Objective To mitigate the excessive elongation of papaya (Carica papaya L.) seedlings at the nursery stage, this study examined the effects of different concentrations of paclobutrazol (PBZ) and chlormequat chloride (CCC) on seedling dwarfing and physiological indices.

Method Taking seedlings of papaya cultivar 'Guire-1' as experimental materials, with foliar spraying of clear water as the control (CK), three concentration gradients of 300, 800 and 1 300 mg/L were set. Treatments A1-A3 and B1-B3 were established by foliar spraying with PBZ and CCC at the above-mentioned concentrations, respectively. Morphological indices including plant height and stem diameter, photosynthetic rate, SPAD value, root-shoot ratio, soluble sugar content, soluble protein content, POD activity, SOD activity and MDA content of seedlings were determined.

Result The plant height of papaya seedlings under treatments A1 and B3 were 18.21 and 18.52 cm, significantly reduced by 46.25% and 45.34% compared with CK, respectively. Their stem diameters were 8.70 and 8.76 mm, significantly increased by 91.21% and 92.53% compared with CK, respectively. There was no significant difference in root-shoot ratio between treatments A1 and B3, whereas both values were significantly higher than other treatments. Treatment A3 exhibited the lowest plant height, stem diameter and root-shoot ratio. The photosynthetic rate under treatments A1 and B3 reached 10.23 and 10.75 μmol/(m²·s), significantly increasing by 42.88% and 50.14% compared with CK, respectively. Their SPAD value under treatments A1 and B3 were 71.83 and 71.33, significantly increasing by 33.59% and 32.66% compared with CK, respectively. POD activity and soluble sugar content showed no significant difference between treatments A1 and B3, but both were significantly higher than other treatments. The SOD activity under treatment A1 was 6 454.82 U/g, which was significantly higher than other treatments. The soluble protein content under treatment B3 was 8.79 mg/g, significantly greater than that of other treatments. Treatment A3 had the significantly highest MDA content, together with the lowest SOD activity, soluble sugar content and soluble protein content. Principal component analysis extracted two principal components with a cumulative variance contribution rate of 89.08%.The first principal component mainly reflected stem diameter, root-shoot ratio, photosynthetic rate, POD activity and soluble sugar content, which could be categorized as the growth-stress resistance comprehensive factor. The second principal component mainly represented plant height, SOD activity and soluble protein content, which could be classified as the osmoregulation-antioxidant synergy factor. Comprehensive evaluation using membership function values revealed that treatment B3 ranked first, followed by A1.

Conclusion Foliar application of 1 300 mg/L CCC exhibited the best effects on dwarfing and physiological characteristics of papaya seedlings.