Sustainable nutrient management for improved productivity and fruit quality in apple (Malus domestica)

Nutrient management is a critical component to achieving sustainable productivity and premium quality of fruit tree crops such as apple. The macronutrients nitrogen (N), phosphorus (P), and potassium (K) are generally supplied as synthetic fertilisers in apple orcharding and are well known to affect fruit yield and quality. However, N uptake efficiency is generally low for apple trees, ranging between 10‚Äö-40%, and studies on N, P and K nutrient management often report inconsistent results from various application rate and timing treatments. This thesis examined the uptake and remobilisation of N of mature apple trees in response to timing of fertiliser application, the contribution of N from residue as a source of N, how different rates of N, P and K interact to affect macro- and micro-nutrient uptake and fruit quality, and the effects of N and K application rates on the development of fruit cells in relation to the physiological disorder, bitter pit. Nitrogen uptake, allocation and remobilisation in mature apple trees were examined using \\(^{15}\\)N-isotope labelled fertiliser applications at pre-harvest, post-harvest, or 50:50 split between pre-and post-harvest timings in an orchard established on Dermosol in Tasmania, Australia. Whole trees were excavated at winter dormancy (June 2018) and commercial harvest (March 2019) in the following season to track the uptake and partitioning of N through the apple tree. Nitrogen uptake was most efficient when supplied at pre-harvest (32.0%) than other application timings (~17.0%). Leaf concentration of N derived from fertiliser, a good indicator of N uptake, concomitantly increased only when pre-harvest N was applied. Despite pre-harvest treated trees allocating more than half of the N taken up into fruit and leaves (65.2 %), an equal amount of fertiliser derived N (~3.5 g N tree\\(^{-1}\\)) was stored into perennial organs for pre- and post-harvest treatments reflecting greater uptake efficiency from pre-harvest treatments. Consequently, subsequent spring remobilisation of N was not affected by N application timing of the previous season. Instead, a decline in root N status (4.5 g tree\\(^{-1}\\)) and a reciprocal increase in branch N status (3.4 g tree\\(^{-1}\\)) were observed between winter dormancy and the fruit harvest of the following season. The significant finding of this study that pre-harvest N application provides adequate storage N to support early spring growth the following season with no detriment to fruit quality shifts the long-held paradigm of post-harvest fertiliser application to boost tree reserves. To better understand the contribution of tree (leaf and prunings) residue to N mineralisation, the decomposition dynamics of apple leaf foliage and pruned branches were examined over two growing seasons using a combination of \\(^{15}\\)N-labelled and non-labelled residue material on Dermosol in Tasmania, Australia. The loss of residue biomass was non-linearly associated with time, whilst the rate of biomass loss was higher in leaf than branch residue. The total N content of both residue types also had a negative linear association with time, where N content of leaf residue [0.092 mg (residue g)\\(^{-1}\\) week\\(^{-1}\\)] loss reduced at a higher rate than branch residue [0.006 mg (residue g)\\(^{-1}\\) week\\(^{-1}\\)] which remained relatively unchanged throughout the experiment period. However, total N derived from residue, as determined by \\(^{15}\\)N atom percentage, was released in a non-linear association with time, and in general, at a higher rate than for branch residue. Over two growing seasons, ~96.2% of N derived from leaf residue was released compared to branch residue which released ~63.3%. The uptake efficiency of N released from both residue types recovered in potted apple trees was 7.7 and 79.4%, respectively. In an orchard with 1667 trees ha\\(^{-1}\\) which requires approximately 20-80 kg N ha\\(^{-1}\\) yr\\(^{-1}\\) from fertilizer application, the amount of apple tree residue shed from one season contributed an insignificant amount (<1%) of the annual fertilizer N demand of trees over two seasons. Fertiliser treatments investigating the influence of N, P and K interactions at two different rates were tested on mature 'Gala' trees in a commercial orchard established on Dermosol in a one-year trial in Tasmania, Australia. Nutrient uptake and status were determined by leaf petiole and leaf lamina assessments at 8, 17 and 23 weeks after full bloom (WAFB) with fruit nutrient status and fruit quality attributes assessed at harvest. Synergistic impacts were found on the interaction between N and K application rates on the uptake of molybdenum (Mo) at 8 WAFB, and between N and P application rates on Zn uptake at 17 WAFB. In contrast, the interaction between N and P, and K and P had an antagonistic impact on the uptake of Mo at 8 and 23 WAFB, respectively. Increasing N application rates was associated with increased leaf N but decreased leaf P after 17 WAFB. Increasing P application rates was associated with increased leaf and fruit B at harvest. The remaining leaf and fruit nutrient levels examined were not affected by the N, P, and K application rates and no influences on fruit yield and quality were observed suggesting that at least over one season the soil and/or apple tree reserves provide buffering capacity against excessive or deficient nutrient availability. The interaction of different N and K application rates on fruit cell and bitter pit development, and fruit quality was also examined in a one-year trial in Washington State, the United States. Mature 'Honeycrisp' trees planted in Pogue soil, a cultivar that is large in fruit size and susceptible to bitter pit development, were studied. Two weeks prior to fruit harvest, structural properties of fruit cells were examined using synchrotron X-ray micro-computed tomography. Fruit porosity and cell size were reduced with increasing N and K application rates, while cell number increased. At fruit harvest, a moderate negative relationship (R\\(^2\\) = 0.089, p = 0.0429) between fruit firmness and N rates was observed whilst other fruit quality attributes and occurrence of bitter pit were not affected by N and K application rates. After 10-weeks storage at 0 vÄv¿C, there was some indication (R\\(^2\\) = 0.137, p = 0.0331) that higher N and K application rates resulted in increased incidence of bitter pit. Leaf and fruit calcium (Ca) , K and magnesium (Mg) status were also examined at 6, 12, 18 (fruit harvest) WAFB. Different N and K application rates did not result in differences in leaf nutrients examined, except leaf Ca at 12 WAFB, where leaf Ca was reduced with higher N and K application rates. Fruit Ca increased with N application rate at all three examined time points and increased with K application rate at 12 and 18 WAFB. Whilst both fruit K and Mg decreased with K application rate at 6 WAFB only. These studies improved the understanding of the physiology of tree N uptake and internal N cycling after various timings of fertiliser application, and the dynamics of tree residue decomposition. Complex interactions arising from N, P and K applications were demonstrated in relation to the acquisition and accumulation of mineral micronutrients. Application rates of N and K can affect the fruit firmness, Ca content, and cell properties of apple cultivar susceptible to bitter pit. These findings provide further knowledge of the tree physiological response and interactions to supply of mineral nutrients, which have important implications for N budgeting and sustainable nutrient management for apple orcharding.