Leaf Longevity Research Articles

Coordination of leaf N, anatomy, photosynthetic capacity, and hydraulics enhances evergreen expansive potential

Key message Reduced leaf longevity, N-fixation, and enhanced hydraulic capacity combined support greater shifts in seasonal photosynthetic capacity of an expansive understory evergreen woody species relative to co-occurring less expansive evergreen species.

A model of seasonal foliage dynamics of the subtropical mangrove species Rhizophora stylosa Griff. growing at the northern limit of its distribution

Progress of forest production in response to the environment requires a quantitative understanding of leaf area development. Therefore, it is necessary to investigate the dynamics of seasonal crown foliage in order to understand the productivity of mangroves, which play an important role in the subtropical and tropical coastlines of the world. Crown foliage dynamics of the mangrove Rhizophora stylosa were studies to reveal patterns of leaf recruitment, survival and seasonal leaf area growth. Flushing of leaves occurred throughout the year, but both flushing and leaf area growth pattern of leaves varied with season. Maximum flushing occurred in summer, but leaf areas did not differ significantly with season. The half-expansion period is longer, and the intrinsic rate of increase was lower in winter. Summer flushed leaves grew faster at their initial stage and reached their maximum area over a shorter period of time. The difference in temperature and air vapor pressure deficit (VPD) between summer and winter contributed to the present dynamics of foliage patterns. The mean leaf longevity was estimated to be 13.1 month. The crown foliage area was almost stable throughout the year. Homeostatic control of the crown foliage area may be accompanied by the existence of ecophysiological mechanisms in R. stylosa. Integrating crown foliage dynamics into forest models represents an important step towards incorporating physiological mechanisms into the models for predicting growth responses to environmental changes and for understanding the complex responses of tree growth and litter production.

Life-history traits predict perennial species response to fire in a desert ecosystem.

The Mojave Desert of North America has become fire-prone in recent decades due to invasive annual grasses that fuel wildfires following years of high rainfall. Perennial species are poorly adapted to fire in this system, and post-fire shifts in species composition have been substantial but variable across community types. To generalize across a range of conditions, we investigated whether simple life-history traits could predict how species responded to fire. Further, we classified species into plant functional types (PFTs) based on combinations of life-history traits and evaluated whether these groups exhibited a consistent fire-response. Six life-history traits varied significantly between burned and unburned areas in short (up to 4 years) or long-term (up to 52 years) post-fire datasets, including growth form, lifespan, seed size, seed dispersal, height, and leaf longevity. Forbs and grasses consistently increased in abundance after fire, while cacti were reduced and woody species exhibited a variable response. Woody species were classified into three PFTs based on combinations of life-history traits. Species in Group 1 increased in abundance after fire and were characterized by short lifespans, small, wind-dispersed seeds, low height, and deciduous leaves. Species in Group 2 were reduced by fire and distinguished from Group 1 by longer lifespans and evergreen leaves. Group 3 species, which also decreased after fire, were characterized by long lifespans, large non-wind dispersed seeds, and taller heights. Our results show that PFTs based on life-history traits can reliably predict the responses of most species to fire in the Mojave Desert. Dominant, long-lived species of this region possess a combination of traits limiting their ability to recover, presenting a clear example of how a novel disturbance regime may shift selective environmental pressures to favor alternative life-history strategies.

New maize hybrids had larger and deeper post-silking root than old ones

Since new maize (Zea mays L.) varieties have higher grain yield, a larger root system with longer total root length and more deep roots is expected for more water and nutrient uptake and shoot biomass accumulation. To compare the differences in root dry weight (DW), total root length, vertical root distribution and post-silking root mortality in the 0–60cm soil profile between old and new maize varieties with different leaf longevity, a two-year field experiment with six maize varieties with early-senescing (old varieties) and stay-green leaves (new varieties) was conducted. Although the new varieties had larger root DW at silking, which corresponded to their larger shoot biomass, the total root length and vertical root distribution of all six varieties were the same in the 0–60cm soil profile at the same stage. At maturity, the difference in root DW between the old and the new varieties enlarged, and the total root length of the new varieties exceeded that of the old varieties, with more deep roots. Longer post-silking root length and more roots in deeper soil layers of the new varieties is beneficial for water acquisition in the deep soil layer and nutrient interception such as nitrate in leachate.

Leaf phenological characters of main tree species in urban forest of Shenyang.

BackgroundPlant leaves, as the main photosynthetic organs and the high energy converters among primary producers in terrestrial ecosystems, have attracted significant research attention. Leaf lifespan is an adaptive characteristic formed by plants to obtain the maximum carbon in the long-term adaption process. It determines important functional and structural characteristics exhibited in the environmental adaptation of plants. However, the leaf lifespan and leaf characteristics of urban forests were not studied up to now.MethodsBy using statistic, linear regression methods and correlation analysis, leaf phenological characters of main tree species in urban forest of Shenyang were observed for five years to obtain the leafing phenology (including leafing start time, end time, and duration), defoliating phenology (including defoliation start time, end time, and duration), and the leaf lifespan of the main tree species. Moreover, the relationships between temperature and leafing phenology, defoliating phenology, and leaf lifespan were analyzed.FindingsThe timing of leafing differed greatly among species. The early leafing species would have relatively early end of leafing; the longer it took to the end of leafing would have a later time of completed leafing. The timing of defoliation among different species varied significantly, the early defoliation species would have relatively longer duration of defoliation. If the mean temperature rise for 1°C in spring, the time of leafing would experience 5 days earlier in spring. If the mean temperature decline for 1°C, the time of defoliation would experience 3 days delay in autumn.InterpretationThere is significant correlation between leaf longevity and the time of leafing and defoliation. According to correlation analysis and regression analysis, there is significant correlation between temperature and leafing and defoliation phenology. Early leafing species would have a longer life span and consequently have advantage on carbon accumulation compared with later defoliation species.

Plant functional groups of species in semiarid ecosystems in Brazil: wood basic density and SLA as an ecological indicator

Deciduousness in arid and semiarid environments is an important adaptive strategy in tree and shrub species to prevent extensive leaf desiccation and nutrient loss during the drought. The loss of leaves benefits some species, since the seasonality of water resources and high temperatures restrict the survival of plants in xeric environments. The aim of this study was to characterize functional groups of caatinga species from soft traits by testing the hypothesis that the four soft trait measured will show a new pattern and thereby characterize plant functional groups, or if the analysis of the chosen traits will converge with deciduousness. The soft traits measured did not form groups based on deciduousness. Three statistically different groups of species could be identified after statistical analysis: (1) low wood basic density; (2) low specific leaf area and high leaf thickness; and (3) high wood basic density. The most important attributes of the first two Principal Component Analysis axis were wood basic density and specific leaf area explaining 40.8 % and 37.4 % of the variation respectively. Albeit no link has been found between SLA and deciduousness, this study represents an additional evidence that perhaps the severity of savanna environments limits leaf longevity, meaning that differences between evergreen and deciduous species are less important.

Leaf habit of Late PermianGlossopteristrees from high-palaeolatitude forests

The leaf longevity of trees, deciduous or evergreen, plays an important role in climate feedbacks and plant ecology. In modern forests of the high latitudes, evergreen trees dominate; however, the fossil record indicates that deciduous vegetation dominated during some previous warm intervals. We show, through an integration of palaeobotanical techniques and isotope geochemistry of trees in one of the earliest polar forests (Late Permian, c . 260 Ma, Antarctica), that the arborescent glossopterid taxa were both deciduous and evergreen, with a greater abundance of evergreen trees occurring in the studied forests. These new findings suggest the possibility that deciduousness was a plastic trait in ancient polar plants, and that deciduous plants, migrating poleward from lower latitudes, were probably better adapted to high-disturbance areas in environments that were light-limited. Supplementary material: Wood anatomy descriptions (supplemental file 1), stable carbon isotope data for tree rings (supplemental files 2–4), and method and parameters for modelled Δ 13 C (supplemental file 5) are available at www.geolsoc.org.uk/SUP18744.

Controls on fallen leaf chemistry and forest floor element masses in native and novel forests across a tropical island

Litter chemistry varies across landscapes according to factors rarely examined simultaneously. We analyzed 11 elements in forest floor (fallen) leaves and additional litter components from 143 forest inventory plots systematically located across Puerto Rico, a tropical island recovering from large‐scale forest clearing. We assessed whether three existing, independently‐derived, landscape classifications (Holdridge life zone, remotely sensed forest type (leaf longevity combined with geology generalized to karst vs. non‐karst), and plot‐based measures of forest assemblage) would separate observed gradients. With principal component and regression analyses, we also tested whether climate‐, landscape‐ (geology, elevation, aspect, percent slope, slope position, distance from coast), and stand‐scale (tree species composition, basal area, density, stand age) variables explained variation in fallen leaf chemistry and stoichiometry. For fallen leaves, C, Ca, Mg, Na, and Mn concentrations differed by Holdridge life zone and C, P, Ca, Mn, Al, and Fe concentrations differed by forest type, where leaf longevity distinguished C and Ca concentrations and geology distinguished C, P, Ca, Mn, Al, and Fe concentrations. Fallen leaf C, P, Ca, and Mn concentrations also differed, and N concentrations only differed, by forest assemblage. Across several scales, fallen leaf N concentration was positively related to the basal area of putatively N2‐fixing tree legumes, which were concentrated in lower topographic positions, providing for the first time a biological explanation for the high N concentrations of fallen leaves in these locations. Phosphorus concentrations in fallen leaves by forest assemblages also correlated with the basal area of N2‐fixing legumes, and P and N concentrations decreased with mean age of assemblage. Fallen leaves from younger (<50 yr, 86% of the plots) and often novel forests had higher P, Fe, and Al and lower C concentrations and lower C/P and N/P ratios than fallen leaves from older forests, the latter due to a decrease in P rather than changes in N. These findings suggest that both N and P availability may currently be greater on the island than pre‐deforestation, and substantiate the unique roles that state factors play in contributing to the spatial heterogeneity of fallen leaf chemistry.

Autophagy, plant senescence, and nutrient recycling

Large numbers of publications have appeared over the last few years, dealing with the molecular details of the regulation and process of the autophagy machinery in animals, plants, and unicellular eukaryotic organisms. This strong interest is caused by the fact that the autophagic process is involved in the adaptation of organisms to their environment and to stressful conditions, thereby contributing to cell and organism survival and longevity. In plants, as in other eukaryotes, autophagy is associated with longevity as mutants display early and strong leaf senescence symptoms, however, the exact role of autophagy as a pro-survival or pro-death process is unclear. Recently, evidence that autophagy participates in nitrogen remobilization has been provided, but the duality of the role of autophagy in leaf longevity and/or nutrient recycling through cell component catabolism remains. This review aims to give an overview of leaf senescence-associated processes from the physiological point of view and to discuss relationships between nutrient recycling, proteolysis, and autophagy. The dual role of autophagy as a pro-survival or pro-death process is discussed.

Turf wars: experimental tests for alternative stable states in a two‐phase coastal ecosystem

Alternative stable states have long been thought to exist in natural communities, but direct evidence for their presence and for the environmental switches that cause them has been scarce. Using a combination of greenhouse and field experiments, we investigated the environmental drivers associated with two distinctive herbaceous communities in coastal ecosystems in New Zealand. In a mosaic unrelated to micro-topography, a community dominated largely by native turf species (notably Leptinella dioica, Samolus repens, and Selliera radicans) alternates with vegetation comprising exotic (i.e., nonnative) pasture species (notably Agrostis stolonifera, Holcus lanatus, Lolium perenne, and Trifolium repens). The species of these two communities differ in functional characters related to leaf longevity and growth rate, and occupy soils of differing nitrogen levels. Both spatial and environmental factors influenced the species composition locally. Reciprocal transplants of soil, with and without associated vegetation, showed that a native turf community developed when sward or soil from either community was bounded by turf, and a pasture community developed when sward or soil from either community was surrounded by pasture. In artificial mixed communities in the greenhouse, turf was able to invade the pasture community where the vegetation was clipped to simulate grazing, and also where Trifolium was removed and/or salt spray was applied. The pasture community invaded the turf where Trifolium was present or nitrogen was added. These results were supported by trends in experimentally manipulated field plots, where the amount of turf cover increased when nitrogen was kept low and when salt spray was applied, whereas pasture cover increased in the absence of salt spray. Thus, persistence of the native turf community is dependent on grazing, both directly and via its effect on keeping nitrogen levels low by excluding the exotic, nitrogen-fixing Trifolium, and by exposing the vegetation to salt spray. If any of these factors change, there could be a state change to pasture dominance that might be resistant to reversion to turf. Managing such coastal herbaceous communities therefore requires an understanding of the environmental and species characteristics that maintain alternative states.

The plant microbiome and its importance for plant and human health.

To study plant-associated microorganisms has a long history that reaches back to Lorenz Hiltner's definition of the rhizosphere in 1904 (Hartmann et al., 2008). Today, we know that microorganisms colonizing plant surfaces and inner tissues play an eminent role in shaping of our planet—from our natural vegetation to intense agricultural production systems up to human health. Plant-associated microorganisms have to be considered as key drivers for plant health, productivity, community composition, and ecosystem functioning. For this e-book “The plant microbiome and its importance for plant and human health” we collected 18 articles, including reviews, original, and opinion articles that highlight the current knowledge regarding plant microbiomes, their specificity, diversity, and function as well as all aspects studying the management of plant microbiomes to improve plant performance and health. The contribution of the single articles of this research topic to these questions is discussed in detail in the mini-review and 1st chapter of the book by Berg et al. (2014a). Overall the presented articles confirm that the plant-associated microbiome has greatly expanded the metabolic repertoire of plants and often increase resource uptake and provide novel nutritional and defense pathways. Thus, the plant microbiome has a direct impact on plant functional traits, such as leaf longevity, specific leaf area, leaf nutrient levels, and shoot/root ratio. By providing novel nutritional and defense pathways and by modifying biochemical pathways, the plant associated microbiome can enhance or decrease species coexistence and consequently influence not only a single plant but complete ecosystems. Thus, future breeding strategies may take the importance of plant-microbe interactions more into account than in the past, to obtain plants that generate high yields and are more tolerate to the constraints of global change. Studies related to raw-eaten vegetables are a special show case in this e-book. Here the plant-associated microbiome does not only influence plant performance but strongly contributes to human health. As those microbes are also part of our diet they can either improve human health (Blaser et al., 2013) or cause heavy outbreaks of infectious diseases by transferring possible pathogens (Van Overbeek et al., 2014). Interestingly, the gathered manuscripts indicate that microbiomes of different environments are not isolated but show interplay. For example, the microbiome of vegetables, humans as well as build environment such as hospitals seems to be well-connected (Berg et al., 2014b). Thus, maintaining microbial diversity in the different environments is an important issue to avoid pathogen outbreaks, which can be often explained by microbial imbalances and poorness (Van Elsas et al., 2012), confirming basic theories of ecology that a loss of native species enhances the probability of invasive species to colonize new environments. Therefore, to maintain and support microbial diversity is of interest to stabilize ecosystems and their resilience toward biotic and abiotic stressors. Biotechnological solutions like probiotics, prebiotics, and synbiotics for plants as well as humans can provide support for the indigenous microbiome (De Vrese and Schrezenmeir, 2008). The most significant recent advances in plant microbiology involve interdisciplinary approaches that link different methodological approaches including omics-technologies. Due to the new methods available and interdisciplinary research cooperation we have the chance to solve many problems of a changing world, but also to address basic hypotheses and questions of microbial ecology and host microbe interactions. Integrating epigenetics in multi-omics techniques opens existing opportunities for new discoveries (Chen et al., 2014). Therefore, we think this comprehensive e-book especially the many reviews can contribute to hold the current knowledge in our hand. This is a very exciting but also challenging time for all researchers in this field. Major advances will come rapidly!

The effect of irradiance on the carbon balance and tissue characteristics of five herbaceous species differing in shade-tolerance.

The carbon balance is defined here as the partitioning of daily whole-plant gross CO2 assimilation (A) in C available for growth and C required for respiration (R). A scales positively with growth irradiance and there is evidence for an irradiance dependence of R as well. Here we ask if R as a fraction of A is also irradiance dependent, whether there are systematic differences in C-balance between shade-tolerant and shade-intolerant species, and what the causes could be. Growth, gas exchange, chemical composition and leaf structure were analyzed for two shade-tolerant and three shade-intolerant herbaceous species that were hydroponically grown in a growth room at five irradiances from 20 μmol m−2 s−1 (1.2 mol m−2 day−1) to 500 μmol m−2 s−1 (30 mol m−2 day−1). Growth analysis showed little difference between species in unit leaf rate (dry mass increase per unit leaf area) at low irradiance, but lower rates for the shade-tolerant species at high irradiance, mainly as a result of their lower light-saturated rate of photosynthesis. This resulted in lower relative growth rates in these conditions. Daily whole-plant R scaled with A in a very tight manner, giving a remarkably constant R/A ratio of around 0.3 for all but the lowest irradiance. Although some shade-intolerant species showed tendencies toward a higher R/A and inefficiencies in terms of carbon and nitrogen investment in their leaves, no conclusive evidence was found for systematic differences in C-balance between the shade-tolerant and intolerant species at the lowest irradiance. Leaf tissue of the shade-tolerant species was characterized by high dry matter percentages, C-concentration and construction costs, which could be associated with a better defense in shade environments where leaf longevity matters. We conclude that shade-intolerant species have a competitive advantage at high irradiance due to superior potential growth rates, but that shade-tolerance is not necessarily associated with a better C-balance at low irradiance. Under those conditions tolerance to other stresses is probably more important for the performance of shade-tolerant species.

Ontogenetic transition in leaf traits: a new cost associated with the increase in leaf longevity

Aims Recent work has identified a worldwide ‘economics’ spectrum of correlated leaf traits that mainly reflects the compromises between maximizing leaf longevity and short-term productivity. However, during the early stages of tree growth different species tend to exhibit a common strategy, because competition for soil water and nutrients forces the maximization of short-term productivity owing to the need for rapid growth during the most vulnerable part of the tree’s life cycle. Accordingly, our aim here was to compare the variations that occur during ontogeny in the different leaf traits (morphology and leaf chemical composition) of several coexisting Mediterranean woody species differing in their leaf life spans and to test our hypothesis that tree species with a long leaf life span should exhibit larger shifts in leaf characteristics along ontogeny. Methods Six Mediterranean tree species differing in leaf life span, selected from three plots located in central-western Spain, were studied during three growth stages: seedlings, juveniles and mature trees. Leaf life span, leaf morphology (leaf area, dry weight, thickness and mass per unit area) and chemical composition (N and fibre concentrations) were measured in all six species. The magnitude of the ontogenetic changes in the different traits was estimated and related to the mean leaf longevity of the different species. Important Findings Along ontogeny, strong changes were observed in all variables analysed. The early growth stages showed lower leaf thickness, leaf thickness and mass per unit area and N, cellulose and hemicellulose concentrations than mature trees, but a higher lignin content. However, these changes were especially marked in species with a longer leaf life span at maturity. Interspecific differences in leaf life span, leaf morphology and chemical composition were stronger at the mature stage than at the seedling stage. We conclude that greater plasticity and more intense strategy shifts along ontogeny are necessarily associated with long leaf life span. Our results thus provide a new aspect that should be incorporated into the analysis of the costs and benefits associated with the different strategies related to leaf persistence displayed by the different species. Accordingly, the intensity of the alterations in leaf traits among different growth stages should be added to the suite of traits that change along the leaf economics spectrum.

Leaf longevity as a normalization constant in allometric predictions of plant production.

In metabolic scaling theory the size-dependence of plant processes is described by a power function of form Y=Y o M θ where Y is a characteristic such as plant productivity that changes with plant size (M) raised to the θ th power and Y o is a normalization constant that adjusts the general relationship across environments and species. In essence, the theory considers that the value of θ arises in the size-dependent relationship between leaf area and vascular architecture that influences plant function and that Y o modulates this general relationship to account for ecological and evolutionary effects on the exchange of resources between plant and environment. Enquist and colleagues have shown from first principles that Y o is a function of carbon use efficiency, the carbon fraction of a plant, the area-specific carbon assimilation rate of a leaf, the laminar area of a leaf, and the mass of a leaf. We show that leaf longevity provides a functional integration of these traits that can serve as a simpler normalization in scaling plant productivity for individual species and potentially for mixed-species communities as well.

Linking ethylene to nitrogen-dependent leaf longevity of grass species in a temperate steppe

Leaf longevity is an important plant functional trait that often varies with soil nitrogen supply. Ethylene is a classical plant hormone involved in the control of senescence and abscission, but its role in nitrogen-dependent leaf longevity is largely unknown. Pot and field experiments were performed to examine the effects of nitrogen addition on leaf longevity and ethylene production in two dominant plant species, Agropyron cristatum and Stipa krylovii, in a temperate steppe in northern China. Nitrogen addition increased leaf ethylene production and nitrogen concentration but shortened leaf longevity; the addition of cobalt chloride, an ethylene biosynthesis inhibitor, reduced leaf nitrogen concentration and increased leaf longevity. Path analysis indicated that nitrogen addition reduced leaf longevity mainly through altering leaf ethylene production. These findings provide the first experimental evidence in support of the involvement of ethylene in nitrogen-induced decrease in leaf longevity.

Scaling climate change experiments across space and time

Scaling climate change experiments across space and time

Temporal Variations in Leaf Miner, Butterfly, and Stem Borer Infestations of Cycas micronesica in Relation to Aulacaspis yasumatsui Incidence

Four phytophagous insects are among the major threats to Cycas micronesica K.D. Hill on Guam, and the temporal comparisons of infestation levels may inform horticultural and conservation decisions. Incidence of Aulacaspis yasumatsui Takagi, Erechthias Meyrick sp., Chilades pandava Horsfield, and Dihammus marianarum Aurivillius infestations on Cycas micronesica plants were recorded every 6 months from 2004 to 2013 to determine if the incidence of A. yasumatsui was related to patterns of the other three insects. Dihammus marianarum infestations reached ephemeral maximum and minimum levels ≈2 years after A. yasumatsui infestations reached maximum and minimum levels. Erechthias sp. infestations disappeared in early 2006 when Cycas micronesica leaves were being killed by acute A. yasumatsui infestations. Erechthias sp. infestations increased thereafter as A. yasumatsui incidence declined in response to biological control. Chilades pandava infestations appeared to increase and decrease inversely with A. yasumatsui infestations throughout the years. Aulacaspis yasumatsui may be indirectly affecting D. marianarum damage by direct control of changes in overall tree health. Aulacaspis yasumatsui may be indirectly influencing Erechthias sp. through direct control over leaf longevity. The C. pandava and A. yasumatsui populations appear to exhibit direct competition with inverse patterns of incidence.

How do leaf veins influence the worldwide leaf economic spectrum? Review and synthesis

Leaf vein traits are implicated in the determination of gas exchange rates and plant performance. These traits are increasingly considered as causal factors affecting the 'leaf economic spectrum' (LES), which includes the light-saturated rate of photosynthesis, dark respiration, foliar nitrogen concentration, leaf dry mass per area (LMA) and leaf longevity. This article reviews the support for two contrasting hypotheses regarding a key vein trait, vein length per unit leaf area (VLA). Recently, Blonder et al. (2011, 2013) proposed that vein traits, including VLA, can be described as the 'origin' of the LES by structurally determining LMA and leaf thickness, and thereby vein traits would predict LES traits according to specific equations. Careful re-examination of leaf anatomy, published datasets, and a newly compiled global database for diverse species did not support the 'vein origin' hypothesis, and moreover showed that the apparent power of those equations to predict LES traits arose from circularity. This review provides a 'flux trait network' hypothesis for the effects of vein traits on the LES and on plant performance, based on a synthesis of the previous literature. According to this hypothesis, VLA, while virtually independent of LMA, strongly influences hydraulic conductance, and thus stomatal conductance and photosynthetic rate. We also review (i) the specific physiological roles of VLA; (ii) the role of leaf major veins in influencing LES traits; and (iii) the role of VLA in determining photosynthetic rate per leaf dry mass and plant relative growth rate. A clear understanding of leaf vein traits provides a new perspective on plant function independently of the LES and can enhance the ability to explain and predict whole plant performance under dynamic conditions, with applications towards breeding improved crop varieties.

Salicylic acid 3-hydroxylase regulates Arabidopsis leaf longevity by mediating salicylic acid catabolism

The plant hormone salicylic acid (SA) plays critical roles in plant defense, stress responses, and senescence. Although SA biosynthesis is well understood, the pathways by which SA is catabolized remain elusive. Here we report the identification and characterization of an SA 3-hydroxylase (S3H) involved in SA catabolism during leaf senescence. S3H is associated with senescence and is inducible by SA and is thus a key part of a negative feedback regulation system of SA levels during senescence. The enzyme converts SA (with a Km of 58.29 µM) to both 2,3-dihydroxybenzoic acid (2,3-DHBA) and 2,5-DHBA in vitro but only 2,3-DHBA in vivo. The s3h knockout mutants fail to produce 2,3-DHBA sugar conjugates, accumulate very high levels of SA and its sugar conjugates, and exhibit a precocious senescence phenotype. Conversely, the gain-of-function lines contain high levels of 2,3-DHBA sugar conjugates and extremely low levels of SA and its sugar conjugates and display a significantly extended leaf longevity. This research reveals an elegant SA catabolic mechanism by which plants regulate SA levels by converting it to 2,3-DHBA to prevent SA overaccumulation. The research also provides strong molecular genetic evidence for an important role of SA in regulating the onset and rate of leaf senescence.

Vegetative and reproductive phenological traits of Rhizophora mucronata Lamk. and Sonneratia alba Sm.

Mangrove phenology is important in understanding the past, present and future response of mangrove species to impacts of climate change. Our study is the first long term direct observation of the phenology of Rhizophora mucronata and Sonneratia alba in Kenya. Objective of the study was to determine, interpret and document the timing of the various phenoevents and phenophases, and to establish relationships between phenology and the climatic variables. Phenological traits were investigated in six monospecific mangrove stands in Gazi Bay, south of Mombasa, for 2 years. Leaf emergence, leaf fall, flower bud, flower and fruit initiation data were recorded every fortnight in 54 shoots of 9 trees at each site. Continuous leaf emergence and loss characterized by multimodal peaks was observed for the two species. Leaf emergence and leaf fall peaked in the wet months and was reduced in the dry months. There was a relationship between the leaf emergence and drop with the reproductive phenology in the two species. Mean leaf longevity for R. mucronata and S. alba was 12.8±1.2 and 4.9±0.5 months respectively. The reproductive cycle took approximately 16–20 months in R. mucronata and 4–5 months in S. alba. Bud initiation in R. mucronata was seasonal and occurred in October and September. Buds were observed for 8–11 months developing slowly on the shoots. Shifts were observed in the timing of flower initiation, and the flowering period lasted in total for 4–5 months. The time period from flower appearance to the developmental start of immature propagules lasted about 1 month at reforested sites and up to 5 months in stands of natural vegetation. In R. mucronata initiation of immature propagules was not seasonal and varied among the sites. Fruiting in S. alba was short and seasonal without overlap, and shifts were observed in the timing of flowering, flowering and fruiting peaks. Abscission of reproductive parts started in July and in June, respectively, in the years 2005 and 2006. Fruiting was observed starting in August in both years, and fruiting peak was reached in October in 2005 and one month earlier in 2006. In R. mucronata vegetative and reproductive phenophases significantly correlated with climatic variables, whereas in S. alba only leaf emergence and leaf fall correlated with temperature. The flowering plasticity in the reproductive phenology of the two species indicates possible sensitivity to certain climatic and environmental triggers. Our results also indicate that R. mucronata trees have a distinctly higher investment in the reproductive cycle than S. alba.