Aj Gracie

Incidence and type of cracking in sweet cherry (Prunus avium L.) are affected by genotype and season

Rain induced fruit cracking in sweet cherries takes 3 distinct forms: stem end cuticular fractures, calyx end cuticular fractures, and large cracks usually deep into the pulp on the cheek of the fruit. A 4-year study of sweet cherry varieties from a commercial orchard in Tasmania, Australia, was conducted to investigate the incidence of crack type and its relative likelihood, as influenced by both genotype and season. Although all 3 crack types developed in the 3-week period before commercial harvest, the extent of cracking was strongly controlled by season. While initial development of cracks coincided with rainfall, no relationship between amount of rain and incidence of cracking was found for crack type. A significant relationship was found between the tangential stress experienced by fruit skin from fruit at harvest maturity and the incidence of cracking recorded in the orchard. No other fruit property (pulp osmotic potential, fruit diameter, weight) explained the differences in incidence of cracking in the field between seasons or varieties. The results suggest that management of cracking needs to consider both varietal and seasonal factors. The development of turgor in maturing fruit also needs further investigation.

Post-pollination capsule development in Eucalyptus globulus seed orchards

Low capsule set is a major factor limiting seed production in Eucalyptus globulus seed orchards. Trials were conducted in E. globulus seed orchards in Tasmania, Australia, to identify the timing of capsule development and abortion, as well as the influence of pollination type, the number of ovules fertilised and weather events on capsule set. Controlled pollination (CP), mass supplementary pollination (MSP), open pollination (OP) and isolated unpollinated control (UP) treatments were performed on 21 genotypes in an orchard in southern Tasmania in 2004–2005 and on six genotypes in a higher-altitude orchard in north-western Tasmania in 2005–2006. No capsules were set in the UP control treatment, and capsule set was significantly lower following CP than OP and MSP. The major period of capsule abortion occurred between 20 and 80 days after pollination for all pollination methods across both sites, coinciding with the period of capsule growth. A positive correlation between the number of fertilised ovules per aborted capsule and the length of time capsules were held on the tree was recorded. Given that capsule abortion occurred during a period of fruit growth and that capsules with the lowest number of fertilised ovules aborted first, it is argued that fertilisation level and the level of resource competition are major factors determining capsule abortion.

Paternal and maternal effects on the response of seed germination to high temperatures in Eucalyptus globulus

ContextHigh temperature stress in nurseries germinating Eucalyptus globulus seed is an important problem affecting germination synchrony and rate. Where there is a risk of high-temperature stress, then the choice of female parent may be important. This issue is particularly relevant to the production of full-sib families from mass-supplementary pollination where there may be opportunities for seed producers to manipulate the directionality of the crossing done in seed orchards.AimsThis study aimed to quantify the maternal versus paternal influence of seed sensitivity to high temperature stress during germination.MethodsA diallel crossing scheme involving four genotypes was used to test the relative importance of male and female genetic influences on the germination and development of E. globulus seed and their response to high temperature stress. Seed was germinated at optimum (25°C) and supra-optimal (32°C and 37°C) temperatures, and six traits describing the proportion and rate of seed germination and early seedling development were assessed.ResultsBoth paternal and maternal effects affected the germination response, arguing for at least some influence of the nuclear genotype of the embryo. However, the response to high temperature stress was more influenced by the maternal than paternal parent.ConclusionBoth the male and female genotype may affect various aspects of seed germination and early seedling development independent of seed size; however, some facets of the germination response will be mainly affected by the female parent.

Rhizome dormancy and shoot growth in myoga (Zingiber mioga Roscoe).

Abstract After the senescence of above ground plant parts the rhizomes of myoga undergo a period of dormancy. The length of this dormancy period and the requirement for chilling to break dormancy are examined in this paper. Myoga rhizome segments were capable of initiating shoot growth 6 weeks after excision from a recently senesced dormant mother plant. A period of chilling was not required for shoot growth, but chilling at 4°C for 3 weeks immediately after excision resulted in shoot development in newly divided myoga rhizome segments which would otherwise not have sprouted until after a 6-week dormancy phase. Longer storage periods before planting and increased duration of chilling treatments decreased the variability in the sprouting response of rhizome segments. Chilling at 4°C for 2–4 weeks was the most effective treatment for promoting uniform sprout emergence and subsequent flower production. Shoot growth rate and the number of vegetative shoots produced by the rhizome was not affected by chilling treatments. Increasing the length of chilling, promoted earlier sprouting and flowering, but may have decreased the weight of the flower buds produced.

The impact of flower density and irrigation on capsule and seed set in Eucalyptus globulus seed orchards

Low capsule set is a major factor limiting the productivity of Eucalyptus globulus seed orchards. This study tested the effect of flower density, as well as two common irrigation techniques on capsule and seed set. Ramets with high flower density had significantly lower capsule set (69.7%) than those with low flower density (81.7%). In a regulated deficit irrigation trial, the non-irrigated ramets set a higher proportion of capsules (63.6%) than the ramets that received conventional irrigation (CI) (51.4%). In a partial root zone drying (PRD) trial, capsule set was highest in the absence of irrigation (74.7%) followed by the PRD treatment (67.8%) and then CI (53.7%). The CI treatment tended to produce the highest number of seed per capsule. Increased water availability resulted in increased vegetative growth, which was associated with higher levels of abortion in developing capsules but those surviving tended to have higher seed set. It is argued that the observed effects of irrigation and flower density can be explained by resource competition between vegetative and reproductive growth as well as competition among reproductive structures themselves.

Relationship between growth rate and the development of hollow stem in broccoli

Boron deficiency is widely accepted as the dominant cause of hollow stem in broccoli (Brassica oleracea L. var. italica Plenck), although high growth rate has also been mentioned in the literature as a possible cause of the disorder. In this study, we investigated the role of growth rate and boron in the development of hollow stem. Two experiments were conducted with broccoli cv. Marathon, using planting density to manipulate growth rate. In the first experiment, plants were grown at 3 planting densities (19 512, 32 520, and 69 686 plants/ha) and either supplemented or not supplemented with foliar boron applications (0.35 kg B/ha) on two occasions. In the second experiment, broccoli plants at 2 planting densities (32 520 and 100 000 plants/ha) were treated in factorial combination from inflorescence initiation onwards with complete trace element fertiliser (1.5% B), 57% light interception, and paclobutrazol (480 g a.i./ha). The incidence and severity of hollow stem were assessed using digital image analysis and were found to increase with plant spacing but were not affected by boron application. The probability of hollow stem occurring increased with absolute growth rate in the first trial, but not the second. Absolute growth rate, measured as dry weight accumulation, was suppressed independently of planting density by the shading treatment, but did not influence the incidence or severity of hollow stem. The application of paclobutrazol did not influence absolute growth rate but reduced the severity of hollow stem at the lowest planting density. Our data support the hypothesis that the rate and pattern of growth are involved in the development of hollow stem, and that a boron deficiency is not the major causal factor.

The relative contribution of the male and female to the variation in reproductive success in Eucalyptus globulus.

Abstract Low and variable capsule and seed set is a major factor limiting seed production in Eucalyptus globulus seed orchards. This study identified the relative contribution of the maternal and paternal parent to reproductive success in E. globulus, and examined the genetic basis to the observed variation. Reproductive success was measured in terms of the number of viable seeds obtained per flower crossed. Data on the reproductive success of numerous genotypes from the Furneaux, Strzelecki Ranges and Otways races were obtained from: (i) 12 years of operational full-sib crossing; (ii) a designed fullsib diallel mating scheme; and (iii) capsule retention trials conducted over three seasons at two Tasmanian seed orchards on the same ramets of different genotypes. Analysis of the sparse operational data revealed that both male and female factors significantly affected reproductive success, accounting for 5.0% and 7.9% of the variation respectively. The more precise diallel crossing revealed that a large, and significant, proportion 55%, of the variation in reproductive success between crosses at a single site was explained by the female parent. The male parent explained only 6.7%, but this effect was not statistically significant and was related to variation in in vitro pollen germination. The significant female effect was found to be consistent at the genotype level across seasons, sites and pollination techniques suggesting a genetic basis to the variation. It is argued that selection of genetically fecund females is a key consideration in reducing costs of manual pollination for breeding and deployment purposes.

Effects of climatic factors, fluctuating water availability and partial defoliation on the diurnal radial growth pattern of carrot (Daucus carota) taproots

Carrot taproots are prone to radial longitudinal tissue fractures (splitting). The longitudinal alignment of the fractures is consistent with the tangential stress induced by radial growth in the annular cambium. In this study linear variable differential transformers were used to monitor the diurnal radial growth pattern of mature carrot taproots. The diurnal growth pattern was characterised by rapid expansion during the night (dark) period and small shrinkage during the light period. The magnitude of the daily shrinkage was positively correlated with both vapour pressure deficit and global radiation. A positive correlation between the rate of taproot expansion during the night period and global radiation level the preceding day was also recorded. Water stress reduced the taproot night time expansion rate, but the taproot of water stressed plants expanded rapidly when watered to field capacity. Partial defoliation of plants at midday resulted in immediate expansion of the taproot, however, the daily growth rates of the partially defoliated plants was less than untreated control plants over the following 3 days. It was concluded that the short-term changes in taproot diameter could be used to infer changes in tangential stresses and therefore taproot splitting propensity.

The relationship of the female reproductive success of Eucalyptus globulus to the endogenous properties of the flower

Low capsule and seed set is a major factor limiting seed production in Eucalyptus globulus seed orchards. Controlled pollination studies showed that the reproductive success (number of seeds produced per flower pollinated) was primarily determined by the female. We aimed to identify the factors contributing to the differences in reproductive success between female genotypes in terms of the physical and anatomical properties of the flower. We studied pairs of genotypes of high and low reproductive success from each of three races (Furneaux Group, Strzelecki Ranges and Western Otways) growing in a seed orchard. Controlled pollinations were performed on six females and along with flower physical measurements, pollen tube growth and seed set were assessed. Overall tree reproductive success was positively correlated with flower size, ovule numbers, style size, cross-sectional area of conductive tissue within the style (all of which were inter-correlated) and the proportion of pollen tubes reaching the bottom of the style. Significant positive correlations of reproductive success and flower physical properties between different ramets of the same genotypes across seasons suggests a genetic basis to the variation observed. The majority of pollen tube attrition occurred within the first millimetre of the cut style and appeared to be associated with differences in style physiology. When examined as pairs within races the difference in reproductive success for the Western Otways pair was simply explained by differences in flower size and the number of ovules per flower. Physical features did not differ significantly for the Strzelecki Ranges pair, but the proportion of pollen tubes reaching the bottom of the style was lower in the less reproductively successful genotype, suggesting an endogenous physiological constraint to pollen tube growth. The difference in reproductive success between the females from the Furneaux Group was associated with a combination of these factors.

Partial defoliation treatments to reduce carrot (Daucus carota L.) taproot splitting

Carrot taproots with longitudinal tissue fractures (splits) are commonly deemed unsaleable. This study examined the use of partial leaf defoliation treatments to reduce the incidence of taproots that fracture during growth of the crop (growth splitting) and susceptibility to splitting during harvest (harvest splitting). Two leaf-trimming treatments, vertical and vertical plus horizontal leaf trimming, were compared with untreated control plots. The vertical plus horizontal trimming was the most effective treatment at reducing both harvest and growth splitting. Harvest splitting was significantly reduced by partial leaf defoliation the day prior to commercial harvest. The effectiveness of the treatments declined with time from treatment application. Growth splitting occurred mainly in the weeks prior to commercial harvest and the application of partial defoliation treatments during that period significantly reduced their incidence in crops predisposed to the disorder. A positive correlation between taproot splitting susceptibility and growth splitting was also identified, supporting previous assumptions that both harvest and growth splitting were induced by the same underlying mechanism. The leaf trimming treatments did not preclude mechanical harvesting.