*3.3. Effect of Light Intensity on Photosynthetic Characteristics and Carbohydrate Accumulation*

The net photosynthetic rate (*P*n), transpiration rate (*T*r) and stomatal conductance (*g*s) gradually increased with an increase in light intensity, whereas the reverse occurred in the intercellular CO<sup>2</sup> concentration (*C*<sup>i</sup> ) of alfalfa plants. The main factors influencing *P*<sup>n</sup> were *g*<sup>s</sup> and *C*<sup>i</sup> , both of which are indispensable for determining the primary cause of change in *P*<sup>n</sup> [36,41]. These results suggest that increased *P*<sup>n</sup> under high light intensity could be due to increased stomatal opening, which would increase net CO<sup>2</sup> assimilation and water vapor exchange, thus promoting photosynthesis [42]. Transpiration acts as a driving force behind the absorption and transportation of water and inorganic irons to the above-ground part of the plant [43]. Additionally, loss of water through the stomata is an important heat dissipation mechanism [37]. We found that under optimum light conditions (L400 and L500), the *P*n, *T*<sup>r</sup> and *g*<sup>s</sup> of alfalfa can be increased and the *C*<sup>i</sup> reduced, which in turn enhanced photosynthesis in alfalfa plants.

As in previous studies [44], we found that sucrose, starch and total soluble sugar contents in alfalfa leaves were significantly improved with increased light intensity. Our results indicate that increased light (L400 and L500) increased specific leaf weight and the net leaf-level photosynthetic rate, which improved the number of photosynthates stored in the leaves. However, low light intensity (L100) could cause carbohydrate loss due to inhibition of photosynthesis and inhibit plant growth. Low light intensity decreased electron transfer and net photosynthetic rates, thereby exerting a negative impact on accumulation of photosynthetic products by the seedlings [19]. Furthermore, carbohydrates also serve as carbon reserves (e.g., sucrose and starch) and are stored in plant organs. Sucrose is one of the main sources of carbon and energy in plants. In our study, sucrose content was significantly higher under high light than low light, suggesting that plants grown in high light possessed stronger photosynthesizing leaves (source tissues) that in turn increased the sucrose produced by photosynthesis for supplying the demand of growing tissues [1]. In addition, starch reserves provide an immediate available energy source that may act as a buffer when environmental conditions are not optimal for photosynthesis (e.g., shade and cloudy days) [45,46]. Less carbon was partitioned to starch synthesis at low light intensity than at high light intensity [47,48]. Our results agree with those of Dayer et al. [45] and Jian et al. (2019) [49], who found that the assimilate demand of plants exceeds the photosynthetic rate under shaded conditions, suggesting that degradation of

starch reserves into soluble sugars could be used to support metabolism during a period of moderate shading stress [50].
