Peter M. Hirst

Increase in fruit size of a spontaneous mutant of ‘Gala’ apple (Malus×domestica Borkh.) is facilitated by altered cell production and enhanced cell size

Fruit size regulation was studied in the apple cultivar ‘Gala’ and a large fruit size spontaneous mutant of ‘Gala’, ‘Grand Gala’ (GG). GG fruits were 15% larger in diameter and 38% heavier than ‘Gala’ fruits, largely due to an increase in size of the fruit cortex. The mutation in GG altered growth prior to fruit set and during fruit development. Prior to fruit set, the carpel/floral-tube size was enhanced in GG and was associated with higher cell number, larger cell size, and increased ploidy through endoreduplication, an altered form of the cell cycle normally absent in apple. The data suggest that the mutation in GG promotes either cell production or endoreduplication in the carpel/floral-tube cells depending on their competence for division. Ploidy was not altered in GG leaves. During fruit growth, GG fruit cells exited cell production earlier, and with a DNA content of 4C suggesting G2 arrest. Cell size was higher in GG fruits during exit from cell production and at later stages of fruit growth. Final cell diameter in GG fruit cortex cells was 15% higher than that in ‘Gala’ indicating that enhanced fruit size in GG was facilitated by increased cell size. The normal progression of cell expansion in cells arrested in G2 may account for the increase in cell size. Quantitative RT-PCR analysis indicated higher MdCDKA1 expression and reduced MdCYCA2 expression during early fruit development in GG fruits. Together, the data indicate an important role for cell expansion in regulating apple fruit size.

Fruit colour responses of ‘Granny Smith’ apple to variable light environments

Abstract Fifteen sequences of sunlight exposure and exclusion were imposed on fruit of ‘Granny Smith’ apple (Malus domestica Borkh.) by covering individual fruit with opaque paper bags. Fruit were harvested at commercial maturity and fruit background colour, blush, soluble solids concentration, flesh firmness, and starch pattern index were measured. The following year, seven of the same sequences were repeated and fruit colour was measured at 10-day intervals beginning from fruit drop until commercial harvest. A strong linear relationship between fruit colour lightness and hue (visually perceived colour) was evident in both years. Fruit became lighter as greenness decreased while an increasing yellowness only slightly increased lightness. Longer durations of light exclusion resulted in lighter coloured fruit, but for a given duration of shading, fruit exposed 1 month before harvest were lighter than fruit which remained shaded until harvest. All fruit became lighter in colour with time, although the rate ...

Seasonal variation in the onset and duration of flower development in 'Royal Gala' apple buds

Summary The formation of bud scales, transition leaves, true leaves, bracts and flower primordia were observed in buds removed from non-flowering sites on one year old shoots of apple ‘Royal Gala’. Buds were sampled from trees in Hawkes Bay, New Zealand (latitude 398S) during six successive seasons. The onset of flower development varied between seasons from 72 to 99.d after full bloom. Floral development was poorly synchronized within the populations of sampled buds, lasting 22 to 50.d. Differences in the onset and duration of flower development could not be related to seasonal differences in heat accumulation from full bloom. There was significant variation between seasons in the minimum number of appendages observed within buds that exhibited doming, which indicates floral development. In the 1998/99 season, doming was observed in buds that had only 15.7 appendages, whereas in the 1999/2000 season the minimum number of appendages within a bud that had initiated flowers was 17.5. When the data from all seasons were considered, the rate of appendage formation within developing buds was not consistently related to heat accumulation.

Crops In Silico: Generating Virtual Crops Using an Integrative and Multi-scale Modeling Platform

Multi-scale models can facilitate whole plant simulations by linking gene networks, protein synthesis, metabolic pathways, physiology, and growth. Whole plant models can be further integrated with ecosystem, weather, and climate models to predict how various interactions respond to environmental perturbations. These models have the potential to fill in missing mechanistic details and generate new hypotheses to prioritize directed engineering efforts. Outcomes will potentially accelerate improvement of crop yield, sustainability, and increase future food security. It is time for a paradigm shift in plant modeling, from largely isolated efforts to a connected community that takes advantage of advances in high performance computing and mechanistic understanding of plant processes. Tools for guiding future crop breeding and engineering, understanding the implications of discoveries at the molecular level for whole plant behavior, and improved prediction of plant and ecosystem responses to the environment are urgently needed. The purpose of this perspective is to introduce Crops in silico (cropsinsilico.org), an integrative and multi-scale modeling platform, as one solution that combines isolated modeling efforts toward the generation of virtual crops, which is open and accessible to the entire plant biology community. The major challenges involved both in the development and deployment of a shared, multi-scale modeling platform, which are summarized in this prospectus, were recently identified during the first Crops in silico Symposium and Workshop.

Bud development and floral morphogenesis in four apple cultivars

Summary Buds were sampled from non-flowering spurs that had developed on 1 year-old wood of four apple (Malus domestica Borkh.) cultivars during the 1998-99 growing season in order to determine the effect of cultivar on appendage formation, doming and flower morphogenesis. Cultivars differed in their pattern of appendage formation over time. The rate of appendage formation was highest in ‘Fuji’ and ‘Pacific Rose™’ and lowest in ‘Braeburn’ during the first 60 d after bloom (DAB). A high proportion of buds were floral in all cultivars at the end of the growing season (75–100% depending on cultivar). However, the probability of observing doming was never greater than 0.13, indicating that flower morphogenesis proceeded rapidly once buds were committed to floral development. The four cultivars each exhibited a unique pattern of floral development, as determined by fitting response probabilities to each of five ordinal stages of development with time. Doming occurred significantly earlier in ‘Fuji’ buds (peaking 86 DAB) than in buds of the other cultivars (peaking 104–112 DAB). Doming was initiated at the same time in buds of ‘Braeburn’, ‘Royal Gala’ and ‘Pacific Rose™, but was completed earlier in ‘Braeburn’ than in ‘Royal Gala’. The timing of floral commitment was not related either to the time of flowering, or to the time of fruit maturity of the cultivar. These observations indicate that the timing of specific events during flower morphogenesis differed between cultivars.

Optimizing planting density for production of high-quality apple nursery stock in China

In China, apple (Malus × domestica Borkh.) nursery stock is generally of low quality because of extremely high planting density. The objective of this study was to determine the optimum planting density of 2-year-old grafted apple trees. Tree growth (height, trunk diameter, leaf area index) increased as density decreased. Trees grown at high densities (14.3–50 plants/m2) were the shortest with the smallest trunk diameters and leaf areas, whereas trees grown at lower densities (4.8–10 plants/m2) were generally largest in terms of height, diameter and leaf area. Trees grown at lower densities tended to have higher bud dry weight, leaf dry weight, nitrogen content, total soluble sugar concentration and total non-structural carbohydrate content. Higher levels of these parameters were generally observed with tree densities at or below 10 plants/m2. Therefore we conclude that 10 plants/m2 is the optimum density for maximizing the number of trees produced per unit land area while maintaining tree quality of nursery stock.

Pruning strategies for restructuring top‐dominant central leader ‘Granny Smith’ apple trees

Abstract Eighteen‐year‐old, top‐dominant central leader trees of ‘Granny Smith’ apple (Malus domestica Borkh.) were restructured during the 1987 and 1988 dormant seasons into either palmette central leader (initially) or pyramid central leader forms. In the palmette form, pruning was restricted to the total removal of the uppermost east‐ and west‐facing fruiting scaffolds in 1987, and the shortening of all middle scaffolds and the upper north‐facing scaffold in 1988. In the pyramid form, major scaffold limbs were subjectively removed or shortened to reestablish the desired pyramid shape over the two dormant seasons. Both forms were also renewal pruned annually to totally remove pendant fruiting laterals and spur‐bearing wood over 4 years old. In addition, unmodified top‐dominant trees that were either renewal pruned or containment pruned (primarily heading cuts) were maintained for comparison with the restructuring treatments. The restructuring treatments resulted in improved light penetration to the lowe...

Pollen Tube Growth and Fruit Set in Apple

Pollination is an essential prerequisite for the production of many fruit and seed crops, including apple. In apple, successful fertilization requires pollen transfer to the stigma, pollen germination, and successful pollen tube growth resulting in fruit set. Precise selection of the most effective pollinizers for commercial orchards is not possible however, until these processes are more fully understood. The present study was undertaken to compare pollinizers in terms of pollen tube growth and fruit set. On trees of ‘Honeycrisp’, ‘Fuji’, and ‘Gala’ from which bees were excluded, flowers were hand-pollinated using pollen collected from crabapple (‘Ralph Shay’ or Malus floribunda), ‘Delicious’ and ‘Golden Delicious’. Flowers were harvested at one, two, three, and four days after pollination (DAP). Pollen source had a significant influence on pollen germination on the stigmatic surface, number of pollen tubes penetrating the stigma, distance of pollen tube growth down the style, and pollen tubes reaching the base of the style. In ‘Honeycrisp’ and ‘Gala’, ‘Golden Delicious’ pollen grew the fastest, followed by ‘Delicious’ and crabapple. Neither ‘Ralph Shay’ nor Malus floribunda were effective pollinizers for ‘Honeycrisp’ and resulted in low fruit set suggesting incompatibility may be involved. However, both these crabapples were effective pollinizers for ‘Fuji’ and ‘Gala’. These results indicate that pollen source can have a tremendous impact on pollen tube growth and fruit set. The physiological basis for these effects is not clear, but implications for pollinizer selection are obvious. Pollination is an essential process for fruit set, fruit growth, fruit quality, and seed set of most apple cultivars. The first step of successful apple pollination is the transfer of pollen to the stigmatic surface (typically vectored by bees) followed by an adhesion of pollen grains to the papilla cells of the stigmatic surface (Dresselhaus and Franklin-Tong, 2013; Selinski and Scheibe, 2014). The deposited pollen hydrates and germinates and then pollen tubes penetrate the stigma and grow down the style. Pollen recognition occurs both on the stigmatic surface and within the style (Dresselhaus and FranklinTong, 2013). Once pollen reaches the base of the style, one sperm nucleus enters the egg cell to produce the embryo (resulting in seed set), and the other fuses with the two polar nuclei to produce the endosperm. Pollen source and temperature have a tremendous influence on the rate of pollen tube growth. Jackson (2003) and Petropoulou and Alston (1998) proposed that the proportion of the stigmatic surface covered by the germinated pollen of apples and pears depends on the pollen donor and environmental temperature at the time of pollination. They reported that ‘Spartan’ pollen had a higher germination percentage than that of ‘Cox’s Orange Pippin’ at 8 to 10 C and ‘Idared’ at 14 to 16 C. Linear correlations between pollen germination on the stigmatic surface and temperature were reported from 13 to 29 C (Yoder et al., 2009) and from 6 to 33.5 C (Jefferies and Brain, 1984). Almost all apple cultivars are reported to be either self-incompatible or semicompatible and require cross-pollination to set fruit in marketable quantities (Garratt et al., 2013; Matsumoto, 2014). For commercial production, at least two cross-compatible cultivars with synchronous flowering are recommended (Garratt et al., 2013; Goldway et al., 2012). Matsumoto et al. (2008) reported a significant decline in fruit set with increasing distance between pollinizer and main cultivars, and suggested pollinizers should be planted not more than 10 m from the target cultivars (Matsumoto et al., 2008). Pollen source is considered to be one of the most influential factors affecting the fruit set and therefore orchard design. Pollen density on the stigmatic surface is positively correlated with pollen tube growth and fruit growth rate (Zhang et al., 2010). Pollen tube growth down the style has been widely studied, but pollen source effects on pollen tube growth are not well understood. The objective of this study was to determine the impact of pollen source on the pollen tube growth and fruit set. The effects on the seed set, fruit quality, and subsequent return bloom will be reported in a subsequent manuscript. Materials and Methods This experiment was conducted in 2013 and repeated in 2014 at the Samuel G. Meigs Horticulture Facility in Lafayette, IN, USA. Three commercial apple cultivars were used: ‘Honeycrisp’/M.7 planted in 2003, ‘Fuji’/B.9 planted in 2001 and ‘Gala’/B.9 planted in 2001. These cultivars were chosen because of their economic importance and also because they represent a range of genetic potential for biennial bearing, so that the effects on flowering could be compared. Selected flowers of each cultivar were hand-pollinated using one of three pollen sources: crabapple [‘Ralph Shay’ (2013) and Malus floribunda (2014)], ‘Delicious’ and ‘Golden Delicious’. Since ‘Ralph Shay’ crabapple was found to be a poor pollinizer of ‘Honeycrisp’ in 2013, it was replaced by Malus floribunda in 2014. All cultivars were pollinated over a 2-hour period on the same day. Temperature data were collected by a weather station located less than one mile from experimental plots. There was little variation in air temperature between 2013 and 2014 during the pollination period and during the days of sample collection, so it seems reasonable to assume any differences between the years were not due to temperature (Fig. 1). Trees were spaced 5.0 · 3.0 m and trained to a vertical axis. General orchard management, including pest management, was according to commercial guidelines for Indiana (Midwest Fruit Workers Group, 2013). Trees were not chemically thinned but were hand thinned to approximate a light commercial crop load to ensure that crop load was not a limiting factor. The experiment was designed as a completely randomized design where two uniform adjacent trees were selected for each cultivar in late April of 2013. The same trees were used in 2014. Trees were netted in late April, before flower opening, to exclude bees, and thus, prevent cross-pollination. Care was taken to ensure the netting was secure and close visual examination did not observe any pollinators inside the netting. At the tight cluster stage of floral development, a total of 60 flower clusters were randomly selected on each cultivar, distributed between the two selected trees. Of the 60 flowers, 20 were randomly assigned to each of the three pollinizer treatments. There was not an unpollinated control because unpollinated flowers were presumed to result in very low fruit set. At late pink (popcorn) stage, just before the flower completely opened, all anthers were removed to prevent selfpollination, and flowers were hand-pollinated using a small brush. Pollen was applied liberally to ensure the amount of pollen was not limiting. Only king flowers were pollinated for uniformity of the experiment, and all lateral flowers as well as all nontagged flowers on the tested trees were either manually removed or dropped off. Pollen used for the Received for publication 14 Nov. 2016. Accepted for publication 8 June 2017. We thank Tristand Tucker and the staff at the Samuel G. Meigs Horticulture Facility, Purdue University, Leon Combs (Virginia Tech) for their technical advice, and USAID for financial support through the Strengthening Afghan Agriculture Faculties (SAAF) program. Graduate student. Professor. Corresponding author. E-mail: hirst@purdue.edu. 1054 HORTSCIENCE VOL. 52(8) AUGUST 2017 experiment was collected from earlier flowering orchards in southern Indiana (2013) or from branches placed in the greenhouse to force flower opening (2014). Pollen was tested in the laboratory in petri dishes before pollination to examine viability. Following the methods of Yoder et al. (2009), pollen was placed on a medium of 1% agarose, 10% sucrose, and 10 ppm boric acid at room temperature for 24 h. The proportion of germination was visually observed under a light microscope. All pollen used in these experiments had high germination rates (>80%). The methods of Yoder et al., (2009) were modified to evaluate pollen germination on the stigmatic surface, number of pollen tubes penetrating the stigma, the length of the longest pollen tube growing down the style, and number of pollen tubes that reached the base of the style. Briefly, five hand-pollinated flowers were collected from each treatment at one, two, three, and four DAP. The flowers were then placed in a solution of 5% sodium sulfite, boiled for 15 min and then refrigerated in the same solution until the time of microscopic examination. Later, five pistils from each flower were detached from the ovary, rinsed with distilled water, and placed in a water-soluble solution of 0.01% aniline blue stain in 0.067MK2HPO4 on microscope slides. Detached pistils were squashed between two microscope slides. The pistils were then incubated in the dark at room temperature for 24 h, and the length of each pollen tube was measured by viewing it in epifluorescence mode through a blue filter (350/50 460/50) using a 10· 0.45 NA objective. The position of each pistil’s end was located based on xy coordinates provided by an encoded, motorized xy stage (Nikon Ti-SER) on a Nikon Ti-E microscope. Length was defined as the linear distance between each endpoint. Images of pistils were acquired using a 10· 0.45 NA objective and Nikon DS-Ri1 color camera on a Nikon 90i epifluorescence microscope using blue (350/50 460/50) and red (560/40 630/60) filter sets. Individual images of each pistil were manually assembled into a montage (Fig. 2). The collected data comprised the rating of pollen tube growth on the stigmatic surface (0% to 100% of stigmatic surface covered by germinated pollen), number of visible pollen tubes penetrating the stigma, average length of the longest pollen tube growing down the st

Multiple Applications of Lime Sulfur for Fruit Thinning of ‘Fuji’ and ‘Hongro’ Apple Trees

The thinning effects of lime sulfur either alone or combined with fish oil on two apple cultivars ‘Fuji’/M.9 and ‘Hongro’/M.9 were investigated during two consecutive seasons. Either 1% or 2% lime sulfur at full bloom decreased the amounts of both terminal and axillary fruits compared with unsprayed ‘Fuji’ control trees in the first year. However, 1% or 2% lime sulfur did not decrease terminal fruit set in the second year. Only, triple applications of 1% lime sulfur significantly reduced the number of terminal and axillary fruits in both years. Multiple application of 1% lime sulfur was the most effective in thinning of ‘Hongro’. In the first year, double or triple applications of 1% lime sulfur was effective in thinning terminal or axillary fruit, but one time applications of 1% or 2% lime sulfur was not reduced axillary fruit in ‘Hongro’ apple trees. In the second season, all treatments reduced terminal or axillary fruit in ‘Hongro’ clusters, except single application of 1% lime sulfur. Fish oil did not always improve the thinning effects of lime sulfur in ‘Fuji’ or ‘Hongro’ apple trees. None of treatments caused significant russeting in the harvested fruits. Most of thinning treatments had little effect on ‘Fuji’ or ‘Hongro’ fruit quality in terms of fruit weight, firmness, soluble solids concentration or titratable acidity.

IMapple — Functional structural model of apple trees

Computer simulation of tree development can provide important insight into real world tree behavior. Numerous models have been developed ranging from simple equations to complicated 3D geometrical models that allow for simulation of acquisition and allocation of carbohydrates. We introduce IMapple (Interactively Modeled Apple), a functional structural apple tree model that builds on the source-sink model of L-peach. Our model integrates accurate biological functionality, as well as high quality visual geometric details. The details are modeled by using subdivision surfaces that allows for fast generation of high precision geometry and direct illumination of leaves is simulated by sampling the sky and the sun trajectory. The main advantage of the IMapple is that allows for an interactive experimentation by the way it manages data. Input data can be provided interactively or by using real-world samples and the model responds immediately. Alternatively, hypothesized data can be input by drawing functional dependencies, for example the resource assimilation rate as a function of light interception can be input by drawing a graph showing the real or assumed relationship. Moreover, our model supports a variety of convenient user interactions, such as branch pruning and flower thinning. We have collected a large data set over the last few years that serves as inputs to our model. IMapple is fast and it allows for interactive or near-interactive simulation of apple trees. For example, a ten years-old tree grown with a time step of one hour can be simulated in under five minutes using a PC. Moreover, our system allows for a simulation of multiple trees and their mutual interaction. We demonstrate the results by simulating growth and fruiting of Golden Delicious apple trees realistically with data collected from a planting at the Purdue Meigs research farm.