Leaf Longevity Research Articles

Determinants of some leaf characteristics of Australian mangroves

Leaf characteristics reflecting the size, lifespan (longevity), moisture content (degree of succulence) and complexity of structure of 20 mangrove species were studied over several years at 13 locations along the tropical and subtropical Australian coast. These characteristics were found to fall generally within the ranges of those for woody species from other ecosystems. With the exception of one species, it was found that leaf longevity was related inversely to leaf moisture content, increasing from nearly 6 months in more succulent species to over 2 years in less succulent species. This suggested that more succulent leaves are less complex in their structure because they have less well-developed ability to compartmentalize salt. There was a tendency also for leaf longevity to increase in species with larger leaves. These findings were consistent with the general view for land plants that leaf longevity is greater in species that have developed tolerance to environmental stress, salt particularly in the case of mangroves. Leaf tissue in such species is more robust or complex and requires greater metabolic resources in its construction; the plant is then advantaged by retaining the tissue for longer periods. Classification of the species considered here, based on their leaf longevity, moisture content and complexity, identified phylogenetically related species groupings that reflected these leaf longevity effects.

Accurate representation of leaf longevity is important for simulating ecosystem carbon cycle

Leaf longevity and leaf turnover rate are important plant traits relating to plant growth, leaf photosynthesis and respiration, plant canopy dynamic and many other factors, but dynamic global vegetation models only set inaccurate values for these factors. In this study, we firstly investigated the leaf longevities at major vegetation types based on 418 field measurements from around the world. By replacing the default leaf longevities in the Lund-Potsdam-Jena model (LPJ) with the revised values, we examined the changes in simulated carbon cycle caused by the revised parameters. The results suggested that the compiled observations of leaf longevity were significantly different from the default values in LPJ. Both the vegetation production and respiration of the simulated natural ecosystems showed significant changes based on the revised leaf longevity compared with the predictions developed using default model parameters. Of all of the variables, the aboveground and belowground litter pools showed the largest changes (about 10% globally). Globally, the default parameters induced a significant overestimation of terrestrial carbon sink by 3%, compared with a simulation using the revised parameters. Furthermore, the uncertainties in leaf longevity caused various uncertainties (5–30%) in the simulated carbon fluxes and carbon pools. The offset of biases in intermediate variables might result in rational final model outputs. Overall, more accurate leaf longevities are critical for simulating the vegetation distribution and ecosystem carbon cycle.

The effect of photosynthesis parameters on leaf lifespan

Leaf longevity (LL) varies widely among higher plants. LL is a sum of the functional component LLf (the duration of active photosynthesis) and the nonfunctional component LLn (duration of the period during which photosynthesis does not occur). LLn corresponds to the period of winter dormancy in the case of evergreen boreal species. The photosynthetic potential of the leaf (PPL)—that is, the maximal possible amount of CO2 fixed during the leaf lifespan—is inferred from the dynamics of the maximal rate of photosynthesis (Pa) during LLf. Pa reaches a peak value (Pa max) during leaf “maturation.” The photosynthetic potential depends on the functional lifespan to a greater degree than on the maximal rate of photosynthesis. The PPL/LLf ratio determines the rate of realization of the photosynthetic potential during the lifetime of the leaf. LLf is strongly and positively correlated to LL, and LL can therefore be considered a parameter determining the rate of PPL realization, along with LLf (for a first approximation). The prolonged LLf characteristic of evergreen species gives them an advantage—a higher photosynthetic potential than that of deciduous species. Consequently, PPL is realized more slowly in evergreen species than in deciduous species. An increase of LLf and LL is accompanied by an increase of leaf construction cost (LCCa) and a decrease of the photosynthesis rate, with the decrease per unit dry weight (Pm) being much more pronounced than that per unit leaf area (Pa). This points to a much higher contribution of cell wall weight to the total dry weight of longlived leaves of evergreen species than to that of short-lived leaves of deciduous species. Unidirectional changes of PPL and LCCa stabilize leaf payback (LP). Species with a short (long) LLf and high (low) rate of PPL realization are characteristic of early (late) succession and habitats with favorable (unfavorable) environmental conditions for photosynthesis and growth, since these species are more competitive under said conditions. Species with a high rate of PPL realization have an advantage in competition under conditions favorable for photosynthesis and growth, since they use environmental resources for rapid growth and expansion. The rate of photosynthetic potential realization characterizes the aging rate of the leaf.

Estimation of leaf biomass and leaf longevity of hinoki cypress (Chamaecyparis obtusa Endlicher) in the suburban secondary forest in Kyoto, Japan.

Estimation of leaf biomass and leaf longevity of hinoki cypress (<i>Chamaecyparis obtusa</i> Endlicher) in the suburban secondary forest in Kyoto, Japan.

C2-substituted aromatic cytokinin sugar conjugates delay the onset of senescence by maintaining the activity of the photosynthetic apparatus

Cytokinins are plant hormones with biological functions ranging from coordination of plant growth and development to the regulation of senescence. A series of 2-chloro-N6-(halogenobenzylamino)purine ribosides was prepared and tested for cytokinin activity in detached wheat leaf senescence, tobacco callus and Amaranthus bioassays. The synthetic compounds showed significant activity, especially in delaying senescence in detached wheat leaves. They were also tested in bacterial receptor bioassays using both monocot and dicot members of the cytokinin receptor family. Most of the derivatives did not trigger cytokinin signaling via the AHK3 and AHK4 receptors from Arabidopsis thaliana in the bacterial assay, but some of them specifically activated the ZmHK1 receptor from Zea mays and were also more active than the aromatic cytokinin BAP in an ARR5::GUS cytokinin bioassay using transgenic Arabidopsis plants. Whole transcript expression analysis was performed using an Arabidopsis model to gather information about the reprogramming of gene transcription when senescent leaves were treated with selected C2-substituted aromatic cytokinin ribosides. Genome-wide expression profiling revealed that the synthetic halogenated derivatives induced the expression of genes related to cytokinin signaling and metabolism. They also prompted both up- and down-regulation of a unique combination of genes coding for components of the photosystem II (PSII) reaction center, light-harvesting complex II (LHCII), and the oxygen-evolving complex, as well as several stress factors responsible for regulating photosynthesis and chlorophyll degradation. Chlorophyll content and fluorescence analyses demonstrated that treatment with the halogenated derivatives increased the efficiency of PSII photochemistry and the abundance of LHCII relative to DMSO- and BAP-treated controls. These findings demonstrate that it is possible to manipulate and fine-tune leaf longevity using synthetic aromatic cytokinin analogs.

Stand structure, phenology and litterfall dynamics of a subtropical mangrove Bruguiera gymnorrhiza

We evaluated the phenology and litterfall dynamics of the mangrove Bruguieragymnorrhiza (L.) Lamk along the Okukubi River, Okinawa Island, Japan. Over 3 years, this species showed the highest litterfall of leaves and stipules in summer and the lowest litterfall in winter. From Kendall’s coefficient of concordance, the monthly changes in leaf, stipule, and branch were strongly and significantly concordant among years. Leaf and stipule litterfall could be governed by monthly maximum wind speed, monthly day length, and monthly mean air temperature. Branch litterfall depended on monthly maximum wind speed and monthly rainfall, and increased exponentially with increasing monthly maximum wind speed. Mean total litterfall was 11.8 Mg ha−1 yr−1, with the largest component being leaf litterfall (65.8 %). Annual leaf litterfall per plot was almost constant regardless of the tree density of the plot. Mean leaf longevity was 18 months. Flower and mature propagule litterfall might be influenced by monthly mean air temperature, monthly day length and monthly mean air temperature. The average development periods from flower buds to flowers and flower buds to mature propagules were 1 and 8 months, respectively. Except for leaf and branch, all vegetative and reproductive organ litterfall had clear annual cycles. B.gymnorrhiza showed a positive correlation between leaf production and reproductive organ production.

A comparative analysis of transcriptomic, biochemical, and physiological responses to elevated ozone identifies species-specific mechanisms of resilience in legume crops.

Current concentrations of tropospheric ozone ([O3]) pollution negatively impact plant metabolism, which can result in decreased crop yields. Interspecific variation in the physiological response of plants to elevated [O3] exists; however, the underlying cellular responses explaining species-specific differences are largely unknown. Here, a physiological screen has been performed on multiple varieties of legume species. Three varieties of garden pea (Pisum sativum L.) were resilient to elevated [O3]. Garden pea showed no change in photosynthetic capacity or leaf longevity when exposed to elevated [O3], in contrast to varieties of soybean (Glycine max (L.) Merr.) and common bean (Phaseolus vulgaris L.). Global transcriptomic and targeted biochemical analyses were then done to examine the mechanistic differences in legume responses to elevated [O3]. In all three species, there was an O3-mediated reduction in specific leaf weight and total non-structural carbohydrate content, as well as increased abundance of respiration-related transcripts. Differences specific to garden pea included a pronounced increase in the abundance of GLUTATHIONE REDUCTASE transcript, as well as greater contents of foliar glutathione, apoplastic ascorbate, and sucrose in elevated [O3]. These results suggest that garden pea may have had greater capacity for detoxification, which prevented net losses in CO2 fixation in an elevated [O3] environment.

Junggaropitys, a new gymnosperm stem from the Middle–Late Triassic of Junggar Basin, Northwest China, and its palaeoecological and palaeoclimatic implications

A novel well-preserved woody stem Junggaropitys dalongkouensis gen. et sp. nov. was collected from the Middle–Late Triassic sediments of the Karamay Formation in the Junggar Basin, Xinjiang Uygur Autonomous Region, Northwest China. The permineralized wood is characterized by a small homogeneous pith, endarch primary xylem and Protophyllocladoxylon-type secondary xylem. The quantitative growth-ring analyses of Junggaropitys dalongkouensis indicate that the species is evergreen, and the leaf longevity may be 3–6years. The combination of the fossil flora and growth-ring analyses suggests that the Junggaropitys dalongkouensis developed under irregularly stressed warm-temperate conditions.

SENESCENCE-SUPPRESSED PROTEIN PHOSPHATASE Directly Interacts with the Cytoplasmic Domain of SENESCENCE-ASSOCIATED RECEPTOR-LIKE KINASE and Negatively Regulates Leaf Senescence in Arabidopsis.

Reversible protein phosphorylation mediated by protein kinases and phosphatases plays an important role in the regulation of leaf senescence. We previously reported that the leucine-rich repeat receptor-like kinase SENESCENCE-ASSOCIATED RECEPTOR-LIKE KINASE (AtSARK) positively regulates leaf senescence in Arabidopsis (Arabidopsis thaliana). Here, we report the involvement of a protein serine/threonine phosphatase 2C-type protein phosphatase, SENESCENCE-SUPPRESSED PROTEIN PHOSPHATASE (SSPP), in the negative regulation of Arabidopsis leaf senescence. SSPP transcript levels decreased greatly during both natural senescence and SARK-induced precocious senescence. Overexpression of SSPP significantly delayed leaf senescence in Arabidopsis. Protein pull-down and bimolecular fluorescence complementation assays demonstrated that the cytosol-localized SSPP could interact with the cytoplasmic domain of the plasma membrane-localized AtSARK. In vitro assays showed that SSPP has protein phosphatase function and can dephosphorylate the cytosolic domain of AtSARK. Consistent with these observations, overexpression of SSPP effectively rescued AtSARK-induced precocious leaf senescence and changes in hormonal responses. All our results suggested that SSPP functions in sustaining proper leaf longevity and preventing early senescence by suppressing or perturbing SARK-mediated senescence signal transduction.

Pipe-model ratio distributions and branch foliage biomass: differences between two sympatric spruce species

The foliage biomass–sapwood relationship (the pipe model) is critical for tree growth and is used in tree growth models for understanding the implications of this structural relationship on the allocation of resources. In this research, we compared this relationship for two commercially important and sympatric species, black spruce (Picea mariana (Mill.) B.S.P.) and white spruce (Picea glauca (Moench) Voss). At locations in eastern Canada, 57 black and 50 white spruce trees were destructively sampled to obtain foliage biomass, crown structure, and tree stem measures. Using a model-based approach, we compared foliage biomass–branch basal area and foliage biomass–sapwood relationships at the tree and disk (i.e. along the tree stem) levels (i.e. pipe-model ratios) between these two species. We found that (i) branch foliage biomass–branch basal area was greater for black spruce than white spruce and (ii) pipe-model ratios along the tree stem given tree size were greater for black spruce than for white spruce. We attributed these differences to: (i) greater shade tolerance and leaf longevity of black spruce; (ii) slower growth rates of black spruce; and (iii) differing hydraulic strategies and mechanical requirements.

Leaf dynamics in growth and reproduction of Xanthium canadense as influenced by stand density.

Background and Aims Leaf longevity is controlled by the light gradient in the canopy and also by the nitrogen (N) sink strength in the plant. Stand density may influence leaf dynamics through its effects on light gradient and on plant growth and reproduction. This study tests the hypothesis that the control by the light gradient is manifested more in the vegetative period, whereas the opposite is true when the plant becomes reproductive and develops a strong N sink.Methods Stands of Xanthium canadense were established at two densities. Emergence, growth and death of every leaf on the main stem and branches, and plant growth and N uptake were determined from germination to full senescence. Mean residence time and dry mass productivity were calculated per leaf number, leaf area, leaf mass and leaf N (collectively termed ‘leaf variables’) in order to analyse leaf dynamics and its effect on plant growth.Key Results Branching and reproductive activities were higher at low than at high density. Overall there was no significant difference in mean residence time of leaf variables between the two stands. However, early leaf cohorts on the main stem had a longer retention time at low density, whereas later cohorts had a longer retention time at high density. Branch leaves emerged earlier and tended to live longer at low than at high density. Leaf efficiencies, defined as carbon export per unit investment of leaf variables, were higher at low density in all leaf variables except for leaf number.Conclusions In the vegetative phase of plant growth, the light gradient strongly controls leaf longevity, whereas later the effects of branching and reproductive activities become stronger and over-rule the effect of light environment. As leaf N supports photosynthesis and also works as an N source for plant development, N use is pivotal in linking leaf dynamics with plant growth and reproduction.

Differences in the environmental control of leaf senescence of four Quercus species coexisting in a Mediterranean environment

Aims of study: Our aim is to check the effect of different environmental factors on the leaf senescence of four Quercus species with different leaf longevities, to help us better understand the implications of climate change on leaf demography.Area of study: The study was carried out in two sites of theprovince of Salamanca (central-westernSpain), both sites showing differences in their temperatures and soil water availability.Material and Methods: Over four years (2007-2010) we monitored the number of leaves of the different cohorts labelled on five specimens of each species at both sites to elaborate life-tables and calculate mortality rates. Mortality rates were then related to several other variables measured during the same period: air temperature, soil water availability, precipitation, predawn water potentials (Ypd) and leaf N resorption.Main results: In the two deciduous species maximum daily temperatures and the time during which their values remain above a certain threshold (between 11 and12ºC of maximum daily temperature) are the main factors controlling the timing of leaf abscission. In the evergreen species abscission of old leaves showed no relationship with the environmental factors analyzed. By contrast, mortality rates of old leaves were related to seasonal N resorption values, with the maximum mortality of old leaves coinciding in time with the maximum withdrawal of N from shed leaves and also with the emergence of the new leaf cohort.Research highlights: The increase in the duration of the leaves of the two deciduous species, as a result of the delayed senescence by warmer autumnal temperatures, could contribute to reducing the differences in the length of the productive leaf life with respect to the evergreen species. This could improve the competitive capacity of deciduous species as opposed to that of evergreen species, and thus alter their respective distribution patterns.Keywords: climate change; deciduous; evergreen; leaf abscission; temperature; water availability.

English

The response of ‘Ventana’, an early flowering cultivar, and ‘Camarosa’, an intermediate flowering cultivar, to nursery late-season nitrogen (N) applications and digging date were studied in strawberry (Fragaria x ananassa Duch). Two experiments were conducted. In the first experiment, runner plants dug on September 20 and October 11 from a high-latitude nursery in California, were established in growth chambers set at 25°/15oC day/night temperature, 12-h photoperiod, and grown for 90 days. Compared to the first experiment, in the second experiment plants received extra N (foliar-applied) in the nursery in late summer, and runner plants were not grown in GC but in open field (Irvine, California). In the second experiment, runner plants were dug on Sept 20 and Oct 2. In both experiments, plants dug in September were exposed to ~100 chilling units (CU: hours ≤7.2°C) and plants dug in October were exposed to ~300 CU. As a result, October-dug plants had greater crown and root dry weight, and greater concentration of starch and total nonstructural carbohydrates (TNC) in leaves, crowns and roots, compared to September-dug plants. In control plants, from September to October, root TNC concentration increased in ‘Camarosa’ from ~6% to ~11%, and in ‘Ventana’ from ~14% to ~21%, and leaf N concentration ranged from 1.47 to 1.81% in ‘Camarosa’, and from 1.60 to 1.96% in ‘Ventana’. Late-summer N applications increased plant N concentration and early-season yields. Late-summer nursery N applications reduced dead leaf biomass (DLB) and dead leaf area (DLA) in both cultivars, although ‘Ventana’ had lower DLB and DLA than ‘Camarosa’. ‘Ventana’ had a greater leaf number and flowered earlier, and had greater early fruit production than ‘Camarosa’. The genetic earliness of ‘Ventana’ would be correlated with the potential of the plant for accumulation of higher initial levels of leaf N and root TNC, and for having greater leaf longevity, compared to ‘Camarosa’. Key words: Fragaria x ananassa Duch., foliar urea, carbohydrates, chilling, plant maturity, early flowering cultivars.

Heat shock transcription factors involved in seed desiccation tolerance and longevity retard vegetative senescence in transgenic tobacco.

Transcription factors normally expressed in sunflower seeds delayed vegetative senescence induced by severe stress in transgenic tobacco. This revealed a novel connection between seed heat shock factors, desiccation tolerance and vegetative longevity. HaHSFA9 and HaHSFA4a coactivate a genetic program that, in sunflower (Helianthus annuus L.), contributes to seed longevity and desiccation tolerance. We have shown that overexpression of HaHSFA9 in transgenic tobacco seedlings resulted in tolerance to drastic dehydration and oxidative stress. Overexpression of HaHSFA9 alone was linked to a remarkable protection of the photosynthetic apparatus. In addition, the combined overexpression of HaHSFA9 and HaHSFA4a enhanced all these stress-resistance phenotypes. Here, we find that HaHSFA9 confers protection against damage induced by different stress conditions that accelerate vegetative senescence during different stages of development. Seedlings and plants that overexpress HaHSFA9 survived lethal treatments of dark-induced senescence. HaHSFA9 overexpression induced resistance to effects of culture under darkness for several weeks. Only some homoiochlorophyllous resurrection plants are able to withstand this experimental severe stress condition. The combined overexpression of HaHSFA9 and HaHSFA4a did not result in further slowing of dark-induced seedling senescence. However, combined expression of the two transcription factors caused improved recovery of the photosynthetic organs of seedlings after lethal dark treatments. At later stages of vegetative development, HaHSFA9 delayed the appearance of senescence symptoms in leaves of plants grown under normal illumination. This delay was observed under either control or stress treatments. Thus, HaHSFA9 delayed both natural and stress-induced leaf senesce. These novel observations connect transcription factors involved in desiccation tolerance with leaf longevity.

Leaf and stem economics spectra drive diversity of functional plant traits in a dynamic global vegetation model.

Functional diversity is critical for ecosystem dynamics, stability and productivity. However, dynamic global vegetation models (DGVMs) which are increasingly used to simulate ecosystem functions under global change, condense functional diversity to plant functional types (PFTs) with constant parameters. Here, we develop an individual- and trait-based version of the DGVM LPJmL (Lund-Potsdam-Jena managed Land) called LPJmL- flexible individual traits (LPJmL-FIT) with flexible individual traits) which we apply to generate plant trait maps for the Amazon basin. LPJmL-FIT incorporates empirical ranges of five traits of tropical trees extracted from the TRY global plant trait database, namely specific leaf area (SLA), leaf longevity (LL), leaf nitrogen content (Narea ), the maximum carboxylation rate of Rubisco per leaf area (vcmaxarea), and wood density (WD). To scale the individual growth performance of trees, the leaf traits are linked by trade-offs based on the leaf economics spectrum, whereas wood density is linked to tree mortality. No preselection of growth strategies is taking place, because individuals with unique trait combinations are uniformly distributed at tree establishment. We validate the modeled trait distributions by empirical trait data and the modeled biomass by a remote sensing product along a climatic gradient. Including trait variability and trade-offs successfully predicts natural trait distributions and achieves a more realistic representation of functional diversity at the local to regional scale. As sites of high climatic variability, the fringes of the Amazon promote trait divergence and the coexistence of multiple tree growth strategies, while lower plant trait diversity is found in the species-rich center of the region with relatively low climatic variability. LPJmL-FIT enables to test hypotheses on the effects of functional biodiversity on ecosystem functioning and to apply the DGVM to current challenges in ecosystem management from local to global scales, that is, deforestation and climate change effects.

Impact of African Weaver Ant Nests [Oecophylla longinoda Latreille (Hymenoptera: Formicidae)] on Mango [Mangifera indica L. (Sapindales: Anacardiaceae)] Leaves

Oecophylla ants are appreciated for their control of pests in plantation crops. However, the ants´ nest building may have negative impacts on trees. In this study we tested the effect of ant densities and nest building on the leaf performance of mango trees. Trees were divided into three groups: trees without ants, trees with low and trees with high ant densities. Subsequently, the total number of leaves, the proportion of leaves used for nest construction, and tree growth was compared between these groups. The percentage of leaves used for nests was between 0.42-1.2 % (mean = 0.7%±0.02) and the total number of leaves and tree growth was not significantly different between trees with and without ants. Further, leaf performance was compared between shoots with and without ant nests and between leaves in or outside ant nests. The number of leaves and lost leaves per shoot, leaf size, leaf condition (withered), leaf longevity and hemipteran infection was compared between groups. In the dry season nest-shoots held more leaves than shoots without nests despite nest-shoots showed more lost leaves. Leaves in nests were smaller than other leaves, more likely to wither and more often infested with scales. However, smaller nest-leaf size was probably due to the ants´ preference for young leaves and the higher incidence of withering resulting as leaves in nests cannot fall to the ground. In conclusion, the costs associated to ant nests were low and did not affect the overall number of leaves per tree nor tree growth.

Carbon acquisition strategies uncoupled from predictions derived from species life-cycle

We compare carbon use strategies of congeneric annual (P. gracilis and P. angustifolia) and perennial (P. mendocina and P. pinetorum) Physaria (Brassicaceae), to evaluate the relevance of eco-physiological traits as determinants of differences in growth and to add information on the current debate on the agro-ecological suitability of perennial species for grain production in low resource ecosystems. Because of differences in growth and in seed-output previously found within this genus, we hypothesized that C acquisition strategies would not be fully coupled with predictions derived from a species life-cycle. Further, we expected to find different suites of traits related to C-acquisition among perennial species of Physaria. We found species with high (P. gracilis and P. pinetorum) and low (P. mendocina and P. angustifolia) relative growth rate (RGR) and biomass. The variation in RGR was linked to differences in specific leaf area (SLA) and allocation to leaves (leaf mass ratio, LMR) and roots (root mass ratio, RMR), but not to the species life-cycle. Physaria gracilis had high allocation to leaf area (leaf area ratio, LAR), LMR, high SLA, and low RMR and carbohydrates reserves. The slow-growth strategy found in P. mendocina was linked to low LAR, low SLA and large below-ground allocation. The other species showed intermediate strategies between these two. The sets of traits present in P. gracilis and in P. mendocina are extremes in the C acquisition–conservation trade-off, and may allow them to cope with low resource environments in different ways. SLA, LMR and RMR were the main determinants of RGR, while total non-structural carbohydrates (TNC) and leaf longevity were linked to the life-cycle. Taken together these characters define the intermediate growth strategies of P. angustifolia and P. pinetorum. These intermediate strategies were not dependent on the species life cycle and support the hypothesis of uncoupled relationships between growth, C acquisition strategies and life-cycle.

Age-Triggered and Dark-Induced Leaf Senescence Require the bHLH Transcription Factors PIF3, 4, and 5

Leaf senescence can be triggered and promoted by a large number of developmental and environmental factors. Numerous lines of evidence have suggested an involvement of phytochromes in the regulation of leaf senescence, but the related signaling pathways and physiological mechanisms are poorly understood. In this study, we initially identified phytochrome-interacting factors (PIFs) 3, 4, and 5 as putative mediators of leaf senescence. Mutations of the PIF genes resulted in a significantly enhanced leaf longevity in age-triggered and dark-induced senescence, whereas overexpressions of these genes accelerated age-triggered and dark-induced senescence in Arabidopsis. Consistently, loss-of-function of PIF4 attenuated dark-induced transcriptional changes associated with chloroplast deterioration and reactive oxygen species (ROS) generation. ChIP-PCR and Dual-Luciferase assays demonstrated that PIF4 can activate chlorophyll degradation regulatory gene NYE1 and repress chloroplast activity maintainer gene GLK2 by binding to their promoter regions. Finally, dark-induced ethylene biosynthesis and ethylene-induced senescence were both dampened in pif4, suggesting the involvement of PIF4 in both ethylene biosynthesis and signaling pathway. Our study provides evidence that PIF3, 4, and 5 are novel positive senescence mediators and gains an insight into the mechanism of light signaling involved in the regulation of leaf senescence.

Cohort-specific tuning of foliage physiology to interacting stresses in evergreens.

Leaf economics spectrum is the basic concept linking leaf carbon gain strategies, fast vs slow turnover, to foliage functional traits (Wright et al. 2004). At the heart of the economics spectrum is the reverse correlation among traits responsible for leaf longevity and physiological activity; thus, high leaf longevity in evergreens is associated with traits improving leaf robustness such as high leaf dry mass per unit area and density reflecting enhanced biomass investment in support biomass (Wright et al. 2004). Increased support investments inevitably result in low photosynthetic capacity per dry mass placing evergreens towards the slow return end of the leaf economics spectrum (Wright et al. 2004, Reich 2014). The economics spectrum describes the basic co-variation patterns among leaf functional traits across plant functional types, but it does not implicitly consider several potentially confounding sources of variation such as leaf age (e.g., Niinemets 2014). Evergreens characteristically support multiple leaf age classes with potentially contrasting structural, chemical and physiological characteristics, but due to practical reasons, often only the leaves from the newest fully mature leaf flush are included in analyses involving evergreens. However, older leaves contribute a major proportion of total canopy leaf area in many evergreens (Whitehead et al. 1994, Niinemets et al. 2005), implying that age-specific differences in foliage performance play a major role in whole canopy carbon gain. While the basic age-dependent modifications in foliage functional traits have been studied in evergreens (Kitajima et al. 1997a, Radoglou and Teskey 1997, Ellsworth 2000, Niinemets and Lukjanova 2003, McGarvey et al. 2004, Niinemets et al. 2005), there is limited information on responses of different-aged leaves to environmental stresses with a few exceptions (e.g., Mulkey et al. 1992, Ogaya and Penuelas 2006). Many evergreen species also have a complex phenology of foliage development characterized by multiple foliage flushes during the same growing season. In this issue of Tree Physiology, the study of Morales et al. (2014) conducted in a Chilean temperate rain forest with the evergreen relatively shade-intolerant species Eucryphia cordifolia Cav. demonstrates that seasonality in environmental conditions can strongly affect foliage growth and development as well as physiological potentials of fully mature leaves. They observed major differences in leaf structural, chemical and physiological traits among leaf flushes produced during a given growing season (Figure 1, Morales et al. 2014). Such flush-specific differences in foliage characteristics have been observed in several warm temperate and subtropical conifers (Whitehead et al. 1994, Teskey 1997, Niinemets et al. 2002, McGarvey et al. 2004) and in evergreen angiosperms in climates with seasonal distribution of precipitation (Mulkey et al. 1992, Kitajima et al. 1997b), and collectively emphasize the importance of the timing of leaf flush in foliage functioning. The study of Morales et al. (2014) further found that differences among leaf cohorts in E. cordifolia were amplified in the low-light environment and under water stress (Figure 1). Although stress led to immediate physiological adjustments such as changes in stomatal conductance, the scaling among foliage physiological potentials (maximum carboxylase activity of Rubisco) and leaf nitrogen content was also altered (Figure 1). Such modifications in foliage functioning to stress during foliage growth suggest that the timing of leaf flush is an important driver altering the leaf economics spectrum in Commentary Tree Physiology Advance Access published November 25, 2014

A general method for calculating the optimal leaf longevity from the viewpoint of carbon economy.

According to the viewpoint of the optimal strategy theory, a tree is expected to shed its leaves when they no longer contribute to maximisation of net carbon gain. Several theoretical models have been proposed in which a tree was assumed to strategically shed an old deteriorated leaf to develop a new leaf. We mathematically refined an index used in a previous theoretical model [Kikuzawa (Am Nat 138:1250-1263, 1991)] so that the index is exactly proportional to a tree's lifelong net carbon gain. We also incorporated a tree's strategy that determines the timing of leaf expansion, and examined three kinds of strategies. Specifically, we assumed that a new leaf is expanded (1) immediately after shedding of an old leaf, (2) only at the beginning of spring, or (3) immediately after shedding of an old leaf if the shedding occurs during a non-winter season and at the beginning of spring otherwise. We derived a measure of optimal leaf longevity maximising the value of an appropriate index reflecting total net carbon gain and show that use of this index yielded results that are qualitatively consistent with empirical records. The model predicted that expanding a new leaf at the beginning of spring than immediately after shedding usually yields higher carbon gain, and combined strategy of the immediate replacement and the spring flushing earned the highest gain. In addition, our numerical analyses suggested that multiple flushing seen in a few species of subtropical zones can be explained in terms of carbon economy.