Cytokinin signalling regulates two-stage inflorescence arrest in Arabidopsis

To maximise their reproductive success, flowering plants must correctly time their entry into and exit from the reproductive phase (flowering). While much is known about the mechanisms that regulate the initiation of flowering, the regulation of end-of-flowering remains largely uncharacterised. End-of-flowering in Arabidopsis thaliana consists of the quasi-synchronous arrest of individual inflorescences, but it is unclear how this arrest is correctly timed with respect to environmental stimuli and ongoing reproductive success. Here we show that Arabidopsis inflorescence arrest is a complex developmental phenomenon which includes a decline in size and cessation of activity in the inflorescence meristem (IM), coupled with a separable developmental arrest in all unopened floral primordia (floral arrest); these events occur well before the visible arrest of the inflorescence. We show that global removal of inflorescences can delay both IM arrest and floral arrest, but that local fruit removal only delays floral arrest, emphasising the separability of these processes. We test a role for cytokinin in regulating inflorescence arrest, and find that cytokinin treatment can delay arrest. We further show that gain-of-function cytokinin receptor hypersensitive mutants can delay floral arrest, and also IM arrest, depending on the expression pattern of the receptor; conversely, loss-of-function mutants prevent extension of flowering in response to inflorescence removal. Collectively, our data suggest that the dilution of cytokinin among an increasing number of sink organs leads to end-of-flowering in Arabidopsis by triggering IM and floral arrest, conversely meaning that a lack of reproductive success can homeostatically extend flowering in compensation.

Fruit Size and Structure of Zoochorous Trees: Identifying Drivers for the Foraging Preferences of Fruit-Eating Birds in a Mexican Successional Dry Forest

Tropical dry forests (TDFs) are affected by land-use changes. These modifications impact their composition and arboreal structure, as well as the availability of food for several bird groups. In this study, we evaluated the foraging preferences in zoochorous trees of fruit-eating birds during the dry season of the year in three successional stages (early, intermediate, and mature) of TDFs in southern Mexico. The fruits of these trees are important in the diet of several birds during the dry season, a period during which food resources are significantly reduced in TDFs. We estimated foliar cover (FC) and foliage height diversity (FHD) of zoochorous trees in 123 circular plots. These variables were recognized as proxies of food availability and tree productivity. Foraging preferences were evaluated at the community level, by frugivore type, and by bird species. We evaluated the effect of the structural variables and the fruit size of zoochorous plants on fruit removal by birds and related the bird body mass and fruit size removed in the successional gradient. A total of 14 zoochorous tree species and 23 fruit-eating bird species were recorded along the successional gradient. Intermediate and mature stages showed greater fruit removal. The birds removed mainly B. longipes fruits across the three successional stages. The FHD and fruit size were important drivers in the selection of zoochorous trees and fruit removal by fruit-eating birds. Fruit size and bird body mass were positively related along the successional gradient. The results suggest that fruit removal by fruit-eating birds in the successional gradient can promote the demographic dynamics of several zoochorous tree species, especially of Bursera spp. along the TDFs.

Same Season and Carry-Over Effects of Source-Sink Adjustments on Grapevine Yields and Non-structural Carbohydrates

The grapevine (Vitis vinifera L.) is managed to balance the ratio of leaf area (source) to fruit mass (sink). Over cropping in the grapevine may reveal itself as spontaneous fruit abortion, delayed ripening, or as alternate bearing. The aim of this work was to study the same season and carry-over effects of manipulating source to sink ratios on grapevine phenology, leaf gas exchange, yield components, berry soluble solids accumulation, and reserve carbohydrate and soluble sugar concentration in roots. Cabernet Sauvignon grapevines were subjected to defoliation (33, 66, and 100% of the leaves retained) and fruit removal treatments (33, 66, and 100% of clusters retained) arranged in a factorial design. Results from two seasons of source-sink manipulations were substantially different. In both seasons defoliation treatments affected season-long net carbon assimilation (AN) and stomatal conductance (gs) where the less leaves were retained, the greater the AN and gs, and fruit removal had no impact on leaf gas exchange. In the first season, leaf area to fruit mass was hardly related to berry soluble solids and in the second season they were strongly correlated, suggesting a degree of acclimation. Defoliation treatments had great impacts on berry size, berries per cluster, and total soluble solids in both years. Fruit removal treatments only had effects on berry mass and berries per cluster in the first season, and only on berry soluble solids in the second. The predominant effect of defoliation (carbon starvation) cascaded onto reducing root starch content, root mass and delaying of veraison and leaf senescence, as well as harvest which was delayed up to 9 weeks with 33% of the leaves retained. In a third season, where grapevines grew without treatments, defoliation treatments had resultant carryover effects, including reduced leaf area, number of berries per cluster, clusters per vine, and yield, but not on leaf gas exchange dependent on previous seasons' severity of defoliation. Balancing source-to-sink ratio is crucial to obtain an adequate speed of ripening. However, this was the culmination of a more complex whole-plant regulation where the number of leaves (source strength) outweighed the effects of fruits (sink strength).

Vegetative and Reproductive Response to Fruit Load in Two Jojoba (Simmondsia chinensis) Cultivars

Jojoba (Simmondsia chinensis) is a wax crop cultivated mainly in arid and semi-arid regions. This crop has been described as an alternate-bearing plant, meaning that it has a high-yield year (“on-year”) followed by a low-yield year (“off-year”). We investigated the effect of fruit load on jojoba’s vegetative and reproductive development. For two consecutive years, we experimented with two high-yielding cultivars—Benzioni and Hazerim—which had opposite fruit loads, i.e., one was under an on-year load, while the other was under an off-year load simultaneously. We found that removing the developing fruit from the shoot during an off-year promotes further vegetative growth in the same year, whereas in an on-year, this action has no effect. Moreover, after fruit removal in an on-year, there was a delay in vegetative growth renewal in the consecutive year, suggesting that the beginning of the growing period is dependent on the previous year’s yield load. We found that seed development in the 2018 season started a month earlier than in the 2017 season in both cultivars, regardless of fruit load. This early development was associated with higher wax content in the seeds. Hence, the wax accumulation rate, as a percentage of dry weight, was affected by year and not by fruit load. However, on-year seeds stopped growing earlier than off-year seeds, resulting in smaller seeds and an overall lower amount of wax per seed.

CHANGES IN THE BIOCHEMICAL COMPONENTS IN KIWI FRUITS DUE TO THE PERIOD OF THEIR COLLECTION

On the basis of the Institute of Agriculture of Academy of Sciences of Abkhazia (Sukhum) there is a collection of sweet Actinidia, research on the introduction and selection of kiwi is actively being conducted, the forms of trellises, planting schemes, crown formation, optimal harvest times, etc. are being studied. Since 2003, research has been conducted to obtain varieties of sweet Actinidia, characterized by less pubescence of the fruit or its complete absence. This article discusses the changes in the biochemical composition of kiwi fruits, depending on the timing of their collection. Objects of research are promising kiwi varieties with hairless skin grown in number lecture plantation IAASA Gulripshyi district of the Abkhazia Republic: Pobeditel, Otchara, Apsny, Gulripshskyj. The fruits were selected in two terms – during the physiological maturity period (the second decade of November) and during the consumer maturity period (the first decade of January). The research was carried out in the Laboratory of Plant Physiology and Biochemistry (Subtropical Scientific Centre, Sochi, Russia Federation) using classical methods. The fruits of the new varieties of kiwi without pubescence skin were characterized by a more pronounced dark color of the peel (compared to the classic varieties), small-fruited (no more than 6.0 cm in size), multi-seeded and lack of pubescence. The flesh of the fruit is greenish-yellow in color with a rich sweet taste and a delicate consistency, different from the pubescent varieties of kiwi. Among the studied varieties cv. Pobeditel was distinguished by a light green and sweet-tasting pulp, with a large number of seeds. The Otchara variety was distinguished by a slightly noticeable pubescence of the fruit and a very thin skin. The flesh of the fruit is light green, loose, with a large number of seeds. In the variety of the fruits are larger, with a large number of seeds. The flesh is light green with a brownish tinge compared to the Gulripshskyj variety, which has a lighter flesh, also with more seeds. It is shown that valuable biochemical components (soluble solids, sugars and organic acids) accumulate more than others in the fruits of the Pobeditel variety. The variation (V, %) of these traits in fruits depends on the variety and the harvest period. By the second harvest period (January), all fruits showed a decrease in the content of ascorbic acid, in the varieties Apsny, Otchara and Gulripshskyj to 40–47 % of the November amount, in the Pobeditel variety by January, the amount of vitamin C was 23 % of the original. The content of ascorbic acid is most dependent on the harvest period, the coefficient of variation depending on the variety is 12.03 % (Gulripshskyj) – 42.31 % (Pobeditel). Fruits taken at a later date (January) contain a higher amount of dry matter, which indicates that the assimilation processes in the fruit continue until January. In the fruits taken in January, synthetic maturation processes continue, as a result of which there is an active consumption of organic acids, and the sugar-acid index leaf towards increasing the sweetness of kiwi fruits. According to the content of ascorbic acid, November fruit removal is more preferable, since fruits contain more ascorbic acid, therefore, have a greater nutritional value.

Fruit presence induces polar auxin transport in citrus and olive stem and represses IAA release from the bud

In many fruit trees, heavy fruit load in one year reduces flowering in the following year, creating a biennial fluctuation in yield termed alternate bearing AB). In subtropical trees, where flowering induction is mostly governed by the accumulation of chilling hours, fruit load is thought to generate a signal (AB signal) that blocks the perception of the cold induction. Fruit removal during a heavy-fruit-load year (On-Crop) is effective at inducing flowering only if performed one to a few months prior to onset of the flowering induction period. We previously showed that following fruit removal, content of the auxin indoleacetic acid (IAA) in citrus buds is reduced, suggesting that the hormone plays a role in the AB signal. Here, we demonstrate that fruit presence generates relatively strong polar auxin transport (PAT) in citrus and olive stems. Upon fruit removal, PAT is reduced and allows auxin release from the bud. Furthermore, using immunolocalization, hormone and gene expression analyses, we show that in citrus, IAA level in the bud and, specifically, in the apical meristem is reduced upon fruit removal. Overall, our data provide support for the notion that fruit presence generates an auxin signal in the bud which may affect flowering induction.