Potential analysis and systematic evaluation of Xixiang 55 as a rootstock for passion fruit

【Objective】To screen out superior rootstock varieties suitable for grafting propagation of passion fruit, the application potential of Xixiang 55 as a rootstock, was evaluated and the suitable stion (rootstock-scion) combinations were identified.【Methods】Using Xixiang 55 as the experimental material and the conventional rootstock variety Changhuang 511 as the control, the following experiments were conducted: (1) Comparative evaluation of rootstock potential between two passion fruit varieties: By comparing the thousand-seed weight, germination rate, and germination index of seeds from the two rootstock varieties, the dynamic changes and growth rates of plant height and stem diameter were examined; the biomass-related traits including total fresh weight, total dry weight, shoot dry weight, and root dry weight were measured; the root/shoot ratio and seedling vigor index, as well as root morphological traits such as root length, root surface area, root volume, and root absorption efficiency index of seedlings from the two rootstock varieties were compared, and differences in seedling growth traits between the two passion fruit varieties were comprehensively evaluated. (2) Disease resistance evaluation of grafted seedlings on two rootstock varieties: Using grafted seedlings on the two rootstock varieties with Qinguo 9 as the scion, disease resistance was assessed under natural infection conditions in orchards with continuous cropping, poor management, and severe incidence of stem base rot, root rot, and phytophthora blight. (3) Comparative evaluation of grafting compatibility and yield among different rootstock-scion combinations: A total of 12 stion combinations were established by grafting Xixiang 55 and Changhuang 511 with the scion varieties Hexiang 5 (J1, J2), Tainong 1 (T1, T2), DM-1 (D1, D2), Xixiang 5 (X1, X2), Qinguo 9 (Q1, Q2), and Xianghuang 1 (X1, X2) (nomenclature detailed in Table 1; the same below). Differences among the combinations were compared in terms of grafting survival rate, post-grafting seedling survival, new shoot length, new shoot diameter, rootstock diameter, scion diameter, and scion-to-rootstock diameter ratio. In addition, six endogenous hormones in leaves—zeatin riboside, gibberellin, indole-3-acetic acid, abscisic acid, jasmonic acid, and the hormonal balance index— were measured. Field performance of the grafted seedlings was further evaluated, including single fruit weight, edible rate, soluble solids content, and yield. Correlation analysis and principal component analysis were conducted to comprehensively evaluate the different stion combinations.【Results】(1) Xixiang 55 significantly outperformed Changhuang 511 in thousand-seed weight, germination index, plant height, plant height growth rate, stem diameter, root length, root surface area, and root volume, and was significantly superior in stem diameter growth rate and root absorption efficiency index, indicating that Xixiang 55 reached the grafting standard 6-10 days earlier than Changhuang 511. No significant differences were observed in germination rate, total fresh weight, total dry weight, shoot dry weight, root dry weight, root-shoot ratio, or seedling vigor index, indicating that the overall seedling quality of Xixiang 55 and Changhuang 511 was comparable. (2) In the disease resistance trials, no significant differences were observed between Xixiang 55 and Changhuang 511 in the incidence of stem base rot, root rot, or phytophthora blight, with both exhibiting strong disease resistance. (3) In the comparative evaluation of grafting compatibility and yield among different stion combinations, 17 traits were measured, including graft survival rate, new shoot length, new shoot diameter, rootstock diameter, scion diameter, scion-torootstock diameter ratio, zeatin riboside, gibberellin, indole-3-acetic acid, abscisic acid, jasmonic acid, hormonal balance index, single fruit weight, edible rate, soluble solids content, and yield. The results indicated that DM- 1, Xixiang 5, and Qinguo 9 performed best on the Xixiang 55 rootstock, while Hexiang 5 and Xianghuang 1 performed best on the Changhuang 511 rootstock; Tainong 1 showed no significant differences between the two rootstocks. (4) Correlation analysis showed that new shoot length, stem diameter, and scion-to- rootstock diameter ratio were significantly or highly significantly and positively correlated with grafting compatibility. Jasmonic acid and the hormonal balance index promoted rootstock-scion growth and enhanced grafting compatibility, while zeatin riboside favored graft survival. In contrast, abscisic acid inhibited grafting compatibility and survival rate. Furthermore, better grafting compatibility was associated with stronger seedling vigor and higher yield. (5) Principal component analysis was performed on 11 key traits, extracting four principal components with a cumulative contribution rate of 82.047%, and the principal component P1 mainly comprised JA, HBI, yield, and new shoot length. P2 mainly comprised new shoot diameter, IAA, ABA, and the scion-to-rootstock ratio. P3 mainly comprised survival rate and ZR and P4 mainly comprised GA. (6) Based on the comprehensive evaluation from the principal component analysis, the ranking of scores was as follows: X1 ＞ D1 ＞ Q1 ＞ D2 ＞ X2 ＞ J2 ＞ H2 ＞ T2 ＞ T1 ＞ J1 ＞ Q2 ＞ H1.【Conclusion】This study provides a theoretical basis and practical reference for rootstock selection and optimization of grafting combinations in passion fruit. Based on the comprehensive evaluation, it is recommended that Xixiang 5, DM-1, and Qinguo 9 be grafted onto Xixiang 55 rootstock; Xianghuang 1 and Hexiang 5 be grafted onto Changhuang 511 rootstock; Tainong 1 may be grafted onto either Xixiang 55 or Changhuang 511 rootstock.