Plant Gene and Trait 2025, Vol.16, No.4, 162-172 http://genbreedpublisher.com/index.php/pgt 164 3.3 Key metrics for light environment assessment It is very important to scientifically assess the light environment of multi-layer systems. Common light assessment indicators include: rPPFD (relative value of photosynthetic photon flux density), daylight integral (DLI), total photon flux (TPF), uniformity of light distribution, and light quality (that is, the color composition of light). Among them, rPPFD is the most core indicator, used to see how much effective light can be received at each layer height (Zhu et al., 2020). DLI and TPF represent the total amount of light absorbed by crops over a period of time, which is directly related to the growth rate and yield of crops. If one wants to understand whether the light distribution is uniform, three-dimensional modeling or radiation models can be used for analysis. The influence of different planting methods on the light distribution can be clearly seen at a glance. In addition, the quality of light is also very important. For instance, red light, blue light and far-red light all have regulatory effects on the growth of leaves, flowering and fruiting of crops. Nowadays, LED lights can adjust these spectra to help crops grow better and have higher yields (Gerovac et al., 2016). 4 Physiological Responses of Understory Crops to Shading 4.1 Photosynthetic adaptation under low light In the shaded environment under the forest, the photosynthesis of crops will undergo obvious changes. Generally speaking, shading can reduce the net photosynthetic rate, stomatal conductance and transpiration rate of leaves, while increasing the CO₂ concentration between cells. This indicates that the decline in photosynthesis is mainly not caused by stomatal limitation, but by non-stomatal factors (Raai et al., 2020; Wang et al., 2021; Zhang et al., 2024). To adapt to weak light, crops usually increase chlorophyll, especially chlorophyll b, which can better absorb blue light (Gong et al., 2022; Sun et al., 2023). Some varieties also enhance the expression of some genes related to photosynthesis, such as the Lhcb1 protein, which helps to better capture light energy under weak light. Meanwhile, shading also activates the antioxidant enzyme system, enhancing the resistance of crops to photosuppression and oxidative stress (Ren et al., 2022). 4.2 Morphological plasticity In an environment with insufficient light, the appearance of crops under the forest will also undergo many changes. For example, the stems become longer, the leaves thinner, the leaf area larger, and the petioles elongated. This enables them to receive sunlight more easily (Gong et al., 2022; Gatti et al., 2023). Like the vine plant Mikania micrantha, the stem nodes grow longer and the epidermal cells stretch under weak light, which is conducive to rapid growth. The internal structure of the leaves will also change. For example, the palisade tissue and spongy tissue become loose, which is more conducive to absorbing scattered light. In addition, shading can also affect the hormone levels within plants. For example, changes in auxin, gibberellin and brassinolide can promote the growth of leaves and stems (Jiang et al., 2020). 4.3 Impact on reproductive development and crop quality Shading can also affect the flowering and yield of crops. Very often, shading can prolong the entire growth cycle of crops and postpone the flowering and maturation time. In terms of yield, shading usually leads to a reduction in the dry matter of crops, and the number and weight of grains also decrease, resulting in a decline in total yield (Liang et al., 2020). However, there are also some crops that do not reduce yield under moderate shading conditions (such as 30% shading), and sometimes the protein content even increases (Raai et al., 2020). In addition, shading can also affect some nutritional indicators and stress resistance. For example, starch content, protein content and antioxidant capacity may also be affected (Laub et al., 2021; Wang et al., 2021). 5 Species-Specific Shade Tolerance in Understory Crops 5.1 Classification of crops by shade tolerance The shade tolerance of under-forest crops can be judged by many indicators. Some are classified based on expert experience, while others use the growth conditions in the field as a reference. The commonly used approach nowadays is to examine the physiological characteristics of crops, such as photosynthetic efficiency and leaf features. Studies in North America and Europe have classified plants into several categories: those that are particularly shade-tolerant (such as some ferns and mosses), those that are moderately shade-tolerant (some
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