Helen L. Boldingh

Carbohydrate levels are important regulators of the growth and yield of tree crops. Current methods for measuring foliar carbohydrate concentrations are time consuming and laborious, but rapid imaging technologies have emerged with the potential to improve the effectiveness of tree nutrient management. Carbohydrate concentrations were predicted using hyperspectral imaging (400–1000 nm) of leaves of the evergreen tree crops, avocado, and macadamia. Models were developed using partial least squares regression (PLSR) and artificial neural network (ANN) algorithms to predict carbohydrate concentrations. PLSR models had R² values of 0.51, 0.82, 0.86, and 0.85, and ANN models had R² values of 0.83, 0.83, 0.78, and 0.86, in predicting starch, sucrose, glucose, and fructose concentrations, respectively, in avocado leaves. PLSR models had R² values of 0.60, 0.64, 0.91, and 0.95, and ANN models had R² values of 0.67, 0.82, 0.98, and 0.98, in predicting the same concentrations, respectively, in macadamia leaves. ANN only outperformed PLSR when predicting starch concentrations in avocado leaves and sucrose concentrations in macadamia leaves. Performance differences were possibly associated with nonlinear relationships between carbohydrate concentrations and reflectance values. This study demonstrates that PLSR and ANN models perform well in predicting carbohydrate concentrations in evergreen tree-crop leaves.

Avocado fruits are considered unusual because of the large amounts of oil and seven-carbon (7-C) carbohydrates (mannoheptulose and perseitol) in the fruit’s flesh and skin. The fruit may be held on the tree unripe until required for marketing, and in some producing regions, this may extend past the next flowering period. This prolonged period on the tree is associated with increased oil content and decreased 7-C carbohydrates. There has been relatively less research into soluble hexose sugars and starch. In this research, the inter-relationships between fruit maturation, storage, and ripening have been investigated for both 7-C and six-carbon non-structural carbohydrates using ‘Hass’ fruit harvested from the same trees between 11 and 14 months after flowering. Significant differences were identified in both fruit flesh and skin for most compounds, affected by maturity, storage, and ripening. It is concluded that the non-structural carbohydrate composition of ‘Hass’ fruit is variable, with significant changes occurring associated with maturation, storage, and ripening. The compositions of the flesh and skin tissues are not consistently proportionate. Maturation provides the initial baseline composition from which any further change through storage or ripening can occur. The changes with maturation appear to be associated with the tree’s phenology, with tree-to-tree differences in the timing or degree of change.

This work aims to understand how Vitis vinifera (Chardonnay) vines prioritise the export and distribution of recently fixed photoassimilate between root tissue, fruit, and defence, following the elicitation of a defence response. Jasmonic acid (JA) and its methyl ester, MeJA, are endogenous plant hormones, known collectively as jasmonates, that have signalling roles in plant defence and consequently are often used to prime plant defence systems. Here, we use exogenous jasmonate application to mature source leaves of Chardonnay grapevines to elucidate the prioritisation strategy of carbon allocation between plant defence and growth. Our results demonstrate that jasmonate application to Chardonnay leaves can elicit a defence response to Botrytis cinerea, but the effect was localised to the jasmonate-treated area. We found no evidence of a systemic defence response in non-treated mature leaves or young growing tissue. JA application reduced the photosynthetic rate of the treated leaf and reduced the export rate of recently fixed carbon-11 from the leaf. Following JA application, a greater proportion of available recently fixed carbon was allocated to the roots, suggesting an increase in sink strength of the roots. Relative sink strength of the berries did not change; however, an increase in berry sugar was observed seven days after JA treatment. We conclude that the data provide evidence for a “high sugar resistance” model in the mature treated leaves of the vine, since the export of carbon was reduced to ensure an elevated defence response in the treated leaf. The increase in berry sugar concentration seven days after treatment can be explained by the initial prioritisation of a greater portion of the exported carbon to storage in the roots, making it available for remobilisation to the berries once the challenge to defence had passed.

Plant metabolomics within field-based food production systems is challenging owing to environmental variability and the complex architecture and metabolic growth cycles of plants. Kiwifruit cultivars of Actinidia chinensis are vigorous perennial vines grown as clones in highly structured orchard environments, intensively managed to maximize fruit yield and quality. To understand the metabolic responses of vines to orchard management practices, we needed to better understand the various sources of metabolic variability encountered in the orchard. Triplicate composite leaf, internode and fruit (mature and immature) samples were collected from each of six Actinidia chinensis var. deliciosa ‘Hayward’ and A. chinensis var. chinensis ‘Zesy002’ kiwifruit vines at three times during the growing season and measured by LC-MS. In general, there was more variation in metabolite concentrations within vines than between vines, with ‘Hayward’ showing a greater percentage of within-vine variability than ‘Zesy002’ (c. 90 vs. 70% respectively). In specific tissues, the sampler, infection by Pseudomonas syringae var. actinidiae and the rootstock also influenced metabolite variability. A similar pattern of metabolic variability was observed from quantitative analysis of specific carbohydrates and phytohormones. High within-vine metabolic variability indicates that it is more important to obtain sufficient replicate samples than to sample from multiple vines. These data provide an objective basis for optimizing metabolite sampling strategies within kiwifruit orchards.

Consumer acceptance of fruit is determined by size, flavour and ripeness. In this study we investigated how altering the carbohydrate supply to Actinidia chinensis var. chinensis ‘Zesy002’ kiwifruit altered the balance between growth and accumulation of metabolites. Canes were phloem girdled and fruit thinned to a leaf-to-fruit ratio (L:F) of either 2 (Low carbohydrate) or 6 (High carbohydrate) at either 38 (Early) or 86 (Late) days after anthesis (DAA) and compared with ungirdled control canes with a L:F of 3. Fruit growth, metabolite accumulation, cytokinin concentrations and maturation were monitored and the sensory attributes of ripe fruit were assessed. The final weight of Early-High and Late-High carbohydrate fruit was 38% and 16% greater compared with control fruit. High carbohydrate fruit had increased starch, soluble sugar and cytokinin concentrations and fruit began to mature earlier and those with a Low carbohydrate had decreased concentrations and matured later compared with control fruit. Control fruit were described by consumers as more acidic and under-ripe compared with those from Early-High carbohydrate canes, but as sweeter than those from Low carbohydrate canes. This study showed that carbohydrate supply can have a major impact on the growth, sugar accumulation and maturity of ‘Zesy002’ fruit sinks.

Following cell division, fruit growth is characterized by both expansion through increases in cell volume and biomass accumulation in cells. Fruit growth is limited by carbon starvation; however, the mechanism controlling fruit growth under restricted carbohydrate supply is poorly understood. In a previous study using red-fleshed kiwifruit, we showed that long-term carbon starvation had detrimental effects on carbohydrate, anthocyanin metabolism, and fruit growth. To elucidate the mechanisms underlying the reduction in fruit growth during kiwifruit development, we integrated phytohormone profiling with transcriptomic and developmental datasets for fruit under high or low carbohydrate supplies. Phytohormone profiling of the outer pericarp tissue of kiwifruit showed a 6-fold reduction in total cytokinin concentrations in carbon-starved fruit, whilst other hormones were less affected. Principal component analysis visualised that cytokinin composition was distinct between fruit at 16 weeks after mid bloom, based on their carbohydrate supply status. Cytokinin biosynthetic genes (IPT, CYP735A) were significantly downregulated under carbon starvation, in agreement with the metabolite data. Several genes that code for expansins, proteins involved in cell wall loosening, were also downregulated under carbon starvation. In contrast to other fleshy fruits, our results suggest that cytokinins not only promote cell division, but also drive fruit cell expansion and growth in kiwifruit.

Kiwifruit (Actinidia spp.) is a recently domesticated fruit crop with several novel‐coloured cultivars being developed. Achieving uniform fruit flesh pigmentation in red genotypes is challenging. To investigate the cause of colour variation between fruits, we focused on a red‐fleshed Actinidia chinensis var. chinensis genotype. It was hypothesized that carbohydrate supply could be responsible for this variation. Early in fruit development, we imposed high or low (carbon starvation) carbohydrate supplies treatments; carbohydrate import or redistribution was controlled by applying a girdle at the shoot base. Carbon starvation affected fruit development as well as anthocyanin and carbohydrate metabolite concentrations, including the signalling molecule trehalose 6‐phosphate. RNA‐Seq analysis showed down‐regulation of both gene‐encoding enzymes in the anthocyanin and carbohydrate biosynthetic pathways. The catalytic trehalose 6‐phosphate synthase gene TPS1.1a was down‐regulated, whereas putative regulatory TPS7 and TPS11 were strongly up‐regulated. Unexpectedly, under carbon starvation MYB10, the anthocyanin pathway regulatory activator was slightly up‐regulated, whereas MYB27 was also up‐regulated and acts as a repressor. To link these two metabolic pathways, we propose a model where trehalose 6‐phosphate and the active repressor MYB27 are involved in sensing the carbon starvation status. This signals the plant to save resources and reduce the production of anthocyanin in fruits.

The elevation of anthocyanin contents in fruits and vegetables is a breeding target for many crops. In some fruit, such as tomato, higher anthocyanin concentrations enhance storage and shelf life. In contrast, highly anthocyanic red-fleshed apples (Malus x domestica) have an increased incidence of internal browning flesh disorder (IBFD). To determine the mechanisms underlying this, ‘Royal Gala’ cultivar apples over-expressing the anthocyanin-related transcription factor (TF) MYB10 (35S:MYB10), which produces fruit with highly pigmented flesh, were compared with standard ‘Royal Gala’ Wild Type (WT) grown under the same conditions. We saw no incidence of IBFD in WT ‘Royal Gala’ but the over-expression of MYB10 in the same genetic background resulted in a high rate of IBDF. We assessed concentrations of potential substrates for IBDF and a comparison of metabolites in these apples showed that anthocyanins, chlorogenic acid, pro-cyanidins, flavon-3-ols, and quercetin were all higher in the MYB10 lines. For the flavol-3-ols sub-group, epicatechin rather than catechin was elevated in MYB10 lines compared with the control fruit. Internal ethylene concentrations were measured throughout fruit development and were significantly higher in 35S:MYB10 lines, and ethylene was detected at an earlier developmental stage pre-harvest. Expression analysis of key genes associated with ethylene biosynthesis (aminocyclopropane-1-carboxylic acid synthase and oxidase; ACS and ACO) and polyphenol oxidase (PPO) showed the potential for increased ethylene production and the mechanism for enhanced PPO-mediated browning. The expression of a transcription factor of the ethylene response factor (ERF) class, ERF106, was elevated in red flesh. Analysis of transcriptional activation by MYB10 showed that this transcription factor could activate the expression of apple ACS, ACO, and ERF106 genes. Our data show a link between the elevation of anthocyanin-related transcription factors and an undesirable fruit disorder. The accelerated advancement of maturity via premature ethylene induction has implications for the breeding and storage of these more highly pigmented plant products.

Alternate bearing of ‘Hass’ avocado ( Persea americana Mill.) trees is a phenomenon across avocado‐growing regions around the world. In New Zealand, the cycle can be more complex as the fruit often remains on the trees for over 12 months, potentially resulting in competition for resources between fruit from the previous year and the flowers. This work looks at the impact of the presence of a 12‐month‐old heavy crop on the accumulation of boron and carbohydrates to flowers. By sequencing and measuring the expression of the avocado Trehalose Phosphate Synthesis ( TPS1 ) gene, we report on the effects of cropping cycle on the delivery of carbohydrates to the flower. Differences in sugar delivery rate to the flower reflected by differences in TPS1 expression were not reflected in the carbohydrate and boron contents of open flowers, and it is suggested that flowers require a certain amount of sugars to accumulate to enable their development and to reach a stage where they are able to open as a female flower.