Plant Gene and Trait 2025, Vol.16, No.4, 162-172 http://genbreedpublisher.com/index.php/pgt 165 herbaceous and shrub species), and those that are less shade-tolerant (mostly sun-loving tree species). The degree of shade tolerance is generally indicated by numbers from 1 to 9, where 1 is the most shade-tolerant and 9 is the most shade-afraid (Feng et al., 2018). The study also found that the “low light abundance index” is a relatively practical method for evaluating the shade tolerance of undergrowth plants. In addition, factors such as seed size, leaf shape (coniferous or broad-leaved), and growth rate are also related to shade tolerance. 5.2 Mechanisms of shade avoidance and acclimation The responses of undergrowth crops to shade are usually divided into two categories. One type is the “shade-avoiding type”. Crops of this type will rapidly elongate their stems and leaves under weak light, striving to obtain as much sunlight as possible. Another type is the “shade-tolerant type”, which can adapt to weak light by improving photosynthetic efficiency, increasing chlorophyll, and enhancing water use efficiency (Figure 1) (Martinez-Garcia and Rodriguez‐Concepcion, 2023). Shade-tolerant crops usually do not grow taller blindly but maintain a high carbon utilization efficiency and survival rate (Schmiege et al., 2020; Cifuentes and Moreno, 2022). This type of crop also shows particular performance in terms of root distribution, leaf size and lifespan. Some crops can also better adapt to low light by increasing the nitrogen content in leaves and raising the ratio of chlorophyll to nitrogen (Kang et al., 2022). Figure 1 Summary of some differential responses associated with either shade avoidance or shade tolerance strategies (Adopted from Martinez-Garcia and Rodriguez‐Concepcion, 2023) Image caption: (a) Plant responses known to differ between strategies are described in the various laboratory conditions used: white (W) or control, proximity shade and canopy or deep shade. (b) Aspect of seedlings of Arabidopsis thaliana and Cardamine hirsuta grown under W (control or unshaded) or W enriched with FR (W + FR, proximity of canopy shade) illustrating the differences in their hypocotyl length. FR, far-red light (Adopted from Martinez-Garcia and Rodriguez‐Concepcion, 2023)
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