Effects of canopy manipulation and environment on carbon resource allocation to flowering and fruit set in apple

In commercial apple production, variability in yield and fruit quality among trees, orchards and seasons may have significant negative impact on profitability of individual orchards, and ultimately, the global competitiveness of the national industry. Although the Australian and New Zealand apple industries differ widely in many aspects, they both recognise the need for increased production of high quality fruit in order to expand export markets. In spring, competition among flowers, developing fruit, and shoots for limited resource supply induces fruitlet abscission, reduces potential fruit size, and may negatively affect fruit quality and induce biennial bearing. In commercial apple production, natural flowering and fruit set generally result in crop loads that are too high to achieve premium fruit size and quality targets. Orchardists seek to reduce flower and fruit numbers to their commercial target as soon as possible in spring. However, current standard technology relies on chemical thinning which does not give reliable or predictable responses. Apart from increasing risk, this leads to delays in achieving final fruit numbers, which results in loss of potential fruit size and quality. The mechanisms regulating flowering, fruit set and abscission in perennial crops such as apple are complicated and not fully understood. Improved knowledge of the physiology of these processes is required if commercial thinning practices are to be improved and consistently high yields of high quality fruit are to be achieved. Artificial spur extinction (ASE) provides a useful tool to investigate the physiology of flowering, fruit set and abscission. ASE is a crop load management tool that uses hand-thinning of whole buds in late dormancy to set targeted floral bud densities consistently on every limb. The process retains buds on spurs and short shoots in well illuminated areas of the canopy and spaces them evenly along the limb. Chemical thinning is not used. In this thesis, removal of floral and vegetative sinks as well as potential leaf area (carbohydrate source) using ASE, provided a means to investigate the role of carbohydrate source-sink relationships in annual flowering and fruit set. The research was conducted on ‘Gala’ strains, in five different commercial production environments through Australia and New Zealand over four seasons. This allowed examination of influence of environment on flowering and fruit set in this genotype. This thesis is presented in six chapters. The first provides background to the investigation, identifies the primary hypotheses tested and gives an outline to the thesis structure. The second to fifth chapters describe specific investigations that were undertaken and discuss the results in the context of current scientific knowledge and commercial technology. The last chapter provides a summary of outcomes, and, with a general discussion of the conclusions arrived at in the investigative research, discusses the relevance of the outcomes of the research to scientific knowledge and commercial technology. In the first research chapter, trees managed using ASE were compared with trees thinned after final fruit drop (Control) on five sites through New Zealand and Australia over four seasons. In Control trees flowering and fruit set was unpredictable and highly variable among trees within sites, among sites, and between seasons. Reducing floral bud density using ASE greatly reduced variability in flowering and fruit set and allowed predictive fruit set models to be developed for each site. Results led to investigations of the role of carbohydrates in fruit set and development during the early part of the season. These investigations are discussed in the ensuing three research chapters. Using two treatments, ASE and flower cluster thinning before bloom (FCT), and comparing these treatments with trees in which flower numbers were not altered (Control), allowed investigation of the effects of early sink removal (ASE and FCT vs Control), and altered leaf area and access to stored carbohydrates (ASE vs FCT) on fruit set. The effects of these factors on fruit development were also investigated by ensuring all treatments had the same final crop load, using post fruit drop hand-thinning to set either 4 or 6 fruit per cm2 branch basal cross-sectional area (BCA). Reduction of floral bud density through FCT or ASE increased within-bud fruit set and led to greater harvest mean fruit weight. These results lead to a hypothesis that these treatments improved carbon availability within floral spurs during early-season development, and that the means by which this occurred differed between ASE and FCT. In FCT, reducing the density of flower clusters (sink size) without altering leaf area may have increased the availability of newly synthesised carbon to remaining sinks, improving fruit set and development. In ASE, removal of, and uniform spatial distribution of remaining buds may have improved irradiance of remaining fruiting spurs, thereby increasing photosynthate availability to the developing fruit within the spurs. These conclusions were examined in investigations on seasonal light interception and the role of stored carbohydrates in ASE canopies, which are discussed in chapters four and five. ASE could be thought likely to reduce total early-season leaf area and light interception, as it greatly reduces total bud numbers on the tree. Fractional light interception was measured on ASE and Unmodified trees from shortly after dormancy, through one whole season. Where ASE was set at bud densities producing commercial crop loads (`4 and 6` `buds` `per` `cm_2` `BCA`), early season light interception was not affected (6 buds per cm2 BCA) or only marginally reduced (`-2%` `4` `buds` `per` `cm_2` `BCA`) very early in the season, but increased (+4%) over most of the season. In ASE managed trees, increased irradiance of remaining shoots, which are closely associated with flowers and developing fruit, was thought to lead to greater within-bud fruit set. Early removal of floral clusters in ASE managed trees is likely to have reduced competition for carbon resources during early fruit development thereby increasing harvest mean fruit weight. However, increased fruit weight may also have occurred through greater development of bourse shoots on ASE canopies increasing carbon supply to developing fruit later in the season. The contribution of carbohydrate reserves to fruit set in apple is unclear. In canopies where ASE is imposed, reduced competition among developing buds for limited carbohydrate reserves may contribute to increased within-bud fruit set compared with unmodified canopies. Early post-harvest defoliation was used to manipulate carbohydrate reserve concentration and investigate the effect on changes in carbohydrate concentration and fruit set the following spring. Carbohydrate concentration in roots, shoots and spurs showed that reducing reserve carbohydrate concentration in these tissues had very little direct impact on fruit set the following season. This led to the conclusion that newly synthesised photosynthates play a much greater role than reserves in supplying carbohydrate to young flowers and fruit, and consequently play a greater role in determining fruit set. Increased within-bud fruit set and improved harvest fruit weight observed in ASE treatments in this series of studies support the hypothesis that removal of competing sinks in late dormancy improved availability of limited carbohydrate resources to remaining sinks. However, contribution from newly synthesised carbohydrates in early spring seemed to play a much greater role in fruit set than availability of stored carbohydrates that were re-mobilised in spring. Consequently, light interception by the canopy, and in particular illumination of leaves closely associated with fruit (e.g. on the same shoot), are likely to play a large role in within-bud fruit set and fruit development; a concept supported in the literature. This might suggest that as many buds should be retained in the canopy as possible in order to maximise early-season canopy light interception. However, on the contrary, this thesis showed that even when about 50% of buds were removed using ASE, whole canopy light interception was increased the following season. Because increased fruit set and fruit weight were also observed in ASE treatments, it was concluded that improved irradiance of leaves on remaining fruit-bearing shoots in ASE more than compensated for the reduced early spring leaf area over the whole canopy. Highly variable fruit set and loss of potential size and fruit quality are a significant limitation in current commercial crop loading technologies, which rely on chemical thinning. Removal of floral sinks and improved irradiance following ASE treatment, greatly improved the reliability and predictability of fruit set, even in seasons where low light intensity, frost, or pollination may have reduced fruit set. Thus ASE provides a technology highly suited to replacing current commercial technologies and that is more likely to fit with sector aspirations of consistently reliable annual production of high quality fruit