Supplementary far-red light enhances the quality and root development of double-root-cutting grafted watermelon seedlings

Cuinan Wu¹Yaya Wang²Kai Cao¹Encai Bao¹Haiyan Ding³Xiaoting Zhou⁴Yifan Dong⁵Shipu Xu^6*Xue Wu^1*

• ¹Jiangsu Academy of Agricultural Sciences (JAAS), Nanjing, China
• ²Xinjiang Agricultural University, Ürümqi, Xinjiang Uyghur Region, China
• ³Dali University, Dali, Yunnan, China
• ⁴Sichuan Agricultural University, Ya'an, Sichuan, China
• ⁵University of Nottingham, Nottingham, England, United Kingdom
• ⁶Key Laboratory of Smart Agricultural Technology (Yangtze River Delta), Shanghai, China

The regeneration of roots is crucial for the survival and healthy development of double-root-cutting (DRC) grafted watermelon seedlings. However, the methods to effectively enhance root regeneration in rootstock of DRC watermelons remain unclear. In this study, supplementary far-red light (FR) was applied on DRC grafted watermelon seedlings to evaluate its impact on seedling growth and rooting, using dark (CK) as control. It was discovered that supplementary FR light substantially promoted root development in the rootstock, reducing the time required for root regeneration and boosting root biomass. The transcriptome profiling data indicated that genes associated with sugar catabolism, oxidative stress, and auxin signaling were markedly upregulated in roots by FR light at 4 d post-grafting. The FR0.3 (red/far-red=0.3) light significantly enhanced the expression of genes involved in hyperoxide scavenging (CmAPX1, CmPOD1, CmCAT1), sugar transportation (CmSWEET12, CmBST2, CmSCP1), and auxin response (CmAUX28, CmIAA11, CmSAUR20) compared with control. Moreover, FR0.3 light treatment notably decreased the reactive oxygen species content, improved antioxidant enzyme activities in roots compared with control. However, despite increased gene expression, peroxidase and catalase, do not contribute substantially to ROS scavenging at the protein activity level under FR0.3 compared with other light quality. In addition, the sugars content and hexokinase activity responded differently to light quality. Of which, starch, sucrose and hexokinase were significantly increased by FR0.3 light at 4 d post-grafting while glucose content was the significantly highest only at 8 d post-grafting under FR0.3 treatment. The results demonstrate that supplementary FR light significantly promoted rooting and growth of DRC grafted watermelon seedlings. Specifically, FR light can induce root regeneration in the rootstock, potentially by alleviating oxidative stress during grafting and providing a relatively stable plant environment through the synergistic effects of sugar metabolism, antioxidant enzymes system, and auxin accumulation in the roots through the regulation of antioxidants, sugar metabolism, and auxin-related gene transcription. The findings from this study demonstrate a practical method to enhance the quality of grafted watermelon seedlings.