Effect of Foliar Application of Paclobutrazol on Growth and Yield of Tomato Cultivated Inside the Protected House in Low Country of Sri Lanka

This study aimed to develop a cultivation protocol for tomato ( Solanum lycopersicum ) under 50% shade conditions in protected environments, targeting the mitigation of heat stress effects through the foliar application of Paclobutrazol (PBZ) to regulate plant growth while maintaining optimal yield. Six treatments, T1 (0 ppm, control), T2 (25 ppm), T3 (50 ppm), T4 (100 ppm), T5 (200 ppm), and T6 (400 ppm) were arranged in a Randomized Complete Block Design (RCBD) with five replicates. Foliar applications were conducted at 14-day intervals, and data on vegetative and reproductive traits were collected at corresponding intervals. Statistical analysis was performed using ANOVA, and mean comparisons were conducted via Dunnett’s test at a 5% significance level. The results demonstrated that PBZ significantly influenced vegetative and reproductive parameters of tomato showing a significant with control, for the plant height increment ( p < 0.0001 ), Internodal lengths in between 4–5 node ( p < 0.0001 ) and 7–8 node ( p < 0.001 ), leaf area in both 7th and 14th leaves ( p < 0.0001 ) and stem diameter at 2.5cm and 7.5 cm from the base of the plant ( p < 0.0001) .However, the number of leaves per plant remained unaffected ( p > 0.05 ). Reproductive parameters including number of flowers/plant ( p  <  0.001 ), number of fruits/plant ( p  =  0.002) , weight/fruit ( p  =  0.003) , and diameter of fruit ( p  <  0.001 ) were also significantly influenced by PBZ treatments. The 25 ppm PBZ treatment emerged as the optimal concentration, reducing plant height by 25.4% without compromising yield. These findings demonstrate that low-dose PBZ application at 14-day intervals can effectively regulate growth and sustain reproductive output, providing a viable agronomic strategy for tomato cultivation in shaded, high-temperature environments.