Apple Fruit Set Research Articles

Effects of reflective material deployed at bloom for 1, 2 or 5 months on ‘WA 38’ apple fruit set, quality and yield

The self-thinning nature of the apple variety ‘WA 38’ was previously characterized in Washington State; the self-regulating abscission of early-stage fruitlets lasts up to 8 weeks after full bloom and can result in the natural drop of 80 to 90% of the original flowers. Several approaches can be pursued to increase the fruit set, but in the present study, the focus was centered on A) alleviating the competition between young fruitlets during the early weeks of fruit development to reduce the loss by natural abscission and B) increasing the number of flower buds the following year. The study was set up in a 2018 planting of ‘WA 38’/G.935 orchard without hail-nets trained to V-trellis-system (4,975 trees/ha) in Central Washington (USA). The treatment consisted of the deployment, at early king bloom, of 80% reflective white material (RM, Extenday®) in the orchard inter-rows. The hypothesis relied upon the fact that enhanced light (mainly diffuse) could increase leaf C assimilation to support and nourish a higher proportion of fruitlets and decrease the natural abscission. The RM treatments had three different durations: reflective cover removed one month after installation (“RM for 1 M”), two months after (“RM for 2 M”) and until harvest (“RM until harvest”). They were compared against untreated control (“No RM (CTRL)”) in a 2-year trial. RM treatments did not result in a significant positive impact on fruit retention, net photosynthetic rate, yield, or return bloom. Trees subjected to whole-season RM had altered canopy and soil microclimates with drier and warmer canopies and cooler soil with higher moisture compared to untreated control. In the second season, a greater proportion of large apples (264-339 g or 72-64 apples/40-lb-box) was graded in RM until harvest treatment, indicating a positive influence of the enhanced light treatment in C allocation to fruit.

Effect of different growth substances on fruit set, yield and quality of custard apple (Annona squamosa L.)

Effect of different growth substances on fruit set, yield and quality of custard apple (Annona squamosa L.)

Organic Zn and nano-Zn amino acids chelates modulate quality growth attributes and antioxidants activity for biofortified apple (Malus x domestica Borkh.) production

Zinc biofortification increase nutritional content of crops to combat Zn deficiency in human diets. Foliar Zn bio-organics can significantly improve Zn content of fruit crops. This method is efficient, can be tailored to specific crop needs, and is crucial for enhancing the nutritional quality of food crops, contributing to better human health outcomes. In order to evaluate the efficacy of foliar sprays in promoting plant growth and development, the study evaluated the comparative efficacy of zinc amino acid complexes with zinc sulphate in apple (Malus × domestica Borkh.) cv. Gala Schniga Schnico during the years 2022 and 2023. Different Zn nutrient sources namely, Zn glycinate (Zn-Gly), Zn gluconate (Zn-Glu), Zn amino acid chelate (Zn-AAC), nano Zn amino acid chelate (NZn-AAC) were applied at the rate of 200 and 400 mg/L each and conventional ZnSO4 at 1250 mg/L with three replicates. Foliar application of the Zn nutrient sources was executed at four stages i.e., 15 days before petal fall, petal fall, 15 days after petal fall, and after harvest. The results revealed that NZn-AAC at 400 mg kg-1 registered the maximum increase in vegetative growth and flowering traits. The superior treatment also exhibited highest fruit set (62.90 %) and fruit yield (5.62 kg/tree) in apple. Fruit quality parameters, in terms of TSS, total sugars, ascorbic acid and anthocyanin contents were increased by 22.9, 17.8, 50.2, and 16.6 per cent, respectively through superior treatment. Fruit flesh and peel nutrient content in terms of P, K, Ca, Mg and Zn also showed improvement through NZn-AAC400 compared to control. Maximum decrease in phytic acid (1.09 %) and increase in ursolic acid (273.95 µg/mL) were also reported with NZn-AAC400. The differences between Zn nutrient sources were analysed using PCA, which explained 95.70 per cent (PC1), 97.81 per cent (PC2), 99.11 per cent (PC3), 99.42 per cent (PC4) and 99.63 per cent (PC5) of the cumulative variance based on Eigen value (>1). In accordance with the results, Zn amino acid complexes emerged as beneficial environment-friendly Zn source for improving growth, fruit yield, quality with higher Zn bio-availability compared to ZnSO4.

Localised and tree total crop loads influence trunk growth, return fruit set, yield, and fruit quality in apples

Localised fruit thinning strategies must be investigated to improve precision crop load management in narrow-canopy, multileader apple trees. This study aimed to determine the effects of within-leader and tree total crop load on leaders’ and trunk’s growth, fruit set, yield, and fruit quality in ‘Ruby Matilda’ apples (marketed as Pink Lady®) over three years. Different crop loads were imposed on two leaders (primary and secondary) of bi-axis trees. Leader and trunk relative growth rate, return fruit set, yield, and fruit quality parameters at harvest were measured. High within-leader crop loads led to a significant increase in yield and reductions in trunk growth, return fruit set, and deterioration of fruit quality parameters except for flesh firmness and starch index. Similar trends were observed in whole-tree relationships. High crop load in secondary leaders had moderate negative effects on trunk growth, yield, and fruit mass of primary leaders; it only marginally affected their return fruit set and had no significant effect (p > 0.05) on their fruit quality. A crop load of 6.8 fruit no. cm−2 of leader cross-sectional area was estimated to achieve a relatively consistent return fruit set within the same leader. At a whole-tree level, a similar crop load (6.9 fruit no. cm−2 of trunk cross-sectional area) produced a consistent return fruit set despite its higher variability. These crop loads produced high yields (120 and 111 t ha−1, respectively) and good quality fruit. Using individual leaders as management units is recommended to simplify operations and reduce variability.

Supplemental nesting habitat increases bee abundance in apple orchards

Abstract Identifying the resources that limit bee populations is essential for both bee conservation and pollination management in agroecosystems. Land‐use change typically leads to decreased habitat availability for wild pollinators, including loss of nesting habitat, which is an essential but often‐overlooked resource for wild bees. Cavity‐nesting bees, such as many Osmia spp. (Hymenoptera: Megachilidae), occupy holes in wood or reeds to build their nests; due to their nesting habits, they are frequently scarce in agricultural settings. Nevertheless, under the right circumstances, these bees can be ideal pollinators of apple and other orchard crops. Artificial nesting structures (‘bee hotels’ or ‘trap‐nests’) are often used to study cavity‐nesting bees and have been proposed as tools for bee conservation. To evaluate the effects of additional nesting habitat on the local abundance of pollinators, we selected 24 sites in apple orchards in eastern Canada in 2021 and 2022. Each site comprised two study plots: one in which we installed artificial nesting structures for cavity‐nesting bees, and one without added nesting habitat (control). Pollinator surveys were conducted in both plot types to measure pollinator visits to apple blossoms and, after apple bloom, to flowers in the undergrowth and/or in shrubs. Numbers and size of developing fruit were also recorded. Both Osmia bees and overall pollinator numbers were significantly higher in the treatment with trap‐nests (respectively, 33% and 22% higher during the 2021 growing season, and 113% and 30% higher during apple bloom across the 2 years of the study); however, there was no consistent difference in apple size or fruit set between the two treatments. Thus, trap‐nests locally increased the numbers of cavity‐nesting bees as well as total pollinator numbers, but they had little effect on apple yield, likely because apple production was not pollinator limited in the years of this study. Synthesis and applications. These findings suggest that bee populations in apple orchards can be limited by nesting resources such that addition of constructed nesting habitat could be an effective means of increasing pollinator numbers in orchards, and potentially, supporting bee conservation.

Commercial-Scale Use and Influence of 1-aminocyclopropane-1-Carboxylic Acid as a Rescue Thinner on Fruit Set and Early Fruit Growth in Apples

Only a limited number of post-bloom thinners is available for apple (Malus domestica Borkh.) at the present time. The influence of 1-aminocyclopropane-1-carboxylic acid (ACC) in two strains of ‘Gala’, one strain of ‘Delicious’, and one strain of ‘Fuji’ apples, when fruitlet diameter was about 20 mm, on fruit set and fruit size at hand-thinning in Southwest Idaho in the Intermountain West region, USA was studied. Applications of ACC reduced fruit set in ‘Schlect Spur Delicious’ in 7 and 30 days after application. Acc applications reduced fruit set in ‘Gala’ and ‘Fuji’ apples 30 days after application. Fruit weight after 30 days of ACC application (hand thinning time) in trees receiving ACC was higher than those in Untreated Control and the difference was significant only in ‘Schlect Spur Delicious’ apple. Application of ACC at 300 mg·L−1 or 365 mg·L−1 in this study did not have any advese effects on foliage or fruit skin in any of the tested cultivars in this project.

Challenges to interpreting internal and external factors limiting apple fruit growth, set and abscission

Challenges to interpreting internal and external factors limiting apple fruit growth, set and abscission

The influence of severity and time of foliar damage on yield and fruit quality in apple (Malus domestica Borkh.)

Leaf damage/defoliation caused by pests or phytotoxic damage following chemical application can result in loss of leaf area at any time during the growing season. Four trials were conducted to examine the impact of severity and time of foliar damage on fruit set and quality in apple. Trees tolerated light to moderate leaf damage throughout the season with no significant effect on fruit set and thus crop load. Where 75% or more of the leaf surface was lost, fruit set was reduced and fruit quality was affected. Complete loss of leaves up to three weeks after full bloom (wAFB) resulted in very low or no fruit set, but from 9 wAFB crop load was not affected by leaf damage. Loss of terminal shoot leaves had no effect on fruit set, but spur leaves were critical early in the season with the impact lessening from 6 wAFB. This study has demonstrated that low levels of foliar damage have little effect on fruit size or quality, but high levels can impact on both set and quality. Fruit quality was reduced following leaf damage during the period 6–12 weeks after full bloom which is the critical period for cell division in the fruit as well as flower initiation for the following year. High levels of leaf damage also reduced fruit size in spite of reduced crop load levels, most likely the result of a reduction in carbohydrate assimilation during the cell division phase of fruit development.

Apple pollination is ensured by wild bees when honey bees are drawn away from orchards by a mass co-flowering crop, oilseed rape

Over the past two decades, the cultivated area of oilseed rape (Brassica napus L. or OSR), a mass-flowering crop, has markedly increased in Europe in response to bioenergy demands. As well as representing a major shift in floral composition across the landscape, mass-flowering OSR may alter pollination services to other simultaneously blooming crops, either decreasing pollination via competition for pollinators or facilitating it via pollinator spill-over. Apple (Malus domestica Borkh.) is an economically important, obligately insect-pollinated fruit crop that co-flowers with OSR. Using twelve independent apple orchards varying in the percentage of OSR in the surrounding landscape, we investigated the effect of OSR on pollinators and pollination of co-blooming apple. We collected bees with pan traps and quantified flower visitors during transect walks in both crops and we experimentally measured pollination service provision to apple as fruit and seed set. We confirm that apples are highly dependent on animal pollination and report pollination limitation in our apple orchards. Honey bees were the numerically dominant visitors of apple flowers observed during transect walks. Though their numbers dropped with an increasing percentage of OSR in the landscape, the number of bumble bees visiting apple flowers remained stable and those of other wild bees rose. The pan trapped Shannon diversity of bees remained constant. We could not detect an effect of OSR in the landscape on apple fruit set or seed set, both of which remained stable. Local wild bee populations might compensate for the loss of honey bees in the provision of pollination services in apple, providing especially effective pollination. Our results underscore not only the dominant role of bees in apple pollination but also the importance of wild bee conservation for providing pollination insurance and stability of apple crop yields under changing agricultural policies and cropping practices.

Improving Habitat Quality at the Local and Landscape Scales Increases Wild Bee Assemblages and Associated Pollination Services in Apple Orchards in China

Bees provide key pollination services for a wide range of crops. Accumulating evidence shows the effect of semi-natural habitats at the landscape level and local management practices on bee diversity in fields. However, most of the evidence is derived from studies in North America and Europe. Whether this paradigm is applicable in China, which is characterized by smallholder-dominated agricultural landscapes, has rarely been studied. In this study, we aimed to investigate how bee diversity affected apple production, and how landscape and local variables affected bee diversity and species composition on the Northern China Plain. The results showed that bees significantly increased apple fruit set compared to bagged controls. Wild bee diversity was positively related to apple seed numbers. Higher seed numbers reduced the proportion of deformed apples and thus increased fruit quality. Wild bee abundance was positively correlated with flowering ground cover, and both the abundance and species richness of wild bees were positively affected by the percentage of semi-natural habitats. We conclude that apple quality can benefit from ecological intensification comprising the augmentation of wild bees by semi-natural habitats and flowering ground cover. Future pollination management should therefore reduce the intensification level of management at both the local and landscape scales.

Effects of metamitron on fruit set and photosynthetic biological characteristics of apples.

With dwarfing interstock Fuji apples as the test materials and water treatment as the control (CK), we examined the fruit thinning effect and its influences on leaves' photosynthesis by spraying 200, 300, and 400 mg·L-1 metamitron during the young fruit period to solve artificial fruit thinning problems (time-consuming, much labor, and low efficiency). The results showed that metamitron application could significantly reduce the inflorescence and flowers' fruit-setting rate by 16.5%-22.8% and 50.9%-53.9%, respectively. The treatment of 300 mg·L-1 metamitron had the strongest fruit thinning effect, with a single fruit rate of 46.6% and a double fruit rate of 18.3%. As a photosynthesis inhibitor, metamitron application reduced the chlorophyll content of leaves and strongly affected photosynthesis. The inhibitory effect on chlorophyll content disappeared after 15 days of the treatment, while that on the net photosynthetic rate disappeared gradually after 11 days of the treatment. The application of metamitron significantly reduced the maximum quantum yield of PSⅡ reaction center (Fv/Fm), actual photochemical efficiency (ΦPSⅡ), photochemical quenching coefficient (qP) and non-photochemical quenching coefficient (NPQ), with such inhibitory effect having been lasted for 15 days. OJIP analysis showed that metamitron caused damage to the apple leaves' oxygen-evolving complex, especially limiting the transfer of electrons in the PSⅡ reaction center from QA to QB. Metamitron treatment increased Wk, and significantly decreased ψo, RC/CSm, and PIabs. Besides, 300 mg·L-1 metamitron had the most significant effect. Our results showed that metamitron destroyed the structure of the PSⅡ reaction center of apple leaves and hindered the transfer of electrons from the donor to the receptor of PSⅡ. Consequently, the photosynthetic rate was affected, and the young fruits fell off due to the lack of accumulation of photosynthetic products.

Ethephon - a best alternative for hand thinning to improve fruit set, colour and shelf life of apple

Ethephon - a best alternative for hand thinning to improve fruit set, colour and shelf life of apple

Bio efficacy of Yara Vita Zintrac (39.55 Zn) on growth, yield and quality of apple (Malus domestica)

The present investigation was carried out at two locations viz., RHR&TS, Bajaura (Kullu) and RHR&TS, Mashobra (Shimla) to test the bio-efficacy of YaraVita Zintrac on growth and quality of apple. The experiment was laid out in Randomized Block Design with five treatment and four replications. Results showed that at RHR&TS, Mashobra fruit set, fruit weight and yield were significantly improved by Zinc EDTA @ 3.29 g/l at petal fall and just after harvest (T3), whereas, fruit firmness, TSS, sugars content were significantly increased by 1.5 ml YaraVita Zintrac/l of water (T2). At RHR&TS, Bajaura, 1.5 ml YaraVita Zintrac/l of water (T2) at petal fall and just after harvest was found significantly effective in improving the fruit set, yield and fruit quality of apple.

Bee abundance and soil nitrogen availability interactively modulate apple quality and quantity in intensive agricultural landscapes of China

Bees provide important pollination services for crops, but pollination limitation is a common problem in agricultural landscapes worldwide. To promote ecological intensification in fruit production, more knowledge is needed concerning the interacting effects of insect pollination services and soil fertility on crop quality and quantity. We investigated the effects of three pollination treatments (open, self and hand pollination) on apple quantity and quality parameters. We also analyzed the effects of bee abundance (wild bees and managed honeybees (Apis mellifera)) and soil nitrogen on fruit quantity and quality, and the responses of bee abundance and species richness to landscape metrics. Apple fruit set and yield of open pollinated flowers increased by 57 % and 25 t/ha (compared to bagged controls), respectively. Hand pollination further enhanced yields by 7 t/ha (compared to open pollination; i.e. to 39 t/ha), indicating pollination limitation in the orchards. Seed number was highest in open pollinated fruits, and increased with bee abundance if soil nitrogen was low, but decreased with bee abundance at high nitrogen levels, possibly due to higher flower density resulting in pollinator dilution effects. Higher seed numbers reduced the proportion of deformed apples and thus increased fruit quality. The percent of surrounding semi-natural habitats positively affected species richness of wild bees in apple orchards. We conclude that yield and quality of apples may benefit from ecological intensification comprising the augmentation of wild bees by semi-natural habitat and lowering of fertilizer inputs.

Calcium Polysulfides-Thiosulfate, Ammonium Thiosulfate, and NC-99 Blossom Thinners Impact Fruit Set of Apples, Peaches, and Plums

Crop load adjustment, using blossom and/or post-bloom thinners is a crucial practice to ensure production of commercially acceptable fruit size and yield efficiency in apples and stone fruit. In a study with two apple, one peach and one plum cultivars, application of calcium polysulfides and thiosulfate mixture (lime sulfur), ammonium thiosulfate (ATS), and NC-99 (Natural-cal), applied during bloom, reduced the fruit set of apples and/or certain stone fruit. In most cases, higher concentrations or double applications of a bloom thinner resulted in more blossom thinning which led into lower fruit set. Double applications of ATS at a rate of 15 mL.L-1 or a single application of this chemical at a rate of 25 mL.L-1 resulted in adequate levels of thinning in ‘Fuji’ apple. Double applications of ATS at a rate of 25 mL.L-1 resulted in excess thinning on ‘Fuji’ apple. In ‘Gala’ apple, a single application of NC-99 at 3% or 6%, with and without refined oil, or double applications of lime sulfur, each time at 5%, significantly reduced fruit set in 2013. In ‘Jonathan’ apple, applications of ATS at 25 mL.L-1 significantly reduced fruit set and increased fruit size. Application of lime sulfur at 6% with or without refined oil significantly reduced fruit set in ‘Elberta’ peach. In plums, application of lime sulfur at 6% with 1% refined oil significantly reduced fruit set, while application of this mixture at 3% lime sulfur (3% lime sulfur plus 1% oil) did not affect fruit set.

Revisiting the role of carbohydrate reserves in fruit set and early-season growth of apple

In apple (Malus x domestica Borkh.), carbohydrate reserves are considered essential for maintaining tree functions during dormancy and for supporting spring growth. The contribution of carbohydrate reserves to fruit set and early-season tree development is unclear. In ‘Royal Gala’/‘M9’ trees, we used artificial spur extinction (ASE) to remove buds in late dormancy and reduce sink numbers. Final fruit numbers were set using hand thinning after final fruit drop. These trees were compared with trees where bud numbers where unmodified and fruit numbers were only modified using post fruit drop hand thinning (PDHT). On both these treatments, either early or natural defoliation was used to manipulate carbohydrate reserve replenishment and alter carbohydrate reserve concentration the following spring. This treatment combination allowed investigation of the interaction of early-season sink demand and altered carbohydrate reserve supply on fruit set. Early defoliation reduced total non-structural carbohydrate concentration in roots the following winter by 44 % and concentration of starch in spurs by 60 % compared with natural defoliation. However, neither defoliation treatment affected fruit set the following spring, implying that at these concentrations, stored carbohydrates in these tissues played little role in fruit set. This conclusion was supported by increase in total non-structural carbohydrate concentration in roots and spurs during the period 2–4 weeks after bloom, when limitations in carbohydrate resources are most likely to affect fruit set. ASE reduced the number of developing floral sinks in spring by 50 % and increased within-bud fruit set compared with PDHT trees, without affecting concentration of total non-structural and soluble carbohydrates in root or spur tissues sampled before final fruit set. In ASE, improved irradiance of spurs may increase fruit set through increased availability of photosynthates to developing fruit. Therefore, supply of newly synthesised carbohydrates probably plays a greater role in determining fruit set than remobilised, carbohydrates. The increase in soluble carbohydrates in roots during fruit set, challenged that concept that storage sinks only accumulate carbohydrates during periods of low carbohydrate demand from other sinks.

Gibberellic acid induced parthenocarpic \u2018Honeycrisp\u2019 apples (Malus domestica) exhibit reduced ovary width and lower acidity

Fruit set and development are dependent on auxin, gibberellin, and cytokinin, which cause parthenocarpic development in many species when applied ectopically. Commercial sprays containing these hormones are used to improve apple fruit set, size, and shape, but have been implicated negatively in other aspects of fruit quality. We applied gibberellic acid (GA3), synthetic auxin (NAA), and the auxin-transport inhibitor NPA to ‘Honeycrisp’ apple flowers. Fruit retention and size were quantified throughout development, and seed number and fruit quality parameters were measured at maturity. GA3 alone caused the development of seedless parthenocarpic apples. At maturity, GA3-treated apples were narrower due to reduced ovary width, indicating that GA3 induced normal growth of the hypanthium, but not the ovary. GA3-treated fruits were also less acidic than hand-pollinated controls, but had similar firmness, starch, and sugar content. To further understand the regulation of parthenocarpy, we performed tissue-specific transcriptome analysis on GA3-treated, NAA-treated, and control fruits, at 18 days after treatment and again at maturity. Overall, transcriptome analysis showed GA3-treated and hand-pollinated fruits were highly similar in RNA expression profiles. Early expression differences in putative cell division, cytokinin degradation, and cell wall modification genes in GA3-treated ovaries correlated with the observed shape differences, while early expression differences in the acidity gene Ma1 may be responsible for the changes in pH. Taken together, our results indicate that GA3 triggers the development of parthenocarpic apple fruit with morphological deviations that correlate with a number of candidate gene expression differences.

Precision Crop Load Management of Apple (Malus x domestica Borkh.) without Chemicals

Fruit thinning is an important management practice in commercial apple production. The standard industry practice for crop load management in many countries is based on bloom and/or post-bloom chemical thinning (CT) followed up with hand thinning. However, the response to CT is unpredictable and there is an increasing awareness of the environmental impact of many chemicals. Hence there is a need to find alternate environmentally acceptable methods for managing crop load. Artificial bud extinction (ABE), a thinning method that imitates natural bud extinction by manually removing buds before bud break, has been suggested as a potential tool to replace chemical thinning, but there have been no studies comparing ABE and chemical thinning. Trials were established in Tasmania, Australia to determine how ABE technology compares with best practice CT programs in terms of yield, fruit quality, and cost of implementation. Results from these trials demonstrated consistent fruit set of both Gala and Fuji apple under ABE management compared with conventional management. Fruit weight was increased in all ABE treatments from 5% up to 38%. The four studies presented here have demonstrated that ABE is a feasible alternative to chemical thinning, improving reliability of crop load management with increased predictability of fruit size and yield. Trees are significantly thinned before flowering, controlling biennial bearing. In addition, bud position is optimised, fruit is well spaced and light distribution into the canopy is enhanced. In terms of costs, implementation of ABE is comparable to managing crop load with CT programs but has the advantage that crop load management costs reduce in subsequent years after the initial tree set-up. ABE is also suitable for use in organic apple orchards.

Apple fruit set is influenced by altered floral bud density but not by reduced carbohydrate reserves

The extent to which carbohydrate reserves contribute to apple (Malus × domestica Borkh.) fruit set is unclear. On ‘Royal Gala’/‘M9’ trees, we used postharvest early defoliation (12 days after harvest) to alter carbohydrate reserve accumulation compared with natural leaf fall. Superimposed over this, late winter bud thinning using artificial spur extinction (ASE, 6 buds cm(-1) branch cross sectional area) to manipulate the number of developing sinks in spring was compared with unmodified trees. Reducing bud density using ASE did not affect the concentration of total non-structural or soluble carbohydrates in root or spur tissues sampled at intervals from dormancy to final fruit set. However, ASE increased within-bud fruit set compared with unmodified trees. Early defoliation did not affect fruit set compared with natural defoliation, but reduced concentrations of total non-structural carbohydrates in roots in winter by 44% (264 to 147 mg g(-1) dry weight) and concentrations of starch in spurs in winter by 60% (19 to 7.7 mg g(-1)). Between dormancy and 1 week before bloom (WBB), total non-structural carbohydrate concentration declined in roots of naturally defoliated trees by 31%, and in spurs of naturally and early defoliated trees by 37 and 46% (respectively). In trees defoliated early, concentrations of total non-structural carbohydrates increased from their minimum at 1WBB in spurs and 2 weeks after bloom (WAB) in roots. In naturally defoliated trees total non-structural carbohydrates did not increase in roots during the measurement period, but increased in spurs between 5 and 9 WAB. Lack of differences in fruit set among defoliation treatments despite differences in non-structural carbohydrate concentration in spurs and roots implied that at concentrations observed here, stored carbohydrates in these tissues had little role in fruit set. This was supported by observations that total non-structural carbohydrate concentration declined before full bloom, not during fruit set, and total non-structural carbohydrate concentration increased during the period 2 to 4 WAB, when limitations in carbohydrate resources are thought most likely to affect fruit set.

Modeling pollen tube growth of ‘Gala’ and ‘Fuji’ apples

Abstract Environmental factors like air temperature directly affect the pollen tube growth which in turn can regulate fruit set and crop load in apple. The estimation of the pollen tube length and the time when the pollen tube reaches the stylar base is useful information for managing fruit load, site selection and the improvement of technologies such as mechanical pollination. Therefore, the objective of this study was to develop a model to predict pollen tube length based on air temperature and style penetration (%). Flowers of two apple cultivars, ‘Fuji’ and ‘Gala’, were emasculated, hand pollinated with ‘Red Delicious’ commercial pollen and placed in growth chambers at one of eight constant temperatures (4.5, 7, 11.5, 14.5, 21.5, 22.6, 28 and 30 °C). Pollen tube lengths of three flowers were measured at different five times after pollination for each evaluated temperature using a fluorescent microscope and measurements were adjusted to logistic and logarithmic curves as a function of time after pollination. Derivatives of the adjusted curves allowed the estimation of the hourly pollen tube growth rates, which in turn were fitted to a response surface as a function of temperature and style penetration. Finally, predicted pollen tube length was obtained by integrating hourly growth rates from the adjusted response surface. The assessment statistics showed an adequate performance of the model with a root mean square error (RMSE) of 0.95 mm for ‘Fuji’ and 1.87 mm for ‘Gala’. However, predicted pollen tube lengths were underestimated when the observed pollen tube lengths were short, mainly related to growth at low temperatures. For this reason, future upgrades should adjust the hourly pollen tube growth rates at low temperatures as well as set apart the temperature effects on fully and semi-compatible crosses.