifolia showed a significantly lower α (p < 0.05) in shade conditions.…”

Nutrients limit photosynthesis in seedlings of a lowland tropical forest tree species

“…These results suggest that increased N supply is allocated to enzymes of the light harvesting complex and carboxylation reactions. Other significant responses of photosynthetic processes to nutrient addition include a strong effect of P, and to a lesser extent K, on g s indicating that stomatal function may be limited by nutrient availability and important for maximizing carbon gain in light-limited environments with rapidly changing light conditions (Chazdon and Pearcy 1986;Valladares et al 1997). Our results also indicate that enhanced photosynthesis may play a role in the increased growth of tree seedlings, saplings and poles in response to fertilization measured at this study site (Wright et al 2011;Santiago, unpublished data).…”