Dynamics of Fruit Growth in ‘Conference’ Pear as Affected by Root Pruning, Irrigation and Climatic Conditions

In 2002, daily fluctuations in trunk diameter and fruit diameter on ‘Conference’ pear trees were measured with linear voltage displacement transducers. On an average sunny day in July, trunk diameter started to decrease around 10 a.m. when air temperature increased. This continued until 8 p.m. when air temperature started to decline. Fruit diameter growth of ‘Conference’ ceased as soon as trunk diameter started to decrease and resumed immediately after trunk diameter started to increase. When fruit growth took place, the rate of diameter increase was fairly constant and was ~ 0.04 mm/h. Root pruning without irrigation significantly affected tree growth but not fruit growth. Fruit size increase was almost linear from early May up to harvest at the beginning of September and averaged 3.9 mm/week. Relationships between fruit growth, climatic condition, soil water availability and cultural practices are discussed. INTRODUCTION Root pruning has become common practice for controlling shoot growth in pear trees in the Netherlands since the ban on the use of the chemical growth retardant chlormequat (CCC, 2-chloroethyltrimethylammonium) in 2001. Although many fruit growers successfully apply this method of mechanical growth control, it is a risky method. Too strong pruning may result in reduced fruit size or decrease in fruit quality due to insufficient uptake capacity for water and nutrients by the remaining root system. As root-pruned trees are more likely to suffer from drought stress, the recommendation is to root prune only in orchards in which trees can be irrigated (Wertheim, 2004). In order to optimize irrigation and growth control, more information is needed on the dynamics of fruit growth as related to water availability in the soil and the evaporation by the canopy of the tree. A technique to monitor the water demand of the tree would be helpful for establishing the relationships between water availability, water uptake and evaporation of trees before and after root pruning. Depending on the amount of water stress experienced by the tree, the grower might decide to carry out additional root pruning, supply water to the trees, or try to reduce the evaporation of the trees by sprinkling water on the leaves. Trunk diameter fluctuations have been shown to be a good parameter for the water status of a tree. During daytime, the diameter of the trunk starts shrinking when the tree begins to transpire. Trunk diameter increases again as soon as the transpiration of the tree diminishes from the end of the day to the next morning. The degree of shrinkage of the trunk during daytime has been used in several species of fruit trees as a parameter to control irrigation (Huguet et al., 1992; Bonany et al., 2000). This paper describes the first experiments on the use of linear voltage displacement transducers (LVDT's) for monitoring daily fluctuations and growth of trunks and fruits of pear trees in an orchard. In addition, effects of climatic and soil water conditions and root pruning on the dynamics of trunk and fruit growth are presented. 555 Proc. VIII IS on Orchard Systems