Delineating chilling and forcing periods is one of the challenging topics in understanding how temperatures drive the timing of budburst and bloom in fruit tree species. Here, we investigated this question on olive trees, using flowering data collected over six years on 331 cultivars in the worldwide collection of Marrakech, Morocco. Using a Partial Least Squares approach on a long-term phenology (29 years) of ‘Picholine Marocaine’ cultivar, we showed that the relevance of delineating the chilling and forcing periods depends more on the variability of inter-annual temperatures than on the long-term datasets. In fact, chilling and forcing periods are similar between those delineated by using datasets of 29 years and those of only 6 years (2014–2019). We demonstrated that the variability of inter-annual temperatures is the main factor explaining this pattern. We then used the datasets of six years to assess the chill and heat requirements of 285 cultivars. We classified Mediterranean olive cultivars into four groups according to their chill requirements. Our results, using the Kriging interpolation method, indicated that flowering dates of most of these cultivars (92%) were governed by both chilling and forcing temperatures. Our investigations provided first insights to select adapted cultivars to global warming.

Tree crop load affects carbon acquisition as well as processes like floral induction or fruit growth. Nevertheless little is known about the impact of local variations of source/sink relationships on these processes variability within the tree. This study aims at investigating the effects of local manipulation of source/sink relationships through leaf or fruit removal on photosynthesis and fruit growth. Experiments were conducted in 2016 and 2017 on adult ‘Golden’ trees planted in the south of France. On trees in either high or low crop loads, 11 leaf or fruit removal treatments were set up early June. Treatments were performed at shoot or branch scales or one side of Yshape trees. Photosynthesis was measured in June and August on the foliated parts of leafremoval-treatment trees and on both the fructified and de-fructified parts of fruit-removaltreatment trees. At harvest, mean fruit weight was evaluated on the different parts of the trees. This study suggests a strong impact of the tree crop load on photosynthesis without any clear impact of the local fruit presence. Moreover, fruit weight was decreased on the defoliated parts when defoliation was performed on branches or on one side of the trees. This could suggest that trees are not able to exchange assimilates at long distances but also that defoliation caused a decrease in water transpiration with possible consequences on phloem-xylem fluxes. Conversely, our results show that long distant transport occurs between fructified and de-fructified parts of the tree. This is probably due to the very low demand of carbon in de-fructified parts which allows carbon fluxes to the fructified parts. This study gives new knowledge on the effects of source/sink distances on carbon acquisition and transport suggesting that the distance effects could differ depending on the process. These experiments will be used to calibrate a model built to simulate carbon transport within trees.

Conversely to many other woody perennial crops, the Vitis vinifera grapevine does not display self-supporting and limited-in-space aerial architectures, but rather develops extended shoot systems relying on external mechanical supports. This behavior results from both structural factors, i.e. stem anatomy, bud and phytomer organisation, and also specificities in the modulation of primary growth and branching, i.e. phyllotaxis, apical dominance and acrotony. To mitigate the most limiting biological properties for cultivation, the grapevine domestication need a range of practices to facilitate plant management and improve agronomic performances. The structure and the functioning of the shoot system regulate not only the potential of biomass accumulation and source/sink balance and but also the canopy microclimate with effects on fruit quality and organ fungus susceptibility. This paper reviews the main biological processes and management practices that regulate grapevine shoot system architecture and development, revisiting the associated terminology.

Tree crop load affects carbon assimilation, storage and fruit growth. Nevertheless, the impact of local variations in source/sink relationships on the within-tree variability of these processes are not fully understood. This study aims at investigating the effect of source/sink manipulations on these processes through leaf or fruit removal performed at different scales of tree organization. Experiments were performed in 2016 and 2017 on adult 'Golden delicious' apple trees planted in the south of France. On trees in either high or low crop loads, 11 different leaf or fruit removal treatments were set up in early June, at shoot or branch scales or one side of Y-shape trees. Photosynthesis was measured in August on the different parts of trees subjected to the different local treatments. Measured leaves and the wood supporting them were sampled for starch concentration. At harvest, mean fruit weight was determined for fruits in different locations on the trees. Our results show a strong impact of the tree crop load on photosynthesis without any clear effect of the local fruit presence. As observed for photosynthesis, starch concentration in leaves was strongly associated with the tree crop load without any local variation depending on local treatment. Conversely, in woody parts starch concentration was affected by the local treatment with greater starch concentrations in de-fruited or foliated parts of trees. Results on fruit weight suggest that assimilates could not be exchanged over long distances in the absence of leaves, whereas, long distant transport did occur between fruiting and non-fruiting parts of the tree. This study gives new insights on the effects of source-sink ratios and distances on functional (carbon acquisition and transport) and developmental (fruit growth) variables suggesting that the distance effects between sources and sinks could differ depending on the experimental setup.

In plants, organs are inter-dependent for growth and development. Here, we aimed to investigate the distance at which interaction between organs operates and the relative contribution of within-tree variation in carbohydrate and hormonal contents on floral induction and fruit growth, in a fruit tree case study. Manipulations of leaf and fruit numbers were performed in two years on ‘Golden delicious’ apple trees, at the shoot or branch scale or one side of Y-shape trees. For each treatment, floral induction proportion and mean fruit weight were recorded. Gibberellins content in shoot apical meristems, photosynthesis, and non-structural carbohydrate concentrations in organs were measured. Floral induction was promoted by leaf presence and fruit absence but was not associated with non-structural content in meristems. This suggests a combined action of promoting and inhibiting signals originating from leaves and fruit, and involving gibberellins. Nevertheless, these signals act at short distance only since leaf or fruit presence at long distances had no effect on floral induction. Conversely, fruit growth was affected by leaf presence even at long distances when sink demands were imbalanced within the tree, suggesting long distance transport of carbohydrates. We thus clarified the inter-dependence and distance effect among organs, therefore their degree of autonomy that appeared dependent on the process considered, floral induction or fruit growth.

In plants, carbon source-sink relationships are assumed to affect their reproductive effort. In fruit trees, carbon source-sink relationships are likely to be involved in their fruiting behavior. In apple, a large variability in fruiting behaviors exists, from regular to biennial, which has been related to the within-tree synchronization vs desynchronization of floral induction in buds. In this study, we analyzed if carbon assimilation, availability and fluxes as well as shoot growth differ in apple genotypes with contrasted behaviors. Another aim was to determine the scale of plant organization at which growth and carbon balance are regulated. The study was carried out on 16 genotypes belonging to three classes: (i) biennial, (ii) regular with a high production of floral buds every year and (iii) regular, displaying desynchronized bud fates in each year. Three shoot categories, vegetative and reproductive shoots with or without fruits, were included. This study shows that shoot growth and carbon balance are differentially regulated by tree and shoot fruiting contexts. Shoot growth was determined by the shoot fruiting context, or by the type of shoot itself, since vegetative shoots were always longer than reproductive shoots whatever the tree crop load. Leaf photosynthesis depended on the tree crop load only, irrespective of the shoot category or the genotypic class. Starch content was also strongly affected by the tree crop load with some adjustments of the carbon balance among shoots since starch content was lower, at least at some dates, in shoots with fruits compared with the shoots without fruits within the same trees. Finally, the genotypic differences in terms of shoot carbon balance partly matched with genotypic bearing patterns. Nevertheless, carbon content in buds and the role of gibberellins produced by seeds as well as the distances at which they could affect floral induction should be further analyzed.

In temperate trees, growth resumption in spring time results from chilling and heat requirements, and is an adaptive trait under global warming. Here, the genetic determinism of budbreak and flowering time was deciphered using five related full-sib apple families. Both traits were observed over 3 years and two sites and expressed in calendar and degree-days. Best linear unbiased predictors of genotypic effect or interaction with climatic year were extracted from mixed linear models and used for quantitative trait locus (QTL) mapping, performed with an integrated genetic map containing 6849 single nucleotide polymorphisms (SNPs), grouped into haplotypes, and with a Bayesian pedigree-based analysis. Four major regions, on linkage group (LG) 7, LG10, LG12, and LG9, the latter being the most stable across families, sites, and years, explained 5.6-21.3% of trait variance. Co-localizations for traits in calendar days or growing degree hours (GDH) suggested common genetic determinism for chilling and heating requirements. Homologs of two major flowering genes, AGL24 and FT, were predicted close to LG9 and LG12 QTLs, respectively, whereas Dormancy Associated MADs-box (DAM) genes were near additional QTLs on LG8 and LG15. This suggests that chilling perception mechanisms could be common among perennial and annual plants. Progenitors with favorable alleles depending on trait and LG were identified and could benefit new breeding strategies for apple adaptation to temperature increase.

In many perennial fruit trees, flowering in the year following a year with heavy fruit load can be quite limited. This biennial cycle of fruiting, termed alternate bearing, was described 170 years ago in apple (Malus domestica). Apple inflorescences are mainly found on short branches (spurs). Bourse shoots (BS) develop from the leaf axils of the spur. BS apices may terminate ~100 days after flowering, with formation of next year's inflorescences. We sought to determine how developing fruit on the spur prevents the adjacent BS apex from forming an inflorescence. The presence of adjacent fruit correlated with reaccumulation of transcript encoding a potential flowering inhibitor, MdTFL1-2, in BS apices prior to inflorescence initiation. BS apices without adjacent fruit that did not flower due to late fruitlet removal, neighbouring fruit on the tree, or leaf removal, also reaccumulated the MdTFL1-2 transcript. Fruit load and gibberellin (GA) application had similar effects on the expression of MdTFL1-2 and genes involved in GA biosynthesis and metabolism. Some apple cultivars are less prone to alternate bearing. We show that the response of a BS apex to different numbers of adjacent fruit differs among cultivars in both MdTFL1-2 accumulation and return flowering. These results provide a working model for the further study of alternate bearing, and help clarify the need for cultivar-specific approaches to reach stable fruit production.

Under the assumption that flower induction is inhibited in the trees carrying heavy fruit load or induced by the absence of fruit, we artificially placed in situation of alternation adult apple trees of the Gala variety to study the differentially expressed genes in their apical meristems, from the period of floral induction to floral differentiation. Analysis of these genes by qRT-PCR and microarray enabled to identify the key biological processes involved. Indeed, classes of differentially expressed genes suggest that the meristems are in contrasting physiological states resulting from various metabolic, hormonal and redox states. In addition, several genes known to be involved in the control of floral induction, such as TEMPRANILLO (TEM1), FLORAL TRANSITION AT MERISTEM (FTM1) and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) were differentially expressed. These results represent a foundation toward the understanding of molecular processes leading to the inhibition vs. promotion of floral induction in fruiting trees.

BACKGROUND:The transition from vegetative to floral state in shoot apical meristems (SAM) is a key event in plant development and is of crucial importance for reproductive success. In perennial plants, this event is recurrent during tree life and subject to both within-tree and between-years heterogeneity. In the present study, our goal was to identify candidate processes involved in the repression or induction of flowering in apical buds of adult apple trees.RESULTS:Genes differentially expressed (GDE) were examined between trees artificially set in either 'ON' or 'OFF' situation, and in which floral induction (FI) was shown to be inhibited or induced in most buds, respectively, using qRT-PCR and microarray analysis. From the period of FI through to flower differentiation, GDE belonged to four main biological processes (i) response to stimuli, including response to oxidative stress; (ii) cellular processes, (iii) cell wall biogenesis, and (iv) metabolic processes including carbohydrate biosynthesis and lipid metabolic process. Several key regulator genes, especially TEMPRANILLO (TEM), FLORAL TRANSITION AT MERISTEM (FTM1) and SQUAMOSA PROMOTER BINDING PROTEIN-LIKE (SPL) were found differentially expressed. Moreover, homologs of SPL and Leucine-Rich Repeat proteins were present under QTL zones previously detected for biennial bearing.CONCLUSIONS:This data set suggests that apical buds of 'ON' and 'OFF' trees were in different physiological states, resulting from different metabolic, hormonal and redox status which are likely to contribute to FI control in adult apple trees. Investigations on carbohydrate and hormonal fluxes from sources to SAM and on cell detoxification process are expected to further contribute to the identification of the underlying physiological mechanisms of FI in adult apple trees.

This research successfully used image-based spectral indices acquired in the field to assess variability of response to drought in a tree mapping population and to detect the related genetic determinisms.An apple tree population, grafted on the same rootstock, was submitted to contrasting summer water regimes over two years. Aerial images acquired in visible, near- and thermal-infrared at three dates each year allowed calculation of vegetation and water stress indices. Tree vigour and fruit production were also assessed. Linear mixed models were built accounting for date and year effects on several variables and including the differential response of genotypes between control and drought conditions.Broad-sense heritability of most variables was high and 18 quantitative trait loci (QTLs) independent of the dates were detected on nine linkage groups of the consensus apple genetic map. For vegetation and stress indices, QTLs were related to the means, the intra-crown heterogeneity, and differences induced by water regimes. Most QTLs explained 15-20% of variance.Airborne multispectral imaging proved relevant to acquire simultaneous information on a whole tree population and to decipher genetic determinisms involved in response to water deficit.