Manipulation of canopy architecture and possible vigour control mechanisms in kiwifruit : a thesis presented in partial fulfilment of the requirements for the degree of Doctor of Philosophy in Plant Physiology at Massey University, Manawatū, New Zealand

Abstract

Dwarfing or vigour-controlling rootstocks have been used in many fruit trees to reduce scions growth, improve precocity and yield efficiency, but they are not currently available for kiwifruit. Therefore, there is a strong need to evaluate the vigour-controlling rootstocks and/or other growth manipulation techniques for controlling excessive growth of kiwifruit. In this study, the initial growth and architecture of ‘Hayward‘ scions may have been modified by the inter-specific hybrid kiwifruit rootstocks, during the first- and second- year of growth following grafting. Rootstocks modified the trunk cross-sectional area and proleptic bud break of the ‘Hayward‘ primary shoots. The lengths of long and short proleptic shoots of the scions from particular rootstocks were also slightly reduced, thus reducing the total length of proleptic shoots on grafted scions. In the field, inter-specific hybrid kiwifruit rootstocks affected the duration and compactness of scions bud break. The most notable effect of hybrid rootstocks was on the growth rate of long proleptic axillary shoots of scions during early spring growth with ‘Hayward‘ scions on particular rootstocks had the slowest growth rate compared to other rootstocks. Rootstocks may affect scions floral precocity, with ‘Hayward‘ scions on particular kiwifruit rootstocks tended to produce higher flower numbers when they were first planted on the field. There was a strong trend that rootstocks affected the proportion of long shoots and this effect had contributed to the differences in the proportion of non-terminated and terminated shoots of the scions. Auxin transport inhibitor, 1-N-naphthylphthalamic acid (NPA) applied to the stem junction at graft-union on some rootstocks had decreased the length, node number and cross-sectional area of scion primary shoots. However, NPA treatment on particular rootstocks did not affect the growth of scion primary shoots on some of the rootstocks, suggesting that restriction of IAA did not influence the level of IAA transported from shoot to root system of those kiwifruit rootstocks. NPA reduced the leaf size of scions, indicating that sufficient IAA is needed for the leaf growth of kiwifruit, but it may be regulated by the rootstocks. The transport and uptake of radioactivity of IAA in the stem segment varied between the rootstocks, suggesting that the level of IAA in the stem tissues of inter-specific hybrid kiwifruit rootstocks may vary depending on the vigour and genetics of the kiwifruit rootstocks. Restriction of IAA by inverting a single piece of bark (180-degree orientation) and grafted back to the main stem did not completely reduce the vigour of young ‘Hort16A‘ vines. However, the growth and vigour of young ‘Hort16A‘ vines in terms of total length, total node number and total leaf area were greatly reduced when grafted three rings of bark from other cultivars in an inverted orientation. In the field, the bark grafting treatments along with girdling were evaluated to regulate the characteristics of ‘Hayward‘ fruits. All treatments did not consistently produce similar effects in each season and year. Comparison between treatment, season (i.e. early and late summer) and year indicated that the treatment effects on fruit fresh weight, dry weight and dry matter concentration were only evident in the first harvesting year, and the effects were lessened in the following year. In this study, four distinct phenotypes were found from the kiwifruit seedlings population based on their main primary shoots; i) Long Multiple Stems (LMS), ii) Short Multiple Stems (SMS), iii) Long Single Stem (LS), and iv) Short Single Stem (SS). Gibberellins (GA3+GA4+7) treatment on these phenotypes at an early stage of bud break has transformed the morphology and characteristics of proleptic axillary shoots. The mean total number of proleptic and sylleptic axillary shoots (i.e. branching) was increased with gibberellins treatment, suggesting that gibberellins can promote meristematic activity by regulating both apical and sub-apical meristem of kiwifruit shoots.