The importance of seed reserves for seedling performance: an integrated approach using morphological, physiological, and stable isotope techniques

Cotyledon loss of Astragalus membranaceus hindered seedling establishment through mineral element reallocation and carbohydrate depletion

Tissue loss of plants caused by herbivores is very common in nature. As the storage and first photosynthetic organ, the loss of cotyledon severely impacts dicot seedling establishment and the subsequent growth. However, it is still not clear how plants adjust their metabolic strategy in response to cotyledon loss. In this study, we employed ICP-OES, GC and LC-MS to examine the effects of cotyledon removal (RC1: remove one cotyledon, RC2: remove two cotyledon) on mineral element distribution and metabolite changes in a traditional Chinese herbal plant, Astragalus membranaceus. The results showed that cotyledon removal had a greater effect on shoot than root growth. Specifically, RC2 revealed a more serious impact on shoot growth than RC1. Microelement Mn and Na in shoot increased more in RC2 than RC1. Macroelement K and microelement B in root increased in RC2. The metabolite results in shoot showed that sugars related to galactose metabolism reduced while amino acids significantly increased in RC2. In root, sugars related to fructose and mannose metabolism decreased in both RC1 and RC2 while most flavonoids increased in RC2. It can be concluded that cotyledon removal triggered different metabolic strategies in both root and shoot. In shoot, more Mn was absorbed to improve the lowered photosynthetic efficiency. Meanwhile, increased Na may have promoted carbohydrate consumption and amino acid synthesis, thereby maintaining shoot growth. In root, K and B participation in cell division and expansion increased, as well as the delivery and metabolism of carbohydrates, to maintain root system growth.

Cloning capacity helps seeds of Garcinia xanthochymus counter animal predation

Seed predators have the potential to act as agents of natural selection that influence seed traits and seed fates, which in turn affect the whole plant population dynamic. Accordingly, plants deploy a variety of mechanisms (e.g., resistance and tolerance strategies) to lessen the impact of predation on seed crop or on an individual seed. In this study, we described a novel mechanism, seed cloning strategy, in a tropical plant species in countering animal predation. By conducting field- and laboratory-based germination experiments, we found that both rodent damaged and artificially damaged seed fragments of a large-seeded tree Garcinia xanthochymus (Clusiaceae) could successfully germinate and establish as seedlings. Tissue culture experiments revealed that G. xanthochymus has no endosperm in seeds, and its seed fragments own strong capacity of differentiation and cloning. Seed damage negatively affected seedling growth and germination, but the seed germination rate was remarkably high. Our study suggests that, seed cloning capacity, adopted by the large-seeded tree G. xanthochymus may act as a novel strategy counteract for seed predation and would play a significant role in stabilizing the mutualism between plant and animals.

The karrikin signaling regulator SMAX1 controls Lotus japonicus root and root hair development by suppressing ethylene biosynthesis

An evolutionarily ancient plant hormone receptor complex comprising the α/β-fold hydrolase receptor KARRIKIN INSENSITIVE 2 (KAI2) and the F-box protein MORE AXILLARY GROWTH 2 (MAX2) mediates a range of developmental responses to smoke-derived butenolides called karrikins (KARs) and to yet elusive endogenous KAI2 ligands (KLs). Degradation of SUPPRESSOR OF MAX2 1 (SMAX1) after ligand perception is considered to be a key step in KAR/KL signaling. However, molecular events which regulate plant development downstream of SMAX1 removal have not been identified. Here we show that Lotus japonicus SMAX1 is specifically degraded in the presence of KAI2 and MAX2 and plays an important role in regulating root and root hair development. smax1 mutants display very short primary roots and elongated root hairs. Their root transcriptome reveals elevated ethylene responses and expression of ACC Synthase 7 (ACS7), which encodes a rate-limiting enzyme in ethylene biosynthesis. smax1 mutants release increased amounts of ethylene and their root phenotype is rescued by treatment with ethylene biosynthesis and signaling inhibitors. KAR treatment induces ACS7 expression in a KAI2-dependent manner and root developmental responses to KAR treatment depend on ethylene signaling. Furthermore, in Arabidopsis, KAR-induced root hair elongation depends on ACS7 Thus, we reveal a connection between KAR/KL and ethylene signaling in which the KAR/KL signaling module (KAI2-MAX2-SMAX1) regulates the biosynthesis of ethylene to fine-tune root and root hair development, which are important for seedling establishment at the beginning of the plant life cycle.

Mining of favorable alleles for seed reserve utilization efficiency in Oryza sativa by means of association mapping

Background

Wet direct-seeded rice is a possible alternative to conventional puddled transplanted rice; the former uses less water and reduces labor requirements. Improving seed reserve utilization efficiency (SRUE) is a key factor in facilitating the application of this technology. However, the QTLs controlling this trait are poorly investigated. In this study, a genome-wide association study (GWAS) was conducted using a natural population composed of 542 accessions of rice (Oryza sativa L.) which were genotyped using 266 SSR markers. Large phenotypic variations in SRUE were found in the studied population.

Results

The average SRUE over 542 accessions across two years (2016 and 2017) was 0.52 mg.mg^− 1, ranging from 0.22 mg.mg-¹ to 0.93 mg.mg^− 1, with a coefficient of variation of 22.66%. Overall, 2879 marker alleles were detected in the population by 266 pairs of SSR markers, indicating a large genetic variation existing in the population. Using general linear model method, 13 SSR marker loci associated with SRUE were detected and two (RM7309 and RM434) of the 13 loci, were also detected using mixed linear model analyses, with percentage of phenotypic variation explained (PVE) greater than 5% across two years. The 13 association loci (P < 0.01) were located on all chromosomes except chromosome 11, with PVE ranging from 5.05% (RM5158 on chromosome 5) to 12% (RM297 on chromosome 1). Association loci RM7309 on chromosome 6 and RM434 on chromosome 9 revealed by both models were detected in both years. Twenty-three favorable alleles were identified with phenotypic effect values (PEV) ranging from 0.10 mg.mg^− 1 (RM7309–135 bp on chromosome 9) to 0.45 mg.mg^− 1 (RM297–180 bp on chromosome 2). RM297–180 bp showed the largest phenotypic effect value (0.44 mg.mg^− 1 in 2016 and 0.45 mg.mg^− 1 in 2017) with 6.72% of the accessions carrying this allele and the typical carrier accession was Manyedao, followed by RM297–175 bp (0.43 mg.mg^− 1 in 2016 and 0.44 mg.mg^− 1 in 2017).

Conclusion

Nine novel association loci for SRUE were identified, compared with previous studies. The optimal parental combinations for pyramiding more favorable alleles for SRUE were selected and could be used for breeding rice accessions suitable for wet direct seeding in the future.

Correlated metabolic and elemental variations between the leaves and seeds of oak trees at contrasting geologically derived phosphorus sites

The leaves and seeds of plants frequently function as the source and sink organs for distinct metabolites, which can interactively vary in response to adverse site conditions. Subtropical soils are typically characterized as having deficient phosphorus (P), calcium (Ca), and magnesium (Mg), with enriched aluminum (Al) and iron (Fe), while Al and manganese (Mn) are toxic at low pH. It remains largely unknown how leaf- and seed-sourced metabolites are synergistically linked to adapt to P-variable soils for trees in subtropical areas. Here we quantified the metabolic and elemental profiling in the mature leaves and immature seeds of Quercus variabilis at contrasting geologically-derived phosphorus sites in subtropical China. The results revealed that carbon (C) and nitrogen (N) based metabolites (primarily sugars and organic acids), as well as enzyme- and protein/nucleic acid-related elements (N, P, Mg, and Mn) played important roles toward characterizing the profiling of metabolites and ionomes in leaves and seeds at two site types, respectively. These metabolites (sugars, amino acids, and fatty acids) and elements (N, P, Mg, and Mn) of seeds were closely related to the sugars, organic acids, and elements (N, P, Mg, and Mn) of leaves at the two site types. For the most part, the content of N and P in the soil affected the accumulation of materials (such as, starchs and proteins) in seeds, as well as N and P assimilation in leaves, by influencing C- and N-containing metabolites in leaves. These results suggested that correlated disparities of C- and N-containing metabolites, along with enzyme- and protein/nucleic acid-related elements in both leaves and seeds played important roles in plants to facilitate their adaptation to nutrient-variable sites in subtropical zones.

Light Is More Important Than Nutrient Ratios of Fertilization for Cymodocea nodosa Seedling Development

Restoration of seagrass beds through seedlings is an alternative to the transplantation of adult plants that reduces the impact over donor areas and increases the genetic variability of restored meadows. To improve the use of Cymodocea nodosa seedlings, obtained from seeds germinated in vitro, in restoration programs, we investigated the ammonium and phosphate uptake rates of seedlings, and the synergistic effects of light levels (20 and 200 μmol quanta m-2 s-1) and different nitrogen to phosphorus molar ratios (40 μM N:10 μM P, 25 μM N:25 μM P, and 10 μM N:40 μM P) on the photosynthetic activity and growth of seedlings. The nutrient content of seedlings was also compared to the seed nutrient reserves to assess the relative importance of external nutrient uptake for seedling development. Eighty two percent of the seeds germinated after 48 days at a mean rate of 1.5 seeds per day. All seedlings under all treatments survived and grew during the 4 weeks of the experiment. Seedlings of C. nodosa acquired ammonium and phosphate from the incubation media while still attached to the seed, at rates of about twice of adult plants. The relevance of external nutrient uptake was further highlighted by the observation that seedlings' tissues were richer in nitrogen and phosphorus than non-germinated seeds. The uptake of ammonium followed saturation kinetics with a half saturation constant of 32 μM whereas the uptake of phosphate increased linearly with nutrient concentration within the range tested (5 - 100 μM). Light was more important than the nutrient ratio of fertilization for the successful development of the young seedlings. The seedlings' photosynthetic and growth rates were about 20% higher in the high light treatment, whereas different nitrogen to phosphorus ratios did not significantly affect growth. The photosynthetic responses of the seedlings to changes in the light level and their capacity to use external nutrient sources showed that seedlings of C. nodosa have the ability to rapidly acclimate to the surrounding light and nutrient environment while still attached to the seeds. C. nodosa seedlings experiencing fertilization under low light levels showed slightly enhanced growth if nourished with a balanced formulation, whereas a slight increase in growth was also observed with unbalanced formulations under a higher light level. Our results highlight the importance of high light availability at the seedling restoration sites.

Quantification of root water uptake in soil using X-ray computed tomography and image-based modelling

Spatially averaged models of root-soil interactions are often used to calculate plant water uptake. Using a combination of X-ray computed tomography (CT) and image-based modelling, we tested the accuracy of this spatial averaging by directly calculating plant water uptake for young wheat plants in two soil types. The root system was imaged using X-ray CT at 2, 4, 6, 8 and 12 d after transplanting. The roots were segmented using semi-automated root tracking for speed and reproducibility. The segmented geometries were converted to a mesh suitable for the numerical solution of Richards' equation. Richards' equation was parameterized using existing pore scale studies of soil hydraulic properties in the rhizosphere of wheat plants. Image-based modelling allows the spatial distribution of water around the root to be visualized and the fluxes into the root to be calculated. By comparing the results obtained through image-based modelling to spatially averaged models, the impact of root architecture and geometry in water uptake was quantified. We observed that the spatially averaged models performed well in comparison to the image-based models with <2% difference in uptake. However, the spatial averaging loses important information regarding the spatial distribution of water near the root system.

Isotopic variation among Amazonian floodplain woody plants and implications for food-web research

Isotopic variation within food sources adds uncertainty to models intended to reconstruct trophic pathways. Understanding this variation is pivotal for planning sampling protocols for food-web research. This study investigates natural variation in C and N stable isotopes among plant species in two western Amazon flooded forests with contrasting watershed biogeochemistry (white-water várzea-forest and black-water igapó-forest). Our objectives were to compare δ13C and δ15N of leaves and fruits between sites; assess the magnitude of within-site variation in δ13C and δ15N of leaves (várzea: 28 spp., igapó: 10 spp.) and fruits (várzea: 22 spp., igapó: 22 spp.); determine within-plant variation in δ13C and δ15N of leaf, wood and fruit tissues; and test whether inter-specific variation in δ13C and δ15N influence the results of a mixing model predicting the contribution of terrestrial C sources to an aquatic consumer. Mean δ13C values of leaves and fruits were not statistically different between the two sites despite regional differences in biogeochemistry and floristic composition. In contrast, mean δ15N of leaves and fruits were significantly lower at the várzea than at the igapó site. The high floristic diversity of both forests was reflected in large within-site interspecific variation in both δ13C and δ15N. Paired comparisons revealed that δ13C of wood and fruits and δ15N of fruits were generally greater than values obtained for leaves from the same plant. The predicted contribution of different carbon sources to the consumer biomass changed between models as a function of source variability. We discuss implications of source variation for designing sampling protocols, interpreting isotopic signatures, and establishing trophic links between plants and consumers. Our findings highlight the importance of in situ sampling to establish reliable primary production baselines for local food webs.

Changes and their possible causes in δ13C of dark-respired CO2 and its putative bulk and soluble sources during maize ontogeny

The issues of whether, where, and to what extent carbon isotopic fractionations occur during respiration affect interpretations of plant functions that are important to many disciplines across the natural sciences. Studies of carbon isotopic fractionation during dark respiration in C3 plants have repeatedly shown respired CO2 to be (13)C enriched relative to its bulk leaf sources and (13)C depleted relative to its bulk root sources. Furthermore, two studies showed respired CO2 to become progressively (13)C enriched during leaf ontogeny and (13)C depleted during root ontogeny in C3 legumes. As such data on C4 plants are scarce and contradictory, we investigated apparent respiratory fractionations of carbon and their possible causes in different organs of maize plants during early ontogeny. As in the C3 plants, leaf-respired CO2 was (13)C enriched whereas root-respired CO2 was (13)C depleted relative to their putative sources. In contrast to the findings for C3 plants, however, not only root- but also leaf-respired CO2 became more (13)C depleted during ontogeny. Leaf-respired CO2 was highly (13)C enriched just after light-dark transition but the enrichment rapidly decreased over time in darkness. We conclude that (i) although carbon isotopic fractionations in C4 maize and leguminous C3 crop roots are similar, increasing phosphoenolpyruvate-carboxylase activity during maize ontogeny could have produced the contrast between the progressive (13)C depletion of maize leaf-respired CO2 and (13)C enrichment of C3 leaf-respired CO2 over time, and (ii) in both maize and C3 leaves, highly (13)C enriched leaf-respired CO2 at light-to-dark transition and its rapid decrease during darkness, together with the observed decrease in leaf malate content, may be the result of a transient effect of light-enhanced dark respiration.

Acorn cotyledons are larger than their seedlings' need: evidence from artificial cutting experiments

Although the consequences of cotyledon removal have been widely studied in oaks producing large acorns, we have little knowledge of at what level cotyledons can be removed without affecting acorn survival and seedling development. In this study, we aimed to test the hypothesis that the amount of energy reserves in cotyledons is more than the demands of seedlings and that large acorns can tolerate seed predation and/or attract seed predators for seed dispersal. Acorn germination rates were not affected even when 60% of cotyledons were cut off at the basal end, suggesting that the energy reserves contained in cotyledons are not essential for acorn survival. Post-cut acorn mass, more than initial acorn mass, appear to be a better predictor of seedling performance, indicating that the energy reserves in cotyledons are sufficient for seedlings. Acorns with large masses sustained cotyledon damage better than small ones with respect to seedling performance. Large acorns were more likely to be dispersed and cached by animals, implying that producing large acorns is more important for oaks to manipulate seed predators and dispersers rather than provide a seedling with cotyledonary reserves.

Retention of cotyledons is crucial for resprouting of clipped oak seedlings

Although resprouting plays an important role in facilitating persistence of damaged seedlings, the functional attributes of cotyledons and taproots during resprouting of 1-year oak seedlings are not well explored. In this study, cotyledons were removed from Quercus mongolica seedlings to explore resprouting in response to simulated disturbance as a function of shoot clipping, and to examine the resprouting ability in relation to timing of clipping and cotyledon removal. Isotope labeling experiments were also performed to evaluate contribution of the cotyledons and taproots to resprouting. Regardless of timing of shoot clipping, seedlings successfully resprouted provided their cotyledons were not detached. Clipped seedlings were less likely to resprout when cotyledons were removed. Seedlings clipped at earlier development stage exhibited higher resprouting capacity than those clipped at later stage. Cotyledon removal, more than timing of clipping, decreased the dry masses of newly-resprouted shoots. However, no significant influences of cotyledon removal and timing of clipping were found on the dry masses of roots, suggesting the importance of cotyledons for resprouting. Roots became functional and accumulated more soil nitrogen after shoot clipping and cotyledon removal, representing a double security-based strategy for the clipped seedlings to resprout despite the importance of cotyledons.

Influence of pericarp, cotyledon and inhibitory substances on sharp tooth oak (Quercus aliena var. acuteserrata) germination

In order to explore the mechanism of delayed and uneven germination in sharp tooth oak (Quercus aliena var. acuteserrata) (STO), mechanical scarification techniques were used to study STO root and shoot germination and growth. The techniques used were: removing cup scar (RS), removing the pericarp (RP), and cutting off 1/2 (HC) and 2/3 (TC) cotyledons. Germination percentage and root and shoot length for Chinese cabbage (Beassica pekinensis) seeds (CCS) were also investigated for CCS cultivated in a Sanyo growth cabinet watered by distilled water and 80% methanol extracts from the acorn embryo, cotyledon and pericarp with concentrations of 1.0 g, 0.8 g, 0.6 g and 0.4 g dry acorn weight per ml methanol. The results showed that the majority of roots and shoots from acorns with RP and HC treatment emerged two weeks earlier, more simultaneously, and their total emergences were more than 46% and 28% higher, respectively. TC accelerated root and shoot emergence time and root length, but root and shoot germination rate and shoot height had no significant difference from the control. Positive consequences were not observed on all indices of RS treatment. The germination rates of CCS watered by 1.0 g·ml⁻¹ methanol extracts from the embryo and cotyledon were significantly lower than those from the pericarp, and all concentrations resulted in decreased growth of root and shoot. Methanol extracts from pericarp significantly reduced root length of CCS, but presented little response in germination percentage and shoot length. The inhibitory effect was gradually increased with the increasing concentration of the methanol extract. We conclude that both the mechanical restriction of the pericarp and the presence of germination inhibitors in the embryo, cotyledon and pericarp are the causes for delayed and asynchronous germination of STO acorns.

Correlated responses of root growth and sugar concentrations to various defoliation treatments and rhythmic shoot growth in oak tree seedlings (Quercus pubescens)

BACKGROUND AND AIMS

To understand whether root responses to aerial rhythmic growth and contrasted defoliation treatments can be interpreted under the common frame of carbohydrate availability; root growth was studied in parallel with carbohydrate concentrations in different parts of the root system on oak tree seedlings.

METHODS

Quercus pubescens seedlings were submitted to selective defoliation (removal of mature leaves, cotyledons or young developing leaves) at appearance of the second flush and collected 1, 5 or 10 d later for morphological and biochemical measurements. Soluble sugar and starch concentrations were measured in cotyledons and apical and basal root parts.

KEY RESULTS

Soluble sugar concentration in the root apices diminished during the expansion of the second aerial flush and increased after the end of aerial growth in control seedlings. Starch concentration in cotyledons regularly decreased. Continuous removal of young leaves did not alter either root growth or apical sugar concentration. Starch storage in basal root segments was increased. After removal of mature leaves (and cotyledons), root growth strongly decreased. Soluble sugar concentration in the root apices drastically decreased and starch reserves in the root basal segments were emptied 5 d after defoliation, illustrating a considerable shortage in carbohydrates. Soluble sugar concentrations recovered 10 d after defoliation, after the end of aerial growth, suggesting a recirculation of sugar. No supplementary recourse to starch in cotyledons was observed.

CONCLUSIONS

The parallel between apical sugar concentration and root growth patterns, and the correlations between hexose concentration in root apices and their growth rate, support the hypothesis that the response of root growth to aerial periodic growth and defoliation treatments is largely controlled by carbohydrate availability.

Remobilization of acorn nitrogen for seedling growth in holm oak (Quercus ilex), cultivated with contrasting nutrient availability

The relative contribution of nitrogen (N) reserves from seeds or uptake by the roots to the growth and N content of young seedlings has received little attention. In this study, we investigated the contribution of N from the acorn or uptake by the roots to the N content of holm oak (Quercus ilex L.) seedlings and determined if remobilization of acorn N was affected by nutrient availability in the growing media. Q. ilex seedlings were cultivated for 3 months, until the end of the second shoot flush of growth, with three N fertilization rates: 8.6 mM N, 1.4 mM N or no fertilization. Fertilizer N was enriched in (15)N. Between 62 and 75% of the N contained in high and low fertilized seedlings, respectively, at the end of the second flush of growth was derived from the acorn. However, the dependence on acorn N was greater during the early root growth and first shoot flush of growth and decreased during the second shoot flush of growth, with root uptake contributing 32-54% of plant new N in this latter developmental stage in high and low fertilized plants, respectively. Fertilization rate did not affect the amount of N taken up during the earliest developmental stages, but it increased it during the second shoot flush of growth. Fertilization increased the mass of the shoot segment formed during the second shoot flush of growth and reduced the root mass, with no effect on whole plant growth. Remobilization of acorn N was faster in unfertilized plants than in fertilized plants. It is concluded that the holm oak seedlings depend greatly upon acorn N until the end of the second shoot flush of growth, that significant root N uptake starts at the beginning of the second shoot flush of growth and that acorn N remobilization is a plastic process that is accelerated under extremely low substratum nutrient content.

No trade-off between seed size and number in the valley oak Quercus lobata

We examined the relationship between acorn mass and number in valley oaks (Quercus lobata) over 4 years in central coastal California. Despite considerable variation in acorn size among both trees and years, trees produced acorns of the same size relative to other trees in different years. Across years, the relationship between acorn mass and acorn crop size was generally positive, even after controlling for environmental conditions and differences in individual tree size and quality. Life-history trade-offs in valley oaks are primarily between current and future reproduction and indirectly between concurrent growth and reproduction, not between seed size and number, and are probably related to this species' mast-seeding behavior. Phenotypic trade-offs in long-lived plants such as oaks exhibit complex patterns of life-history covariation and deserve greater attention, both theoretically and empirically.

Plasticity and stress tolerance override local adaptation in the responses of Mediterranean holm oak seedlings to drought and cold

Plant populations of widely distributed species experience a broad range of environmental conditions that can be faced by phenotypic plasticity or ecotypic differentiation and local adaptation. The strategy chosen will determine a population's ability to respond to climate change. To explore this, we grew Quercus ilex (L.) seedlings from acorns collected at six selected populations from climatically contrasting localities and evaluated their response to drought and late season cold events. Maximum photosynthetic rate (A(max)), instantaneous water use efficiency (iWUE), and thermal tolerance to freeze and heat (estimated from chlorophyll fluorescence versus temperature curves) were measured in 5-month-old seedlings in control (no stress), drought (water-stressed), and cold (low suboptimal temperature) conditions. The observed responses were similar for the six populations: drought decreased A(max) and increased iWUE, and cold reduced A(max) and iWUE. All the seedlings maintained photosynthetic activity under adverse conditions (drought and cold), and rapidly increased their iWUE by closing stomata when exposed to drought. Heat and freeze tolerances were similarly high for seedlings from all the populations, and they were significantly increased by drought and cold, respectively; and were positively related to each other. Differences in seedling performance across populations were primarily induced by maternal effects mediated by seed size and to a lesser extent by idiosyncratic physiologic responses to drought and low temperatures. Tolerance to multiple stresses together with the capacity to physiologically acclimate to heat waves and cold snaps may allow Q. ilex to cope with the increasingly stressful conditions imposed by climate change. Lack of evidence of physiologic seedling adaptation to local climate may reflect opposing selection pressures to complex, multidimensional environmental conditions operating within the distribution range of this species.

Divergence in delta(13)C of dark respired CO(2) and bulk organic matter occurs during the transition between heterotrophy and autotrophy in Phaseolus vulgaris plants

Substantial evidence has been published in recent years demonstrating that postphotosynthetic fractionations occur in plants, leading to (13)C-enrichment in heterotrophic (as compared with autotrophic) organs. However, less is known about the mechanism responsible for changes in these responses during plant development. The isotopic signature of both organic matter and respired CO(2) for different organs of French bean (Phaseolus vulgaris) was investigated during early ontogeny, in order to identify the developmental stage at which isotopic changes occur. Isotopic analyses of metabolites and mass balance calculations helped to constrain the metabolic processes involved. At the plant scale, apparent respiratory fractionation was constantly positive in the heterotrophic phase (c. 1 per thousand) and turned negative with autotrophy acquisition (down to -3.08 per thousand). Initially very close to that of the dry seed (-26.83 +/- 0.69 per thousand), isotopic signatures of organic matter and respired CO(2) diverged (in opposite directions) in leaves and roots after onset of photosynthesis. Respired CO(2) reached values up to -20 per thousand in leaves and became (13)C-depleted down to -29 per thousand in roots. It was concluded that isotopic differences between organs occurred subsequent to metabolic changes in the seedling during the transition from heterotrophy to autotrophy. They were especially related to respiration and respiratory fractionation.

Timing of cotyledon damage affects growth and flowering in mature plants

Although the effects of herbivory on plant fitness are strongly linked to age, we understand little about how the timing of herbivory at the seedling stage affects growth and reproduction for plants that survive attack. In this study, we subjected six north-western European, dicotyledonous grassland species (Leontodon autumnalis, Leontodon hispidus, Plantago lanceolata, Plantago major, Trifolium pratense and Trifolium repens) to cotyledon removal at 7, 14 and 21 d old. We monitored subsequent growth and flowering (number of inflorescences recorded, and time taken for first flowers to open) over a 107 d period. Cotyledon removal reduced growth during establishment (35 d) for all species, and a further three exhibited reduced growth at maturity. Four species developed fewer inflorescences, or had delayed flowering after cotyledon removal. Although early damage (7 d old) had the greatest long-term effect on plant performance, responses varied according to the age at which the damage occurred and the species involved. Our results illustrate how growth and flowering into the mature phase is affected by cotyledon damage during different stages of seedling ontogeny, and we highlight the ways in which ontogenetic variation in seedling tolerance of tissue loss might impact upon plant fitness in mature plant communities.

Cotyledon damage at the seedling stage affects growth and flowering potential in mature plants

Seedling herbivory is an important selective filter influencing patterns of plant community composition. Nevertheless, while many of the mechanisms governing seedling selection by herbivores are well established, the effects of tissue loss at the seedling stage on subsequent plant development are poorly understood. Here we examined how the removal of 50 or approximately 100% of cotyledon area from 7-d-old chalk grassland seedlings affected subsequent plant growth and flowering over a 100-d period. Cotyledon damage had a significant effect on growth during the establishment phase for six of the nine species. For two species, significant effects on plant growth were manifest in 100-d-old plants. Of the five species that flowered, three developed fewer inflorescences or flowered later as a consequence of cotyledon damage suffered as a seedling. Our results show that, in addition to the direct effect of herbivory on seedling mortality, more subtle sublethal effects may also influence plant establishment. Reduced growth as a result of cotyledon damage may have implications for plant competition during the establishment phase, and on subsequent reproductive success at maturity.