Environmental and Hormonal Physiology of Postharvest Needle Abscission in Christmas Trees

Several of the conifer species are increasingly adopted as Christmas trees worldwide. These species have become integral parts of the horticultural economies of North American and European countries. Postharvest characteristics such as needle abscission/retention, color, fragrance and rehydration abilities vary with species and these complex physiological traits are strongly modulated by hormonal and environmental factors. A large body of research indicates that prevalence of low temperature before harvest evokes cold acclimation responses that involve an increase in complex sugar concentrations, alterations in membrane structures and enhancements in scavenging abilities promoting postharvest needle retention. Adverse postharvest environmental factors, for example, high temperature and vapor pressure deficit are found to increase water stress, cause dehydration and accelerate needle abscission and/or discoloration. Postharvest water stress/cellular dehydration is one of the fundamental biophysical signals that triggers a cascade of hormonal changes, leading to needle abscission. Abscissic acid levels increase during cold acclimation as well as prior to abscission indicating a complicated and paradoxical role in abscission. Ethylene levels increase before abscission and are well proven to instigate the needle fall. Concentrations of cytokinins, auxins and polyamines decline postharvest. However, their interactive roles with other phytohormones orchestrating the abscission process still remain elusive. This review presents and discusses our current knowledge of the physiological aspects of pre-and postharvest environmental factors on needle abscission.     

Characterization of phytohormonal and postharvest senescence responses of balsam fir (Abies balsamea (L.) Mill.) exposed to short-term low temperature

To fulfill the US Thanksgiving and Christmas tree markets, balsam fir (Abies balsamea (L.) Mill.) is generally harvested before the cold season, anecdotally leading to premature needle senescence. Accordingly, we tested the hypothesis that LT exposure before harvest induces specific hormonal changes and delays postharvest senescence and/or abscission in balsam fir. Two hundred and six seedlings exposed to two temperature treatments for 48?h, LT at 5?°C and controls at 22?°C were severed off roots and monitored for their postharvest needle senescence. Root and shoot (needles and buds) tissues were examined for major endogenous hormone metabolites. LT increased shoot ABA (2,007?ng?g^?1 DW) by 2.5× and decreased GA₄₄ (9.84?ng?g^?1 DW) by 3.5× over those in roots. LT did not alter cytokinins, auxins or any root hormonal concentration. With auxins, only IAA, IAA-Asp, IAA-Leu and IAA-Glu were detected and the concentrations of IAA and IAA-Asp in shoots were lower than those found in roots. Among cytokinins, shoot c-ZR (58.95?ng?g^?1 DW) and t-ZR (4.17?ng?g^?1 DW) were 3× higher than those in roots. Apart from GA₄₄, GA₉ (136.76?ng?g^?1 DW) was abundant in shoots. The PBL and PNL were 46 and 1.2?%, irrespective of treatments. LT seedlings held needles 11?days longer than the controls (122?days). In balsam fir, short-term LT exposure augmented ABA and decreased GA₄₄ levels in shoots and delayed postharvest needle senescence.     

Temperature and Photoperiod Influence Postharvest Needle Abscission of Selected Balsam Fir (Abies balsamea L. (Mill.)) Genotypes by Modulating ABA Levels

Seasonal changes in the concentration of abscisic acid (ABA) in current-year needles of two different genotypes (AB-NSD-004 and AB-NSD-184) were monitored in branches collected from 20-year-old balsam fir (Abies balsamea L. (Mill.)) trees over a period of 11 months. A significant genotype × harvesting time interaction was observed for endogenous ABA levels and postharvest needle retention duration (NRD). A consistent pattern of seasonal variation in ABA concentration was observed in both genotypes, with the highest amount of ABA (7,887 ng g^?1 DW) accumulating in April and May. The highest levels of ABA coincided with the lowest postharvest NRD regardless of genotype. Nevertheless, genotypes differed in their ABA concentrations. Branches of genotype AB-NSD-184 sampled during August exhibited 170 days of NRD whereas those collected in May and June registered the lowest NRD of around 40 days. There was a significant negative correlation (P < 0.05) between endogenous ABA concentrations and postharvest NRD in genotype AB-NSD-184. Also, an inverse relationship was observed between the average daily photoperiod and the postharvest NRD (R ² = 0.35; P = 0.000) in the same genotype. Together with average daily temperature, the R ² value for this correlation reached the highest (0.75; P ≤ 0.00). Genotypes differed in their physiological responses to environmental cues and thus differed in their postharvest qualities. Average daily photoperiod and maximum daily temperature are strongly linked to the postharvest NRD through modulating endogenous ABA concentration.     

Endogenous and exogenous ethylene induces needle abscission and cellulase activity in post-harvest balsam fir (Abies balsamea L.)

Post-harvest needle loss is a major problem for balsam fir and other Christmas tree species. Recent evidence has implicated ethylene as a signal responsible for post-harvest needle abscission, but enzymological changes remain unknown. The objective of this study was to identify and quantify cellulase activity associated with endogenous and exogenous ethylene-induced abscission. An experiment was designed with three treatments (control, endogenous ethylene, or exogenous ethylene) with five replicates. Key response variables include needle retention duration, xylem pressure potential, ethylene evolution rate, and cellulase activity. Two complimentary methods were used to assess cellulase activity: a cellulose plate digestion and zymography. The results confirm ethylene as a signal for post-harvest abscission and identify ethylene-induced cellulase. Ethylene evolution was typically between 15 and 16 μL g⁻¹ h⁻¹, but there was no difference among the three treatments. However, exogenous ethylene significantly decreased needle retention by 60% and resulted in a sixfold decrease in xylem pressure potential. In addition, cellulase activity increased by 8- and 12-fold in endogenous and exogenous ethylene-induced abscission, respectively, compared to the control. Identification of ethylene-induced cellulase activity has increased our understanding of the post-harvest needle abscission process and confirms ethylene’s role as a signal molecule.     

Ethylene triggers needle abscission in root-detached balsam fir

Post-harvest needle abscission is a major challenge for Christmas tree and greenery industries. It was hypothesized that ethylene triggers abscission in balsam fir. Three experiments were conducted to test this hypothesis. In experiment 1, 70 balsam fir branches were collected, placed in water, and ethylene evolution was observed over time. In experiment 2, a 2 × 5 factorial experiment was designed to determine the effect of exogenous ethylene and an ethylene receptor blocker, 1-methylcyclopropene (1-MCP), on needle abscission. In experiment 3, a 2 × 6 factorial experiment was designed to determine the effect of exogenous ethylene and an ethylene inhibitor, aminoethoxyvinylglycine (AVG), on needle abscission. It was found that ethylene evolution was the highest 1–2 days prior to needle abscission, which was consistent in untreated branches and branches exposed to exogenous ethylene. Exposure to exogenous ethylene significantly decreased needle retention by 63%. When ethylene receptors were blocked by 1-MCP, needle retention increased by 147% despite the presence of ethylene and increased by 73% in the absence of ethylene when compared to the respective controls. When endogenous ethylene synthesis was inhibited by AVG, there was no improvement in needle retention in the presence of ethylene, but there was a 113% increase in needle retention in the absence of exogenous ethylene. Ethylene is strongly implicated as the signal triggering abscission in root-detached balsam fir.     

Experimental cloud immersion and foliar water uptake in saplings of Abies fraseri and Picea rubens

Key message

Frequent cloud immersion events result in direct uptake of cloud water and improve plant water potentials during daylight hours in saplings of two dominant cloud forest species.

Abstract

In ecosystems with frequent cloud immersion, the influence on plant water balance can be important. While cloud immersion can reduce plant water loss via transpiration, recent advances in methodology have suggested that many species also absorb water directly into leaves (foliar water uptake). The current study examines foliar water uptake and its influence on daily plant water balance in tree species of the endangered spruce–fir forest of the southern Appalachian Mountains, USA. These mountain-top communities are considered relic, boreal forests that may have persisted because of the benefits of frequent cloud immersion. We examined changes in needle water content, xylem water potentials, and stable isotope values in saplings of the two dominant tree species, Abies fraseri and Picea rubens before and after a 24 h period of experimental cloud immersion. Both species exhibited foliar water uptake following immersion, evidenced by substantial changes in stable isotope values of extracted needle water that reflected the composition of the fog water. In addition, total needle water content improved 3.7–6.4 % following experimental submersion and xylem water potentials were significantly greater (up to 0.33 MPa) in cloud-immersed plants over control plants. These results indicate that foliar water uptake may be an adaptive strategy for utilizing cloud water and improving overall tree vigor in these most southerly distributed boreal species.     

Impact of Irrigation Strategies on Abscisic Acid and its Catabolites Profiles in Leaves and Berries of Baco noir Grapes

To understand the relationship among soil and plant water status, plant physiology, and the hormonal profiles associated with it, abscisic acid (ABA) and its catabolites [phaseic acid (PA), dihydrophaseic acid (DPA), 7-hydroxy-ABA, 8′-hydroxy-ABA, neophaseic acid, and abscisic acid glucose ester (ABA-GE)] in leaves and berries from wine grape cultivar Baco noir (Folle blanche × Vitis riparia) were analyzed. The experiment was conducted during the growing seasons 2006 and 2007 in an irrigation trial set up in a commercial vineyard located in Niagara-on-the-Lake, ON, Canada. ABA and its metabolites were quantified using liquid chromatography with ion trap combined with electrospray ionization-mass spectrometry. The hormonal profile indicated a direct relationship between the amount of ABA and climatic factors. The ABA varied between 582 and 4,026 ng g^?1 dry matter (DM), DPA between 417 and 562 ng g^?1, and ABA-GE between 337 and 2,764 ng g^?1 DM. At many sampling times PA in the leaves was undetectable, and its highest concentration (260 ng g^?1 DM) was at beginning of July 2007. ABA followed different catabolic pathways depending on the plant water status. ABA was likely catabolized by conjugation to form ABA-GE in treatments at higher water deficit levels, whereas in treatments with high water status, the oxidation pathway leading to DPA or PA was likely preferred. The ABA and ABA-GE concentrations in the berries at harvest showed high correlation with soil and plant water status.     

Pine needle growth and fine structure after prolonged acid rain treatment in the subarctic

The growth and morphology of Scots pine needles were studied in a long-term acid rain experiment in the far north of Finnish Lapland. Pine trees 5 m tall of age 50–70 years were exposed, by spraying the foliage and soil from a height of 2 m, to either clean water (IC) or acidified water over the period 1985–1992, the acidification site being divided into sub-areas in which the precipitation contained two levels of either sulphuric (Sm, Sh) or nitric (Nm, Nh) acid, or both (SNm, SNh). The treatments with medium and high sulphate-S over eight consecutive years yielded a total sulphur deposition of 3·4 and 17·1 gm⁻², respectively, and those with medium and high nitrate-N a total nitrogen deposition of 1·1 and 5·9 g m⁻². Needles were collected for light and electron microscopy, growth measurements and morphometry. Growth in branch height had decreased by about 40% after 6 years of SNm or SNh treatment, and needle growth by 15% in the SNh trees as compared with the irrigated control trees (IC), although decreases were statistically significant only with respect to the non-irrigated control trees (DC). Growth of branches and needles was slightly better in the Nh treatment than in the IC group. The areas of the whole needle, the mesophyll and the phloem decreased in response to SNh treatment as compared with IC or DC, and a statistically significant decrease of about 30–40% was seen in the area of the xylem in comparison with DC. Cellular damage was observed following the acid treatments, especially with a high acid load. The damage was manifested in collapse of the cellular compartments, increases in lipid accumulations and swelling or disorganization of the protoplast. Increased vacuolization of the cytoplasm, plasmalemma irregularities and chilling-type damage to the mitochondria were also observed.     

Effects of residue 5-point mutation and N-terminus hydrophobic residues on temporin-SHc physicochemical and biological properties

Temporin-SHc (FLSHIAGFLSNLF_amide) first isolated from skin extraction of the Tunisian frog Pelophylax saharica, which shows potent antimicrobial activity against Gram-positive bacteria and is highly active against yeasts and fungi without hemolytic activity at antimicrobial concentrations. The peptide adopts well-defined α-helical conformation when bound to SDS micelles. In this study, we explored the effects of residue at position 5 and the N-terminus hydrophobic character on the hydrophilic/polar face of temp-SHc, on its biological activities (antimicrobial and hemolytic) and biophysical properties (hydrophobicity, amphipathicity and helicity). Antibacterial and hemolytic properties of temporin-SHc derivatives depend strongly on physicochemical properties. Therefore, slight decreasing amphipathicity together with hydrophobicity and helicity by the substitution Ile⁵ → Leu decreased antimicrobial potency approximately twofold without changing of hemolytic activity. It is noteworthy that a conservative amino acid substitution decreases the antimicrobial activity, underlining the differences between Leu/Ile side chains insertion into the lipid bilayer. While the modification of N-terminal hydrophobic character by four residue inversion decreased amphipathicity (twofold) of (4-1)L₅temp-SHc and resulted in an increase in antibacterial activity against E. coli, E. faecalis and C. parapsilosis of at least fourfold, its therapeutic potential is limited by its drastic increase of hemolysis (LC₅₀ = 2 μM). We found that the percentage of helicity of temp-SHc analog is directly correlated to its hemolytic activity. Last, the hydrophobic N-terminal character is an important determinant of antimicrobial activity.     

Cytokinin Biochemistry in Relation to Leaf Senescence: IV. Cytokinin Metabolism in Soybean Explants

When [³H]dihydrozeatin riboside and [³H]zeatin riboside were supplied to soybean (Glycine max L.) explants (comprising one leaf, associated pods, and subtending stem) via the xylem at mid to late podfill, 0.1% of the supplied ³H was extracted from the seeds. The distribution of ³H in the explants was similar to that bound previously following uptake of [³H]zeatin riboside at earlier stages of pod development. Metabolites formed in the explants from ³H-labeled zeatin, zeatin riboside, and dihydrozeatin riboside were identified and related to the endogenous cytokinins shown to be present. When zeatin riboside and zeatin were supplied for 1 hour, zeatin nucleotide was the principal metabolite formed and this appeared to be the precursor of the other metabolites detected subsequently. Explants supplied with zeatin riboside or dihydrozeatin riboside for 1 hour, and then transferred to water for 20 to 24 hours, yielded leaf blades in which the main metabolites were O-glucosyldihydrozeatin, adenosine, and adenine. The metabolism of zeatin riboside in blades of explants at pre-podfill, early podfill, and mid to late podfill did not differ appreciably. The results are discussed in relation to leaf senescence and seed development.     

Hydraulic limitation on maximum height of Pinus strobus trees in northern Minnesota,USA

Key message

Using comparisons within and between trees, the authors show evidence for hydraulic limitation of tree height in a humid-climate species that is far from the global maximum tree height.

Abstract

We measured water status and two indicators of drought stress as a function of height within the canopies of four tall (32–35 m) eastern white pines (Pinus strobus) at an old-growth site in northern Minnesota, USA. Pre-dawn and midday xylem pressure potential measured on terminal shoots (Ψ _shoot), needle length, and foliar carbon isotope discrimination (δ ¹³C) all showed within-canopy gradients consistent with increasing drought stress with height. Midday Ψ _shoot near tree tops was ?1.8 MPa, close to values associated with stomatal closure for other temperate conifers. Pre-dawn Ψ _shoot decreased with height at >2× the gradient in gravitational potential. δ ¹³C was strongly correlated with height and weakly correlated with light. Needles were 15–25 % shorter at canopy top compared to the bottom of the canopy. Midday Ψ _shoot and needle length showed significant differences in regression model coefficients from tree to tree. The patterns are consistent with hydraulic constraints on height growth of white pine at this site.     

Simulated herbivory advances autumn phenology in Acer rubrum

To determine the degree to which herbivory contributes to phenotypic variation in autumn phenology for deciduous trees, red maple (Acer rubrum) branches were subjected to low and high levels of simulated herbivory and surveyed at the end of the season to assess abscission and degree of autumn coloration. Overall, branches with simulated herbivory abscised ～7 % more leaves at each autumn survey date than did control branches within trees. While branches subjected to high levels of damage showed advanced phenology, abscission rates did not differ from those of undamaged branches within trees because heavy damage induced earlier leaf loss on adjacent branch nodes in this treatment. Damaged branches had greater proportions of leaf area colored than undamaged branches within trees, having twice the amount of leaf area colored at the onset of autumn and having ～16 % greater leaf area colored in late October when nearly all leaves were colored. When senescence was scored as the percent of all leaves abscised and/or colored, branches in both treatments reached peak senescence earlier than did control branches within trees: dates of 50 % senescence occurred 2.5 days earlier for low herbivory branches and 9.7 days earlier for branches with high levels of simulated damage. These advanced rates are of the same time length as reported delays in autumn senescence and advances in spring onset due to climate warming. Thus, results suggest that should insect damage increase as a consequence of climate change, it may offset a lengthening of leaf life spans in some tree species.     

Evidence for Cytokinin Involvement in Rhizobium (IC3342)-Induced Leaf Curl Syndrome of Pigeonpea (Cajanus cajan Millsp.)

A uniquely abnormal shoot development (shoot tip-bending, leaf curling, release from apical dominance, and stunted growth) in pigeonpea (Cajanus cajan Millsp) induced by a nodulating Rhizobium strain, IC3342, is thought to be due to a hormonal imbalance. Amaranthus betacyanin bioassay indicated that xylem exudate and leaf extracts from pigeonpea plants with Rhizobium-induced leaf curl symptoms contained high concentrations of cytokinin relative to those in normal plants. Radioimmunoassay (RIA) of samples purified with high performance liquid chromatography revealed that zeatin riboside (ZR) and dihydrozeatin riboside (DZR) concentrations in xylem sap from plants with leaf curl symptoms were 7 to 9 times higher than those in the sap from symptomless, nodulated plants. The sap from symptomless plants nodulated by a Curl⁻ mutant had ZR and DZR concentrations comparable to those in the normal plant sap. RIA indicated that the respective concentrations of zeatin and N⁶-isopenteny-ladenine in culture filtrates of the curl-inducing strain IC3342 were 26 and 8 times higher than those in filtrates of a related normal nodulating strain (ANU240). Gas chromatographic-mass spectrometric analyses revealed similar differences. Gene-specific hybridization and sequence comparisons failed to detect any homology of IC3342 DNA to Agrobacterium tumefaciens or Pseudomonas savastanoi genetic loci encoding enzymes involved in cytokinin biosynthesis.     

An anatomical assessment of branch abscission and branch-base hydraulic architecture in the endangered Wollemia nobilis

Background and Aims: The branch-base xylem structure of the endangered Wollemia nobiliswas anatomically investigated. Wollemia nobilis is probablythe only extant tree species that produces only first-orderbranches and where all branches are cleanly abscised. An investigationwas carried out to see if these unusual features might influencebranch-base xylem structure and water supply to the foliage. Methods: The xylem was sectioned at various distances along the branchbases of 6-year-old saplings. Huber values and relative theoreticalhydraulic conductivities were calculated for various regionsof the branch base. Key Results: The most proximal branch base featured a pronounced xylem constriction.The constriction had only 14–31 % (average 21 %)of the cross-sectional area and 20–42 % (average28 %) of the theoretical hydraulic conductivity of themore distal branch xylem. Wollemia nobilis had extremely lowHuber values for a conifer. Conclusions: The branch-base xylem constriction would appear to facilitatebranch abscission, while the associated Huber values show thatW. nobilis supplies a relatively large leaf area through a relativelysmall diameter ‘pipe’. It is tempting to suggestthat the pronounced decline of W. nobilis in the Tertiary isrelated to its unusual branch-base structure but physiologicalstudies of whole plant conductance are still needed.     

Indirect monitoring of root activity in soybean cultivars under contrasting moisture regimes by measuring electrical capacitance

The applicability of root electrical capacitance (EC) measurement for in situ investigation of root activity and drought tolerance was tested in soybean cultivars. Well-watered and drought-stressed plants were grown in pots with repeated EC measurements, followed terminally by harvest to determine root dry mass (RDM), shoot dry mass (SDM), root/shoot ratio (RSR) and leaf area (LA). EC measurement showed the cultivar differences in root growth and biomass production. EC increased till the beginning of flowering, then became nearly constant. Terminal EC was highly correlated with RDM for non-stressed (R ² = 0.844) and stressed plants (R ² = 0.936). Drought reduced the EC of cultivars by 28.8–50.5 %, consistently with the corresponding changes of SDM (25.5–49.1 %) and LA (23.6–51.5 %), but considerably exceeded the loss of RDM (12.6–47.3 %) in some cultivars. The reason is drought increased the RSR (by 3.9–21.9 %), leading to decreased water uptake, and thus EC per unit of RDM. This was confirmed by the significantly decreased slope of EC–RDM regression line from 0.437 to 0.317 nF g^?1 RDM calculated for well-watered and drought-stressed plants, respectively. As EC referred to root uptake activity, it was better indicator of the actual root status than RDM. EC measurement was adequate for monitoring the cultivar-specific differences in root growth and for estimation of biomass loss caused by drought. By supplementing the conventional methods, this in situ technique could be useful for various fields of agriculture, including cultivar selection or stress tolerance studies.     

Canopy transpiration and water fluxes in the xylem of the trunk of Larix and Picea trees — a comparison of xylem flow,porometer and cuvette measurements

Summary Leaf gas exchange, transpiration, water potential and xylem water flow measurements were used in order to investigate the daily water balance of intact, naturally growing, adult Larix and Picea trees without major injury. The total daily water use of the tree was very similar when measured as xylem water flow at breast height or at the trunk top below the shade branches, or as canopy transpiration by a porometer or gas exchange chamber at different crown positions. The average canopy transpiration is about 12% lower than the transpiration of a single twig in the sun crown of Larix and Picea. Despite the similarity in daily total water flows there are larger differences in the actual daily course. Transpiration started 2 to 3 h earlier than the xylem water flow and decreased at noon before the maximum xylem water flow was reached, and stopped in the evening 2 to 3 h earlier than the water flow though the stem. The daily course of the xylem water flow was very similar at the trunk base and top below the lowest branches with shade needles. The difference in water efflux from the crown via transpiration and the water influx from the trunk is caused by the use of stored water. The specific capacitance of the crown wood was estimated to be 4.7 x 10^-8 and 6.3 x 10^-8 kg kg^-1 Pa^-1 and the total amount of available water storage was 17.8 and 8.7 kg, which is 24% and 14% of the total daily transpiration in Larix and Picea respectively. Very little water was used from the main tree trunk. With increasing transpiration and use of stored water from wood in the crown, the water potential in the foliage decreases. Plant water status recovers with the decrease of transpiration and the refilling of the water storage sites. The liquid flow conductance in the trunk was 0.45 x 10^-9 and 0.36 x 10^-9 mol m^-2s^-1 Pa^-1 in Larix and Picea respectively. The role of stomata and their control by environmental and internal plant factors is discussed.     

Impact of phloem girdling on leaf gas exchange and hydraulic conductance in hybrid aspen

We investigated phloem-xylem interactions in relation to leaf hydraulic capacity in hybrid aspen (Populus tremula L. × P. tremuloides Michx.) by using phloem girdling method. Removal of bark tissues (phloem girdling) at the branch base resulted in a substantial decline in stomatal conductance (g_S), net photosynthetic rate (P_N), and leaf hydraulic efficiency, and in increase of leaf water potential (Ψ_L). Although gS declined more than P_N (83 versus 78 %), the ratio of intercellular to ambient CO₂ concentrations (c_i/c_a) increased from 0.67 to 0.87 in three days after girdling. Girdling induced a decrease in leaf hydraulic conductance (K_L) on average by 43 % (P = 0.006). The changes in gS and leaf conductance to water vapour were co-ordinated with K_L only in girdled branches whereas intrinsic water-use efficiency was invariant to K_L. The declines in K_L with girdling were not accompanied by changes in potassium ion concentration ([K⁺]), electrical conductivity, or pH of xylem sap. The results suggest that phloem girdling at the branch base does not influence the recirculation of ions between the phloem and xylem in hybrid aspen and the decrease of K_L in response to the manipulation is not related to changes in [K⁺] and total ionic content of xylem sap.     

Chitosan and a fungal elicitor inhibit tracheary element differentiation and promote accumulation of stress lignin-like substance in Zinnia elegans xylogenic culture

We investigated the effect of elicitors on xylem differentiation and lignification using a Zinnia elegans xylogenic culture system. Water-soluble chitosan and a fungal elicitor derived from Botrytis cinerea were used as elicitors. Elicitor addition at the start of culturing inhibited tracheary element (TE) differentiation in a concentration-dependent manner, and 30 μg mL^?1 of chitosan or 16.7 μg mL^?1 of the fungal elicitor strikingly inhibited TE differentiation and lignification. Addition of chitosan (at 50 μg mL^?1) or the fungal elicitor (at 16.7 μg mL^?1) during the culturing period also inhibited TE differentiation without inhibiting cell division, except for immature TEs undergoing secondary wall thickening. Elicitor addition after immature TE appearance also caused the accumulation of an extracellular lignin-like substance. It appears that elicitor addition at the start of culturing inhibits the process by which dedifferentiated cells differentiate into xylem cell precursors. Elicitor addition during culturing also appears to inhibit the transition from xylem cell precursors to immature TEs, and induces xylem cell precursors or xylem parenchyma cells to produce an extracellular stress lignin-like substance.     

Application of a Rapid and Sensitive Method for Hormonal and Vitamin E Profiling Reveals Crucial Regulatory Mechanisms in Flower Senescence and Fruit Ripening

Knowledge of ripeness and regulation of postharvest processes is an important tool to prevent loss of commercial value in both fruit and cut flower markets. The joint analysis of hormones and vitamin E levels can reveal complex interactions between hormones and oxidative stress as key regulators of postharvest processes. Profiling of both groups of metabolic compounds was performed during the ripening of non-climacteric fruits (red raspberry, Rubus idaeus L.) and senescence of ethylene-insensitive flowers (Dutch Iris, Iris x hollandica L.). After an initial extraction of the sample, without further purification steps, the hormonal profile was analyzed by UPLC-MS/MS and vitamin E levels were measured by HPLC. This methodological approach was very fast and had enough sensitivity for the analysis of small samples. Raspberry fruit maturation was characterized by a decline of cytokinin levels [zeatin, zeatin riboside, 2-isopentenyl adenine, and isopentenyl adenosine (Z, ZR, 2-iP, and IPA, respectively)] and gibberellins (GA₁ in particular). Exogenous application of ABA prevented δ-tocopherol loss during fruit ripening. Iris floral senescence was also under strict hormonal control, also mediated by cytokinins and gibberellins. Z, ZR, 2-iP, GA₉, and GA₂₄ levels decreased in inner tepals, whereas the level of IPA decreased in style-merged-to-stigma tissues, thus suggesting tissue-specific roles for different hormones. α-Tocopherol levels decreased during senescence of inner tepals, hence suggesting enhanced oxidative stress. In conclusion, the rapid and sensitive hormonal and vitamin E profiling presented here can help in understanding the key physiological processes underlying fruit ripening and floral senescence.     

Disbudding treatments on pistachio trees cv. mateur: dry matter accumulation and distribution within fruiting and non-fruiting branches under dry climate

Key message

After applying disbudding treatments, removal of fifty percent of flower bud each year improves dry matter accumulation in fruiting and non-fruiting branches of pistachio trees, which could minimize alternate bearing.

Abstract

Dry matter accumulation and distribution within branches of pistachio trees were investigated during 2005-2008 to determine the effects of fruiting on shoot growth under rain-fed conditions in arid climate. Four treatments were applied: T ₀ normal alternation cycle, T ₁ trees disbudded for 1 year, T ₂ trees disbudded for two successive years, and T ₃ removal of 50 % of all floral buds for each year. Consecutive disbudded treatment (T ₂) allowed a higher growth potential of pistachio trees with reference to normal biennial cycle (T ₀). Individual current shoot of T ₂ accumulated 44 % as much dry matter cm^?1 as those of ‘On’ trees, which have the highest yield. Removal of 50 % of floral buds (T ₃) significantly increased the dry matter accumulation in fruiting branch to reach 57 g in postharvest compared to 42.6 g for the control T ₀. Trees disbudded for 2 years (T ₂) had increased dry matter accumulation in the non-fruiting branch from 3.3 to 16.3 g. Leaves, current shoot, 1-year-old wood and inflorescence buds represented, respectively, 87, 5.3, 5 and 2.7 % of the total dry matter of individual branch of T ₂. In fruiting branches, nuts consisted of 83–87 %, leaves 7–10 %, rachises 4 %, 1-year-old wood 1.6–2 % and current shoot 0.8–1.3 % of the total dry matter. One-year-old wood played a major role as sources and sinks for developing current year shoot, leaves, inflorescence buds and nuts. Removal of 50 % of flower bud (T ₃) improves the dry matter accumulation in fruiting and non-fruiting branches. Thus, under rain-fed conditions, annual pruning could be used to minimize alternate bearing of pistachio.