MartÃnez Fuentes, Amparo
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Hormonal and carbohydrate control of fruit set in avocado 'Lamb Hass'. A question of the type of inflorescence?
[EN] The avocado tree (Persea americana Mill.) has two types of shoots, indeterminate, which maintain vegetative development from an apical bud, and determinate, which do not have vegetative growth. Indeterminate shoots set fewer fruits than determinate ones, and significantly hasten physiological fruitlet abscission. The competition between vegetative and flower development is accepted as the most reasonable hypothesis to explain the differences. However, our results show that from anthesis until fruit set flowers of indeterminate inflorescences, both those remaining on the tree and those abscised, had a higher sucrose and C6 carbohydrate content than flowers of determinate ones and no differences between them were found for C7 carbohydrates, which disagrees with this hypothesis, and indicates that factors other than carbohydrate content are responsible for fruit set in avocado. At anthesis and fruit set stage, gibberellin and cytokinin concentrations (mainly GA1 and tZ, respectively) were significantly higher in flowers of determinate inflorescences than in those of indeterminate ones, indicating their higher ability to set. We conclude that fruit set is hormonally regulated in avocado, irrespective of vegetative growth. The lower fruit set of the indeterminate inflorescences does not depend on the competition for photosynthates due to the apical vegetative growth, since C6 and C7 carbohydrate availability is enough to ensure fruit set, but on their lower content of GA1 and tZ.
Warm temperature during floral bud transition turns off EjTFL1 gene expression and promotes flowering in Loquat (Eriobotrya japonica Lindl)
[EN] The Rosaceae family includes several deciduous woody species whose flower development extends over two consecutive growing seasons with a winter dormant period in between. Loquat (Eriobotrya japonica Lindl.) belongs to this family, but it is an evergreen species whose flower bud initiation and flowering occur within the same growing year. Vegetative growth dominates from spring to late summer when terminal buds bloom as panicles. Thus, its floral buds do not undergo winter dormancy until flowering, but a summer heat period of dormancy is required for floral bud differentiation, and that is why we used loquat to study the mechanism by which this summer rest period contributes to floral differentiation of Rosaceae species. As for the deciduous species, the bud transition to the generative stage is initiated by the floral integrator genes. There is evidence that combinations of environmental signals and internal cues (plant hormones) control the expression of TFL1, but the mechanism by which this gene regulates its expression in loquat needs to be clarified for a better understanding of its floral initiation and seasonal growth cycles. Under high temperatures (>25 & DEG;C) after floral bud inductive period, EjTFL1 expression decreases during meristem transition to the reproductive stage, and the promoters of flowering (EjAP1 and EjLFY) increase, indicating that the floral bud differentiation is affected by high temperatures. Monitoring the apical meristem of loquat in June-August of two consecutive years under ambient and thermal controlled conditions showed that under lower temperatures (<25 & DEG;C) during the same period, shoot apex did not stop growing and a higher EjTFL1 expression was recorded, preventing the bud to flower. Likewise, temperature directly affects ABA content in the meristem paralleling EjTFL1 expression, suggesting signaling cascades could converge to refine the expression of EjTFL1 under specific conditions (T<25 & DEG;C) during the floral transition stage.
Gibberellic acid and flower bud development in loquat (Eriobotrya japonica Lindl.)
The application of gibberellic acid (GA 3) to the whole loquat tree from mid-May to early June and from early August to the onset of flowering, significantly reduced the number of premature flowering shoots per current shoot and per m 3 of canopy, and so reduced the total number of panicles per m 3 of canopy. The number of vegetative shoots per m 3 of canopy was also significantly reduced by applying GA 3. The response depended on the concentration applied and produced optimal results at 50mgl -1. Differences in the number of flowers per panicle and leaves per shoot were not significantly modified by the treatment. Nevertheless, GA 3 applied directly to the developing apex near to flower differentiation reduced the number of flowers per panicle by 25-35% and without modifying the morphological characteristics of the panicle. Results suggest that less sprouting of lateral buds was largely responsible for the reduction in flowering intensity caused by GA 3. Best treatments reduced thinning costs (60%, approximately) of premature flowering shoots, slightly increased fruit diameter and significantly improved fruit colour and juice quality, thus advancing harvest date. © 2011 Elsevier B.V.
Mechanical pruning attenuates alternate bearing in 'Nadorcott' mandarin
[EN] In Citrus, certain species and/or cultivars are prone to alternate bearing, i.e., a year of heavy crop (ON year) is followed by one of low flowering and light crop (OFF year), and vice-versa. To counteract the fruit effect, we propose a guided mechanical pruning initiated in the spring of an ON year and carried out annually. Our hypothesis is that this gives rise to abundant shoots with well-developed leaves that reach the floral bud inductive period ready to receive the inductive signal and, thus, to induce the bud to bloom the following spring. The experiment was carried out during four consecutive years (2013¿16) using a commercial plantation of `Nadorcott¿ mandarin in southwest Spain. Trees were mechanically pruned: hedging (H) or topping by completely removing the upper shoots of the canopy (T) or by cutting ¿ of their length (T¿). Unpruned trees were used as controls. The experiment began in the spring of an ON-year. T and T¿ carried out in the spring restart the meristem and give rise new shoots that develop in summer and autumn. At the inductive period, the leaves on these new shoots showed a CiFT2 gene expression 4- and 3.7-fold higher than their respective old leaves. In the case of the T¿ their buds avoided the inhibitory effect of the fruit and bloomed the following spring even with the fruit in the cut-back shoot. For the two periods of ON-OFF years of our experiment, cumulative yield of the T treated trees barely increased, whereas T¿ significantly increased with respect to the unpruned trees. We conclude that cutting flowering shoots annually in half-length by means of mechanical pruning (T¿), attenuates alternate bearing behaviour in 'Nadorcott' mandarin. In this experiment, cumulative yield increased by 25% with regard to unpruned trees during the four years of the trial.
Soil Temperature Regulates Fruit Color Change in Algerie Loquat: Nutritional and Hormonal Control
[EN] In Rosaceae fruit tree species, fruit and roots grow opposite because of carbohydrate competition, and root activity is thus reduced by fruit growth. In agreement with this, for some of these species soil temperature has been suggested as a factor regulating fruit ripening, but the mechanism with which it works remains unknown. In this study, we reduced loquat root activity by lowering soil temperature, expecting faster fruit growth and advanced fruit ripening. Eight 4-year-old 'Algerie' loquat trees, budded onto seedling rootstock, and grown outdoors in 39-l plastic containers filled with sandy-loamy soil were used. The roots of four trees were cooled by placing the containers in a cooling compartment (9.5 A degrees C), whereas those of the other four trees were maintained at air temperature (16.5 A degrees C). We measured lateral root primordia emergence, fruit diameter and fruit color development, carbohydrates and nitrogen partitioning, as well as GA, CK, IAA, ABA, and JA content. Lowering soil temperature increased carbohydrate translocation to the fruit and reduced root N uptake and translocation to both the canopy and the fruit. Changes in plant hormones were also caused by reduced soil temperature, and fruit color advanced. Loquat fruit ripened 8-10 days earlier when soil temperature was reduced to 9.5 A degrees C.
On how auxin, ethylene and IDA-peptide relate during mature Citrus fruit abscission
[EN] While the ethylene-auxin interactions are well documented in model dicots such as tomato (climacteric fruit) during mature fruit abscission, the process is not clearly understood in citrus (non-climacteric fruit). The mature fruit produces very little ethylene but is sensitive to ethylene treatments to induce abscission. By contrast, auxin treatments delay fruit abscission, but the particular role of auxin in the process is unknown. Since the IDA-HAE/HSL2 ethylene-independent pathway seem to regulate organ abscission in both model and crop species, we proposed that auxin treatment delays citrus fruit abscission by reactivating the basipetal auxin flux and reducing CitIDA3 expression, without modifying ethylene synthesis. Comparing orange (C. sinensis) genotypes which differ in their abscission rate, 'Navelate' vs. 'Valencia Late', we found that the force needed to detach the fruit from the tree (FDF) declines in parallel with 1) an increase of ethylene synthesis and CitIDA3 gene expression, and 2) a reduction of PIN1-like (auxin transporter) gene expresion. Further, auxin (2,4-D) treatment maintains a higher force in the abscission zone upregulating PIN1-like and AUX1-like (auxin transporter) gene expression, and downregulating CitIDA3 gene expression, but without modifying ethylene production. We conclude that the 2,4-D treatment delays citrus mature fruit abscission through an ethylene-independent pathway.
Influence of harvest date on fruit yield and return bloom in Marsh grapefruit trees (Citrus paradisi Macf.) grown under a tropical climate
Grapefruit grown in tropical climates reach a suitable fruit size and juice content for fresh consumption during August – October in the northern hemisphere. However, some tropical plantations delay harvesting until November or December, with the fruit then being used for processing. In our experiments, delaying the harvest from October to December reduced the average mature fruit weight by 10% and increased abscission from 29 to 70 fruit per tree. Juice contents decreased slightly, from 44.5% (w/w) to 43.2% (w/w), while total soluble solids (TSS) contents barely changed, from 10.4 ºBrix to 10.2 ºBrix. Delaying the harvest date also reduced return flowering by 20% in the following Spring, and the number of fruit set by 20%. Mature fruit abscission and reduced flowering were not dependent on weather conditions.The former was spontaneous and due to senescence, while the latter was due to fruit remaining on the tree. Over a 4-year period, our results showed an average reduction of 30% in fruit yield per tree when harvest dates were delayed from October (153 kg tree–1) to December (105 kg tree–1). As juice content and TSS content values were suitable for processing in October, there was no reason to delay the harvest date.
Gibberellin reactivates and maintains ovary-wall cell division causing fruit set in parthenocarpic Citrus species
[EN] Citrus is a wide genus in which most of the cultivated species and cultivars are natural parthenocarpic mutants or hybrids (i.e. orange, mandarin, tangerine, grapefruit). The autonomous increase in GA(1) ovary concentration during anthesis was suggested as being the stimulus responsible for parthenocarpy in Citrus regardless of the species. To determine the exact GA-role in parthenocarpic fruit set, the following hypothesis was tested: GA triggers and maintains cell division in ovary walls causing fruit set. Obligate and facultative parthenocarpic Citrus species were used as a model system because obligate parthenocarpic Citrus sp (i.e. Citrus unshiu) have higher GA levels and better natural parthenocarpic fruit set compared to other facultative parthenocarpic Citrus (i.e. Citrus clementina). The autonomous activation of GA synthesis in C. unshiu ovary preceded cell division and CYCA1.1 up regulation (a G2-stage cell cycle regulator) at anthesis setting a high proportion of fruits, whereas C. clementina lacked this GA-biosynthesis and CYCA1.1 up-regulation failing in fruit set. In situ hybridization experiments revealed a tissue-specific expression of GA20ox2 only in the dividing tissues of the pericarp. Furthermore, CYCA1.1 expression correlated endogenous GA(1) content with GA(3) treatment, which stimulated cell division and ovary growth, mostly in C clementina. Instead, paclobutrazol (GA biosynthesis inhibitor) negated cell division and reduced fruit set. Results suggest that in parthenocarpic citrus the specific GA synthesis in the ovary walls at anthesis triggers cell division and, thus, the necessary ovary growth rate to set fruit. (C) 2016 Elsevier Ireland Ltd. All rights reserved.
The flower to fruit transition in Citrus is partially sustained by autonomous carbohydrate synthesis in the ovary
[EN] Why evergreen fruit tree species accumulate starch in the ovary during flower bud differentiation in spring, as deciduous species do during flower bud dormancy, is not fully understood. This is because in evergreen species carbon supply is assured by leaves during flower development. We suggest the existence of an autonomous mechanism in the flowers which counteracts the competition for photoassimilates with new leaves, until they become source organs. Our hypothesis is that starch accumulated during Citrus ovary ontogeny originates from 1) its own photosynthetic capacity and 2) the mobilization of reserves. Through defoliation experiments, we found that ovaries accumulate starch during flower ontogeny using a dual mechanism: 1) the autotrophic route of source organs activating Rubisco (RbcS) genes expression, and 2) the heterotrophic route of sink organs that hydrolyze sucrose in the cytosol. Defoliation 40 days before anthesis did not significantly reduce ovary growth, flower abscission or starch concentration up to 20 days after anthesis (i.e. 60 days later). Control flowers activated the energy depletion signaling system (i.e. SnRK1) and RbcS gene expression around athesis. Defoliation accelerated and boosted both activities, increasing SPS gene expression (sucrose synthesis), and SUS1, SUS3 and cwINV (sucrose hydrolysis) to maintain a glucose threshold which satisfied its need to avoid abscission.
Fruit Load and Root Development in Field-Grown Loquat Trees (Eriobotrya japonica Lindl)
Photosynthate translocation to the root in loquat trees decreases as fruit develops. Thus, during the most active period of fruit development, that is, from 50 % of its final size to the beginning of fruit color change, which correspond to BBCH growth scale stages 705 and 801, both translocating and reducing carbohydrate concentrations diminish greatly. Concomitantly, the results from our experiment show an increased abscisic acid (ABA) concentration and a decrease in the respiration rate detected by an accumulation of glucose-6-phosphate, which paralleled a reduced indole-3-acetic acid (IAA) concentration in roots. As a consequence, root development was strongly and significantly reduced. Because loquat fruit develops in winter and nonshoot growth takes place at this time, our results show that root development in loquat trees is controlled by the fruit, mediated by competition for carbohydrates and modulated by hormones. The experiment was conducted using field-grown loquat during two consecutive years and by comparing fruiting and defruited trees. Fruits were detached from the trees in the early fruit developmental stage (10 % of final size, 701 BBCH growth scale), and carbohydrate concentrations in leaves, shoot bark, and roots, as well as nitrogen fractions (N-NO 3 -, N-NH 4 +, and N-proteinaceous) and hormone (IAA, zeatin, and ABA) concentrations in roots, were analyzed throughout the period of fruit development. Root development was evaluated by counting the emerging lateral root primordia during the fruit developmental stages BBCH growth scale 701-809 (fruit color fully developed). © 2012 Springer Science+Business Media, LLC.
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