The kiwifruit (Actinidia sp.) vine root system: Responses to vine manipulations

Abstract

The root system is a significant component of whole-plant biomass in kiwifruit vines and is likely to be a considerable carbohydrate sink. Seasonal trends in root growth were investigated to infer seasonal fluctuations in root carbohydrate demand. Root growth peaked in summer and autumn. Observations indicate that peak root demand for carbohydrates occurs when fruit demands are also high. This infers competition between fruit and root growth, disagreeing with observations in other species. Carbohydrate partitioning between fruit and roots was studied in small kiwifruit plants using the tracer 14C. Simultaneous tracer import was observed into fruit and roots. When half the root system was cut off, root system import was reduced. The fruit imported a larger proportion of the available tracer, whilst there was no change in import into the remaining root half. Simultaneous carbohydrate import into fruit and roots demonstrated competition between these sinks. However, increased carbohydrate import into fruit suggests that the fruit has higher priority. The findings of this research suggest that the root system is a more competitive sink in kiwifruit vines than in some other fruit crops. Trunk girdling and root pruning are used to improve kiwifruit vine productivity. The effects of these practices on vine physiology, and root growth and function were investigated. Girdling reduced photosynthesis, with reductions attributed to stomatal limitation and feedback inhibition. Repeat annual girdling reduced root biomass by approximately 59%. Root growth rates were not affected, however, girdled vines produced less root tips. This suggests that new root production is more of a carbon expense than root extension. Root pruning reduced whole-vine hydraulic conductance. As treatment severity increased, reduced photosynthesis was evident, attributed to reduced sink demand. Roots in other species have demonstrated rapid changes in hydraulic conductance in response to environmental stresses. Hydraulic conductance in intact roots of kiwifruit vines did not respond when 80% of root biomass was removed. However, sap flow increased to a rate approximately 50% higher than pre-treatment. Kiwifruit roots exhibited hydraulic redundancy, which may be a stress tolerance mechanism.