Growth,photosynthetic performance and antioxidative response of ‘Hass’ and ‘Fuerte’ avocado (<Emphasis Type="Italic">Persea americana</Emphasis> Mill.) plants grown under high soil moisture

‘Hass’ and ‘Fuerte’ avocado plants were grown under well-watered or waterlogged conditions. Results indicated significant effects on the majority of the allometric parameters in waterlogged plants, with ‘Fuerte’ displaying a more pronounced growth inhibition. Waterlogged conditions caused a progressive and simultaneous decline in net photosynthetic rate and stomatal conductance, earlier in ‘Fuerte’ than in ‘Hass’. Maximal potential quantum yield of PSII was unaffected by the soil water regime and/or variety and leaf water potential values in waterlogged plants were not more negative compared with control plants. ‘Fuerte’ waterlogged plants exhibited increased contents of thiobarbituric acid reactive substances, whereas oxidative injury was not detected in ‘Hass’. Finally, none of the two cultivars displayed valuable antioxidant potential, as evidenced by the decreased activities of the antioxidant enzymes superoxide dismutase, guaiacol peroxidase, glutathione peroxidase, and ascorbate peroxidase.     

Detection of Avocado Sunblotch Viroid in Spain by Double Polyacrylamide Gel Eletrophoresis

Avocado sunblotch viroid (ASBV) was detected in samples of young leaves and flowers of the avocado cultivars ‘Hass’ and ‘Fuerte’ grown in southern Spain. Partially purified nucleic acid preparations, obtained by cellulose chromatography, were analyzed by double polyacrylamide gel electrophoresis. This technique, consisting of two consecutive electrophoresis cycles under nondenaturing and denaturing conditions, followed by silver staining of the gel, allowed the selective and sensitive identification, of ASBV. This is the first report on the occurrence of the causal agent of avocado sunblotch in Spain.     

Some Effects of Daylength and Flower Manipulation on the Floral Cycle of Two Cultivars of Avocado (Persea americana Mill., Lauraceae), A Species showing Protogynous Dichogamy

Floral cycling of the avocado cultivars Fuerte and Hass wasobserved under controlled conditions of 25 °C and daylengthsof 0 h, 1 h (08.00 h-09.00 h), 6 h (08.00 h-14.00 h), 6 h (13.00h-19.00 h), 12 h (08.00 h-20.00 h) and 24 h with a photon fluxdensity of 350 /imol m_–2 s ^–1 (400–700 nm).Normal floral cycling of discrete opening periods of femalestage, followed by closed phase, followed by male stage, occurredunder all conditions except 24 h in Fuerte, and 0 h and 24 hin Hass. Under continuous light the cycle was disrupted andfemale and male stages opened throughout the day, and very fewHass flowers opened at all. No male stage Hass flowers openedunder continuous darkness. Under the shorter daylengths thefloral cycle was condensed, and the timing of the cycle shiftedfollowing alteration in the time of start of the light period.Emasculation of flowers in the female stage prevented reopeningduring the period of opening of the male stage flowers in Fuertebut not in Hass. The effect was localized to the perianth partsadjacent to the removed anther or stamen. Key words: Flowering, Avocado, Dichogamy     

Fruit misshapen in strawberry cultivars (Fragaria×ananassa) is related to achenes functionality

Fruit deformation is a phenomenon commonly observed in commercial strawberry cultivars (Fragaria×ananassa) with a negative impact on the economic benefit of crop production. To better understand this physiopathy, we evaluated fruit size and the relative amount of ‘small’ (<0.4 mm) and ‘big’ achenes (>0.6 mm) in misshapen (MF) and well‐formed (WF) fruits from ‘Camarosa’, ‘Ventana’ and ‘Medina’ strawberry cultivars growing in the field. In ‘Camarosa’, size‐dependent achenes functionality was assessed by analysing achenes germinability and differences in time to ripening between MF and WF. We found that the occurrence of fruit deformation was not only strongly dependent on the cultivar (i.e. genetic factor) but also was promoted by low temperatures (i.e. environmental factor). Regardless of the cultivar, MF showed higher percentage of ‘small’ non‐functional achenes than WF, indicating a failure in achene development, which could be related to the functional integrity of reproductive structures. To test this hypothesis, we performed an experiment by pollinating strawberry flowers with pollen developed under low and mild temperatures. Low temperatures reduced pollen viability, and flowers pollinated with low quality pollen led to a higher amount of MF. However, fruit deformation was not completely explained by differences in pollen quality, neither by differences in pistil maturity and receptivity nor ovule viability, suggesting that non‐functional achenes might result from a disruption in postfertilisation processes such as embryo abortion as a consequence of low temperatures.     

Selection of Highly Informative Microsatellite Markers to Identify Pollen Donors in ‘Hass’ Avocado Orchards

‘Hass’ is the most popular avocado (Persea americana Mill.) cultivar in the world. It has been characterized as a crop requiring cross-pollination. However, the potential extent of self-pollination and the most effective pollen donors (best cross-pollinizing cultivars) have not been determined. In this study, 56 markers were screened against ‘Hass’ and nine commonly used pollinizing cultivars grown in southern California: ‘Bacon,’ ‘Ettinger,’ ‘Fuerte,’ ‘Harvest,’ ‘Lamb Hass,’ ‘Marvel,’ ‘Nobel,’ ‘Sir Prize,’ and ‘Zutano.’ Seventeen microsatellite, i.e., simple sequence repeat (SSR) markers, were found to be very promising for paternity analysis. Four highly informative SSR markers were selected to accurately and unequivocally identify pollen parents of ‘Hass’ fruit from an orchard interplanted with these pollinizing cultivars. From 2003 to 2006, 7,984 ‘Hass’ fruit were analyzed for their paternity. Overall, the pollen parents of 99.55% of the analyzed fruit could be unequivocally identified with a single multiplex polymerase chain reaction (PCR). Only 36 fruits (<0.45%) required a second PCR reaction to reach unequivocal identification of the pollen parents.     

FLORAL BIOLOGY AND REGULATION OF SEED SET AND SEED SIZE IN THE LILY,CLINTONIA BOREALIS

We studied the reproductive ecology of Clintonia borealis, a clonal understory species, in eastern Ontario. Flowers are protogynous and require insect pollination for outcrossing and maximum seed set. Most pollination is done by nectar-gathering bumble bees. We recorded pollen deposition and flowering patterns in 1983. Flowering lasted for about 10 days in mid-June. Stigmatic pollen loads were lowest at the onset of blooming, reflecting the scarcity of male-phase flowers. Pollination increased significantly by peak bloom and subsequently remained high. However, since bees fly mostly between neighboring stems, much of the pollen transferred may have been geitonogamous. In 1983, seed set per flower was not increased by supplementing pollination or by reducing the number of fruits competing for resources per stem. Rather, flowers set more seeds (14%) only if both treatments were performed simultaneously. Seed size was increased by 25% when competitive fruits were removed and by 5% more when pollen was added under the removal treatment. Further work in 1984 showed that pollination effects may be related to changes in pollen source. Selfed flowers set fewer and smaller seeds than outcrossed ones when screened from insects and hand pollinated. Outcrossing distance had little effect on seed set or seed size. Thus, in C. borealis seed number and seed size are limited by a balance between maternal resource availability and the amount of outcrossing provided by pollinators.     

Involvement of Epicatechin in Cultivar Susceptibility of Avocado Fruits to Colletotrichum gloeosporioides after Harvest1

Avocado cultivars were defined as susceptible and resistant to Colletotrichum gloeosporioides depending upon the length of the incubation period of the disease after fruit softening. In the susceptible cultivars Fuerte, Horshim, Vurtz, Rincon, and Benik, epicatechin concentration of the peel decreased to 60-130 μg.g^?1, fr. wt. at fruit softening and symptoms appeared on the same or one day later. In the resistant cultivars Hass, Nabal, Netaim and Pinkerton, epicatechin concentration was still 632–1740 μg.g^?1 fr. wt. when fruit softening and symptoms appeared only 4-10 days later. When susceptible Fuerte fruits became soft the concentration of the antifungal compound 1-acetoxy-2-hydroxy-4-oxo-heneicosa-12,15 diene, had decreased to 120 μg.g^?1 fr. wt. and symptoms appeared. In resistant Hass fruits, the antifungal diene was still 238 μg.g^?1 fr. wt. at fruit softening; and it had further decreased to 159 μg.g^?1 fr. wt. when symptoms appeared, four days later. A modified atmosphere and 0.2 M CaCl₂ infiltration both delayed softening of Fuerte fruits; but symptom appearance on these fruits was related to diene decrease and not to fruit softening. The results are discussed in relation to the hypothesis that the susceptibility of avocado cultivars to post-harvest decay by C. gloeosporioides is related to the degradation of the antifungal diene, catalyzed by avocado lipoxygenase, the activity of which is regulated by the decline of its inhibitor epicatechin.     

Light Microscope Study of Pollen Tube Growth, Fertilization and Early Embryo and Endosperm Development in the Avocado Varieties Fuerte and Hass

Pollen tube growth, fertilization and early embryo and endospermdevelopment were studied using light microscopy in the avocadovarieties Fuerte and Hass. The ovule was penetrated by a pollen tube by 24 h after pollination.On reaching the ovary, the pollen tube grew along the surfaceof the inner ovary wall. It then grew around the funicle, throughthe micropyle in the inner integument and between the papillatecells at the apex of the nucellus. It entered the embryo sacvia a synergid. Sperm nuclei were present in the embryo sacat 48 h after pollination and fusion of the polar and spermnuclei took place before fusion of the egg and sperm. The endospermnucleus was the first to divide and cell wall formation occurredfollowing division. The first division of the zygote occurredat 5 or 6 days after pollination. In the variety Fuerte less than 20 per cent of the 1- and 2-day-oldembryo sacs had been penetrated by a pollen tube although tubeswere often observed in the integument or nucellus. In the varietyHass over 60 per cent of the embryo sacs were penetrated. Inwas concluded that low yields of the variety Fuerte may be partlyattributable to the failure of the pollen tube to penetratethe embryo sac. Persea americana Mill, avocado, pollen tube, fertilization, embryo, endosperm     

Natural plant volatiles as an alternative approach to control stem‐end rot in avocado cultivars

Stem‐end rot is a postharvest disease associated with multiple important fungal pathogens including Lasiodiplodia theobromae. The incidence of stem‐end rot in avocado during postharvest storage affects the shelf life, quality and marketability of the fruit. This study is aimed at the investigation of the antifungal activities of selected natural plant volatiles (vapour phase): citral, octanal, hexanal and thymol against L. theobromae (causal pathogen of stem‐end rot) in vitro and in vivo in “Hass” and “Fuerte” avocados. Hexanal showed a lower inhibitory effect on the radial mycelial growth of L. theobromae in vitro. However, citral at a minimum concentration of 4 μl/L revealed fungicidal activity and completely inhibited the spore germination of L. theobromae. Artificially inoculated “Hass” and “Fuerte” avocados with L. theobromae were exposed to citral (768 μl) and commercial fungicide prochloraz and stored for 6 days at 20°C and 14 days at 10°C separately and thereafter held at 20°C for 3 days to simulate the retail shelf conditions. Although citral in a volatile phase effectively reduced the development of stem‐end rot in both cultivars, its effect was significant in “Fuerte” with 75% reduction in the incidence of stem‐end rot. The biochemical analysis demonstrated an increase in total phenol contents, phenylalanine ammonia‐lyase, chitinase and β‐1, 3 glucanase activity in fruit exposed to citral when compared to the reference treatment prochloraz and the untreated control for both cultivars. Furthermore, fruits exposed to citral retained the ready‐to‐eat firmness and therefore could be considered a potential alternative treatment to control stem‐end rot at the postharvest stage.     

Ethylene-induced proliferation of avocado fruit lenticel cells

Ethylene enhances the proliferation of lenticels of avocado fruit stored in polyethylene bags under low oxygen concentration and high humidity; CO₂ has no effect in this respect. The effect of ethylene is exerted both on fruits still attached to the tree and on those already harvested. Photographs of fruits of three avocado cultivars with proliferated lenticels and scanning electron micrographs of such proliferated cells of the Fuerte cultivar are presented.     

Self‐pollination and parthenocarpic ability in developing ovaries of self‐incompatible Clementine mandarins (Citrus clementina)

This study aimed to determine if self‐pollination is needed to trigger facultative parthenocarpy in self‐incompatible Clementine mandarins (Citrus clementina Hort. ex Tan.). ‘Marisol’ and ‘Clemenules’ mandarins were selected, and self‐pollinated and un‐pollinated flowers from both cultivars were used for comparison. These mandarins are always seedless after self‐pollination and show high and low ability to develop substantial parthenocarpic fruits, respectively. The time‐course for pollen grain germination, tube growth and ovule abortion was analyzed as well as that for carbohydrates, active gibberellins (GA₁ and GA₄), auxin (IAA) and abscisic acid (ABA) content in the ovary. ‘Clemenules’ showed higher pollen grain germination, but pollen tube development was arrested in the upper style 9 days after pollination in both cultivars. Self‐pollination did not stimulate parthenocarpy, whereas both un‐pollinated and self‐pollinated ovaries set fruit regardless of the cultivar. On the other hand, ‘Marisol’ un‐pollinated flowers showed greater parthenocarpic ovary growth than ‘Clemenules’ un‐pollinated flowers, i.e. higher ovule abortion rate (+21%), higher fruit set (+44%) and higher fruit weight (+50%). Further, the greater parthenocarpic ability of ‘Marisol’ paralleled higher levels of GA₁ in the ovary (+34% at anthesis). ‘Marisol’ ovary also showed higher hexoses and starch mobilization, but lower ABA levels (?64% at anthesis). Self‐pollination did not modify carbohydrates or GA content in the ovary compared to un‐pollination. Results indicate that parthenocarpy in the Clementine mandarin is pollination‐independent with its ability to set depending on the ovary hormone levels. These findings suggest that parthenocarpy in fertile self‐incompatible mandarins is constitutively regulated.     

Bee visit rates vary with floral morphology among highbush blueberry cultivars (Vaccinium corymbosum L.)

As the human population has increased, so too has the demand for biotically pollinated crops. Bees (Apoidea) are essential for pollen transfer and fruit production in many crops, and their visit patterns can be influenced by floral morphology. Here, we considered the role of floral morphology on visit rates and behaviour of managed honey bees (Apis mellifera) and wild bumble bees (genus Bombus), for four highbush blueberry cultivars (Vaccinium corymbosum L.). We measured five floral traits for each cultivar, finding significant variation among cultivars. Corolla throat diameter may be the main morphological determinant of visit rates of honey bees, which is significantly higher on the wider flowers of cv. ‘Duke’ than on ‘Bluecrop’ or ‘Draper’. Honey bees also visited cv. ‘Duke’ legitimately but were frequent nectar robbers on the long, narrow flowers of cv. ‘Bluecrop’. Bumble bees were infrequent (and absent on cv. ‘Draper’) but all observed visits were legitimate. Crop yield was highest for the cultivar with the highest combined (honey bee + bumble bee) visit rate, suggesting that aspects of floral morphology that affect pollinator visit patterns should be considered in crop breeding initiatives.     

Mating system in Mexican populations of the annual herb Solanum rostratum Dunal (Solanaceae)

Traditionally, annual colonising species are expected to have high rates of self‐fertilisation, although recent theoretical and empirical studies have shown that cross‐fertilisation can be selected for under heterogeneous pollination environments. Solanum rostratum is a self‐compatible annual herb that colonises disturbed habitats. Despite the lack of physiological mechanisms to prevent self‐fertilisation, pollen transfer between individuals is expected to be favoured because of its complex floral morphology. In previous studies of S. rostratum it has been shown that anther dimorphism within flowers results in precise pollen placement on the pollinator's body, and the presence of mirror‐image floral morphs within plants promotes outcrossing in experimental arrays. However, the mating system of natural populations of S. rostratum has never been assessed, and thus whether it is predominantly selfing or outcrossing remains unknown. We hypothesise that floral and inflorescence morphology of S. rostratum should facilitate cross‐fertilisation, making it a predominantly outcrossing despite its lack of a self‐incompatibility system. To test this hypothesis, we estimated outcrossing rates by genotyping 700 individuals at 13 microsatellite loci, sampled from four populations across a 690‐km transect in the species' native range. We found that populations had mean outcrossing rates of 0.70 ± 0.03, with multiple sires contributing to paternity of each progeny array (average effective number of sires = 8.97 ± 0.57). This indicates that natural populations S. rostratum have relatively high levels of outcrossing, probably facilitated by its floral and inflorescence morphology. We speculate that partial selfing in this species may be an unavoidable consequence of displaying multiple flowers at the same time (geitonogamy), as well as the result of self‐pollen transfer by illegitimate visitors.     

Preparation of Avocado Mitochondria Using Self-Generated Percoll Density Gradients and Changes in Buoyant Density during Ripening

Mitochondria from avocado (Persea americana Mill, var. Fuerte and Hass) can be rapidly prepared at every stage of ripening using differential centrifugation and self-generated Percoll gradients. The procedure results in improved oxidative and phosphorylative properties, especially for mitochondria isolated from preclimacteric fruits.

A gradual change in the buoyant density of avocado mitochondria takes place during ripening. Climacteric and postclimacteric avocado mitochondria have the same buoyant density as other plant mitochondria (potato, cauliflower), whereas mitochondria from preclimacteric fruit have a lower density. The transition in buoyant density occurs during the climacteric rise, and two populations of intact mitochondria (p = 1.060 and p = 1.075) can be separated at this stage. Evidence indicates that the difference in mitochondrial buoyant density between preclimacteric and postclimacteric mitochondria is likely due to interactions with soluble cytosolic components.

     

Contextualising pollination benefits: effect of insecticide and fungicide use on fruit set and weight from bee pollination in lowbush blueberry

Current approaches to determining the value of insect pollinators to crop yield assume that plants are primarily pollen limited. This is particularly relevant in a crop such as lowbush blueberry, Vaccinium angustifolium, where no fruit will set without insect‐mediated cross‐pollination. However, such valuations usually ignore other factors that are necessary to maximise crop yields. We conducted an experiment to test whether yields of lowbush blueberry attributed to pollinator activity increased independently of pest management. The experiment was a 2 × 2 factorial design, incorporating two intensities of pollination (25% or 100% of flowers), and two levels of insect and disease management with recommended fungicide and insecticide sprays (‘full inputs’ or ‘no inputs’). We demonstrated an interaction between these two factors, such that increased fruit set at harvest was only possible if 100% pollination was combined with the ‘full input’ treatment. Furthermore, increases in fruit weight among the remaining treatments were only realised in the ‘full input’ plots. These results suggest that the value accorded to pollinator activity in blueberries is strongly dependent upon pest and disease management of the crop.     

High‐temperature‐induced reductions in cropping in everbearing strawberries (Fragaria × ananassa) are associated with reduced pollen performance

Heat‐induced short‐term reductions in cropping (often referred to as ‘thermodormancy’ by UK strawberry growers) occur frequently in extended‐season everbearing strawberry production in the UK. A 2‐year study evaluated the effects of high temperatures on the performance of pollen in two everbearing strawberry cultivars (cvs ‘Everest’ and ‘Diamante’). Pollen from both cultivars lost its ability to germinate following exposure to a period of high temperature (30/20°C day/night). However, after termination of the high‐temperature episode, germination capacity recovered to levels similar to control plants that were not exposed to high temperature. High temperature also resulted in slower rates of pollen tube growth. As a result, only a small proportion of high‐temperature‐affected flowers developed into fruits. These results suggest that heat‐induced reductions in cropping in everbearing strawberries are associated with reduced pollen performance.     

Effects of salt stress and rhizobial inoculation on growth and nitrogen fixation of three peanut cultivars

Increasing soil salinity represents a major constraint for agriculture in arid and semi‐arid lands, where mineral nitrogen (N) deficiency is also a frequent characteristic of soils. Biological N fixation by legumes may constitute a sustainable alternative to chemical fertilisation in salinity‐affected areas, provided that adapted cultivars and inoculants are available. Here, the performance of three peanut cultivars nodulated with two different rhizobial strains that differ in their salt tolerance was evaluated under moderately saline water irrigation and compared with that of N‐fertilised plants. Shoot weight was used as an indicator of yield. Under non‐saline conditions, higher yields were obtained using N fertilisation rather than inoculation for all the varieties tested. However, under salt stress, the yield of inoculated plants became comparable to that of N‐fertilised plants, with minor differences depending on the peanut cultivar and rhizobial strain. Our results indicate that N fixation might represent an economical, competitive and environmentally friendly choice with respect to mineral N fertilisation for peanut cultivation under moderate saline conditions.     

Reproductive biology of the invasive exotic shrub, Rhododendron ponticum L. (Ericaceae)

The reproductive biology of an exotic species will affect its ability to become naturalized and invasive in non-native habitats. Rhododendron ponticum is an ecologically damaging exotic weed in the British Isles, which spreads predominantly by seed. I investigated how inbreeding and outcrossing affect seed production and germination in a wild population of this species in Ireland. Experimental manipulations revealed low fruit and seed set when insects were excluded from flowers, suggesting that this species has limited capability for spontaneous autogamy. Hand-pollination treatments showed that, although flowers are self-compatible (with self and same plant pollen), higher levels of seed set occur following outcrossing (xenogamy). There was no significant difference in rate of germination of seeds from inbred or outcrossed treatments. The addition of xenogamous pollen to open flowers did not increase fruit or seed set, suggesting that flowers in this population are not pollen limited: native generalist pollinators, mainly bumblebees ( Bombus spp.), are providing an adequate pollinator service. This work demonstrates that outcrossing increases seed set and pollinators are required to facilitate this. Hence, generalist native pollinators can promote invasion by exotic plants. Native pollinators can clearly play an important part in alien species invasion.  © 2007 The Linnean Society of London, Botanical Journal of the Linnean Society , 2007, 155 , 373–381.     

Impact of plant growth regulators spray on fruit quantity and quality of pepper (Capsicum annuum L.) cultivars grown under plastic tunnels

The study aims to investigate the effect of foliar spray with three plant growth regulators (PGRs) p-Chlorophenoxyacetic acid (CPA) at 20 and 40 ppm; Gibberellic acid (GA3) at 20 and 30 ppm, 1-Naphthaleneacetic acid (NAA) at 10 and 20 ppm on the response of fruit set, yield, and fruit quality of some hot pepper cultivars (Chillina, Parbirian, Shampion, and Hyffa) grown in sandy soil under plastic tunnels as compared to the control. Spraying Chillina cultivar GA3 at 30 ppm significantly increased the number of fruits/ plant and fruit set (%), yield/plant, and total yield/fad. In addition, the contents of TSS and Vit C, furthermore, maximum capsaicin content were observed in chili fruits in both seasons. However, the interaction between Chillina cultivar and spraying with GA3 at 20 ppm ranked second in yield and quality. The interaction between Parbirian cultivars and spraying with GA3 at 20 or 30 ppm increased the number of flowers/plants in both seasons. On the other hand, the interaction between Shampion cultivar and spraying with tap water (control) gave the lowest values of the number of flowers/ plants, the number of fruits/ plant and fruit set (%), yield, and its components, and fruit quality in both seasons.     

Pollen tube growth in the self‐compatible sweet cherry genotype, ‘Cristobalina’, is slowed down after self‐pollination

Sweet cherry is a self‐incompatible fruit tree species in the Rosaceae. As other species in the family, sweet cherry exhibits S‐RNase‐based gametophytic self‐incompatibility. This mechanism is genetically determined by the S‐locus that encodes the pollen and pistil determinants, SFB and S‐RNase, respectively. Several self‐compatible sweet cherry genotypes have been described and most of them have mutations at the S‐locus leading to self‐compatibility. However, ‘Cristobalina’ sweet cherry is self‐compatible due to a mutation in a pollen function modifier that is not linked to the S‐locus. To investigate the physiology of self‐compatibility in this cultivar, S‐locus segregation in crosses involving ‘Cristobalina’ pollen, and pollen tube growth in self‐ and cross‐pollinations, were studied. In the crosses with genotypes sharing only one S‐haplotype, the non‐self S‐haplotype was inherited more frequently than the self S‐haplotype. Pollen tube growth studies revealed that the time to travel the whole length of the style was longer for self‐pollen tubes than for cross‐pollen tubes. Together, these results suggest that ‘Cristobalina’ pollen tube growth is slower after self‐pollination than after cross‐pollination. This reproductive strategy would allow self‐fertilisation in the absence of compatible pollen but would promote cross‐fertilisation if cross‐compatible pollen is available, a possible case of cryptic self‐incompatibility. This bet‐hedging strategy might be advantageous for an ecotype that is native to the mountains of the Spanish Mediterranean coast, in the geographical limits of the distribution of this species. ‘Cristobalina’ blooming takes place very early in the season when mating possibilities are scarce and, consequently, self‐compatibility may be the only possibility for this genotype to produce offspring.