The considerable variation in leaf morphology and structure reflects the organ’s phenotypic plasticity . Therefore, leaf characteristics are often used as an indicator of plant acclimation potential and adaptation mechanism . Because excessive irradiance has a detrimental impact on photosynthetic tissues, plants must produce smaller and thicker leaves with higher leaf mass per area under high light conditions. This morphology allows heat dissipation, avoiding damage from overheating and high transpiration rates [35,36]. Conversely, shaded conditions result in increasing area and decreasing thickness of leaves [26,37], with low leaf mass per unit area . Increasing leaf area allows plants to acquire more light for photosynthesis [13,39] and is thus an adaptation to low-light environments . In this study, we observed larger leaf area under 85% shading degree for S. superba and under 60% shading degree for C. lanceolata. Our findings are in line with previous research on Elaeagnus angustifolia leaves, which became smaller and thicker under high light intensity .
Furthermore, leaf mass per unit area decreased with increasing shading degree in both species. In agreement with our results, Alocasia macrorrhiza displays the same adaptations (larger and thinner leaves) to optimize photosynthetic efficiency under low light availability . Shading also resulted in greater LMA for Citharexylum, Dendropanax, Fraxinus, Quercus, and Magnolia . Interestingly, our study revealed between-species differences in the response of mean leaf area to increasing shading degree. Specifically, mean leaf area was greatest at 60% shading degree in C. lanceolata, but at 85% shading degree in S. superba. These traits enhanced the ability of S. superba to tolerate low light condition (shading) compared with C. lanceolata, which concords with a previous study . Our finding is in line with the carbon gain hypothesis that leaf area is higher in shade-tolerant seedlings than in shade-intolerant seedlings , and implies that S. superba is better adapted to shading.
C. lanceolata seedlings have been shown to adapt to shaded conditions through adjusting morphological characteristics . However, seedlings had difficulty maintaining C balance under extremely shaded (95% shading degree) conditions, causing poor growth and survival. The issue of negative C and relatedly NSCs balance under low light is a common problem plants face. For instance, a study made on Pinus koraiensis and Quercus mongolica demonstrated that low light induced carbohydrate deficiency and therefore high seedling mortality, with none surviving at 1% light intensity . Similarly, under extremely shaded conditions, Quercus aliena seedlings had difficulty maintaining C balance and thus experienced mortality . To overcome the lack of an energy source under low light intensity, plants store NSCs to enhance growth and survival [6,7,38,46]. Here, we found that 40% shading degree results in significantly higher soluble sugar, starch, and NSC content for both species. Once under low light intensity, all three variables decreased, presumably as a result of seedlings using their energy stores for growth and also a decrease in C fixation due to light limitation.
Shade-tolerant species should have higher NSCs concentrations than shade-intolerant species , because carbon gain was low in understory and NSC reserves are needed to enhance shade tolerance. Other studies also found that shade-tolerant species tend to have greater NSC reserves. For example, the seedlings of palm Chamaedora elegans (shade-tolerant species) had higher NSC content than seedling of Chrysalidocarpus lutescens (shade-intolerant species) [47,48]; and the shade-tolerant species Acer saccharum seedlings had higher NSC concentrations than seedlings of intermediate light-demanding Betula alleghaniensis . In our study, seedlings of shade-tolerant S. superba had higher NSC content than shade-intolerant C. lanceolate, especially under low light conditions. This result demonstrates that S. superba seedlings had an advantage under shaded conditions and, moreover, could flexibly adjust to a vast range of shade levels. In terms of mechanism, exposure to high light intensity would result in greater C gain than demand, leading to NSCs storage [38,50]. Once light becomes a limiting resource, plants will mobilize NSCs to support growth and survival . Under 85% shading, growth in height, diameter and biomass production of S. superba were considerable higher than other shading treatments (Data not shown). The results support our hypothesis that S. superba produces more NSCs under low light condition than C. lanceolata. This finding agrees with a previous study that demonstrated that shade-tolerant species exhibit higher NSCs content than shade-intolerant species .
Both genetic and environmental factors influence plant nutrient uptake, as demonstrated by interspecific differences, along with intraspecific differences under various habitats . In our study, S. superba and C. lanceolata produce C during photosynthesis and absorb N and P differently under varying shading degree, suggesting species-specific strategies in balancing nutritional metabolism and adapting to environmental stress. Both species had higher C content under intermediate shade condition (40-60% to full light availability), likely due to strong photosynthetic efficiency resulting in heightened synthesis of organic matter and C accumulation. Importantly C content was significantly larger in S. superba than in C. lanceolata. Given previous research linking higher C content with greater photosynthetic efficiency and resilience to adverse environments , our findings imply that S. superba is better adapted to low light condition than C. lanceolata. Our results are consistent with previous studies demonstrating that shade-tolerant plants have higher NSCs accumulation and C pool than shade-intolerant plants [7,46], this is because their photosynthetic machinery is adapted to be more efficient in the low light condition and store more C than plants that are not adapted their photosystems to low light. Higher P and N contents in both species were observed under no shading treatment and 95% shading degree, respectively. These results support that the adaptive strategy to shade might be species specific. P and N are essential macro-elements for plant growth and development, which participate in a number of metabolic processes, such as photosynthetic phosphorylation, ATP production, the production and export of triose-P and ribulose-1, 5-bisphosphate regeneration as well as synthesis of amino acids . This outcome is the vigorous growth under strong photosynthetic ability in full sunlight, leading to greater requirements for proteins and nucleic acids. On the contrary, seedlings of both species may use more N resources to synthesize light-trapping proteins under low light intensity. This is further evidenced in our study that NSC content was negatively correlated with N content and N:P ratio in both species, whereas a positive correlation was observed between NSC and C:N ratio in C. lanceolata and with C content and C:N ratio in S. superba. Our findings are corroborated by previous research showing that plants growing under low light condition will have increased leaf N content and allocate more N to photosynthetic pigments. We observed higher chlorophyll a content in S. superba than in C. lanceolata (data not shown). Due to the prevention of photo-damage, this strategy increases light use efficiency and maintain normal photosynthetic function . The findings give credence to our results that C:N:P stoichiometry varies with shade levels might be species-specific. As a whole, the findings have greater implication for establishment and maintenance of mixed species stand. S. superba is better adapted to low light intensity (shade tolerant), thus it would be advisable to plant S. superba later once the canopy of C. lanceolata is well developed but allowing enough sunlight (up to 40%). Conversely, thinning of dense stands of C. lanceolata to allow sufficient light to reach the understory would be recommended to expedite the natural regeneration and subsequent growth of S. superba.