Effect of shading degree on the grain yield and photosynthetic characteristics of wheat at the grain filling stage in an almond-winter wheat intercropping system

Fruit tree-wheat intercropping is practiced in large parts of southern Xinjiang, a region where agroforestry intercropping is the main type of agricultural production. In the present study, a field experiment was conducted to investigate the effects of the shading degree from fruit tree canopies on the grain yield and photosynthetic characteristics of wheat at the grain filling stage. This study aimed to provide information for the selection of management standards and the optimization of the intercropping system in southern Xinjiang. In the experiment, 'Xindong 20' (Triticum aestivum L. var. Xindong 20), the main winter wheat cultivar in South Xinjiang, was used as the research object, and two treatments (heavy and light shading) were established by cutting the almond (Amygdalus communis L.) canopy of the almond-winter wheat intercropping system; delayed open-central canopy (DC) and a semicircle small-canopy (SC), respectively. Monocultured wheat was used as the control. The yield and yield components, photosynthetic pigments and soluble protein contents, light response curves, chlorophyll fluorescence induction of the flag leaves, and the canopy apparent photosynthetic rate of the intercropped wheat were investigated in three areas: near the canopy west of the almond trees, near the canopy east of the almond trees, and far from the canopy. The results indicated that the shading degree of the tree canopy was closely associated with the canopy size and the distance between the intercropping area and the tree. The daily mean values of photosynthetically active radiation (PAR) in the intercropping areas near and far from the almond canopy were 18.61% and 25.90% with DC, and 56.00% and 64.53% with SC of the natural light intensity during the wheat-filling period, respectively. The content of Chla+b and the Chla/b ratio in the wheat flag leaves were reduced in both shading treatments. When the daily mean strength of PAR was ≤56.00% of the natural light intensity, the soluble protein content in the wheat flag leaves was significantly reduced. When the daily average intensity of PAR was reduced to 25.90% and 18.61% of the natural light, the indices of optical system Ⅱ actual photosynthetic efficiency (ΦSPⅡ), photochemical quenching coefficient (qP), and maximum net photosynthetic rate (P_max) of the flag leaves, as well as the daily mean canopy photosynthetic rate (CAP) value, were significantly reduced. Under the light shading intercropping condition, when the daily mean intensity of PAR reached ≥ 64.53% of the natural light intensity, the CAP value exhibited an obvious increased (compensation phenomenon). In the almond-winter wheat intercropping system, the influence of a weak light environment caused by the tree canopy on wheat photosynthetic capacity and yield was closely related to the degree of shading. Heavy shading resulted in a significant decrease in single-leaf P_n and CAP and caused a significant decrease in yield. Under light shading conditions, the photosynthetic capacity of wheat flag leaves did not change, while CAP had a significant compensation phenomenon and no significant influence on grain yield.