Merle Farinós, Hugo Basilio
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Gibberellic acid in Citrus spp. flowering and fruiting: A systematic review
[EN] Background In Citrus spp., gibberellic acid (GA) has been proposed to improve different processes related to crop cycle and yield. Accordingly, many studies have been published about how GA affects flowering and fruiting. Nevertheless, some such evidence is contradictory and the use of GA applications by farmers are still confusing and lack the expected results. Purpose This review aims to collate, present, analyze and synthesize the most relevant empirical evidence to answer the following questions: (i) how does gibberellic acid act on flowering and fruiting of citrus trees?; (ii) why is all this knowledge sometimes not correctly used by farmers to solve yield problems relating to flowering and fruit set? Methods An extensive literature search to obtain a large number of records about the topic was done. Searches were done in five databases: WoS, Scopus, Google Academics, PubMed and Scielo. The search string used was "Gibberellic acid" AND "Citrus". Records were classified into 11 groups according to the development process they referred to and initial data extraction was done. Records related with flowering and fruit set were drawn, and full texts were screened. Fifty-eight full text records were selected for the final data extraction. Results Selected studies were published from 1959 to 2017 and were published mainly in Spain, USA, Brazil and Japan. Twelve species were studied, and Citrus sinensis, C. reticulata and C. unshiu were the principal ones. Most publications with pre-flowering treatments agreed that GA decreases flowering, while only 3 out of 18 did not observe any effect. In most of these studies, the effect on fruit set and yield was not evaluated. Studies with treatments at full bloom or some weeks later mostly reported increased fruit set. However, these increases did not imply higher yields. The results on yield were highly erratic as we found increases, decreases, no effects or variable effects. Conclusions Despite some limitations, the action of GA related to cell division and growth, stimulating the sink ability of the organ and discouraging its abscission, has been clearly established through reviewed studies. GA applications before flowering counteract the floral induction caused by stress reducing flowering. However, on adult trees under field conditions, reducing flowering by applying GA would be difficult because it would be necessary to previously estimate the natural floral induction of trees. During flowering and fruit set, many problems may arise that limit production. Only when the problem is lack of fruit set stimulus can GA applications improve yields. However, much evidence suggests that the main factor-limiting yield would be carbohydrate availability rather than GA levels. GA applications increased fruit set (often transiently), but this increase did not mean improved yields.
Effect of freezing conservation time on loquat (Eriobotrya japonica) pollen germination
[EN] Aim of study: Several studies point out that storage at -20 ºC is a suitable method for preserving pollen of many species in the long term. Part of those studies indicate the total storage time at which these conditions are optimal. However, we have found a lack of information about the freezing time conditions and incubation temperature of loquat pollen. The main objective of this study was to evaluate the effect of the -20 ºC conservation temperature on loquat (Eriobotrya japonica (Thunb.) Lindl.) pollen. Area of study: The study was conducted in Montserrat (Valencia, Spain). Material and methods: Loquat flowers were collected in November 2017 and stored at -20 ºC for three time periods: 4 (T1), 6 (T2) and 8 (T3) months. Subsequently, pollen grains were incubated at different temperatures for 72 h. We analyzed (i) the effect of freezing conservation time; (ii) the effect of incubation temperature on germination; (iii) the interaction between these two factors. Main results: T1 showed higher germination percentage and tube length values (mean and maximum) than T2 and T3. The highest germination percentage (52.77%) was detected for T1 at an incubation temperature of 25 ºC. The interaction between freezing time and incubation temperature showed more consistent results for T1 than for T2 and T3. Research highlights: This suggests that storing at -20 ºC for more than 4 months affects pollen grain and reduces germination and pollen growth. Therefore, -20 ºC loquat pollen storage should not exceed 4 months.
Insect repellent and chemical agronomic treatments to reduce seed number in Afourer mandarin. Effect on yield and fruit diameter
[EN] Obtaining citrus fruits without seeds is a recurrent objective for farmers as it is one of the most valued characteristics, especially in mandarins. "Afourer" tangor is a highly valuable well-established mandarin, and a high percentage of seeded fruits are produced under cross-pollination conditions. Several agronomic techniques have been suggested to control presence of seeds, such as covering with nets and copper sulfate (CuSO4) and gibberellic acid (GA3) treatments. Natural bee repellents are also proposed to reduce the number of seeds per fruit. In this study, we aimed to compare the effect of several agronomic treatments to reduce the seed number in 'Afourer' mandarin. To this end, we assessed the effect of chemical and bee repellent treatments on the seed number per fruit and the side effect on yield and fruit diameter. Under these experimental conditions the two bee repellents, one based on zinc and one based on Capsicum annuum, were not useful for reducing the seed number per fruit in "Afourer" mandarin. The copper sulfate + GA3 treatment reduced the seed number per fruit by only 35%, and this efficiency was clearly not enough to reduce the seed number for commercial purposes. The most effective method to reduce the seed number per fruit was covering with nets, but this technique led to markedly reduced yields. Yield data was highly variable. Fruit diameter correlated positively with the seed number, but it was a weak relationship as the seed number explained only 15% of fruit size variability (R2 = 0.15)
Ultrastructural Evidence Elucidates the Mode of Action of Sulfur in Preventing Pollen Tube Development in Stigma of Citrus cv. Nadorcott and Other Horticultural Species
[EN] Citrus species have a "wet"-type stigma with abundant exudate, and their style contains numerous canals. For successful seed formation, the process includes pollen grain germination on the stigma, pollen tube development and growth through the stigma and style and, ultimately, successful ovule fertilization. However, preventing the fertilization process can be useful for many agronomic and plant-breeding purposes, such as seedless fruits or for developing new varieties. Several studies have recently shown the inhibition effect of sulfur on pollen tube development inside the Nadorcott mandarin stigma and its effective application to obtain seedless mandarins. However, when applied to the stigma, how can sulfur inhibit pollen tube growth? Moreover, does sulfur have the same effect on other species? The main objective of the present study is to clarify the mode of action of sulfur on the ultrastructure of the Nadorcott mandarin stigma and style. To fulfill this goal, untreated flowers and flowers treated with sulfur were pollinated 24 h later. The treated and untreated stigmas were analyzed and compared with several microscopy techniques. The main results showed that sulfur specifically caused an alteration to the outer layer of stigma papillary cells. This marked alteration resulted in papillary cells losing their functionality due to the deterioration and degradation of their cellular structure. Basal papillae, the stigmatic tissue and stylar canals also underwent major alteration. Sulfur also modified the quantity and uniformity distribution of the stigmatic exudate. All these alterations collectively prevented pollen tube development inside the stigma. These effects have been observed in several Citrus species and varieties, and in some other horticultural species, which suggests a generic (non species-specific) action.
Agronomic treatments to avoid presence of seeds in Nadorcott mandarin II. Effect on seed number per fruit and yield
[EN] Nadorcott is a well-established and appreciated mandarin by the fresh market. However, it produces seeds due to cross-pollination with other compatible varieties, which is quite frequent in most producing countries. Consumers prefer seedless mandarins and, therefore, citrus growers need techniques to avoid seeds forming. This study aims to evaluate the effect of six treatments (ammonium nitrate, potassium nitrate, sulfur, saccharose, methylcellulose, callose) on seed number per fruit when applied to Nadorcott trees. In this way, we evaluate which of them is more efficient and can be used in the future as an agronomic treatment to avoid seeds in mandarins. The effect of treatments on yield and fruit quality is also reported. To fulfill this main objective, a randomized complete block design experiment with three applications at flowering was performed on trees. Of the six tested treatments, only elemental sulfur was able to significantly reduce seed number by 87% compared to the positive control. This is a very novel result because it is the first time that such an effective treatment has been found. The biggest seed number per fruit was obtained for the saccharose treatment. Treatments did not significantly influence yield or fruit quality. These results are entirely consistent with a previous study that evaluated the effect of the same products on pollen tube growth, and they can help to develop new techniques. Nevertheless, more studies are necessary to test, for example, different treatment doses.
Forced Flowering in Mandarin Trees Under Phytotron Conditions
[EN] Phytotron has been widely used to assess the effect of numerous parameters on the development of many species. However, less information is available on how to achieve fast profuse flowering in young fruit trees with this plant growth chamber. This study aimed to outline the design and performance of a fast clear methodology to force flowering in young mandarin trees (cv. Nova and cv. Clemenules) and to analyze the influence of induction intensity on inflorescence type. The combination of a short water stress period with simulated spring conditions (day 13 h, 22 °C, night 11 h, 12 °C) in the phytotron allowed flowers to be obtained only after 68-72 days from the time the experiment began. Low-temperature requirements were adequately replaced with water stress. Floral response was proportional to water stress (measured as the number of fallen leaves): the greater the induction, the larger the quantity of flowers. Floral induction intensity also influenced inflorescence type and dates for flowering. Details on artificial lighting (lumens), photoperiod, temperatures, plant size and age, induction strategy and days for each stage are provided. Obtaining flowers from fruit trees at any time, and also several times a year, can have many advantages for researchers. With the methodology proposed herein, three, or even four, flowering periods can be forced each year, and researchers should be able to decide when, and they will know, the duration of the entire process. The methodology can be useful for: flower production and in vitro pollen germination assays; experiments with pests that affect early fruit development stages; studies on fruit physiological alterations. All this can help plant breeders to shorten times to obtain male and female gametes to perform forced-crosses.
In Vivo Pollen Tube Growth and Evidence of Self-Pollination and Prefloral Anthesis in cv. Macabeo (Vitis vinifera L.)
[EN] Cultivar Macabeo is one of the most planted white grape varieties of northern Spain. A general agreement supports many Vitis vinifera cultivars possibly being self-fertile, although this seems to be a variety-dependent characteristic. No previous information about the mating system of cv. Macabeo was found. This study aimed to analyze its mating system and to compare the in vivo fertilization process with and without artificial cross-pollination. Two treatments were performed: emasculation and cross-pollination. The seed number was counted, and pollen tube growth was observed by microscopy. The results showed that cv. Macabeo is self-fertile and selfing probably occurs before the flower opens. Pollen was found over the stigma of flowers before capfall and ovule fertilization was observed even in emasculated flowers, which suggests that germination and pollen tube growth happened in a very early flower development stage. Cross-pollination increased the presence of the pollen tubes growing inside flowers but was not necessary for fruit set. Ovule fertilization was very fast as 24 h (h) were enough for pollen tubes to reach the end of stylar canals.
Effect of sulfur on pollen germination of Clemenules mandarin and Nova tangelo
[EN] This study aims to elucidate whether sulfur can inhibit citrus pollination by affecting pollen grains. For this, four sulfur-based products (inorganic sulfur, water dispersible granular sulfur, ammonium sulfate, copper sulfate) were tested to evaluate their effect on pollen germination and pollen tube growth of two citrus varieties: Clemenules mandarin (Citrus clementina) and Nova tangelo (Citrus clementina x [Citrus paradisi x Citrus reticulata]). Pollen grains were extracted from the flowers of these two varieties and subsequently placed in Petri dishes with modified BK (boron and potassium) germination medium with six concentrations of the sulfur-based products (0.2, 2, 20, 200, 2,000, 20,000 mg l-1). All the dishes were incubated and the pollen germination rate was calculated. All the sulfur products showed progressive pollen germination inhibition with a rising sulfur concentration. CTC50 (50% cytotoxicity inhibition) was around 20 mg l-1 , with significant differences among treatments. Total pollen germination inhibition took place at 20,000 mg l-1. These results demonstrate that sulfur application can affect citrus pollination.
Effect of temperature on pollen germination for several Rosaceae species: influence of freezing conservation time on germination patterns
[EN] Between February 2018 and April 2018, flowers were collected from eight Rosaceae species. Flowers were kept in a freezer at -20 degrees C for three freezing times (Treatment 1, two months; Treatment 2, four months; Treatment 3, six months). After extracting pollen, in vitro germination was induced in a culture medium and incubated at six different temperatures for 72 h. The percentage of pollen germination, average pollen tube length and maximum pollen tube length were measured. Pollen germination was maximum for all species between 15 degrees C and 30 degrees C. Cydonia oblonga, Malus sylvestris, Prunus avium, Prunus domestica, Prunus dulcis, Prunus persica and Pyrus communis obtained 30-52% pollen germination between 15 degrees C and 20 degrees C. Prunus cerasifera had 40% pollen germination at 30 degrees C. All species studied reached the maximum pollen tube length between 10 degrees C and 25 degrees C. Germination did not change significantly for any of the species with freezing time, but we found significant differences in the three parameters measured between treatments. The highest germination percentages were obtained in Treatment 2 (four months frozen at -20 degrees C), while the maximum pollen tube length was reached in Treatment 1 (two months frozen at -20 degrees C). According to our results, freezing time affected the germination-temperature patterns. This could indicate that studies on the effect of temperature on pollen germination should always be carried out with fresh pollen to obtain more conclusive data.