The considerable variation in leaf morphology and structure reflects the organ’s phenotypic plasticity [32]. Therefore, leaf characteristics are often used as an indicator of plant acclimation potential and adaptation mechanism [33]. Because excessive irradiance has a detrimental impact on photosynthetic tissues, plants must produce smaller and thicker leaves with higher leaf mass per area under highlight conditions. This morphology allows heat dissipation, avoiding damage from overheating and high transpiration rates [2, 34]. Conversely, shaded conditions result in increasing area and decreasing thickness of leaves [22, 31], with low leaf mass per unit area [12]. Increasing leaf area allows plants to acquire more light for photosynthesis [5, 35] and is thus an adaptation to low-light environments [34]. In this study, we observed larger leaf area under 5% and 15% light intensity levels for S, superba and under 40% light intensity for C. lanceolata. Our findings are in line with previous research on Elaeagnus angustifolia leaves, which became smaller and thicker under high light [34].
Furthermore, here we observed decreasing leaf mass per area and increasing leaf area with decreasing light intensity, again 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 [36]. Low light intensity also resulted in greater leaf mass per area for Citharexylum, Dendropanax, Fraxinus, Quercus, and Magnolia [4]. Interestingly, our study revealed between-species differences in the response of mean leaf area to decreasing light intensity. Specifically, mean leaf area was greatest at 40% light intensity in C. lanceolata, but at 15% light intensity in S. superba. The latter species also had significantly larger mean leaf area than C. lanceolata. These traits enhanced the ability of S. superba to tolerate low light intensity (shade) compared with C. lanceolata. Our finding is in line with the carbon gain hypothesis, which proposes that leaf area is higher in shade-tolerant seedlings than in intolerant seedlings [37], and implies that S. superba is better adapted to low light. As a whole, the findings support our first hypothesis that S. superba will have larger leaf area but smaller leaf mass per unit area under low light intensity than C. lanceolata.
Previous studies have shown that C. lanceolata seedlings adapt to shaded conditions through adjusting morphological characteristics [38]. However, seedlings had difficulty maintaining a C balance under extremely shaded (5% sunlight) conditions, causing poor growth and survival. The issue of negative C and relatedly NSC 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 [39]. Similarly, under extremely shaded conditions, Quercus aliena seedlings had difficulty maintaining C balance and thus experienced mortality [40]. To overcome the lack of an energy source under low light intensity, plants store NSC to enhance growth and survival [12, 39, 41, 42]. Here, we found that 60% light intensity 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. In addition, when averaged across all light treatments, the carbohydrate contents were significantly larger in S. superba than in C. lanceolata. Moreover, C. lanceolata had a larger soluble sugar/starch ratio across all light treatments, despite considerable variability as light intensity decreased. This result demonstrates that S. superba seedlings had an advantage under shaded conditions and, moreover, could flexibly adjust to a vast range of light conditions. In terms of mechanism, exposure to high light intensity would result in greater C gain than demand, leading to NSC storage [12, 43]. Once light becomes a limiting resource, plants will mobilize NSC to support growth and survival [44]. The results support our second hypothesis that S. superba produces more NSC under low light intensity than C. lanceolata.
Both genetic and environmental factors influence plant nutrient uptake, as demonstrated by interspecific differences, along with intraspecific differences under various habitats [45]. In our study, S.superba and C. lanceolata used C, N, and P differently under varying light intensities, suggesting species-specific strategies in balancing nutritional metabolism and adapting to environmental stress. Light level, species, and their interaction significantly altered C, N, P content and stoichiometry. Notably, leaf C content decreased with decreasing light intensity. Both species had higher C content under full sunlight, 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 linked higher C content with greater photosynthetic efficiency and resilience to adverse environments [46], our findings imply that S. superba is better adapted to low light than C. lanceolata. Also in agreement with our results, some studies have suggested that shade-tolerant plants have higher NSC accumulation and C pool than non-tolerant plants [41, 42]. In further support of light-dependent changes in strategy, we observed higher P and N contents in both two species under 100% and 5% light intensity, respectively. 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 [47]. 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 where NST 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 intensity will have increased leaf N content and allocate more N to photosynthetic pigments. This strategy increases light use efficiency and maintain normal photosynthetic function [48]. The findings give credence to our third hypothesis where C:N:P stoichiometry varies with light intensity with marked inter-species variability.