Growth of Jackfruit Seedlings under Plant Growth Regulator (PGR) and Shading Treatments

Abstract

The success of jackfruit (Artocarpus heterophyllus Lam.) grafting is strongly influenced by the physiological quality of the rootstock prior to grafting. This study aimed to evaluate the effects of 75% shading and the individual application of 50 ppm α-naphthaleneacetic acid (NAA) and 15 ppm abscisic acid (ABA) on the vegetative growth and physiological characteristics of jackfruit rootstock seedlings. The experiment was conducted using a randomised complete block design with four treatments (control/untreated, 75% shade, 15 ppm ABA, and 50 ppm NAA), five replications, and five seedlings per experimental unit. Morphological parameters, including plant height, stem diameter, branch number, shoot number, and leaf number, were measured biweekly, whereas chlorophyll, glucose, and fructose contents were determined monthly. Stem anatomy was evaluated 7 days after treatment. The results showed that 75% shading consistently promoted vegetative growth by increasing stem diameter, branch number, shoot number, and leaf number while maintaining stable chlorophyll content and carbon metabolism. The application of 50 ppm NAA enhanced plant height and tended to maintain higher chlorophyll content, indicating improved photosynthetic performance and vegetative vigour. In contrast, 15 ppm ABA did not affect vegetative growth but increased soluble sugar accumulation, suggesting its role in maintaining physiological homeostasis and stress adaptation. Stem anatomical observations revealed darker tissues in ABA- and NAA-treated seedlings than in the control, which may indicate greater accumulation of lignin, phenolic compounds, and oxidised secondary metabolites associated with enhanced cell wall development and physiological responses induced by plant growth regulator application. Overall, 75% shading was the most effective treatment for improving the vegetative and physiological quality of jackfruit seedlings prior to grafting.