Introduction: Cotton (Gossypium hirsutum L.) is the most widely used natural fiber and the most important dual-purpose industrial plant in the world. Delayed transplanting of cotton using early cultivars is one of the effective methods to increase the cultivated area and increase the production of this strategic crop. Plant growth regulators can be effective in the source-sink balance in different plants and are progressively used to increase yield in many plants. Cytokinins are one of the most important plant growth regulators, which increase cell division and differentiation, reduce the effect of terminal dominance, increase leaf surface development, increase the number of lateral branches, increase mobility of nutrients, prevent chlorophyll decomposition, and prevent aging. Growth regulators containing mepiquat chloride, such as pix, reduce vegetative growth in cotton by preventing the synthesis of gibberellic acid. Controlling the growth of cotton as a result of the use of pix, allocates the photosynthetic assimilates to the fruit organs and growing bolls, which increases the weight of the bolls.
Materials and Methods: To investigate the effect of foliar application of 6-benzylaminopurine and pix on the growth and yield of cotton (cv. May 344) in delayed transplanting, a factorial experiment was conducted in a randomized complete block design with three replications in Sabzevar during 2020. The factors studied were foliar application of pix at two levels of no foliar application and foliar application at the rate of 1.5 liters per hectare and 6-benzylaminopurine (BAP) foliar application at five concentrations of 0, 5, 25, 50 and 100 mg l^-1. May 344 cotton cultivar was used in this experiment.
Results and Discussion: The results showed that foliar application of pix significantly increased the content of chlorophyll a compared to the no foliar application conditions. Foliar application with concentrations of 50 and 100 mg l^-1 BAP also caused a significant increase in the content of chlorophyll a compared to the no foliar application conditions. The highest content of chlorophyll a was observed in the concentration of 50 mg l^-1 BAP, which had an increase of 17.7% compared to the control treatment. The highest height of the plant (67.9 cm) was obtained by foliar application of 25 mg l^-1 BAP alone, which showed an increase of 15.1% compared to the control. The lowest plant height (51.6 cm) was also observed in the treatment of foliar application of pix along with 100 mg l^-1 BAP. Foliar application of pix along with 25 mg l^-1 BAP produced the highest number of bolls per plant, which was 29.8% higher than the control. The highest average boll weight (5.4 g) and the highest percentage of open bolls (83.8%) were obtained in the foliar application treatment of pix with 100 mg l^-1 BAP, which was not significantly different from the control treatment. Foliar application of 1.5 liters per hectare of pix alone produced the highest seed cotton yield (2754.3 kg ha^-1), which had a 40.2% increase compared to the control. The highest biological yield was produced in the foliar application treatment of PIX along with 5 mg l^-1  of BAP, which had an increase of 26.5% compared to the control treatment. The lowest biological yield was obtained in the treatment of foliar application with 100 mg l^-1  of BAP, which was not significantly different from the control treatment. The highest percentage of lint (52.2%) was observed in the pix foliar application treatment along with 50 mg l^-1 BAP, which showed an increase of 20.3% compared to the control.
Conclusion: According to the results, foliar application of 1.5 liters per hectare of pix is recommended to produce high seed cotton yield and increase the percentage of lint in delayed transplanting cotton.