A preliminary study on the photosynthetic induction response ofQuercus serrata seedlings

To understand the sunfleck utilization of leaves in heliophilic trees within grass canopies, we studied the photosynthetic induction response ofQuercus serrata seedlings grown for 5 months in different microsites in aMiscanthus sinensis canopy. Two phases, a rapid increase in CO₂ uptake and a following slow increase, were recognizabie in the time course of CO₂ uptake in response to an increase of photon flux densities (PFD). When the preceding period of low light became shorter, the period of the two phases became shorter. The capacity of response to a sudden light increase was evaluated by the relative photosynthetic induction efficiency (RPIE) defined as the ratio of integrated carbon gain measured to that calculated by assuming that a steady-state assimilation would be achieved instantaneously after the light increase. RPIEs estimated were negatively related to potential sunfleck PFD of microsite. The leaves of the seedlings grown in the microsite with a lower sunfleck PFD and a shorter sunfleck duration showed a more rapid response to a sudden increase of light. These findings suggest that the leaves ofQ. serrata seedlings growing under a lower sunfleck PFD are able to increase the photosynthetic capacity more rapidly to an increase of PFD.     

玉米冠层内太阳直接辐射三维空间分布的模拟

太阳直接辐射在植物冠层内的空间分布特征影响植物生理生态功能 ,是衡量植物群体结构是否合理的重要指标。利用田间实测的玉米冠层内植株各器官的三维空间坐标进行冠层结构分析 ,将冠层内的植株器官表面划分成小面元 ;根据几何光学中光的直线传播原理 ,利用面元沿太阳光线的平行投影和投影深度排序 (Z- buffer)算法计算冠层内面元受太阳光直接照射的情况 ,建立了太阳直接辐射在玉米冠层内三维空间分布的模拟模型。模型可计算出作物冠层内任选植株的器官表面或冠层内地面上的太阳直射光斑 (Sunflecks)分布 ,也可输出选定空间位置或范围上的太阳直接辐射的分布 ,同时可实现模拟结果的三维可视化。根据此模型的模拟结果可对太阳直接辐射在玉米冠层内的空间分布进行各种分析。利用玉米冠层内光斑的三维分布测定试验 ,在光合有效辐射 (PAR)波段对模型进行了检验。模型适用于任意三维结构可测并可进行面元化划分的植物群体或个体     

Measuring and simulating crown respiration of Scots pine with increased temperature and carbon dioxide enrichment

Acclimation to elevated atmospheric carbon dioxide concentration and temperature of respiration by the foliage in the crown of Scots pine (Pinus sylvestris) trees is measured and modelled. Starting in 1996, individual 20-year-old trees were enclosed in chambers and exposed to either normal ambient conditions (CON), elevated CO2 concentration (EC), elevated temperature (ET) or a combination of EC and ET (ECT). Respiration of individual leaves within the crown was measured in 2000. To extrapolate the response of respiration of individual leaves to the whole crown, a multi-layer model was developed and used to predict daily and annual crown respiration, in which the crown structure and corresponding microclimate data were used as input. Respiration measurements showed that EC led to higher Q10 values (4.6%) relative to CON, but lower basal respiration rates at 20 degrees C [R1.d(20)] (-7.1%) during the main growth season (days 120-240), whereas ET and ECT both reduced Q10 (-12.0 and -9.8%, respectively) throughout the year but increased R1.d(20) (27.2 and 21.6%, respectively) during the period of no-growth, and slightly reduced R1.d(20) (-1.7 and -2.8%, respectively) during the main growth season. Model computations showed that annual crown respiration increased: (1) by 16% in EC, with 92% of this increase attributable to the increase in foliage area; (2) by 35% in ET, with 66% related to the increase in foliage area and 17% to the rise in ambient temperature; and (3) by 27% in the case of ECT, with 43% attributable to the increase in foliage area and 29% to the rise in ambient temperature. Changed respiration parameters for individual leaves, induced by treatments, made only a small contribution to the annual crown respiration compared with the increased foliage area. The effects of changes in crown architecture and nitrogen distribution, caused by treatments, on the daily and annual course of crown respiration are discussed.     

The effects of light acclimation during and after foliage expansion on photosynthesis ofAbies amabilis foliage within the canopy

Variation in the photosynthetic function ofAbies amabilis foliage within a canopy was examined and related to three different processes that affect foliage function: foliage aging, sun-shade acclimation that occurred while foliage was expanding, and reacclimation after expansion was complete. Foliage produced in the sun had higher photosynthesis at light saturation (A _max, mol·m^-2·s^-1), dark respiration (mol·m^-2·s^-1), nitrogen content (g·m^-2), chlorophyll content (g·m^-2), and chlorophylla:b ratio, and a lower chlorophyll to nitrogen ratio (chl:N), than foliage produced in the shade. As sun foliage becomes shaded, it becomes physiologically similar to shade foliage, even though it still retains a sun morphology. Shaded sun foliage exhibited lowerA _max, dark respiration, nitrogen content, and chlorophylla:b ratio, and a higher chl:N ratio than sun foliage of the same age remaining in the open. However, shaded sun foliage had a higher chlorophyll content than sun foliage remaining in the open, even though true shade foliage had a lower chlorophyll content than sun foliage. This anomaly arises because as sun foliage becomes shaded, it retains a higher nitrogen content than shade foliage in a similar light environment, but the two forms have similar chl:N ratios. Within the canopy, most physiological indicators were more strongly correlated with the current light environment than with foliage age or leaf thickness, with the exception of chlorophyll content.A _max decreased significantly with both decreasing current light environment of the foliage and increasing foliage age. The same trend with current light and age was found for the chlorophylla:b ratio. Foliage nitrogen content also decreased with a decrease in current light environment, but no distinct pattern was found with foliage age. Leaf thickness was also important for predicting leaf nitrogen content: thicker leaves had more nitrogen than thinner leaves regardless of light environment or age. The chl:N ratio had a strong negative correlation with the current light environment, and, as with nitrogen content, no distinct pattern was found with foliage age. Chlorophyll content of the foliage was not well correlated with any of the three predictor variables: current light environment, foliage age or leaf thickness. On the other hand, chlorophyll content was positively correlated with the amount of nitrogen in a leaf, and once nitrogen was considered, the current light environment was also highly significant in explaining the variation in chlorophyll content. It has been suggested that the redistribution of nitrogen both within and between leaves is a mechanism for photosynthetic acclimation to the current light environment. Within theseA. amabilis canopies, both leaf nitrogen and the chl:N ratio were strongly correlated with the current light environment, but only weakly with leaf age, supporting the idea that changing light is the driving force for the redistribution of nitrogen both within and between leaves. Thus, our results support previous theories on nitrogen distribution and partitioning. However,A _max was significantly affected by both foliage age and the current light environment, indicating that changes in light alone are not enough to explain changes inA _max with time.     

Leaf Development, Metamorphic Heteroblasty and Heterophylly in Berberis s. l. (Berberidaceae)

Shoot development of temperate and tropical members of Berberis s. l. was examined in order to assess: (1) the homology of the spines along the long shoots and the foliage leaves that form on the short shoots; (2) the occurrence of heterophylly and/or heteroblasty in the genus; and (3) the structural correspondence between cataphylls, spines, and foliage leaves. The 1-5-armed spines have been interpreted as modified compound leaves lacking stipules, as a modified lamina (central spine) with stipules (lateral spines), or less often, as transformed branches, or as epidermal outgrowths. On the other hand, the foliage leaves of the short shoots have been interpreted as leaflets of palmately compound leaves. Our results indicate that there are three distinct leaf types per node: (1) Leaves modified in spines spirally arranged in long shoots; (2) foliage, expanded leaves densely arranged in short shoots; and (3) cataphylls protecting axillary buds. The spines are leaf homologs with a clear distinction between the leaf base with stipules, and a laminar portion modified into the 1-5-armed spine; the lateral spines are not stipules as they arise from the marginal meristem of the laminar portion, and not from the leaf base. The foliage leaves also have stipules flanking the leaf base. Both spiny leaves and foliage leaves develop an articulation between the base and the laminar portion. Cataphylls of the short shoots of Berberis s. str. and those of the reproductive short shoots of Mahonia correspond to the entire leaf base, but those of the renewal (vegetative) shoots of Mahonia are spiny and have an odd vestigial pinnately compound lamina. Heterochrony due to ontogenetic truncation caused by the formation of the terminal inflorescence at the apex of the short shoots could be responsible for the lack of petiole/lamina differentiation in the foliage leaves. The spiny long-shoot/foliose short-shoot system of branching in Berberis s. str. appears to be genetically and phylogenetically fixed and not environment-dependent. This represents a clear example of metamorphic heteroblasty sensu Zotz et al. (Botanical Review 77:109–151, 2011) with further occurrence of heterophylly along the short shoots.     

Modeling the vertical foliage distribution of an individual <Emphasis Type="Italic">Castanopsis cuspidata</Emphasis> (Thunb.) Schottky,a dominant broad-leaved tree in Japanese warm-temperate forest

The vertical foliage distribution of Castanopsis cuspidata (Thunb.) Schottky was examined in trees of various sizes to clarify its variation in relation to tree size and the light environment in a stand. As indices of these parameters, we analyzed crown social position (CSP: percent of stand height) and specific leaf area (SLA). The vertical foliage distribution of trees was expressed by a Weibull function. The variation in the vertical foliage distribution of C. cuspidata could be categorized into three types using crown social position and light environment. In the first type, leaves were concentrated to the top 20% of the tree; such trees are canopy trees that can receive full sunlight. The second type had a large relative crown depth and an asymmetric distribution with the maximum foliage located near the top of the tree; such trees are suppressed trees whose crowns do not receive sufficient light. The third type had a large relative crown depth and a symmetric distribution; such trees occur in high light environments, although their crowns are in the understory layer. The differences in the vertical foliage distribution are related to the strategies used to capture light. Multiple regression analysis showed that CSP and SLA at the top layer of the tree explained successive changes in the vertical foliage distribution. These results will contribute to scaling-up the vertical foliage distribution to the community level in pure stands of C. cuspidata using an individual-based model.     

Influence of sunflecks on the temperature and water relations of two subalpine understory congeners

Summary The temperature and water relations of the herbaceous, understory, congeners Arnica cordifolia and Arnica latifolia were evaluated in relation to the sunfleck dynamics of their respective microhabitats. Arnica cordifolia microhabitats had more frequent, longer, and more intense sunflecks than those of A. latifolia which led to higher leaf temperatures (31°C versus 15°C) and transpirational fluxes (65 g cm^-2 s^-1 versus 16 g cm^-2 s^-1). Stomatal closure did not occur in response to high leaf temperatures and low stem water potentials during natural sunfleck exposures, even though plants were observed to wilt during midday, especially A. cordifolia. Experimentally, an artificial midday sunfleck of about 165 min caused plants of A. cordifolia not to regain turgor after 8 h in shade compared to a sunfleck duration of about 90 min for plants of A. latifolia. However, these sunfleck intervals occurred naturally only during the early morning and late afternoon when solar intensities were minimal. Also, A. cordifolia populations had over twice as many plants that were sunlit (>40% of total) compared with A. latifolia (<20% of total) at any particular time during a day. The small-scale distribution of both species appears tightly coupled to the sunfleck dynamics of their respective microhabitats due to the lack of stomatal action which would reduce transpiration and improve plant water status under sunlit conditions.     

DEVELOPMENTAL CYCLES IN SHOOT GROWTH OF MALE CYCAS CIRCINALIS

Cycles of development in several male shoots of Cycas circinalis were studied by counting sequences of foliage leaves, scale leaves, and cones by observation of either their scar patterns or vascular remains. Although male cones are always produced subsequent to a cycle of foliage leaves and are fairly regularly spaced there is no obvious correlation between their presence and the previous cycles of leaves. Ratios of foliage leaves to scale leaves in each cycle are variable. The primary diameter of the axis is obconical, but a conical stem form results from secondary growth. The result of concentric development of xylem and phloem is positively correlated with stem diameter rather than stem age.     

Bi-directional acclimation of <Emphasis Type="Italic">Cycas micronesica</Emphasis> leaves to abrupt changes in incident light in understory and open habitats

Leaf gas-exchange responses to shadefleck–sunfleck and sun–cloud transitions were determined for in situ Cycas micronesica K.D. Hill plants on the island of Guam to add cycads to the published gymnosperm data. Sequential sunfleck–shadefleck transitions indicated understory leaves primed rapidly but open field leaves primed slowly. Time needed to reach 90% induction of net CO₂ assimilation (P_N) was 2.9 min for understory leaves and 13.9 min for open field leaves. Leaf responses to sun–cloud transitions exhibited minimal adjustment of stomatal conductance, so P_N rapidly returned to precloud values following cloud–sun transitions. Results indicate bi-directional leaf acclimation behavior enables mature C. micronesica trees to thrive in deep understory conditions in some habitats and as emergent canopy trees in other habitats. These data are the first nonconifer gymnosperm data; the speed of gas-exchange responses to rapid light transitions was similar to some of the most rapid angiosperm species described in the literature.     

Seasonal patterns in species richness of herbivores: Macrolepidopteran larvae on Finnish deciduous trees

Abstract. 1. The seasonal distribution of macrolepidopteran species richness on Finnish deciduous trees vaned from positively skewed (peak in spring) to negatively skewed (peak in autumn).
2. The skewness values of species richness had a significant negative correlation ( r = - 0.98) with the duration of the seasonal shoot-growth period of the tree species.
3. Trees which complete their shoot growth early in the season ( Quercus type) produce new leaves only during spring, while trees whose shoot growth continues to autumn ( Populus type) do so throughout the summer.
4. Consequently, there is a difference in the number of available resources in the late summer foliage of different tree species, Trees ceasing leaf production early such as oak ( Quercus robur ) and bird cherry ( Prunus padus ) have one major resource type (mature leaves) in late-season foliage while trees like birches and alders have two Ooung and mature leaves).
5. Because young leaves formed late in the season are preferred to mature ones by some species of herbivores and because other species prefer mature leaves at the same time, the species richness of Populus-type trees is higher later in the season than the species richness of Quercus-type of trees, which have just one type of resource available.     

Heteroblasty in Arabidopsis thaliana (L.) Heynh

 Heteroblasty in Arabidopsis thaliana was analyzed in a variety of plants with mutations in leaf morphology using a tissue-specific β-glucuronidase gene marker. Some mutants exhibited their mutant phenotypes specifically in foliage leaves. The phenotypes associated with the foliage-leaf-specific mutations were also found to be induced ectopically in cotyledons in the presence of the lec1 mutation. Moreover, the features of an emf1lec1 double mutant showed that cotyledons can be partially converted into carpelloids. When heteroblastic traits were examined in foliage leaves in the presence of certain mutations or natural deviations by histochemical analysis of the expression of the tissue-specific marker gene, it was found that ectopic expression of the developmental program for the first foliage leaves in lec1 cotyledons seemed to affect the heteroblastic features of the first set of foliage leaves, while foliage leaves beyond the third position appeared normal. Similarly, in wild-type plants, discrepancies in heteroblastic features, relative to standard features, of foliage leaves at early positions seemed to be eliminated in foliage leaves at later positions. These results suggest that heteroblasty in foliage leaves might be affected in part by the heteroblastic stage of the preceding foliage leaves but is finally controlled autonomously at each leaf position. Received: 9 July 1999 / Accepted: 17 August 1999     

Opposing effects of spring defoliation on late season oak caterpillars

ABSTRACT. 1. The pedunculate oak, Quercus rohur L., suffers high annual levels of spring defoliation in Wytham Woods. near Oxford.
2. This spring defoliation affects late season caterpillars through a variety of damage-induced changes in the leaves.
3. Diurnea fagella (D. & S.), one of the commonest late season caterpillars, shows reduced larval survival and pupal weight on regrowth foliage when compared to undamaged primary foliage.
4. D. fagella also suffer higher larval mortality on naturally damaged primary foliage than they do on undamaged foliage.
5. Despite this, the three commonest late season caterpillar species are more abundant on damaged trees than undamaged ones. and their distributions are biased towards damaged leaves within the canopy.
6. Other factors that may be more important than leaf damage in determining the distribution and abundance of late season caterpillars are discussed. D.fagella larvae spin their larval refuges more rapidly on damaged than undamaged foliage, and this may reduce mortality by natural enemies, or ameliorate adverse effects of weather.     

INTERACTIONS BETWEEN CROWN STRUCTURE AND LIGHT ENVIRONMENT IN FIVE RAIN FOREST PIPER SPECIES

Measurements of light variation among leaves within crowns of five Piper species were compared with estimates of spatial variation in light within understory, forest edge, and clearing habitats to estimate the extent to which crown structure contributes to variation in leaf light environment. Daily photon flux density (PFD) varied greatly within and among crowns. Coefficients of variation for daily PFD among sensors within a single crown ranged from 26 to 79%. Within a single crown located in a clearing, the range in daily PFD among leaves was nearly as great as the range over the entire sample of plants. In the understory, localized sunfleck activity contributed to a high degree of spatial variation in instantaneous and total PFD among leaves within individual crowns. Much of the microsite variation in sunfleck activity, however, reflected environmental conditions within the understory habitat. Within an array of sensors placed next to Piper crowns in the understory, correlations were poor for light sensors spaced only 0.2 m apart, and only 8% of the variance in light readings was explained by measurements made 0.5 m away. In the clearing habitats, microsite heterogeneity among leaves was more strongly influenced by leaf positions within crowns and leaf angles than by spatial heterogeneity within the habitat.     

Acclimatory responses of Arabidopsis to fluctuating light environment: comparison of different sunfleck regimes and accessions

Acclimation to fluctuating light environment with short (lasting 20?s, at 650 or 1,250?μmol photons m(-2)?s(-1), every 6 or 12?min) or long (for 40?min at 650?μmol photons m(-2)?s(-1), once a day at midday) sunflecks was studied in Arabidopsis thaliana. The sunfleck treatments were applied in the background daytime light intensity of 50?μmol photons m(-2)?s(-1). In order to distinguish the effects of sunflecks from those of increased daily irradiance, constant light treatments at 85 and 120?μmol photons m(-2)?s(-1), which gave the same photosynthetically active radiation (PAR) per day as the different sunfleck treatments, were also included in the experiments. The increased daily total PAR in the two higher constant light treatments enhanced photosystem II electron transport and starch accumulation in mature leaves and promoted expansion of young leaves in Columbia-0 plants during the 7-day treatments. Compared to the plants remaining under 50?μmol photons m(-2)?s(-1), application of long sunflecks caused upregulation of electron transport without affecting carbon gain in the form of starch accumulation and leaf growth or the capacity of non-photochemical quenching (NPQ). Mature leaves showed marked enhancement of the NPQ capacity under the conditions with short sunflecks, which preceded recovery and upregulation of electron transport, demonstrating the initial priority of photoprotection. The distinct acclimatory responses to constant PAR, long sunflecks, and different combinations of short sunflecks are consistent with acclimatory adjustment of the processes in photoprotection and carbon gain, depending on the duration, frequency, and intensity of light fluctuations. While the responses of leaf expansion to short sunflecks differed among the seven Arabidopsis accessions examined, all plants showed NPQ upregulation, suggesting limited ability of this species to utilize short sunflecks. The increase in the NPQ capacity was accompanied by reduced chlorophyll contents, higher levels of the xanthophyll-cycle pigments, faster light-induced de-epoxidation of violaxanthin to zeaxanthin and antheraxanthin, increased amounts of PsbS protein, as well as enhanced activity of superoxide dismutase. These acclimatory mechanisms, involving reorganization of pigment-protein complexes and upregulation of other photoprotective reactions, are probably essential for Arabidopsis plants to cope with photo-oxidative stress induced by short sunflecks without suffering from severe photoinhibition and lipid peroxidation.     

Heteroblasty and preformation in mayapple, Podophyllum Peltatum (Berberidaceae): developmental flexibility and morphological constraint

Developmental preformation can constrain growth responses of shoots to current conditions, but there is potential for flexibility in development preceding formation of the preformed organs. Mayapple (Podophyllum peltatum) is strongly heteroblastic, producing rhizome scales, bud scales, and either a single vegetative foliage leaf or two foliage leaves on a sexual shoot. To understand how and when preformation constrains growth responses, we compare (1) how leaf homologs of the renewal shoot differ in development, (2) whether there are differences in shoot development that occur in advance of morphological determination of shoot type, and (3) whether there are points of developmental flexibility in renewal shoot growth prior to preformation of the foliage and floral organs. We use scanning electron microscopy and histology to show that the three vegetative leaves (both types of scale leaves and the vegetative foliage leaf) are similar in the initial establishment of an encircling and overarching leaf base. Differences among them are found in the timing of differentiation of the leaf base and in the relative timing and degree of growth of the lamina and petiole. In contrast, foliage leaves on sexual shoots show less expression of the leaf base and precocious growth of the lamina and petiole. Prior to shoot type determination, there are no morphological differences in the sequence or position of leaf homologs that predict final shoot type. In this colony, leaves at positions 12 and 13, on average, appear to be identical in development until they are between 700 and 800 μm in length, when it becomes possible to distinguish leaves that will become vegetative foliage leaves from additional bud scale leaves on vegetative or sexual shoots. We suggest that late developmental determination of leaves at positions 12 and 13 reflects ontogenetic sensitivity to a transition to flowering. Thus, in mayapple, heteroblasty appears to facilitate developmental flexibility prior to the point where shoot growth becomes constrained by preformation of determined aerial structures.     

Leaf Development in Narcissus pseudonarcissus L.: With twenty Figures in the Text: The Comparative Development of Scale and Foliage Leaves

Details are given of the distribution of cell division and cellelongation in various tissues of the daffodil leaf. The development of the vascular system is also described, andrelated to the intercalary growth of the leaf. The productionof a new longitudinal vascular strand appears to be determinedby the number of cells between the existing strands. The scale and foliage leaves appear to originate from similarprimordia. Their developments diverge when they are about 1mm. long; a scale leaf is developed where most cell divisionsoccur in the sheath, and a foliage leaf is formed where thereis a region of more rapid cell division at the base of the blade.     

Light regime in relation to plant population geometry

A beam-emission experiment using a Monte Carlo technqiue was attempted in a computer model for the random distribution (RD) of foliage. The RD foliage consisted of 500 leaves of the same size, their individual areas being just 1/100 of the unit land area; i.e., a leaf area index (LAI) of RD foliage was equal to 5. Satisfactory agreement of light transmission under a uniform overcast sky was obtained between the observed values in the RD foliage and theoretical anticipations for any leaf inclination angle and/or leaf area density. This result demonstrated the usefulness of the Monte Carlo technique adopted in this experiment. The Monte Carlo experiment was also applied to a patch-like population. For such a foliage, additional effect of lateral incidence were worth noting in contrast with the microclimates in the corresponding infinite foliage. For every leaf inclination angle the mean light flux density below the middle stratum within a patch-like population was not further attenuate with increasing LAI.     

Stomatal Control and Leaf Thermal and Hydraulic Capacitances under Rapid Environmental Fluctuations

Leaves within a canopy may experience rapid and extreme fluctuations in ambient conditions. A shaded leaf, for example, may become exposed to an order of magnitude increase in solar radiation within a few seconds, due to sunflecks or canopy motions. Considering typical time scales for stomatal adjustments, (2 to 60 minutes), the gap between these two time scales raised the question whether leaves rely on their hydraulic and thermal capacitances for passive protection from hydraulic failure or over-heating until stomata have adjusted. We employed a physically based model to systematically study effects of short-term fluctuations in irradiance on leaf temperatures and transpiration rates. Considering typical amplitudes and time scales of such fluctuations, the importance of leaf heat and water capacities for avoiding damaging leaf temperatures and hydraulic failure were investigated. The results suggest that common leaf heat capacities are not sufficient to protect a non-transpiring leaf from over-heating during sunflecks of several minutes duration whereas transpirative cooling provides effective protection. A comparison of the simulated time scales for heat damage in the absence of evaporative cooling with observed stomatal response times suggested that stomata must be already open before arrival of a sunfleck to avoid over-heating to critical leaf temperatures. This is consistent with measured stomatal conductances in shaded leaves and has implications for water use efficiency of deep canopy leaves and vulnerability to heat damage during drought. Our results also suggest that typical leaf water contents could sustain several minutes of evaporative cooling during a sunfleck without increasing the xylem water supply and thus risking embolism. We thus submit that shaded leaves rely on hydraulic capacitance and evaporative cooling to avoid over-heating and hydraulic failure during exposure to typical sunflecks, whereas thermal capacitance provides limited protection for very short sunflecks (tens of seconds).     

Induced sink strength as a prerequisite for induced tannin biosynthesis in developing leaves of Populus

Induced defenses occur predominately in young, developing plant tissues that rely upon carbohydrate import to support their growth and development. To test the hypothesis that the induced production of carbon-based defenses is dependent upon photoassimilate import, we examined the response of developing leaves of hybrid poplar (Populus deltoides 2 P. nigra) saplings to wounding by gypsy moth caterpillars (Lymantria dispar L.) and exogenous jasmonic acid (JA). Growth rates, condensed tannin contents and acid invertase activities were measured for individual leaves and the translocation of ¹³C-labeled resources between orthostichous source-sink pairs was quantified. Results showed a substantial increase in the activity of cell wall invertase in sink leaves wounded by gypsy moth caterpillars and treated with JA. JA-induced sink leaves also imported 3-4 times as much ¹³C-labeled carbon from orthostichous source leaves relative to controls and allocated a significant portion of this imported ¹³C to condensed tannin biosynthesis. Reduced carbohydrate flow to these leaves, caused by source leaf removal, resulted in reduced condensed tannin levels and the emergence of a growth-defense tradeoff. These results indicate that (1) induced sink strength is elicited by insect wounding and JA application in hybrid poplar foliage, (2) imported resources are allocated to the production of carbon-based defenses, and (3) the level of induced defense in leaves can be constrained by the ability of leaves to import carbohydrates from source tissues. Together, these results suggest that within-canopy variations in induced resistance may arise in part because of uneven distribution of resources to induced foliage.     

Photosynthetic responses to light variation in rainforest species

Summary The dependence of net carbon gain during lightflecks (artificial sunflecks) on leaf induction state, lightfleck duration, lightfleck photosynthetic photon flux density (PFD), and the previous light environment were investigated in A. macrorrhiza and T. australis, two Australian rainforest species. The photosynthetic efficiency during lightflecks was also investigated by comparing observed values of carbon gain with predicted values based on steady-state CO₂ assimilation rates. In both species, carbon gain and photosynthetic efficiency increased during a series of five 30-or 60-s lightflecks that followed a long period of low light; efficiency was linearly related to leaf induction state.In fully-induced leaves of both species, efficiency decreased and carbon gain increased with lightfleck duration. Low-light grown A. macrorrhiza had greater efficiency than predicted based on steady-state rates (above 100%) for lightflecks less than 40 s long, whereas leaves grown in high light had efficiencies exceeding 100% only during 5-s lightflecks. The efficiency of leaves of T. australis ranged from 58% for 40-s lightflecks to 96% for 5-s lightflecks.In low-light grown leaves of A. macrorrhiza, photosynthetic responses to lightflecks below 120 mol m^-2 s^-1 were not affected significantly by the previous light level. However, during lightflecks at 530 mol m^-2 s^-1, net carbon gain and photosynthetic efficiency of leaves previously exposed to low light levels were significantly reduced relative to those of leaves previously exposed to 120 and 530 mol m^-2 s^-1.These results indicate that, in shade-tolerant species, net carbon gain during sunflecks can be enhanced over values predicted from steady-state CO₂ assimilation rates. The degree of enhancement, if any, will depend on sunfleck duration, previous light environment, and sunfleck PFD. In forest understory environments, the temporal pattern of light distribution may have far greater consequences for leaf carbon gain than the total integrated PFD.Supported by National Science Foundation Grant BSR 8217071 and USDA Grant 85-CRCR-1-1620