Effects of north-south rows unequal height cultivation with east-west ridges on the canopy light environment and growth of tomatoes in solar greenhouse

East-west ridge cultivation is one planting modes of the row orientation along an east-west axis on the south-facing side in the Northern Hemisphere. Heat accumulation can be used to manipulate the microclimate (light, temperature, and moisture) around plants. Mechanical and simple operations can be realized for vegetable production in solar greenhouses, due to labor cost-saving and high efficiency. However, the shading effects have caused the significant variation in the fruit yields of crops, such as tomatoes on the north and south rows of each ridge. This study aims to explore the effects of north-south rows unequal height cultivation with east-west ridges on canopy light environment and growth of tomatoes in solar greenhouse. An east-west ridge cultivation was adopted with a ridge spacing of 1.8 m (ridge width of 0.8 m + working path width of 1 m). Large-fruited tomato varieties were selected for the equal height cultivation on the north and south rows of each ridge as the control (CK). Three treatments of unequal heights on the north and south rows were set as: five fruit trusses on the south row and six fruit trusses on the north row (DH1), five fruit trusses on the south row and seven fruit trusses on the north row (DH2), and six fruit trusses on both rows with 0.2 m of vine dropping on the south row (DH3). A systematic investigation was made to clarify the effects of these unequal height treatments on the canopy light environment, plant growth, photosynthetic characteristics, fruit yield, and quality of tomatoes in the east-west ridge solar greenhouse during the autumn and winter seasons. The results showed that the unequal height treatments improved the light environment conditions in the upper and middle parts of the canopy on the north row of the ridge. The average light intensity in the upper and middle parts of the canopy on the north row of DH1, DH2, and DH3 increased by 9.17%, 15.37%, and 14.53% at 09:00, 12:00, and 15:00, respectively, compared with the north row of CK. The average light interception of the plant canopy in the DH1, DH2, and DH3 increased by 7.45%, 15.23%, and 10.78%, respectively. The light interception of the plant canopy in DH2 and DH3 increased significantly by 22.17% and 18.66% (P<0.05) at 12:00, respectively. The light environment was conducive to the plant growth. The net photosynthetic rate (Pn) in the north row of DH2 and DH3 increased significantly by 48.9% and 46.7% (P<0.05), respectively. There was increase in the light interception and light energy utilization. The overall fruit yield of DH2 and DH3 increased significantly by 9.20% and 6.88% (P<0.05), respectively. There were no significant differences in the light intensity, light interception, Pn, and yield on the south row of the unequal height treatments (P>0.05), compared with the south row of CK. At the same time, the fruit yield difference between the north and south rows on the same ridge in DH3 decreased by 5.8 to 6.6 percentage points. The light intensity in the canopy of the north side of DH2 and the accumulation of light interception in DH3 were greatly contributed to the soluble sugar content and dry matter content in the fruit. Therefore, the unequal height cultivation of DH2 and DH3 effectively increased the light intensity in the middle of the canopy, the light interception of the plant population, the plant growth and photosynthetic capacity. Thereby the tomato yield and quality were enhanced to avoid the shading effect between the north and south rows of each ridge in the east-west ridge cultivation.