Abstract
Adaptation and acclimation of tree seedlings to different combinations of light and water conditions can determine the species-specific patterns of distribution along environmental gradients and the underlying physiological mechanisms are fundamental to the understanding of such patterns. Seedlings of two Cyclobalanopsis species naturally occurring in southwest China, with distinct distribution and regeneration characteristics, were grown under 100%, 50% and 4% sunlight conditions and traits related to shade and drought tolerance were studied. Particularly, we investigated whether leaf hydraulics, photosynthetic traits and their functional coordination play an important role in determining seedling environmental adaptation and acclimation of the two species. Seedlings of C. helferiana showed characteristics adapted to high irradiance while C. rex had traits adapted to partially shaded environments. Cyclobalanopsis helferiana had significantly higher maximum net photosynthetic rate (Amax), light compensation point and light saturation point than C. rex and the contrasts were particularly large when they were grown under full sunlight. Cyclobalanopsis helferiana showed the highest Amax when grown under 100% sunlight, while C. rex exhibited the highest Amax at 50% sunlight. Similarly, under full sunlight conditions C. helferiana showed significantly higher leaf hydraulic conductance (Kleaf) than C. rex, i.e., 13.37 vs. 7.09 mmol m−2 s−1 MPa−1 (p < 0.01). The correlation between Kleaf and Amax followed a unified positive correlation across different light treatments of both species. Moreover, leaves of C. helferiana showed greater resistance to drought-induced hydraulic dysfunction and to desiccation than C. rex. The contrasts in functional traits between the two Cyclobalanopsis species are consistent with the hypothesis that there is a trade-off between shade tolerance and drought tolerance. Findings of the present study contribute to a deeper understanding of mechanisms of divergence between closely related (congeneric) species with respect to key ecophysiology associated with natural regeneration.
Highlights
The distribution patterns of plants along environmental gradients often show significant interspecific differences, and the underlying mechanisms are important topics of plant ecophysiological investigations [1,2,3,4,5]
This study focuses on investigating the intrinsic differences between the two congeneric species of contrasting adaptations with a focus on leaf-level physiology related to shade and drought tolerance
Under the same light conditions, the the maximum maximum photosynthetic rate of helferiana was higher with the difference large under high growth photosynthetic rate of C. helferiana was higher with the difference large under high light treatments
Summary
The distribution patterns of plants along environmental gradients often show significant interspecific differences, and the underlying mechanisms are important topics of plant ecophysiological investigations [1,2,3,4,5]. Changes in different environmental factors are often coupled in nature; for example, with the change of water conditions, the light environment of forests can change significantly. Trees in dry forests have evolved a series of adaptive characteristics in structure, physiology, phenology, etc. Trees in humid forests usually have strong shade tolerance at the seedling stage, but are sensitive to drought [11,12]. There is a synergistic or trade-off relationship between the adaptability of plants to different environmental limiting factors. The investigation into the interactive effects between different environmental limiting factors would contribute to a more comprehensive understanding of the environmental adaptability of plants [13,14]
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