Leaf lifespan and lifetime carbon balance of individual leaves in a stand of an annual herb, Xanthium canadense

Changes in leaf lifespan, nitrogen resorption, and mean residence time of leaf nitrogen along a soil fertility gradient in an evergreen oak tree

The ability of plants to retain nitrogen (N) for a long period of time is critical to their N use efficiency, growth, and fitness, particularly in infertile environments. The mean residence time of leaf N (MRTL) and its two determinants, leaf lifespan and N resorption efficiency (rN, the fraction of the total leaf N pool that is resorbed during leaf senescence), have been hypothesized to increase plastically with decreasing soil N fertility but this remains to be fully tested. To avoid confusion by random changes in these characteristics in a relatively narrow N fertility range, MRTL, leaf lifespan, and N resorption efficiency were measured in Quercus glauca over a broad N fertility range. In the high to moderate N fertility range, leaf lifespan and rN increased with decreasing N addition rate, and thus the MRTL increased. However, in the moderate to low N fertility range, leaf lifespan increased but rN decreased significantly, so MRTL decreased. The decrease in rN occurred because the senesced leaf N concentration was almost constant at the lower limit while the green leaf N concentration decreased in this range. The hump-shaped quadratic responses of MRTL and rN along the N fertility gradient suggest that incorrect conclusions about the response of these traits to N fertility variation may be drawn from experiments that include only a few fertility levels, and N recycling within leaf canopy alone cannot achieve efficient N use in infertile environments.

Individual architecture and photosynthetic performance of the submerged form of Drosera intermedia Hayne

Drosera intermedia grows in acidic bogs in parts of valleys that are flooded in winter, and that often dry out in summer. It is also described as the sundew of the most heavily hydrated habitats in peatlands, and it is often found in water and even underwater. This sundew is the only one that can tolerate long periods of submersion, and more importantly produces a typical submerged form that can live in such conditions for many years. Submerged habitats are occupied by D. intermedia relatively frequently. The aim of the study was to determine the environmental conditions and architecture of individuals in the submerged form of D. intermedia. The features of the morphological and anatomical structure and chlorophyll a fluorescence of this form that were measured were compared with analogous ones in individuals that occurred in emerged and peatland habitats. The submerged form occurred to a depth of 20 cm. Compared to the other forms, its habitat had the highest pH (4.71-4.92; Me = 4.71), the highest temperature and substrate hydration, and above all, the lowest photosynthetically active radiation (PAR; 20.4-59.4%). This form differed from the other forms in almost all of the features of the plant's architecture. It is particularly noteworthy that it had the largest main axis height among all of the forms, which exceeded 18 cm. The number of living leaves in a rosette was notable (18.1 ± 8.1), while the number of dead leaves was very low (6.9 ± 3.8). The most significant differences were in the shape of its submerged leaves, in which the length of the leaf blade was the lowest of all of the forms (0.493 ± 0.15 mm; p < 0.001) and usually the widest. The stem cross-sectional area was noticeably smaller in the submerged form than in the other forms, the xylem was less developed and collaterally closed vascular bundles occurred. Our analysis of the parameters of chlorophyll fluorescence in vivo revealed that the maximum quantum yield of the primary photochemistry of photosystem II is the highest for the submerged form (Me = 0.681), the same as the maximum quantum yield of the electron transport (Me φE0 = 0.183). The efficiency of energy use per one active reaction center of photosystem II (RC) was the lowest in the submerged form (Me = 2.978), same as the fraction of energy trapped by one active RC (Me = 1.976) and the non-photochemical energy dissipation (DI0/RC; Me = 0.916). The ET0/RC parameter, associated with the efficiency of the energy utilization for electron transport by one RC, in the submerged plant reached the highest value (Me = 0.489). The submerged form of D. intermedia clearly differed from the emerged and peatland forms in its plant architecture. The submerged plants had a thinner leaf blade and less developed xylem than the other forms, however, their stems were much longer. The relatively high photosynthetic efficiency of the submerged forms suggests that most of the trapped energy is utilized to drive photosynthesis with a minimum energy loss, which may be a mechanism to compensate for the relatively small size of the leaf blade.

Supra-optimal leaf area index of a temperate liana Pueraria lobata for competition with Solidago altissima at the expense of canopy photosynthesis

Leaf area index (LAI) measured for the actual plant canopy is higher than the LAI that maximizes canopy photosynthesis (referred to as optimal LAI) because each individual can increase its light interception by unilaterally producing more leaf area. The LAI of an invasive woody vine Pueraria lobata (kudzu) is one of the highest among plant species, sometimes attaining nearly 10 m2 m-2. The high LAI casts heavy shade over neighboring plants, making their survival difficult. Interesting to note is that the high LAI also increases self-shading, thereby decreasing its own photosynthesis processes. In the present study, the influences of the high LAI on light interception and canopy photosynthesis, as well as on the inter-specific competition was investigated on a roadside P. lobata vegetation in Japan. With the aid of a canopy photosynthesis model and a sensitivity analysis, it was revealed that the actual LAI was 2.2-3.0 times higher than the optimal LAI for maximizing canopy photosynthesis. In the following year, a field experiment was conducted where a nearly optimal LAI was maintained throughout the growth period by regularly clipping the leaves of P. lobata. Ultimately, the field results revealed that even with a nearly optimal LAI, P. lobata was outcompeted by a competing alien weed, Solidago altissima (tall goldenrod). These results indicate that the supra-optimal leaf area, rather than maximum canopy carbon gain, makes P. lobata the dominating species in light-competing environments.

Effect of differences in light source environment on transcriptome of leaf lettuce (Lactuca sativa L.) to optimize cultivation conditions

When used in closed-type plant factories, light-emitting diode (LED) illumination systems have the particular advantages of low heat emission and high luminous efficiency. The effects of illumination quality and intensity on the growth and morphogenesis of many plant species have been examined, but improvements are needed to optimize the illumination systems for better plant products with lower resource investments. In particular, new strategies are needed to reduce the wastage of plant products related to leaf senescence, and to better control the ingredients and appearance of leafy vegetables. Although the quality of light is often altered to change the characteristics of plant products, the transcriptional status underlying the physiological responses of plants to light has not been established. Herein, we performed a comprehensive gene expression analysis using RNA-sequencing to determine how red, blue, and red/blue LEDs and fluorescent light sources affect transcriptome involved in the leaf aging of leaf lettuce. The RNA-sequencing profiling revealed clear differences in the transcriptome between young and old leaves. Red LED light caused large variation between the two age classes, while a pure or mixed blue LED light spectrum induced fewer transcriptome differences between young and old leaves. Collectively, the expression levels of genes that showed homology with those of other model organisms provide a detailed physiological overview, incorporating such characteristics as the senescence, nutrient deficiency, and anthocyanin synthesis of the leaf lettuce plants. Our findings suggest that transcriptome profiles of leaf lettuce grown under different light sources provide helpful information to achieve better growth conditions for marketable and efficient green-vegetable production, with improved wastage control and efficient nutrient inputs.

"Diminishing returns" for leaves of five age-groups of Phyllostachys edulis culms

PREMISE

Leaf mass (M) and lamina surface area (A) are important functional traits reported to obey a scaling relationship called "diminishing returns" (i.e., M ∝ A^α>1 ). Previous studies have focused primarily on eudicots and ignored whether the age of leaves affects the numerical value of the scaling exponent (i.e., α).

METHODS

The effect of age was examined using 1623 Phyllostachys edulis leaves from culms differing in age collected in Nanjing, China. The scaling relationships among leaf A, fresh mass (FM), and dry mass (DM) were evaluated using reduced major axis protocols. The bootstrap percentile method was used to test the significance of differences in α-values.

RESULTS

Overall, the numerical values of α exceeded 1.0. The scaling relationship between FM and A was statistically more robust than that between DM and A. The scaling exponents of FM vs. A exhibited a "high-low-high-low-high" numerical trend from the oldest to the youngest age-group. FM increased linearly as culm age decreased; the leaf DM per unit area (LMA) exhibited a parabolic trend across the age-groups.

CONCLUSIONS

"Diminishing returns" is confirmed for all but one age-group of an important monocot species. The relationship between FM and A was statistically more robust than that between DM and A for each age-group. The FM per unit A decreased with increasing age-groups, whereas the middle age-groups had a greater LMA than the oldest and youngest age-groups. These data are the first to show that the age of shoots affects the scaling relationship between leaf mass and area.

Leaf productivity and persistence have been improved during soybean (Glycine max) domestication and evolution

Artificial and natural selection improved the leaf photosynthetic rate of soybean (Glycine max (L.) Merr. subsp. max). This change may be accompanied by unconscious, undesired changes in other leaf traits, such as decreased leaf persistence, if a finite resource was shared by two or more leaf traits-i.e., if they were traded off. We investigated leaf traits related to productivity (leaf photosynthetic rate, leaf nitrogen content, and stomatal conductance) and those related to persistence (leaf lifespan, leaf mass per unit area, and leaf bulk density) in one wild soybean line and three domesticated soybean lines (a landrace, an old cultivar, and a modern cultivar) in a three year experiment. Some leaf trait values increased while others did not change significantly during domestication and evolution. These results indicate that productivity-related leaf traits and persistence-related leaf traits are not negatively correlated. It was also found that the changes in productivity-related leaf traits and persistence-related leaf traits occurred at different times. Our results indicate that the productivity-related leaf traits and the persistence-related leaf traits have been independently selected for in soybean, and that they were not traded off. Combination of high leaf productivity and high leaf persistence would lead to higher lifetime leaf carbon gain and increased soybean yield.

Leaf shedding increases the photosynthetic rate of the canopy in N2-fixing and non-N2-fixing woody species

It has long been hypothesized that timing of leaf shedding is critical for plant fitness but there is little experimental evidence to support the hypothesis. According to an optimality theory, shedding of old leaves increases canopy photosynthesis despite some nitrogen (N) being lost as litterfall, when the ratio of daily photosynthesis to leaf N (N-use efficiency, ε) in old leaves, expressed as a fraction of ε in new leaves, becomes lower than the fraction of leaf N that is resorbed before shedding (RN). This was shown to be true for N-poor plants but not for N-rich plants in a pot experiment; however, the use of planting pots imposes a variety of physical, chemical and biological constraints that could change the experimental results. Here we conducted a 3-year field survey in a cool temperate deciduous forest to examine whether Alnus sieboldiana Matsum. (N2-fixing) and Carpinus tschonoskii Maxim. (non-N2-fixing) shed their leaves to increase canopy photosynthesis in accord with the above criterion. These species often grow sympatrically and were chosen as representatives of N-rich and N-poor plants, respectively. Overall, daily photosynthesis decreased with leaf age, accompanied by small changes in leaf N, resulting in a decrease in ε. In both species, ε of leaves at shedding expressed as a fraction of ε in new leaves was nearly equal to RN in all years, implying that the old leaves were shed to increase canopy photosynthesis. Our results, together with those of previous field surveys, suggested that the timing of leaf shedding is explained by N use in maximizing canopy photosynthesis across broad groups of species.

Changes in leaf area, nitrogen content and canopy photosynthesis in soybean exposed to an ozone concentration gradient

Influences of ozone (O3) on light-saturated rates of photosynthesis in crop leaves have been well documented. To increase our understanding of O3 effects on individual- or stand level productivity, a mechanistic understanding of factors determining canopy photosynthesis is necessary. We used a canopy model to scale photosynthesis from leaf to canopy, and analyzed the importance of canopy structural and leaf ecophysiological characteristics in determining canopy photosynthesis in soybean stands exposed to 9 concentrations of [O3] (37-116 ppb; 9-h mean). Light intensity and N content peaked in upper canopy layers, and sharply decreased through the lower canopy. Plant leaf area decreased with increasing [O3] allowing for greater light intensity to reach lower canopy levels. At the leaf level, light-saturated photosynthesis decreased and dark respiration increased with increasing [O3]. These data were used to calculate daily net canopy photosynthesis (Pc). Pc decreased with increasing [O3] with an average decrease of 10% for an increase in [O3] of 10 ppb, and which was similar to changes in above-ground dry mass production of the stands. Absolute daily net photosynthesis of lower layers was very low and thus the decrease in photosynthesis in the lower canopy caused by elevated [O3] had only minor significance for total canopy photosynthesis. Sensitivity analyses revealed that the decrease in Pc was associated with changes in leaf ecophysiology but not with decrease in leaf area. The soybean stands were very crowded, the leaves were highly mutually shaded, and sufficient light for positive carbon balance did not penetrate to lower canopy leaves, even under elevated [O3].

Leaf age dependent changes in within-canopy variation in leaf functional traits: a meta-analysis

Within-canopy variation in leaf structural and photosynthetic characteristics is a major means by which whole canopy photosynthesis is maximized at given total canopy nitrogen. As key acclimatory modifications, leaf nitrogen content (N A) and photosynthetic capacity (A A) per unit area increase with increasing light availability in the canopy and these increases are associated with increases in leaf dry mass per unit area (M A) and/or nitrogen content per dry mass and/or allocation. However, leaf functional characteristics change with increasing leaf age during leaf development and aging, but the importance of these alterations for within-canopy trait gradients is unknown. I conducted a meta-analysis based on 71 canopies that were sampled at different time periods or, in evergreens, included measurements for different-aged leaves to understand how within-canopy variations in leaf traits (trait plasticity) depend on leaf age. The analysis demonstrated that in evergreen woody species, M A and N A plasticity decreased with increasing leaf age, but the change in A A plasticity was less suggesting a certain re-acclimation of A A to altered light. In deciduous woody species, M A and N A gradients in flush-type species increased during leaf development and were almost invariable through the rest of the season, while in continuously leaf-forming species, the trait gradients increased constantly with increasing leaf age. In forbs, N A plasticity increased, while in grasses, N A plasticity decreased with increasing leaf age, reflecting life form differences in age-dependent changes in light availability and in nitrogen resorption for growth of generative organs. Although more work is needed to improve the coverage of age-dependent plasticity changes in some plant life forms, I argue that the age-dependent variation in trait plasticity uncovered in this study is large enough to warrant incorporation in simulations of canopy photosynthesis through the growing period.

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

BACKGROUND AND AIMS

Leaf longevity is controlled by the light gradient in the canopy and also by the nitrogen (N) sink strength in the plant. Stand density may influence leaf dynamics through its effects on light gradient and on plant growth and reproduction. This study tests the hypothesis that the control by the light gradient is manifested more in the vegetative period, whereas the opposite is true when the plant becomes reproductive and develops a strong N sink.

METHODS

Stands of Xanthium canadense were established at two densities. Emergence, growth and death of every leaf on the main stem and branches, and plant growth and N uptake were determined from germination to full senescence. Mean residence time and dry mass productivity were calculated per leaf number, leaf area, leaf mass and leaf N (collectively termed 'leaf variables') in order to analyse leaf dynamics and its effect on plant growth.

KEY RESULTS

Branching and reproductive activities were higher at low than at high density. Overall there was no significant difference in mean residence time of leaf variables between the two stands. However, early leaf cohorts on the main stem had a longer retention time at low density, whereas later cohorts had a longer retention time at high density. Branch leaves emerged earlier and tended to live longer at low than at high density. Leaf efficiencies, defined as carbon export per unit investment of leaf variables, were higher at low density in all leaf variables except for leaf number.

CONCLUSIONS

In the vegetative phase of plant growth, the light gradient strongly controls leaf longevity, whereas later the effects of branching and reproductive activities become stronger and over-rule the effect of light environment. As leaf N supports photosynthesis and also works as an N source for plant development, N use is pivotal in linking leaf dynamics with plant growth and reproduction.

Contributions of leaf photosynthetic capacity, leaf angle and self-shading to the maximization of net photosynthesis in Acer saccharum: a modelling assessment

BACKGROUND AND AIMS

Plants are expected to maximize their net photosynthetic gains and efficiently use available resources, but the fundamental principles governing trade-offs in suites of traits related to resource-use optimization remain uncertain. This study investigated whether Acer saccharum (sugar maple) saplings could maximize their net photosynthetic gains through a combination of crown structure and foliar characteristics that let all leaves maximize their photosynthetic light-use efficiency (ε).

METHODS

A functional-structural model, LIGNUM, was used to simulate individuals of different leaf area index (LAI(ind)) together with a genetic algorithm to find distributions of leaf angle (L(A)) and leaf photosynthetic capacity (A(max)) that maximized net carbon gain at the whole-plant level. Saplings grown in either the open or in a forest gap were simulated with A(max) either unconstrained or constrained to an upper value consistent with reported values for A(max) in A. saccharum.

KEY RESULTS

It was found that total net photosynthetic gain was highest when whole-plant PPFD absorption and leaf ε were simultaneously maximized. Maximization of ε required simultaneous adjustments in L(A) and A(max) along gradients of PPFD in the plants. When A(max) was constrained to a maximum, plants growing in the open maximized their PPFD absorption but not ε because PPFD incident on leaves was higher than the PPFD at which ε(max) was attainable. Average leaf ε in constrained plants nonetheless improved with increasing LAI(ind) because of an increase in self-shading.

CONCLUSIONS

It is concluded that there are selective pressures for plants to simultaneously maximize both PPFD absorption at the scale of the whole individual and ε at the scale of leaves, which requires a highly integrated response between L(A), A(max) and LAI(ind). The results also suggest that to maximize ε plants have evolved mechanisms that co-ordinate the L(A) and A(max) of individual leaves with PPFD availability.

Lifetime return on investment increases with leaf lifespan among 10 Australian woodland species

• Co-occurring species often differ in their leaf lifespan (LL) and it remains unclear how such variation is maintained in a competitive context. Here we test the hypothesis that leaves of long-LL species yield a greater return in carbon (C) fixed per unit C or nutrient invested by the plant than those of short-LL species. • For 10 sympatric woodland species, we assessed three-dimensional shoot architecture, canopy openness, leaf photosynthetic light response, leaf dark respiration and leaf construction costs across leaf age sequences. We then used the YPLANT model to estimate light interception and C revenue along the measured leaf age sequences. This was done under a series of simulations that incorporated the potential covariates of LL in an additive fashion. • Lifetime return in C fixed per unit C, N or P invested increased with LL in all simulations. • In contrast to other recent studies, our results show that extended LL confers a fundamental economic advantage by increasing a plant's return on investment in leaves. This suggests that time-discounting effects, that is, the compounding of income that arises from quick reinvestment of C revenue, are key in allowing short-LL species to succeed in the face of this economic handicap.

Is analysing the nitrogen use at the plant canopy level a matter of choosing the right optimization criterion?

Optimization theory in combination with canopy modeling is potentially a powerful tool for evaluating the adaptive significance of photosynthesis-related plant traits. Yet its successful application has been hampered by a lack of agreement on the appropriate optimization criterion. Here we review how models based on different types of optimization criteria have been used to analyze traits-particularly N reallocation and leaf area indices-that determine photosynthetic nitrogen-use efficiency at the canopy level. By far the most commonly used approach is static-plant simple optimization (SSO). Static-plant simple optimization makes two assumptions: (1) plant traits are considered to be optimal when they maximize whole-stand daily photosynthesis, ignoring competitive interactions between individuals; (2) it assumes static plants, ignoring canopy dynamics (production and loss of leaves, and the reallocation and uptake of nitrogen) and the respiration of nonphotosynthetic tissue. Recent studies have addressed either the former problem through the application of evolutionary game theory (EGT) or the latter by applying dynamic-plant simple optimization (DSO), and have made considerable progress in our understanding of plant photosynthetic traits. However, we argue that future model studies should focus on combining these two approaches. We also point out that field observations can fit predictions from two models based on very different optimization criteria. In order to enhance our understanding of the adaptive significance of photosynthesis-related plant traits, there is thus an urgent need for experiments that test underlying optimization criteria and competing hypotheses about underlying mechanisms of optimization.

Leaf-trait variation explained by the hypothesis that plants maximize their canopy carbon export over the lifespan of leaves

Measured values of four key leaf traits (leaf area per unit mass, nitrogen concentration, photosynthetic capacity, leaf lifespan) co-vary consistently within and among diverse biomes, suggesting convergent evolution across species. The same leaf traits co-vary consistently with the environmental conditions (light intensity, carbon-dioxide concentration, nitrogen supply) prevailing during leaf development. No existing theory satisfactorily explains all of these trends. Here, using a simple model of the carbon-nitrogen economy of trees, we show that global leaf-trait relationships and leaf responses to environmental conditions can be explained by the optimization hypothesis (MAXX) that plants maximize the total amount of carbon exported from their canopies over the lifespan of leaves. Incorporating MAXX into larger-scale vegetation models may improve their consistency with global leaf-trait relationships, and enhance their ability to predict how global terrestrial productivity and carbon sequestration respond to environmental change.

Why does Viola hondoensis (Violaceae) shed its winter leaves in spring?

PREMISE OF THE STUDY

Viola hondoensis is a perennial herb that inhabits the understory of temperate, deciduous forests. It is an evergreen plant with a leaf life span that is shorter than a year. Its summer leaves are produced in spring and shed in autumn; winter leaves are produced in autumn and shed in spring. Here we asked why the plant sheds its winter leaves in spring, though climate conditions improve from spring to summer. We proposed four hypotheses for the cause of shedding: (1) changes in seasonal environment such as day length or air temperature, (2) shading by canopy deciduous trees, (3) self-shading by taller summer leaves, and (4) competition for nutrients between summer and winter leaves. •

METHODS

To test these hypotheses, we manipulated the environment of winter leaves: (1) plants were transplanted to the open site where there was no shading by canopy trees. (2) Petioles of summer leaves were anchored to the soil surface to avoid shading of winter leaves. (3) Sink organs were removed to eliminate nutrient competition. •

KEY RESULTS

Longevity of winter leaves was extended when shading by summer leaves was eliminated and when sink organs were removed, but not when plants were transplanted to the open site. •

CONCLUSION

We conclude that the relative difference in light availability between summer and winter leaves is a critical factor for regulation of leaf shedding, consistent with the theory of maximization of the whole-plant photosynthesis.

The autophagic degradation of chloroplasts via rubisco-containing bodies is specifically linked to leaf carbon status but not nitrogen status in Arabidopsis

Autophagy is an intracellular process facilitating the vacuolar degradation of cytoplasmic components and is important for nutrient recycling during starvation. We previously demonstrated that chloroplasts can be partially mobilized to the vacuole by autophagy via spherical bodies named Rubisco-containing bodies (RCBs). Although chloroplasts contain approximately 80% of total leaf nitrogen and represent a major carbon and nitrogen source for new growth, the relationship between leaf nutrient status and RCB production remains unclear. We examined the effects of nutrient factors on the appearance of RCBs in leaves of transgenic Arabidopsis (Arabidopsis thaliana) expressing stroma-targeted fluorescent proteins. In excised leaves, the appearance of RCBs was suppressed by the presence of metabolic sugars, which were added externally or were produced during photosynthesis in the light. The light-mediated suppression was relieved by the inhibition of photosynthesis. During a diurnal cycle, RCB production was suppressed in leaves excised at the end of the day with high starch content. Starchless mutants phosphoglucomutase and ADP-Glc pyrophosphorylase1 produced a large number of RCBs, while starch-excess mutants starch-excess1 and maltose-excess1 produced fewer RCBs. In nitrogen-limited plants, as leaf carbohydrates were accumulated, RCB production was suppressed. We propose that there exists a close relationship between the degradation of chloroplast proteins via RCBs and leaf carbon but not nitrogen status in autophagy. We also found that the appearance of non-RCB-type autophagic bodies was not suppressed in the light and somewhat responded to nitrogen in excised leaves, unlike RCBs. These results imply that the degradation of chloroplast proteins via RCBs is specifically controlled in autophagy.

Leaf life span optimizes annual biomass production rather than plant photosynthetic capacity in an evergreen shrub

*Owing to nitrogen (N) translocation towards new leaves, the shedding of old leaves can increase the whole-plant carbon gain. It occurs when their photosynthetic nitrogen use efficiency (PNUE) declines below a given threshold. *Here, we investigated variations in net photosynthetic capacity (A(max)), N resorption and PNUE in populations of Rhododendron ferrugineum presenting different mean leaf life spans (LLS). *Both populations had comparable annual leaf surface area production and A(max) across leaf-age cohorts. Branch photosynthetic capacity was up to 95% higher in the population with the longer LLS mainly because of the high contribution of old leaves to the total leaf area. Despite lower N concentrations, old leaves maintained relatively high A(max) and consequently PNUE that were higher than or similar to the values found in current-year leaves. *As the ratio of PNUE in old to PNUE in new leaves was always higher than the fraction of leaf N resorbed during leaf shedding, we concluded that leaf shedding did not improve plant photosynthetic capacity. We suggest that in R. ferrugineum, leaf shedding is mainly controlled by the leaf storage function and, therefore, that models aiming to explain LLS should not only consider the leaf carbon assimilation function, particularly in nutrient-poor habitats.

Construction costs, payback times, and the leaf economics of carnivorous plants

Understanding how different plant species and functional types "invest" carbon and nutrients is a major goal of plant ecologists. Two measures of such investments are "construction costs" (carbon needed to produce each gram of tissue) and associated "payback times" for photosynthesis to recover construction costs. These measurements integrate among traits used to assess leaf-trait scaling relationships. Carnivorous plants are model systems for examining mechanisms of leaf-trait coordination, but no studies have measured simultaneously construction costs of carnivorous traps and their photosynthetic rates to determine payback times of traps. We measured mass-based construction costs (CC(mass)) and photosynthesis (A(mass)) for traps, leaves, roots, and rhizomes of 15 carnivorous plant species grown under greenhouse conditions. There were highly significant differences among species in CC(mass) for each structure. Mean CC(mass) of carnivorous traps (1.14 ± 0.24 g glucose/g dry mass) was significantly lower than CC(mass) of leaves of 267 noncarnivorous plant species (1.47 ± 0.17), but all carnivorous plants examined had very low A(mass) and thus, long payback times (495-1551 h). Our results provide the first clear estimates of the marginal benefits of botanical carnivory and place carnivorous plants at the "slow and tough" end of the universal spectrum of leaf traits.

Controls on declining carbon balance with leaf age among 10 woody species in Australian woodland: do leaves have zero daily net carbon balances when they die?

* Here, we evaluated how increased shading and declining net photosynthetic capacity regulate the decline in net carbon balance with increasing leaf age for 10 Australian woodland species. We also asked whether leaves at the age of their mean life-span have carbon balances that are positive, zero or negative. * The net carbon balances of 2307 leaves on 53 branches of the 10 species were estimated. We assessed three-dimensional architecture, canopy openness, photosynthetic light response functions and dark respiration rate across leaf age sequences on all branches. We used YPLANT to estimate light interception and to model carbon balance along the leaf age sequences. * As leaf age increased to the mean life-span, increasing shading and declining photosynthetic capacity each separately reduced daytime carbon gain by approximately 39% on average across species. Together, they reduced daytime carbon gain by 64% on average across species. * At the age of their mean life-span, almost all leaves had positive daytime carbon balances. These per leaf carbon surpluses were of a similar magnitude to the estimated whole-plant respiratory costs per leaf. Thus, the results suggest that a whole-plant economic framework, including respiratory costs, may be useful in assessing controls on leaf longevity.

Influence of nitrogen and potassium fertilization on leaf lifespan and allocation of above-ground growth in Eucalyptus plantations

Eucalyptus grandis (W. Hill ex Maiden) leaf traits and tree growth were studied over 3 years after the establishment of two adjacent complete randomized block designs in southern Brazil. In a nitrogen (N) input experiment, a treatment with the application of 120 kg N ha(-1) was compared to a control treatment without N addition, and in a potassium (K) input experiment a control treatment without K addition was compared to a treatment with the application of 116 kg K ha(-1). Young leaves were tagged 9 months after planting to estimate the effect of N and K fertilizations on leaf lifespan. Leaf mass, specific leaf area and nutrient concentrations were measured on a composite sample per plot every 28 days until the last tagged leaf fell. Successive inventories, destructive sampling of trees and leaf litter fall collection made it possible to assess the effect of N and K fertilization on the dynamics of biomass accumulation in above-ground tree components. Whilst the effects of N fertilization on tree growth only occurred in the first 24 months after planting, K fertilization increased the above-ground net primary production from 4478 to 8737 g m(-2) over the first 36 months after planting. The average lifespan of tagged leaves was not modified by N addition but it increased from 111 to 149 days with K fertilization. The peak of leaf production occurred in the second year after planting (about 800 g m(-2) year(-1)) and was not significantly modified (P < 0.05) by N and K fertilizations. By contrast, K addition significantly increased the maximum leaf standing biomass from 292 to 528 g m(-2), mainly as a consequence of the increase in leaf lifespan. Potassium fertilization increased the stand biomass mainly through the enhancement in leaf area index (LAI) since growth efficiency (defined as the ratio between woody biomass production and LAI) was not significantly modified. A better understanding of the physiological processes governing the leaf lifespan is necessary to improve process-based models currently used in Eucalyptus plantations.

Leaf investment and light partitioning among leaves of different genotypes of the clonal plant Potentilla reptans in a dense stand after 5 years of competition

BACKGROUND AND AIMS

While within-species competition for light is generally found to be asymmetric - larger plants absorbing more than proportional amounts of light - between-species competition tends to be more symmetric. Here, the light capture was analysed in a 5-year-old competition experiment that started with ten genotypes of the clonal plant Potentilla reptans. The following hypotheses were tested: (a) if different genotypes would do better in different layers of the canopy, thereby promoting coexistence, and (b) if leaves and genotypes with higher total mass captured more than proportional amounts of light, possibly explaining the observed dominance of the abundant genotypes.

METHODS

In eight plots, 100 leaves were harvested at various depths in the canopy and their genotype determined to test for differences in leaf biomass allocation, leaf characteristics and the resulting light capture, calculated through a canopy model using the actual vertical light and leaf area profiles. Light capture was related to biomass to determine whether light competition between genotypes was asymmetric.

KEY RESULTS

All genotypes could reach the top of the canopy. The genotypes differed in morphology, but did not differ significantly in light capture per unit mass (Phi(mass)) for leaves with the laminae placed at the same light levels. Light capture did increase disproportionately with leaf mass for all genotypes. However, the more abundant genotypes did not capture disproportionately more light relative to their mass than less-abundant genotypes.

CONCLUSIONS

Vertical niche differentiation in light acquisition does not appear to be a factor that could promote coexistence between these genotypes. Contrary to what is generally assumed, light competition among genetic individuals of the same species was size-symmetric, even if taller individual leaves did capture disproportionately more light. The observed shifts in genotype frequency cannot therefore be explained by asymmetric competition for light.

Reproductive yield of individuals competing for light in a dense stand of an annual, Xanthium canadense

In a dense stand, individuals compete with each other for resources, especially for light. Light availability decreases with increasing depth in the canopy, thus light competition becoming stronger with time in the vegetative phase. In the reproductive phase, on the other hand, leaves start senescing, and the light environment, particularly of smaller individuals, will be improved. To study the effect of change in light climate on reproduction of individuals, we established an experimental stand of an annual, Xanthium canadense, and assessed temporal changes in whole plant photosynthesis through the reproductive phase with particular reference to light availability of individuals. At flowering, 83% of individuals were still alive, but only 27% survived to set seeds. Most of the individuals that died in the reproductive phase were smaller than those that produced seeds. Individuals that died at the early stage of the reproductive phase had a lower leaf to stem mass ratio, suggesting that the fate of individuals was determined partly by the pattern of biomass allocation in this period. At the early stage of the reproductive phase, leaf area index (LAI) of the stand was high and larger individuals had higher whole plant photosynthesis than smaller individuals. Although light availability at later stages was improved with reduction in LAI, whole plant photosynthesis was very low in all individuals due to a lower light use efficiency, which was caused by a decrease in photosynthetic N use efficiency. We conclude that light competition was still strong at the early stage of the reproductive phase and that later improvement of light availability did not ameliorate the photosynthesis of smaller individuals.

Does leaf shedding increase the whole-plant carbon gain despite some nitrogen being lost with shedding?

When old leaves are shed, part of the nitrogen in the leaf is retranslocated to new leaves. This retranslocation will increase the whole-plant carbon gain when daily C gain : leaf N ratio (daily photosynthetic N-use efficiency, NUE) in the old leaf, expressed as a fraction of NUE in the new leaf, becomes lower than the fraction of leaf N that is resorbed before shedding (R(N)). We examined whether plants shed their leaves to increase the whole-plant C gain in accord with this criterion in a dense stand of an annual herb, Xanthium canadense, grown under high (HN) and low (LN) nitrogen availability. The NUE of a leaf at shedding expressed as a fraction of NUE in a new leaf was nearly equal to the R(N) in the LN stand, but significantly lower than the R(N) in the HN stand. Thus shedding of old leaves occurred as expected in the LN stand, whereas in the HN stand, shedding occurred later than expected. Sensitivity analyses showed that the decline in NUE of a leaf resulted primarily from a reduction in irradiance in the HN stand. On the other hand, it resulted from a reduction in irradiance and also in light-saturated photosynthesis : leaf N content ratio (potential photosynthetic NUE) in the LN stand.