Carlos Mesejo

Effect of the synthetic auxin 2,4-DP on fruit development of loquat

Application of the butylglycol ester of 2,4-DP increased final fruit size in loquat without causing fruit thinning. Its effectiveness depended on the concentration applied and treatment date. When applied at the onset of the cell enlargement fruit stage, 25 mg l−1 was the most effective treatment. Fruit diameter distribution showed a significant shift to the larger size for treated trees; further, fruit colour break and maturation were encouraged and harvest time was earlier than in untreated trees.

Rootstock influence on the incidence of rind breakdown in ‘Navelate’ sweet orange

Summary The influence of three rootstocks, Carrizo citrange (Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.) Raf.), Cleopatra mandarin (Citrus reshni Hort. ex Tanaka) and sour orange (Citrus aurantium L.) on the incidence and severity of rind breakdown in ‘Navelate’ sweet oranges was studied over seven years in four orchards. The proportion of affected fruit was greater in trees on Carrizo citrange (60% on an average) followed by Cleopatra mandarin (38%) and then on sour orange (9%). There were also significant differences in the severity of the disorder among rootstocks; more than 35% and 20% of fruit from trees on Carrizo citrange and Cleopatra mandarin, respectively, were rejected for commercialization, whereas only 4% of fruit on sour orange was rejected. Cryo-scanning electron microscopy (SEM) of fruit peduncles showed that average diameter of xylem vessel elements of peduncles of fruits on Carrizo citrange rootstock were 6% and 17% larger than those on Cleopatra mandarin and sour orange, respectively. Defoliation of fruit-bearing branches reduced water loss from fruit, maintained fruit peel water potential at a higher value than that of fruit on undefoliated branches and reduced the proportion and severity of affected fruit. It is concluded that fruit-tree water relationships are related to rind breakdown in ‘Navelate’ oranges and low fruit water content might be responsible for the disorder.

Gibberellin reactivates and maintains ovary-wall cell division causing fruit set in parthenocarpic Citrus species

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 GA1 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 GA1 content with GA3 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.

Application of 2,4-dichlorophenoxypropionic acid 2-ethylhexyl ester reduces mature fruit abscission in Citrus navel cultivars

Summary 2,4-dichlorophenoxyacetic acid (2,4-D) isopropyl ester has been used extensively, since the 1950’s, to reduce mature fruit abscission in Citrus navel cultivars. However, this synthetic auxin is no longer registered for this purpose in the European Union (EU). 2,4-dichlorophenoxypropionic acid (2,4-DP) 2-ethylhexyl ester has been registered in the EU for use in citrus growing and can be an effective replacement for 2,4-D to reduce pre-harvest fruit abscission. Use of the 2-ethylhexyl ester of 2,4-DP sprayed before mature fruit abscission significantly reduced fruit drop in sweet orange (Citrus sinensis L. Osbeck) ‘Washington navel’ and ‘Navelate’. The magnitude of the response depended on the concentration applied. At 15 mg l–1, the percentage of abscised fruit was reduced by 50–75% compared to untreated trees, depending on the variety and the orchard. Increasing the concentration applied to 50 mg l–1 did not improve this response. The response to this auxin was as effective as that obtained with 2,4-D applied on the same day at the same concentration (15 mg l–1). 2, 4-DP treatment had no effect on skin colour or on the internal and external characteristics of the fruit at harvest.

Effect of branch scoring on fruit development in loquat

Summary Branch scoring of loquat (Eriobotrya japonica Lindl.) at the 702–704 growth stage of the BBCH-scale increased fruit size at harvest and caused a significant shift in the distribution of fruit diameters to the larger sizes. Fruit from scored trees grew faster and reached commercial colour and flesh softness earlier than fruit from non-scored trees, and therefore could be harvested earlier. No other fruit characteristics were altered significantly by scoring. As the number of fruit was not modified, but fruit weight increased significantly, there was a significant increase in yield on trees scored at this growth stage. Scoring was as effective as ringing at improving fruit development.

Persimmon fruit size and climacteric encouraged by 3,5,6-trichloro-2-pyridyloxyacetic acid

Summary The application of 10 mg 1–1 of 3,5,6-TPA significantly increased final fruit size of ‘Rojo Brillante’ persimmon (Diospyros kaki), the response magnitude depending on the date of treatment. Treatment applied at the onset of cell enlargement encouraged the development of fruit which grew faster, reached ripening earlier and were harvested earlier. Yield was slightly, but not significantly, increased by the treatment.

Fruit Regulates Bud Sprouting and Vegetative Growth in Field-Grown Loquat Trees (Eriobotrya japonica Lindl.): Nutritional and Hormonal Changes

The effects of fruit on bud sprouting and vegetative growth were compared on fruiting and defruited loquat trees from fruit set onward. Carbohydrate and nitrogen content in leaves and bark tissues and hormone concentrations were studied during the fruit development and vegetative growth periods. On defruited trees, a significant proportion of buds sprouted in winter, whereas buds from fruiting trees sprouted only in the spring when fruit reached its final size. Furthermore, when panicles were completely removed in autumn, the buds also sprouted. In addition, fruit directly affected vegetative growth by reducing shoot length. An effect of sink removal (flower or fruit) promoting bud sprouting, regardless of the season, was then demonstrated. Neither soluble sugar concentration nor nitrogen fraction concentration in leaves or bark tissues was related to bud sprouting, but a certain nutritional imbalance was observed during the most active period of fruit development. Moreover, fruit sink activity significantly modified hormone content by increasing indole-3-acetic acid (IAA) and reducing zeatin concentrations, resulting in a higher IAA/zeatin ratio parallel to the lower bud sprouting intensity. Therefore, these changes caused by fruit removal are all related to vegetative growth, but there is no evidence that they are responsible for bud burst.

Auxin and Gibberellin Interact in Citrus Fruit Set

Gibberellins (GA) and auxin (indole-3-acetic acid, IAA) are considered the main compounds involved in the induction of fruit set. Citrus trees flower profusely but exhibit dramatically low fruit set rates and, in particular, seeded orange cultivars also require pollination for fruit to adequately set. Consequently, they represent an excellent model to investigate the interactions between both hormones and their effect on fruit set and development. Unpollinated ovaries from ‘Pineapple’ sweet orange trees were treated with IAA and pollinated ones with TIBA (2,3,5-triiodobenzoic acid, inhibitor of auxin transport), and changes in ovaries were registered shortly after the treatments. The highest IAA levels were found in unpollinated ovaries treated with auxin (twofold increase compared to pollinated ones), and the lowest corresponded to pollinated ones supplemented with TIBA (30% reduction). GA content also differed substantially among samples. In general, expression of the GA-biosynthetic gene GA20ox2 in the ovule and pericarp paralleled the changes in GA20 content in both tissues, and also expression of GA3ox1 and GA1 content but only in the ovule. The levels of these GA in unpollinated ovaries were promoted in response to exogenous IAA, whereas expression of the GA-inactivation gene GA2ox1 and the concentration of the GA-catabolite GA8 were reduced by this treatment. Significantly, treatments with GA3 or IAA to unpollinated ovaries recovered fruit set to the level reached by free pollinated ones. Our study demonstrates that IAA alters GA metabolism in citrus leading to marked changes in the active GA1 levels in ovules and pericarp, mainly through the regulation of GA-biosynthetic genes and the inhibition of the catabolic pathway.

Biosynthesis and Contents of Gibberellins in Seeded and Seedless Sweet Orange (Citrus sinensis L. Osbeck) Cultivars

In this work, we study the capacity to biosynthesize gibberellins (GA) of ovules (either fertilised or unfertilised), developing seeds and pericarp from fruitlets and their relation with fruit set capacity. Experiments were performed in adult, 12-year-old trees of seeded (Pineapple) and seedless parthenocarpic (Washington navel) sweet orange [Citrus sinensis L. Osbeck] cultivars. The activity of GA20-, GA3- and GA2-oxidases and gibberellin levels were measured in the ovules and pericarp of fruitlets in different development states. The results indicate that ovules are the main sites of gibberellin synthesis in fruitlets during the post-anthesis period. The most intense GA1 synthesis—coincident with the highest expression of GA20ox2, GA3ox1 and GA2ox1—was detected in the ovules of the seeded cultivar, probably induced by fecundation and associated with low early fruitlet abscission rates. By contrast, the low activity detected in the sterile cultivar appears to be rather developmentally or constitutively regulated. As a fruitlet develops, the GA1 concentration is augmented in the pericarp in comparison to ovules or developing seeds, and levels therein did not exhibit noticeable differences between varieties. Furthermore, developing seeds from pineapple had higher GA1 content than the unfertilised abortive ovules from Washington navel. Taken together, data suggest a main role for this hormone in the control of fruitlet abscission, and also demonstrate a function in seed development.

Loquat fruit ripening is associated with root depletion. Nutritional and hormonal control

In woody species, it is known that there is a competition for nutrients, water and carbohydrates between root and fruit-shoot systems, however the influence of root development on fruit quality has received little attention. This research aims to identify the network of mechanisms involved in loquat (Eriobotrya japonica Lindl.) fruit ripening in connection with root activity. The study includes root growth rate measurements paralleling the ongoing fruit developmental stages, photosynthate translocation to the root by using (13)CO2 tracing, and nitrogen fractions (N-NH4(+), N-NO3(-), and N-proteinaceous) as well as their upward translocation to the fruit. The role of hormones (IAA, zeatin and ABA) in regulating the responses is also addressed. The experiment was conducted during two consecutive years on adult and 3-year-old loquat trees from early fruit developmental stage (10% of final size, 701 BBCH scale) to fully developed fruit colour (809 BBCH scale). This approach revealed that root development depends on the growing fruit sink strength, which reduces carbohydrates translocation to the roots and prevents them for further elongation. A nitrate accumulation in roots during the active fruit growth period takes place, which also contributes to slowing elongation and paralleled reduced ammonium and proteinaceous nitrogen concentrations. Concomitantly, the concentration of IAA and zeatin were lowest while that of ABA was highest when root exhibited minimum elongation. The depletion in zeatin and nitrogen supply by the roots paralleling the high ABA transport to the fruit allowed for colour break. These results suggest that loquat fruit changes colour by reducing root growth, as fruit increases sugars and ABA concentrations and reduces nitrogen and zeatin concentrations.