The effects of trinexapac-ethyl on Pinus sylvestris and Picea abies

Abstract: The use of so-called micro-plant propagation have become a well-adapted cultivation technique in Swedish forestry nurseries. Although this production method has many advantages, a commonly occurring problem is that the young plants, after trans- plantation in the nursery, have an underdeveloped root system when the shoot part have reached desired length. Currently used methods to control the plant growth in a desired way involves long-night treatment and exposure to different stresses such as drought or nutrient deficiency. Unfortunately, long-night treatment requires expensive equipment and stressing might cause injury to the plants. Alternative methods to control plant growth are therefore sought after by the forestry industry. One of these alternative methods could potentially be the use of plant growth regulators (PGRs) which have been used for a long time in the agricultural sector, mainly to reduce the risk of lodging in cereals. There is however a key difference between the agricultural sector and the forestry industry – all plants grown as crops are members of the angiosperms, unlike the forestry plants which are dominated by gymnosperms. Research on PGRs have so far been focused on angiosperms exclusively. In this study, the effects of the substance trinexapac-ethyl on two economical important species of trees, Scots pine (Pinus sylvestris) and Norway spruce (Picea abies), were examined. Trinexapac-ethyl works by inhibition of multiple steps in the biosynthesis of gibberellin, an important plant hormone for length growth. Trinexapac-ethyl was applied in the form of Moddus Start (250 g L-1 of a.i., Syngenta AG) in dosages of 0.3, 0.6 or 1.2 L ha-1, either all at once or divided into two equally large amounts. There was three different occasions of applications: right after transplantation to cultivation cassettes, when desired shoot length had been reached and four weeks after the first treatment (if any). Both species were analysed by measurements of shoot length and stem diameter, weighing of the root and shoot parts separately and comparing their dry matter proportion, as well as ocular screening of any morphological changes. The frequency of crown buds in P. sylvestris was also determined and P. abies was examined via microscopy analysis. The results indicate none or very modest growth regulating effect of the trinexapacethyl when applied to P. sylvestris. The substance did however have a more profound effect on the P. abies – growth regulating effect was observed for both shoot length and stem diameter in two out of three trial lanes, and all treated groups had an increase in their root/shoot proportion. Meanwhile, microscopy analysis revealed no harmful effects on the buds of P. abies. No other toxic effect of the trinexapac-ethyl could be observed on any of the species. Trinexapac-ethyl therefore seem to have potential as a PGR for the P. abies, but the substance needs further evaluation before any large-scale use.