THE EFFECT OF DIFFERENT RATES OF PROHEXADIONECALCIUM AND GIRDLING ON SHOOT GROWTH AND FRUIT QUALITY WHEN APPLIED TO DIFFERENT PEAR CULTIVARS

Abstract

Prohexadione-Calcium (P-Ca) is a promising new shoot growth retardant that is already registered on apples in North America (Apogee ® ) and in Europe (Regalis ® ). This gibberellin biosynthesis inhibitor with limited persistence and low toxicity was tested on five Pyrus communis cultivars: ‘Rosemarie’, ‘Flamingo’, ‘Early Bon Chretien’, ‘Packham’s Triumph’ and ‘Forelle’. P-Ca was able to reduce shoot growth in all of the cultivars, but there was a marked difference in sensitivity towards different rates of P-Ca between the different cultivars. Fruit set was improved in ‘Rosemarie’, ‘Forelle’ and ‘Early Bon Chretien’, which led to a decrease in final fruit size of ‘Rosemarie’. P-Ca caused a decrease in return bloom in ‘Packham’s Triumph’ and ‘Forelle’. Girdling only reduced shoot growth in ‘Forelle’ but did not improve fruit set in any of the cultivars. Girdling improved final fruit weight of ‘Flamingo’, increased fruit length in ‘Early Bon Chretien’ and increased return bloom in all of the cultivars except ‘Packham’s Triumph’. The five cultivars are categorised according to their sensitivity towards P-Ca: ‘Rosemarie’ > ‘Flamingo’ = ‘Early Bon Chretien’ = ‘Packham’s Triumph’ > ‘Forelle’. INTRODUCTION Controlling excessive vegetative growth in fruit trees is very important (Costa et al., 2002; Forshey and Elfving, 1989; Williams, 1984) as it is a strong sink that competes with the fruit on the tree. This competition is at its strongest during the first 50 days after full bloom when shoot growth is very rapid (Byers and Yoder, 1999; Elfving et al. 2002) and coincides with the cell division stage of fruit growth. This results in a decrease in the number of cells in the fruit and, therefore, fruit size (Cowan et al., 2001; Yamaguchi et al., 2002). Excessive shoot growth also has a negative effect on fruit quality, yield and pest control (Greene, 1999; Miller and Tworkoski, 2003). Shading caused by excessive shoot growth has a negative effect on flower bud induction and the quality of the return bloom (Greene, 1999; Miller and Tworkoski, 2003). One of the main methods to control shoot growth is pruning. Pruning, however, is a very expensive, labour intensive and time-consuming management practice (Byers and Yoder, 1999). Other control measures have been used e.g. ethephon applications, summer pruning, root pruning and dwarfing rootstocks, but all have negative side effects (Greene, 1999). Girdling is also a practice used in pear orchards to control shoot growth, increase fruit set and improve fruit quality (Ingels, 2002; Miller and Tworkoski, 2003; Wilton, 2000). Girdling affects assimilate partitioning and the flow of plant hormones and nutrients throughout the tree (Miller and Tworkoski, 2003). As gibberellins (GA) have been implicated in stem elongation (Owens and Stover, 1999) researchers have been looking at GA biosynthesis inhibitors to counteract these and thus reduce shoot growth (Miller, 2002; Unrath, 1999). Although many of these compounds inhibit shoot elongation (e.g. chlormequat, daminozide) their persistence in the tree is a big disadvantage (Owens and Stover, 1999). Prohexadione-calcium [(P-Ca); BAS-125 (3-oxido-4-propionyl-5-oxo-3cyclohexene-carboxylate)] is a GA biosynthesis inhibitor with low toxicity and limited persistence (Owens and Stover, 1999). The application of P-Ca reduces levels of GA1