Regulating plant growth

Dr Abigail Graceson, Amenity Technical Manager

May 2023

To enable growth, all plants require light, water, air, nutrients, and the right temperature. Given these things and sufficient space in which to grow, plants are able to create food through photosynthesis and to breakdown that food for energy through respiration. Plant growth will be restricted by the absence or reduction of any one of these factors, even if other factors are increased.

Phytohormones enable plants to organise their activity according to these external stimuli to create the right growth response at the right time. There are currently eight recognised phytohormones, two of which, abscisic and jasmonic acid, inhibit various aspects of plant growth and development in response to stressful environmental conditions.

Four phytohormones are primarily responsible for stimulating growth; auxin, cytokinin, gibberellin and brassinosteroid. You may be familiar with the use of auxin mimics such as 2,4-D in herbicides to overstimulate plant growth leading to twisting and distortion of the affected plants and ultimately resulting in plant death.

In turf management we can also have a requirement to restrict growth but to do so without creating stressful environmental conditions which can cause other detrimental effects on turf health. This is where plant growth regulators (PGRs) come in. PGRs can assist us in restricting growth whilst maintaining environmental conditions that lead to good plant health. Gibberellin is the main phytohormone responsible for stem elongation and is also the phytohormone that PGRs target.

Both prohexadione-Ca and trinexapac-ethyl inhibit the biosynthesis of gibberellins at the same point in the metabolic processes of the plant. This restricts plant growth as the plant no longer receives signals to extend its shoot length, resulting in shortened internode distances and reduced shoot growth. Furthermore, because the plant is no longer actively growing upwards it can increase its lateral growth resulting in increased sward density and a bigger rooting system with more fine roots. This change in plant structure is associated with better plant health and stress resilience due to an increase in ability to uptake water and nutrients.

Prohexadione-Ca and trinexapac-ethyl are a salt and an ester respectively but to inhibit biosynthesis of gibberelins, they must be converted to their acidic forms. There are slight differences, however, in the way the acidic forms are activated, leading to variation in the speed of action and required timing for application.

Prohexadione-Ca is a salt containing prohexadione(2-) and calcium(2+) ions. The addition of water triggers a reaction in which the calcium dissociates from the prohexadione activating its ability to regulate plant growth. This process occurs at temperatures as low as 7°C with regulation of plant growth occurring as soon as the plant has taken up the active substance. This means that growth regulation can occur in as little as 4 hours after application. Trials data shows that growth regulation from prohexadione-Ca lasts for between 21-28 days, enabling a spraying programme to be undertaken which fits in with nutritional programmes on either a three, or four, weekly basis.

Trinexapac-ethyl is an ester which must be broken down by metabolic processes in the plant before the trinexapac dissociates from the ethyl and becomes active. Because the plant must be actively growing for the activation of trinexapac to occur, slightly higher temperatures are required for application of this PGR. It also means that it can take up to 2-5 days for growth regulation to occur which may be useful when spraying windows are limited or a delayed response is desirable, for example to fit in with tournament play.

Gibberellin does not only control stem elongation, it is also involved in controlling flower development. Research on turfgrass indicates that PGRs which inhibit gibberellin can also affect the development of seedheads on grasses, particularly if applied prior to seedheads being visible. Because it is active at lower temperatures and can, therefore be applied earlier in the growing season, prohexadione-Ca may help to improve the uniformity, visual quality, and playing quality of playing surfaces by controlling the development of ephemeral species such as Poa annua.

Both prohexadione-Ca and trinexapac-ethyl are useful regulators of plant growth on turfgrass surfaces which can be used throughout the growing season as part of an integrated turf management plan.