Morphological and Physiological Changes Caused by Weak Light in Early Stage Bringing About Photosynthetic Performance Decreasing and Yield Loss in Sweet Potato (Ipomoea batatas L.)

In the relay intercropping of maize and sweet potato, maize brought 40–70 days of shading stress on sweet potato; thus, sweet potato yield was reduced. Morphological and physiological impacts of weak light or shading stress on sweet potato in the early stage are not known. We hypothesised that shading stress would change morphogenesis and physiology of sweet potato in the early stage that leads to low yield. To test this hypothesis, we simulated the shading stress using weak light and applied the shade stress onto two sweet potato cultivars, Yushu-17 and Qianshu-8. Results showed that 45-day weak light caused abnormal growth of sweet potato seedlings. The weak light triggered a smaller diameter, longer internodes and extended length of the main vines on both cultivars. The fresh weight of stems and leaves was less than that under normal light. It was also found that weak light promoted the accumulation of proline (Pro) and malondialdehyde (MDA) that influence osmotic status of the vines. Weak light elevated the activities of both superoxide dismutase (SOD) and catalase (CAT). Although weak light enhanced the content of chlorophyll, it inhibited the net photosynthetic rate (Pn) and sucrose phosphate synthase (SPS), and delayed root development. The yield loss was not reversed by resuming normal light after 60 days of weak light. We conclude that weak light in the early stage impedes normal morphogenesis by disturbing osmotic status and adversely impacting antioxidant and photosynthetic enzymes that led to abnormal growth of the main vines and roots, thus causing yield reduction. These findings may explain the negative impact of the shading stress by maize on the yield of sweet potato in the field.