Plant Growth Retardants: A Case Study

Plant growth regulators, which include both promoters as well as inhibitors, are chemicals designed to manipulate plant growth and development (Arteca 2014). They are applied for specific purposes to affect specific plant responses, e.g. to control plant height, eliminate excessive vegetative growth, and enhance flowering (Grossman 1990). One of their main mode of action, particularly of those called growth retardants, is to reduce stem growth (i.e. shorten the internode length). These substances usually do not interfere with flower development, although late applications or high application rates can delay flowering and affect flower size. Growth retardants inhibit the production of gibberellic acid, which is involved in cellular elongation

1984; Zummo et al. 1984; Reddy et al. 1990; Nichols et al. 2003). MPC suppresses the production of the plant hormone gibberellic acid which is a growth stimulant that induces cell elongation. Suppression of gibberellic acid production results in decreased cell elongation and overall decrease in the vertical and horizontal elongation of stems and branches (York 1983; Kerby 1985). It should be noted that MPC inhibits cell expansion, but not cell division. MPC is not thought to be readily translocated through the plant. The overall effect of MPC on plant growth and development depends on growing conditions after it is applied. Conditions favoring growth typically increase the likelihood of realizing positive plant responses and benefits. Cotton yield responses to MPC have been inconsistent. MPC increased yields in some tests (York 1983), had no effect in others (Heilman, 1985; Stuart et al. 1984), and reduced yield in some other tests (Thomas

Nevertheless, it is often necessary to treat plants with a fungicide after flowering or apply a desiccant before harvest as a usual practice to achieve an economical and stable yield, as well as to maintain oil quality. Considering that the normal sunflower height between flowering and harvest may often reach 250 cm, these operations cannot be performed without damaging the stand. Data on the use of MPC for reduction of sunflower plant height under field conditions are highly limited. The available data concerning MPC use in field sunflower showed MPC at the rate of 25 g/ha reduced sunflower plant height at maturity by 11.7% without adverse effects on achene and oil yields, thus providing a basis for reducing the risk of plant lodging (Koutroubas et al. 2014). However, the same study concluded that the practical importance of the plant height reduction obtained by the foliar application of MPC should be further investigated, including factors like different application schemes, with multiple split applications at different rates or at different crop stages (Koutroubas et al. 2014). Particularly with plant growth regulators, further research is always useful, considering the inconsistent performance of many plant growth regulators and the diverse effects on plant growth related to the timing of application (i.e. the growth stage of crop), the application rate used, and the variable effects of the