Reversion of fruit-dependent inhibition of flowering in Citrus requires sprouting of buds with epigenetically silenced CcMADS19.

In Citrus, the response to environmental floral inductive signals is inhibited by the presence of developing fruits. The mechanism involves epigenetic activation of the CcMADS19 locus (FLC orthologue), encoding a floral repressor. To understand how this epigenetic regulation is reverted to allow flowering in the following season, we have forced precocious sprouting of axillary buds in fruit-bearing shoots, and examined the competence to floral inductive signals of old and new leaves derived from them.

Fruit-dependent epigenetic regulation of flowering in Citrus.

In many perennial plants, seasonal flowering is primarily controlled by environmental conditions, but in certain polycarpic plants, environmental signals are locally gated by the presence of developing fruits initiated in the previous season through an unknown mechanism. Polycarpy is defined as the ability of plants to undergo several rounds of reproduction during their lifetime, alternating vegetative and reproductive meristems in the same individual. To understand how fruits regulate flowering in polycarpic plants, we focused on alternate bearing in Citrus trees that had been experimentally established as fully flowering or nonflowering.

Molecular Characterization and Stress Tolerance Evaluation of New Allotetraploid Somatic Hybrids Between Carrizo Citrange and W. rootstocks.

Polyploidy is one of the main forces that drives the evolution of plants and provides great advantages for breeding. Somatic hybridization by protoplast fusion is used in citrus breeding programs. This method allows combining the whole parental genomes in a single genotype, adding complementary dominant characters, regardless of parental heterozygosity.

Tetraploidy Enhances Boron-Excess Tolerance in Carrizo Citrange (Citrus sinensis L. Osb. × Poncirus trifoliata L. Raf.).

Unlabelled: Tetraploidy modifies root anatomy which may lead to differentiated capacity to uptake and transport mineral elements. This work provides insights into physiological and molecular characters involved in boron (B) toxicity responses in diploid (2x) and tetraploid (4x) plants of Carrizo citrange (Citrus sinensis L. Osb.

An Integrated View of Whole-Tree Hydraulic Architecture. Does Stomatal or Hydraulic Conductance Determine Whole Tree Transpiration?

Hydraulic conductance exerts a strong influence on many aspects of plant physiology, namely: transpiration, CO2 assimilation, growth, productivity or stress response. However we lack full understanding of the contribution of root or shoot water transport capacity to the total water balance, something which is difficult to study in trees. Here we tested the hypothesis that whole plant hydraulic conductance modulates plant transpiration using two different seedlings of citrus rootstocks, Poncirus trifoliata (L.

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).

Flooding impairs Fe uptake and distribution in Citrus due to the strong down-regulation of genes involved in Strategy I responses to Fe deficiency in roots.

This work determines the ffects of long-term anoxia conditions--21 days--on Strategy I responses to iron (Fe) deficiency in Citrus and its impact on Fe uptake and distribution. The study was carried out in Citrus aurantium L. seedlings grown under flooding conditions (S) and in both the presence (+Fe) and absence of Fe (-Fe) in nutritive solution.

Carbon utilization by fruit limits shoot growth in alternate-bearing citrus trees.

Fruit load in alternate-bearing citrus trees is reported to alter shoot number and growth during spring, summer, and autumn flushes, and the source-sink balance, which affects the storage and mobilization of reserve nutrients. The aim of this work was to assess the extent of shoot growth inhibition resulting from the presence of fruits in 'Moncada' mandarin trees loaded with fruit (ON) or with very light fruit load (OFF), and to identify the role of carbohydrates and nitrogenous compounds in the competition between fruits and shoots. Growth of reproductive and vegetative organs was measured on a monthly basis.

Proteomic study of 'Moncada' mandarin buds from on- versus off-crop trees.

A proteomic analysis of buds from mandarin trees with contrasting fruit load (on- and off-crop trees) was carried out during the onset of low-temperature induction. The aim of the study was to find out more about the molecular mechanism relating to alternate bearing in Citrus and its relationship with flowering. The 'Moncada' variety (Clementine 'Oroval'x'Kara' mandarin), displaying remarkable behaviour in alternate production, was used in this study.

Bicarbonate blocks iron translocation from cotyledons inducing iron stress responses in Citrus roots.

The effect of bicarbonate ion (HCO3(-)) on the mobilization of iron (Fe) reserves from cotyledons to roots during early growth of citrus seedlings and its influence on the components of the iron acquisition system were studied. Monoembryonic seeds of Citrus limon (L.) were germinated "in vitro" on two iron-deprived media, supplemented or not with 10mM HCO3(-) (-Fe+Bic and -Fe, respectively).

Metabolic responses to iron deficiency in roots of Carrizo citrange [Citrus sinensis (L.) Osbeck. x Poncirus trifoliata (L.) Raf].

The effects of iron (Fe) deficiency on the low-molecular-weight organic acid (LMWOA) metabolism have been investigated in Carrizo citrange (CC) [Citrus sinensis (L.) Osb. × Poncirus trifoliata (L.

Use or abuse of bioinformatic tools: a response to Samach.

In a recent paper, we described for the first time the effects of fruit on the expression of putative homologues of genes involved in flowering pathways. It was our aim to provide insight into the molecular mechanisms underlying alternate bearing in citrus. However, a bioinformatics-based critique of our and other related papers has been given by Samach in the preceding Viewpoint article in this issue of Annals of Botany.

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.).

Fruit load modulates flowering-related gene expression in buds of alternate-bearing 'Moncada' mandarin.

Background And Aims: Gene determination of flowering is the result of complex interactions involving both promoters and inhibitors. In this study, the expression of flowering-related genes at the meristem level in alternate-bearing citrus trees is analysed, together with the interplay between buds and leaves in the determination of flowering.

Methods: First defruiting experiments were performed to manipulate blossoming intensity in 'Moncada' mandarin, Citrus clementina.

Flooding affects uptake and distribution of carbon and nitrogen in citrus seedlings.

Soil flooding has been widely reported to affect large areas of the world. In this work, we investigated the effect of waterlogging on citrus carbon and nitrogen pools and partitioning. Influence on their uptake and translocation was also studied through ¹⁵N and ¹³C labeling to provide insight into the physiological mechanisms underlying the responses.

Fruit regulates seasonal expression of flowering genes in alternate-bearing 'Moncada' mandarin.

Background And Aims: The presence of fruit has been widely reported to act as an inhibitor of flowering in fruit trees. This study is an investigation into the effect of fruit load on flowering of 'Moncada' mandarin and on the expression of putative orthologues of genes involved in flowering pathways to provide insight into the molecular mechanisms underlying alternate bearing in citrus.

Methods: The relationship between fruit load and flowering intensity was examined first.

Root signalling and modulation of stomatal closure in flooded citrus seedlings.

In this work, we studied the sequence of responses induced by flooding in citrus plants, with the aim of identifying the signals that lead to stomatal closure. One-year-old seedlings of Carrizo citrange, grown in sand under greenhouse conditions, were waterlogged for 35 d and compared with normally watered well-drained plants. Significant decreases in stomatal conductance and transpiration were detected between flooded and control seedlings from a week after the beginning of the experiment.

Relationships between xylem anatomy, root hydraulic conductivity, leaf/root ratio and transpiration in citrus trees on different rootstocks.

The aim of the study was to determine the extent in which leaf and whole plant transpiration (Tp) were influenced by root hydraulic conductance (K(r)), leaf to root ratio and leaf mass. Also, the relationships between the anatomic characteristics of roots and K(r) were investigated. To this end, 9-month-old seedlings of the citrus rootstocks Cleopatra mandarin (CM), Poncirus trifoliata (PT), and their hybrids Forner-Alcaide no 5 (FA-5) and Forner-Alcaide no 13 (FA-13) and 15-month-old trees of Valencia orange budded on these four rootstocks were tested.

Ammonium transport and CitAMT1 expression are regulated by N in Citrus plants.

Citrus seedlings (Citrus sinensis L. Osbeck x Poncirus trifoliata Blanco) were used to describe the effects of different N treatments on the NH4+ influx mediated by high- and low-affinity transport systems (HATS and LATS, respectively) and CitAMT1 gene expression. Results show that Citrus plants favor NH4+ over NO3- influx mediated by HATS and LATS when both N sources are present in the nutrient solution and Citrus plants display a much higher capacity to take up NH4+ than NO3-.

Regulation of nitrate transport in citrus rootstocks depending on nitrogen availability.

Previously, we reported that in Citrus plants, nitrate influx through the plasmalemma of roots cells follows a biphasic pattern, suggesting the existence of at least two different uptake systems, a high and low affinity transport system (HATS and LATS, respectively). Here, we describe a novel inducible high affinity transport system (iHATS). This new nitrate transport system has a high capacity to uptake nitrate in two different Citrus rootstocks (Cleopatra mandarin and Troyer citrange).

Ammonium transport and CitAMT1 expression are regulated by light and sucrose in Citrus plants.

Here the isolation and characterization of CitAMT1 cDNA from citrange Troyer (Citrus sinensis L. OsbeckxPoncirus trifoliata Blanco) is reported, suggesting that this belongs to the AMT gene family, which is involved in the high-affinity transport system (HATS). Results show that in Citrus plants, the HATS is much more dependent on the light conditions and C status of the roots than the low-affinity transport system.

Responses of citrus plants to ozone: leaf biochemistry, antioxidant mechanisms and lipid peroxidation.

The effects of ozone upon 3-year-old trees of Clementina mandarin (Citrus clementina Hort. ex Tan.) cv.