Time Of Leaf Expansion Research Articles

Effect of expansion time and sunlight radiation on the functional and anatomical traits of mango tree leaves

The mango tree (Mangifera indica L.) is cultivated on a large scale in Brazil for economic purposes. Since the leaves, as the main photosynthetic organs, play an essential role in fruit production, the primary objective of the present study was to analyze comparatively the morphologicalcharacteristics of leaves at different stages of expansion and of leaves exposed to different levels of solar radiation. Leaves were collected at the beginning of expansion, during intermediate expansion and when completely expanded, and sun and shade leaves were compared. The individuals were adult plants without flowers and fruits located along the South Lake, Brasília. The leaves were analyzed for area, specific leaf area (SLA), thickness, water content and anatomical traits. Data were analyzed by ANOVA post hoc Tukey test to test the influence of leaf expansion time and by the T-test to determine the effect of radiation (α = 5%). Greater scleromorphy was observed in the completely expanded leaves compared to the leaves at the beginning of expansion, with lower water content and SLA in completely expanded leaves. Higher sclerophyll content was observed in sun leaves than in shade leaves, with lower SLA, greater leaf thickness and greater thickness of adaxial epidermis, palisade parenchyma and mesophyll in sun leaves. Based on these results, greater care is indicated regarding leaf management at the beginning of expansion and the management of shade leaves, since they aremore susceptible to damage from herbivores. The objective of this management is to maximize fruit production.

How tree species identity and diversity affect light transmittance to the understory in mature temperate forests.

Light is a key resource for plant growth and is of particular importance in forest ecosystems, because of the strong vertical structure leading to successive light interception from canopy to forest floor. Tree species differ in the quantity and heterogeneity of light they transmit. We expect decreases in both the quantity and spatial heterogeneity of light transmittance in mixed stands relative to monocultures, due to complementarity effects and niche filling. We tested the degree to which tree species identity and diversity affected, via differences in tree and shrub cover, the spatiotemporal variation in light availability before, during, and after leaf expansion. Plots with different combinations of three tree species with contrasting light transmittance were selected to obtain a diversity gradient from monocultures to three species mixtures. Light transmittance to the forest floor was measured with hemispherical photography. Increased tree diversity led to increased canopy packing and decreased spatial light heterogeneity at the forest floor in all of the time periods. During leaf expansion, light transmittance did differ between the different tree species and timing of leaf expansion might thus be an important source of variation in light regimes for understory plant species. Although light transmittance at the canopy level after leaf expansion was not measured directly, it most likely differed between tree species and decreased in mixtures due to canopy packing. A complementary shrub layer led, however, to similar light levels at the forest floor in all species combinations in our plots. Synthesis. We find that a complementary shrub layer exploits the higher light availability in particular tree species combinations. Resources at the forest floor are thus ultimately determined by the combined effect of the tree and shrub layer. Mixing species led to less heterogeneity in the amount of light, reducing abiotic niche variability.

Convergence in leaf phenology traits of two understorey ferns in the northwestern Iberian Peninsula

Plants control leaf phenology to maximize annual photosynthetic product. Although ferns play an important ecological role in many habitats, especially forests, their phenology traits have been poorly studied. Here, we investigate the leaf phenology of two ferns of the forest understorey and analyse the relationship between the timing of leaf emergence and spore dispersal and the effect of between-year climatic variation. We compared the leafing and sporing phenologies of two ferns with very large (>2 m), overwintering leaves: Culcita macrocarpa and Woodwardia radicans. We regularly monitored individuals of six populations in the northwestern Iberian Peninsula during a 3-year study. We studied eight phenology variables: leafing start date, leafing end date, leaf expansion time, number of new leaves per individual, between-individual synchrony, within-individual synchrony, percentage of fertile leaves and spore release date. We also determined leaf mass per area (LMA) and gathered data on air temperature and humidity. Both C. macrocarpa and W. radicans produce few leaves (~2 leaves individual⁻¹ year⁻¹), which expand simultaneously for a very long period (from midwinter to early summer), are retained for more than 1 year (37 and 19 months, respectively) and have relatively high LMAs. Such traits, together with large leaf size, have also been found in seed plants from the forest understorey and represent adaptations to this light-limited environment. Spores of both study ferns are simultaneously released in late winter, with little between-year variation caused by differences in air humidity. This remarkable similarity between species suggests that the convergence in timing of leaf emergence favours the convergence in timing of spore dispersal.

Earlier snowmelt and warming lead to earlier but not necessarily more plant growth.

Climate change over the past ∼50 years has resulted in earlier occurrence of plant life-cycle events for many species. Across temperate, boreal and polar latitudes, earlier seasonal warming is considered the key mechanism leading to earlier leaf expansion and growth. Yet, in seasonally snow-covered ecosystems, the timing of spring plant growth may also be cued by snowmelt, which may occur earlier in a warmer climate. Multiple environmental cues protect plants from growing too early, but to understand how climate change will alter the timing and magnitude of plant growth, experiments need to independently manipulate temperature and snowmelt. Here, we demonstrate that altered seasonality through experimental warming and earlier snowmelt led to earlier plant growth, but the aboveground production response varied among plant functional groups. Earlier snowmelt without warming led to early leaf emergence, but often slowed the rate of leaf expansion and had limited effects on aboveground production. Experimental warming alone had small and inconsistent effects on aboveground phenology, while the effect of the combined treatment resembled that of early snowmelt alone. Experimental warming led to greater aboveground production among the graminoids, limited changes among deciduous shrubs and decreased production in one of the dominant evergreen shrubs. As a result, we predict that early onset of the growing season may favour early growing plant species, even those that do not shift the timing of leaf expansion.

Leaf damage effects on leaf expansion timing in Mallotus japonicus (Euphorbiaceae)

AbstractFor many plant species, biotic factors determining the timing of leaf expansion have not been elucidated sufficiently. We investigated the effects of leaf damage on the timing of leaf defoliation and on the timing of leaf expansion in Mallotus japonicus (Euphorbiaceae). The degree of leaf herbivory of M. japonicus in the field was examined in summer, with subsequent investigation of the date of leaf defoliation in autumn and that of leaf expansion the following spring. Effects of artificial leaf damage on the timing of leaf defoliation and of leaf expansion were also examined in a greenhouse. In the field, the degree of leaf herbivory sustained by M. japonicus did not affect the timing of defoliation. However, the timing of leaf expansion the next spring advanced earlier, concomitantly with the increase of leaf damage during the previous year. Fifty per cent artificial leaf damage in M. japonicus saplings in summer had no marked effect on the timing of leaf defoliation in autumn. However, the effects of leaf damage on the timing of leaf expansion were apparent the following spring: the damaged plants expanded new leaves ca. 8 days earlier than control plants did. Plants that showed earlier leaf expansion tended to have a higher shoot/root ratio. Our results demonstrate that the plants advance the timing of their leaf expansion in response to leaf damage sustained during the previous year, suggesting that the shoot/root ratio is a determining factor.

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.

Interannual variability of net ecosystem production for a broadleaf deciduous forest in Sapporo, northern Japan

To estimate net ecosystem production (NEP), ecosystem respiration (RE), and gross primary production (GPP), and to elucidate the interannual variability of NEP in a cool temperate broadleaf deciduous forest in Sapporo, northern Japan, we measured net ecosystem exchange (NEE) using an eddy covariance technique with a closed-path infrared gas analyzer from 2000 to 2003. NEP, RE, and GPP were derived from NEE, and data gaps were filled using empirical regression models with meteorological variables such as photosynthetic active radiation and soil temperature. In general, NEP was positive (CO2 uptake) from May to September, either positive or negative in October, and negative (CO2 release) from November to the following April. NEP rapidly increased during leaf expansion in May and reached its maximum in June or July. The four-year averages (± standard deviation) of annual NEP, GPP, and RE were 443 ± 45, 1,374 ± 39, and 931 ± 11…−2 year−1, respectively. The lower annual NEP and GPP in 2000 may have been caused by lower solar radiation in the foliated season. During the foliated season, monthly GPP varied from year to year more than monthly RE. Variations in the amount of incoming solar radiation may have caused the interannual variations in the monthly GPP. Additionally, in May, the timing of leaf expansion had a large impact on GPP. Variations in GPP affected the interannual variation in NEP at our site. Thus, interannual variation in NEP was affected by the incoming solar radiation and the timing of leaf expansion.

The necessity and availability of noise-free daily satellite-observed NDVI during rapid phenological changes in terrestrial ecosystems in East Asia

General, global, long-term, and comprehensive phenological observations are required to evaluate the variability of photosynthetic activities due to environmental changes in terrestrial ecosystems. The observation of seasonal changes and detection of interannual variation in canopy phenology over regional and global scales require satellite data with high temporal resolution (i.e. a daily time step). However, satellite data often include noise caused by snow cover on vegetation, cloud contamination, and atmospheric aerosols. To accurately detect the timing of leaf-expansion and leaf-fall, which occur rapidly, and their rates, it is necessary to examine the observational frequency of noise-free satellite-observed vegetation index data during each phenological period. In this study, we investigated the spatio-temporal distribution of the number of observational days (NUMdays) in the Terra/MODIS (Moderate Resolution Imaging Spectroradiometer)-observed daily high-quality normalized difference vegetation index (NDVIhigh) data with no effects of snow cover, cloud contamination, or atmospheric noise. These data were examined for each month over 10 years in the various ecosystems of East Asia. To ground-truth the relationship between the Terra/MODIS-observed daily NDVIhigh data and canopy surface images, we performed a long-term continuous field study in a cool-temperate deciduous broad-leaved forest in central Japan. During the leaf-expansion and leaf-fall periods, the NUMdays for NDVIhigh data in southern Russia, northeastern China, the Tibetan Plateau, Korea, and maritime Japan was about 3–7 for each month. The NUMdays for NDVIhigh data exceeded 10 for each month in arid regions during the growing season and in the subtropical region including northeastern India, Myanmar, and southwestern China during the dry season. In contrast, the NUMdays for NDVIhigh data was almost 0 for each month in southeastern China throughout the year and in the subtropical region during the southeastern monsoon season (July and August). By considering observations from both the Terra/MODIS and Aqua/MODIS satellites, the NUMdays for NDVIhigh data in the deciduous broad-leaved forest in Japan was increased by 40% compared with only Terra/MODIS satellite observations. Our findings indicate that daily NDVI data from multiple satellites detect the seasonal changes in the various ecosystems of East Asia more accurately than 8-day or biweekly composite NDVI data.

Field experiments to test the use of the normalized-difference vegetation index for phenology detection

Some previous studies have detected the timing of leaf expansion and defoliation using the normalized-difference vegetation index (NDVI), but to examine tree phenology using satellite data, NDVI results should be confirmed using ground-truthing. We examined the relationship between NDVI and tree phenology during leaf expansion and defoliation by simultaneously observing the spectral reflectance of the canopy surface and canopy surface images in a cool-temperate deciduous broad-leaved forest. To define the timing of leaf expansion and defoliation using NDVI, the index should meet three criteria: (1) NDVI should exhibit a monotonous increase or decrease (monotonicity). (2) The relationship between NDVI and the forest canopy's status should be unique (uniqueness). (3) The method is robust against the systematic noise (bias) (robustness). In the spring, NDVI values of 0.2–0.3 (relative values: 0.15–0.28) and 0.6–0.7 (relative values: 0.65–0.78) satisfied all three criteria. NDVI values of 0.6–0.7 can serve as potential criteria for detecting the timing of leaf expansion. In autumn, no NDVI values satisfied all three criteria. Thus, NDVI does not appear to be useful for detecting the timing of defoliation. For an area where evergreen vegetation or snow covers the forest floor in winter, our results suggest that previous uses of NDVI to identify the timing of leaf expansion and defoliation on the basis of the date of the maximum rate of growth or reduction of NDVI and the date with a value midway between the year's maximum and minimum values are misleading.

Water vapour isotopic exchange by epiphytic bromeliads in tropical dry forests reflects niche differentiation and climatic signals

The 18O signals in leaf water (delta18O(lw)) and organic material were dominated by atmospheric water vapour 18O signals (delta18O(vap)) in tank and atmospheric life forms of epiphytic bromeliads with crassulacean acid metabolism (CAM), from a seasonally dry forest in Mexico. Under field conditions, the mean delta18O(lw) for all species was constant during the course of the day and systematically increased from wet to dry seasons (from 0 to +6 per thousand), when relative water content (RWC) diminished from 70 to 30%. In the greenhouse, progressive enrichment from base to leaf tip was observed at low night-time humidity; under high humidity, the leaf tip equilibrated faster with delta18O(vap) than the other leaf sections. Laboratory manipulations using an isotopically depleted water source showed that delta18O(vap) was more rapidly incorporated than liquid water. Our data were consistent with a Craig-Gordon (C-G) model as modified by Helliker and Griffiths predicting that the influx and exchange of delta18O(vap) control delta18O(lw) in certain epiphytic life forms, despite progressive tissue water loss. We use delta18O(lw) signals to define water-use strategies for the coexisting species which are consistent with habitat preference under natural conditions and life form. Bulk organic matter (delta18O(org)) is used to predict the deltaO18(vap) signal at the time of leaf expansion.

Improving the budburst phenology subroutine in the forest carbon model PnET

The timing of leaf expansion in spring and leaf senescence in fall determines growing season length; hence leaf phenology is important in modelling carbon production. Previous work monitoring net ecosystem exchange using eddy flux technology found phenological timing to be a key factor determining ecosystem carbon balance. Growing degree days (GDD; the sum of daily mean temperature above 0 °C) have long been used to track tree phenological behavior. Ecosystem carbon balance simulations should employ models that reliably track leaf phenological activities. Our goals for this study were to evaluate the GDD-based phenological algorithm for the forest carbon simulation model PnET and to develop a robust budburst model for the implementation of PnET in the forests of Ohio. We obtained 11 years (1990–2000) of leaf budburst phenology data from Harvard Forest, MA and found large inter-annual variations on the date of budburst. Contrary to the previous sensitivity analysis on GDD, we used the observed range in the data to inform our new GDD sensitivity analyses. In fact the broad inter-annual variation of the budburst phenology had substantial impacts on the annual net primary production. Moreover, GDD did not provide reliable prediction on the date of budburst although it was employed by the PnET family of models. The predictions of budburst dates were largely improved by incorporating a chilling factor, with 16 of 17 deciduous tree species showing significant linear relationships between predicted and observed dates of budburst. However, this updated phenology model showed geographic specificity that is not suitable for our simulations in southern Ohio. For successful development of a more comprehensive phenological model, locally developed and long-term leaf phenology records are required.

Interspecific variation of plant traits associated with resistance to herbivory among four species of Ficus (moraceae).

To understand the defensive characteristics of interspecies varieties and their responses to herbivory damage, four species of Ficus plants (Ficus altissima, F. auriculata, F. racemosa and F. hispida) were studied. They were similar in life form, but differed in successional stages. Of these, Ficus altissima is a late successional species, F. hispida is a typical pioneer and F. auriculata and F. racemosa are intermediate successional species. We addressed the following questions: (1) What is the difference in plant traits among the four species and are these traits associated with differences in herbivory damage levels? (2) What is the difference in the damage-induced changes among the four species? Herbivory damage was measured in the field on randomly planted seedlings of the four species of the same age. Defences to herbivory were also tested by feeding leaves of the four species to larvae of Asota caricae in the laboratory. A total of 14 characters such as water content, thickness, toughness, pubescence density on both sides, leaf expansion time, lifetime and the contents of total carbon (C), nitrogen (N), phosphorous (P), potassium (K), magnesium (Mg) and calcium (Ca) were measured. Leaf calcium oxalate crystal (COC) density, total Ca and N content, leaf toughness and height were measured to investigate induced responses to artificial herbivory among the four species. and conclusions Herbivory damage in the four studied species varied greatly. The pioneer species, F. hispida, suffered the most severe herbivory damage, while the late successional species, F. altissima, showed the least damage. A combination of several characteristics such as high in content of N, Ca and P and low in leaf toughness, lifetime and C : N ratio were associated with increased herbivore damage. The late successional species, F. altissima, might also incorporate induced defence strategies by means of an increase in leaf COC and toughness.

Effects of Temperature and Leaf Position on Leaf Area Expansion of Kiwifruit ( Actinidia deliciosa) Shoots: Development of a Modelling Framework

This paper describes mathematically the effects of temperature and position on the expansion of leaves along a kiwifruit ( Actinidia deliciosa) shoot, taking into account shoot morphology. The leaves were grouped into three zones along the shoot: initial cluster leaves (first zone); the rest of the leaves that were preformed during the previous season (second zone); and leaves that were initiated during the current season (third zone). After opening of the initial cluster, the leaves appeared at constant rates for each of the two temperature treatments considered. The expansion of individual leaves was modelled by a growth function with the parameters: final area; duration of the growth window; centre of the growth window (timing of expansion); and lower asymptote. Within the first two zones, the pattern of leaf expansion was affected by nodal position, with basal leaves having higher initial rates of expansion than distal leaves. The timing of expansion was linear with respect to the nodal position within each of the zones, with the slope independent of temperature for the first zone. The slopes of the timing of expansion for the second and third zones depended on temperature and were correlated for each temperature treatment. Final leaf area was a function of leaf position in the first zone and a function of timing of leaf expansion for distal leaves starting from leaf 10. Temperature had no effect on final leaf area in the first zone. For the rest of the leaves, temperature affected final leaf area indirectly, through the timing of leaf expansion. The effect of temperature on the growth window of individual leaves within the first zone was less than that for the rest of the leaves. However, simulated values for the total leaf area of shoots using the average shoot growth window showed good agreement with experimental values.

Do small leaves expand faster than large leaves, and do shorter expansion times reduce herbivore damage?

Leaves are most vulnerable to herbivory during expansion. We hypothesised that one factor favouring small leaves could be that smaller‐leaved species have shorter expansion times and are therefore exposed to high levels of herbivory for a shorter period than large leaves. In order to test this hypothesis, leaf expansion time and leaf area loss were measured for 51 species from Sydney, Australia. Strong positive correlations were found between leaf length and area and leaf expansion time, confirming that small leaves do expand in a shorter time than large leaves. The amount of leaf area lost was highly variable (from 0.5 to 90% of total leaf area), but was significantly related to both leaf expansion time and log leaf area. The amount of leaf area lost was not significantly correlated with specific leaf area nor with the presence of distasteful substances in the leaves, but was lower on species with hairy expanding leaves.

Seasonal and interannual variability of energy fluxes over a broadleaved temperate deciduous forest in North America

The components of the surface energy balance were measured for 3 years over a broadleaved deciduous forest using the eddy covariance technique. Within years, the magnitude and distribution of fluxes was controlled by seasonal changes in solar radiation, drought, as well as leaf emergence and senescence. Evapotranspiration increased by a factor greater than five (from about 0.5 to 3 mm day −1) after leaves emerged in spring. Large decreases in sensible heat flux were observed over the same period (6 to 2 MJ day −1) despite increases in solar radiation. The most influential effect on annual fluxes was the occurrence and extent of drought, with lesser control exerted by differences in the timing of leaf expansion and leaf senescence. Average annual evapotranspiration over the period was 567 mm and ranged from 537 to 611 mm. The year with the lowest precipitation, soil moisture content and surface conductance also had the lowest evapotranspiration. Although evapotranspiration was quite sensitive to surface conductance and surface conductance was reduced substantially by drought, the correlation of low surface conductance and high humidity deficit reduced the effects of drought on evapotranspiration. Differences in net radiation among years were only a minor source of variability in evapotranspiration. In addition to surface conductance, other bulk parameters are calculated to describe the general exchange characteristics of this forest.

Exchange of Carbon Dioxide by a Deciduous Forest: Response to Interannual Climate Variability

The annual net uptake of CO 2 by a deciduous forest in New England varied from 1.4 to 2.8 metric tons of carbon per hectare between 1991 and 1995. Carbon sequestration was higher than average in 1991 because of increased photosynthesis and in 1995 because of decreased respiration. Interannual shifts in photosynthesis were associated with the timing of leaf expansion and senescence. Shifts in annual respiration were associated with anomalies in soil temperature, deep snow in winter, and drought in summer. If this ecosystem is typical of northern biomes, interannual climate variations on seasonal time scales may modify annual CO 2 exchange in the Northern Hemisphere by 1 gigaton of carbon or more each year.

The dynamics of photosynthetic acclimation to changes in light quanlity and quality in three Australian rainforest tree species.

Photosynthetic acclimation was studied in seedlings of three subtropical rainforest species representing early (Omalanthus populifolius), middle (Duboisia myoporoides) and late (Acmena ingens) successional stages in forest development. Changes in the photosynthetic characteristics of pre-existing leaves were observed following the transfer of plants between deep shade (1-5% of photosynthetically active radiation (PAR), selectively filtered to produce a red/far-red (R/FR) ratio of 0.1) and open glasshouse (60% PAR and a R/FR ratio of 1.1-1.2), and vice versa. The extent and rate of response of the photosynthetic characteristics of each species to changes in light environment were recorded in this simulation of gap formation and canopy closure/overtopping. The light regimes to which plants were exposed produced significant levels of acclimation in all the photosynthetic parameters examined. Following transfer from high to low light, the light-saturated rate of photosynthesis was maintained near pre-transfer levels for 7 days, after which it decreased to levels which closely approximated those in leaves which had developed in low light. The decrease in photosynthetic capacity was associated with lower apparent quantum yields and stomatal conductances. Dark respiration was the parameter most sensitive to changes in light environment, and responded significantly during the first 4-7 days after transfer. Acclimation of photosynthetic capacity to increases in irradiance was significant in two of the three species studied, but was clearly limited in comparison with that of new leaves produced in the high light conditions. This limitation was most pronounced in the early-successional-stage species, O. populifolius. It is likely that structural characteristics of the leaves, imposed at the time of leaf expansion, are largely responsible for the limitations in photosynthetic acclimation to increases in irradiance.

Asynchronous leaf expansion and shedding in a seasonal environment: Result of a competitive game

A game model was developed to analyze the difference in the timing of leaf expansion and shedding among trees co-occurring in single forests in a seasonal environment. It was assumed that the photosynthetic rate of a tree is affected by the shading effect of its neighboring trees and by the seasonal change in environmental factors.The analyses showed that, if the photosynthetic rate of the tree concerned does not take an unfavorable turn due to the leaf expansion of its neighboring trees, all the trees expand their leaves at the same time when the photosynthetic rate and the maintenance cost balance. However, if the photosynthetic rate of the tree takes an unfavorable turn due to the leaf expansion of its neighboring trees, timing of leaf expansion does not synchronize among trees; instead leafing occurs continuously from the time when the photosynthetic rate and the maintenance cost balance under no-shading by the neighboring trees to the time when these factors balance under full-shading by neighboring trees. In turn, at the end of the growing season, if the photosynthetic rate of the tree does not take a favorable turn due to the leaf shedding of its neighboring trees, all trees shed their leaves at the same time when the photosynthetic rate and the maintenance cost balance. However, if the photosynthetic rate of then tree takes a favorable turn due to the leaf shedding of its neighboring trees, timing of leaf shedding does not synchronize among trees; instead shedding occurs continuously from the time when the photosynthetic rate and the maintenance cost balance under full-shading by the neighboring trees to the time when these factors balance under no-shading by the neighboring trees.

Phyllotaxy and Stage of Leaf and Fruit Development Influence Initiation and Direction of Carbohydrate Export from Sour Cherry Leaves

Abstract The influence of phyllotaxy and stage of leaf and fruit development on the initiation and direction of carbohydrate (CH2O) export from sour cherry (Prunus cerasus L.) leaves was investigated during two different seasons. One-year-old ‘Montmorency’ sour cherry trees on ‘Mahaleb’ rootstock were pruned to a single shoot, the seventh and 10th leaf (from the base) were pulsed with 14CO2, and labeled carbon products were located after 24 hr using autoradiography. In 1983, gross export (EG) from the seventh and 10th leaf was initiated when the area of the seventh leaf reached 8.5 cm2 (27% expansion) and when that of the 10th leaf reached 14 to 21 cm2 (48% to 72% expansion), respectively. EG was generally initiated later in 1985 than in 1983, for the seventh leaf later than 26 cm2 (47% expansion) and for the 10th leaf at 36 cm2 (78% expansion). Leaf size was greater at full expansion in 1985 than in 1983. On defoliated shoots, the 10th leaf (54 ± 16 cm2 full expansion) started export between 24 and 28 cm2 (44% to 52% expansion), whereas control leaves on nondefoliated shoots started export at 36 cm2 (78% expansion). We suggest that, after a leaf has developed the potential for phloem loading, the onset of CH2O export is a function of the CH2O availability in the plant at the time of leaf expansion. Translocation paths followed closely the orthostichy of the exporting leaf. Fruit effects on the direction of translocation were studied in 2-year-old trees. During stages I and III of fruit development, leaves closest to the base showed basipetal translocation only. All leaves during stage II and leaves distal to the midpoint of the shoot during stages I and III showed bidirectional translocation to the shoot apex and the fruits.

Optimal Timing of Leaf Expansion and Shedding in a Seasonally Varying Environment1)

Abstract Optimal utilization of a seasonally varying environment by deciduous trees is studied by analyzing two mathematical models. Both models are constructed from the viewpoint of carbon economy. First, optimal times for leaf expansion and shedding in the seasonal environment that maximize the total net gain of photosynthelic products are obtained. We show that both leaf ageing and leaf mortality select for delayed leaf expansion. We suggest that the delay of leaf expansion for canopy trees is important in allowing spring ephemeral species to survive in the temperate forest floor. Second, a two‐layer model, in which the lower layer of leaves is shaded by the upper, is analyzed to study the effect of self‐shading on the optimal pattern of leaf expansion. We show that the succeeding expansion of leaves is more advantageous than flushing expansion, even in the situation where all leaves are constructed by the products in the previous year.