The use of plant growth regulator mepiquat chloride (MPC) has been a widespread practice for the control of vegetative growth in cotton production for several decades. As a growth retardant, MPC could potentially provide a means of controlling plant height in field sunflower (Helianthus annuus
L.), but this possibility has not been studied. Field experiments over two years were conducted in northern Greece to study the influence of MPC at rates of 37.5 plus 37.5 g ha−1
(named double application) and 37.5 plus 37.5 plus 37.5 g ha−1
(named triple application) on sunflower growth and yield. MPC provided a height reduction of 9.5% (25 cm) with the double application and a height reduction of 14.4% (49.2 cm) with the triple application at maturity. The number of nodes was also reduced, indicating shorter plants, whereas stem width did not show a consistent response. Moreover, MPC resulted in increased crop growth rate with the triple application scheme. MPC reduced nitrogen utilization efficiency in both growing seasons. However, a reduction in achene yield per plant by 19.9% was observed with the triple application scheme. The achene yield reduction resulted by the reduction in the 100-achene weight (22.3%), given that the number of filled achenes was similar to control. None of the application schemes reduced seed nitrogen and oil content. The results from testing application schemes could be a basis for further research on the use of MPC in this crop, e.g., different timing of applications could be tested for benefits to height reduction.
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