Effects of aboveground herbivory on plants with long-term belowground biomass storage

Plant tolerance to herbivory is contingent on multiple traits and adaptive mechanisms, which makes it a complex response with ecological implications. In plants with long-term belowground storage, allocation of biomass to inaccessible parts belowground in response to folivory is a well-recognized tolerance mechanism. In temperate regions, spring growth from buried rootstock is common among winter deciduous plants and is often followed by regrowth after defoliation, both of which draws resources from the stored reserves. We developed a mathematical model to analyze this tolerance response in a winter deciduous plant with long-term belowground biomass when it is defoliated by a specialist insect folivore. The model explores how three closely associated traits—(1) belowground biomass allocation to roots, (2) spring utilization of stored reserves, and (3) post-defoliation regrowth capacity—modulate the persistence and dynamics of the plant and herbivore populations. Model results show that allocation to belowground storage is not only a critical component of tolerance but also influences the herbivore population dynamics in ways that depend on how and when plant biomass is allocated and used. Low belowground biomass allocation and high storage utilization combined with poor photosynthetic growth caused extirpation of the plant population by the defoliating insects. Stable coexistence of the plant at low biomass along with its specialist insect required a moderate amount of post-herbivory belowground allocation. High values of belowground biomass allocation, storage utilization, and photosynthetic growth resulted in sustained cycles of the herbivore and plant populations. Interestingly, utilization of stored reserves had conflicting influence on above and belowground biomass, and strongly affected herbivore population dynamics. Our model thus highlights the complexity of tolerance response when it involves multiple traits and mechanisms as evinced by winter deciduous plants. We close by discussing the implications of our findings for the contributions of defoliating insects to biocontrol programs.     

Life-history variation in the short-lived herb Rorippa palustris: The role of carbon storage

Carbon storage is commonly found among perennials, but only rarely in annuals. However, many short-lived species may behave as annuals or short-lived perennials depending on the date of germination, photoperiod or disturbance. Due to the trade-off between investments into current reproduction vs. survival, these life-history modes presumably differ in carbon allocation. In this study, we aimed to evaluate how carbon storage is affected by germination date and disturbance in an outdoor pot experiment with the short-lived Rorippa palustris. Plants from autumnal and summer cohorts were injured in different ontogenetic stages (vegetative, flowering and fruiting) and the starch content in roots was assessed. Plants from the autumnal cohort invested more carbon into growth and reproduction, whereas plants from the summer cohort invested preferentially into reserves. However, injury changed the allocation pattern: in plants from the autumnal cohort, injury prevented allocation to reproduction and thus injured plants had a larger carbon storage at the end of the season than control plants; injury at the flowering and fruiting stage caused depletion of reserves for regrowth in plants from the summer cohort, resulting in lower starch reserves compared to control plants. We suggest that life-history variation in R. palustris can be caused by changes in its carbon economy: when all resources could not be used for flowering due to weak photoinduction or loss of flowering organs due to injury, part of the resources is stored for over wintering and reproduction in the next year.     

Abiotic stresses increase plant regeneration ability

In disturbed habitats, vegetative regeneration is partly ruled by plant reserves and intrinsic growth rates. Under nutrient-limiting conditions, perennial plants tend to exhibit an increased allocation to storage organs. Under mechanically stressful conditions, plants also tend to increase allocation to below-ground biomass and storage organs. We tested whether those stresses acting differently on plants (nutrient level versus mechanical forces) led to similar effect on storage organs and regeneration ability. We measured, for an aquatic plant species, (1) the size and allocation to storage organs (stems) and (2) the regeneration ability of the storage organs. Plant stems were collected in 4 habitats ranked along a nutrient stress gradient, and having encountered null versus significant mechanical stress (flowing water). All stems were placed in similar neutral conditions and left for a period of 6 weeks before measuring their survival and growth. Dry mass allocation to the storage organ (stem) was higher in stressful habitats. Moreover, stress encountered by plants before the experiment significantly affected regeneration: stems of previously stressed plants (i.e. plants that had grown in nutrient-poor or mechanically stressful habitats) survived better than unstressed ones. Stems of plants having encountered mechanical stress before the experiment had increased growth in nutrient-rich habitats but reduced growth in the poorest habitats. These results demonstrate that regeneration could rely on the level of stress previously encountered by plants. Stress could lead to greater regeneration ability following mechanical failure. The possible mechanisms involved in these results are discussed.     

A non-targeted metabolomics approach to quantifying differences in root storage between fast- and slow-growing plants

Life history theory posits that slower-growing species should invest proportionally more resources to storage, structural (e.g. stems) or defence traits than fast-growing species. Previously, we showed that the slower-growing monocarpic plants had lower mortality rates and higher bolting probabilities after two defoliation events. Here, we consider a mechanistic explanation, that the slower-growing species invested relatively more resources to storage. We compared the relative levels of root storage compounds between eight monocarpic species using metabolomic profiling, and characterized plant growth using a size-corrected estimate of relative growth rate (RGR). Growth rate was negatively correlated with the proportional allocation of root metabolites identified as sucrose, raffinose and stachyose and with amino acids known for their roles in nitrogen storage, particularly proline and arginine. The total amount and concentration of energy-corrected carbohydrates were also negatively correlated with RGR. Our results show for the first time that slower-growing species invest proportionally more of their total root metabolites in carbon- and nitrogen-storage compounds. We conclude that the increased investment in these reserves is an important resource allocation strategy underlying the growth-survival trade-off in plants.     

The influence of nitrogen availability on carbon and nitrogen storage in the biennial Cirsium vulgare (Savi) Ten. II. The cost of nitrogen storage

The cost of nitrogen storage to current growth was examined in relation to N availability in the biennial Cirsium vulgare. Plants were grown outdoors, in sand culture, with continuous diel drip irrigation of fertilization medium containing one of five different N concentrations. Plants grown at the highest N concentration stored twice as much N in their tap roots as did plants grown at the lowest N concentration. In high-N-grown plants, the storage of N reserves occurred during the period of maximum growth, at the same time as tap-root production. At the time of maximum biomass, stored N was also at a maximum. During the period following maximum biomass, no additional storage of N occurred. This pattern was observed despite frequent late-season leaf senescence which resulted in a large pool of potentially mobile N which could have been stored at no cost to growth. In low-N-grown plants, the production of tap-root storage tissue and the filling of that tissue with stored N were staggered. Tap-root production and growth occurred during the period of maximum growth, as in the high-N-grown plants. However, filling of the storage tissue with N occurred late in the growing season, when the pool of mobile N from senescent leaves was large. The utilization of this late-season N source occurred with little or no cost to growth, and this N is labelled, according to previous definitions, as ‘accumulated’. The costs of storing N in plants of the different N treatments were calculated using two models based on different growth constraints. In one model, the cost of N storage was represented as lost growth due to allocation of N to storage, rather than to the photosynthetic shoot (i.e. growth was assumed to be limited by carbon acquisition). In the second model, the storage cost was calculated as lost growth due to allocation of N to storage, rather than to the nitrogen-acquiring fine-root system (i.e. growth was assumed to be limited by nitrogen acquisition). In both models, the total cost of N storage was predicted to decrease as N availability decreased due to smaller storage pool sizes in plants of the low-N treatments. The cost of filling the tap root with stored N as a percentage of the total storage cost was also reduced as N availability decreased due to the occurrence of late-season accumulation. By relying, at least in part, on late-season accumulation, plants grown at the lowest three levels of N availability reduced total storage costs by 15 to 22%. The results demonstrate that plants are capable of adjusting their storage patterns in response to low nitrogen availability such that the costs of storage are reduced.     

Influence of initial organic N reserves and residual leaf area on growth, N uptake, N partitioning and N storage in alfalfa (Medicago sativa) during post-cutting regrowth

BACKGROUND AND AIMS: The influence of initial residual leaf area and initial N reserves on N uptake, final N distribution, and yield in alfalfa regrowing after cutting, were studied. METHODS: The effects of two levels of initial residual leaf area (plants cut to 15 cm, with (L+) or without (L-) their leaves) and two initial levels of N status [high N (HN) or low N (LN)] on growth, N uptake and N partitioning, allocation and storage after 29 d of post-cutting regrowth were analysed. KEY RESULTS: During most of the regrowth period (8-29 d after the initial harvest), HN and L+ plants had higher net N uptake rates than LN and L- plants, respectively, resulting in a greater final mineral N uptake for these treatments. However, the final partitioning of exogenous N to the regrowing shoots was the same for all treatments (67 % of total exogenous N on average). Final shoot growth, total plant N content, and N allocation to the different taproot N pools were significantly lower in plants with reduced initial leaf area and initial N reserve status. CONCLUSIONS: Although both initial residual leaf area and initial N reserves influenced alfalfa regrowth, the residual leaf area had a greater effect on final forage production and N composition in the taproot, whereas the N uptake rate and final total N content in plant were more affected by the initial N reserve status than by the residual leaf area. Moreover, N storage as proteins (especially as vegetative storage proteins, rather than nitrate or amino acids) in the taproot allowed nitrate uptake to occur at significant rates. This suggests that protein storage is not only a means of sequestering N in a tissue for further mobilization, utilization for growth or tissue maintenance, but may also indirectly influence both N acquisition and reduction capacities.     

Non-structural carbohydrate accumulation and use in an understorey rain-forest shrub and relevance for the impact of leaf herbivory

1. Piper arieianum, an evergreen, understorey shrub of lowland moist forests of Central and South America, exhibits marked seasonal variation in reproductive activity even though climatic variation is low at the study site. Despite a lack of climatic seasonality, previous experimental leaf removal suggested that carbohydrate accumulation is seasonal, occurring prior to flowering.
2. We first tested the hypothesis that carbohydrates necessary for reproduction are accumulated prior to flowering, rather than during or after. By measuring non-structural carbohydrate production in the form of glucose and starch we found that the concentration of these reserves is greatest 1–3months before flowering, decreasing by 50% during peak fruit maturation.
3. The hypothesis that reproduction was the cause of this decrease in carbohydrate reserves was then tested by comparing reserves in plants that were prevented from flowering with those that flowered and produced fruit naturally. As predicted, reserves declined more in flowering than in non-flowering plants. A smaller decline in reserves of non-flowering plants was accompanied by greater stem and leaf production, suggesting that stored carbohydrates are also required for growth.
4. Because concentrations of non-structural carbohydrates were similar in roots, stems and leaves, and because the greatest amount of plant biomass is in stems for plants of a range of sizes, stems appear to be the main storage site of carbohydrate reserves in this plant species.
5. These results, together with previous studies, demonstrate that the impact of leaf herbivory on seed production in P. arieianum depends on the timing of that herbivory relative to the accumulation and use of non-structural carbohydrates.     

Effect of abandonment and plant classification on carbohydrate reserves of meadow plants

We studied the effect of cessation of management on carbohydrate reserves of plants in meadows with different environmental characteristics and plant composition. We recorded storage carbohydrates and seasonal changes for 40 plant species. We asked whether there are differences in responses of carbohydrate reserves in forbs versus graminoids and in plants storing starch versus plants storing osmotically active carbohydrates. We analysed belowground organs before the meadows were mown and at the end of the vegetation season in mown versus recently abandoned plots. Whereas starch and fructans were widely distributed, raffinose family oligosaccharides were the main carbohydrate reserves of the Lamiaceae and Plantago lanceolata. Properties of carbohydrate reserves differed between forbs and graminoids but no difference was found between plants storing starch versus osmotically active carbohydrates. Graminoids had lower carbohydrate concentrations than forbs. We observed a positive effect of mowing on carbohydrate concentrations of graminoids in the dry, calcium-rich meadow and higher seasonal fluctuations of these values in the acid, wet meadow, suggesting that local factors and/or the species pool affect carbohydrate reserves. Despite local conditions, graminoids represent a distinct functional group in meadows from the point of view of their storage economy. We suggest that as well as growth, storage processes should also be considered for understanding the functioning of meadow plant communities.     

Resource availability and the trichome defenses of tomato plants

We conducted two experiments to determine how resource availability influenced allocation by tomato (Lycopersicon esculentum) to trichomes, and how different patterns of trichome allocation by plants grown in different resource environments might then influence the behavior of tobacco hornworm (Manduca sexta) caterpillars. In the first experiment we used high and low levels of light and water, and then, using scanning electron microscopy, determined trichome densities on the leaves and stems. We sampled leaves and stems at several places throughout the plant to determine whether there were within-plant differences in allocation to trichomes. The results of the first experiment showed that resource availability influenced allocation to trichome growth. Patterns in high and low-light supported both the growth-differentiation balance hypothesis (GDBH) and the carbon-nutrient balance hypothesis (CNBH). However, the GDBH was not supported by differences among water treatments. Contrary, to predictions of the GDBH, plants with intermediate growth did not have the highest trichome densities, and plants with similar growth differed in trichome density. Possible biological and artifactual explanations are discussed. The first experiment also showed that there was within-plant variation in allocation to trichomes, and that plant resource availability may influence within-plant variation in allocation to trichomes. In the second experiment, we grew plants in high and low-light, and then monitored the behavior of tobacco hornworms on the stems of these plants in the laboratory. This experiment demonstrated that the light environment that tomato plants were grown in influenced the resting behavior of caterpillars. Furthermore, it demonstrated that both glandular and non-glandular trichomes impeded caterpillars from searching for food. Overall, this study indicated that plant resource availability can influence allocation to trichome defenses, and that these differences may affect insect herbivores.     

Energy acquisition and retention by age‐0 and age‐1 paddlefish Polyodon spathula (Walbaum, 1792) in relation to size,growth, and rearing conditions in two Great Plains reservoirs and hatchery ponds

The objective of this study was to investigate lipid accumulation and storage in age‐0 and age‐1 paddlefish Polyodon spathula (Walbaum, 1792) in relation to age, stock, year, and growth. Juvenile paddlefish were collected from three locations in North Dakota and Montana, USA, during July and August of 2011 and 2012 and proximate analysis was used to determine lipid content. RNA/DNA ratios were used as an index of growth rates. Differences in age‐based lipid accumulation and storage in juvenile paddlefish suggest a split allocation between growth and lipid storage, with growth being the highest initial priority and emphasis on energy storage occurring at a larger size, later in life. Differences in lipid allocation between stocks indicate that allocation is influenced by hatchery/wild rearing conditions. Differences within and between year‐classes are consistent with field evidence observed in 2012 of a strong 2011 year‐class, and indicate that during productive times, paddlefish may allocate energy to both body growth and lipid reserves, and that allocation differs among years. The lack of a relationship between RNA/DNA ratio and lipid does not support a physiologically exclusive allocation strategy between growth and lipid. Evidence from this and other studies suggests rather that an emphasis on growth, some energy storage, and a large rostrum size in relation to overall fish length in age‐0 and age‐1 fish, may be adaptive in avoiding predation while accruing necessary energy reserves for overwintering. Although this study also provides reference information regarding proximate composition of wild and hatchery origin juvenile paddlefish, much more study is needed into the relationships among growth, low and high lipid groups, lipid allocation in juvenile paddlefish as well as the existence and timing of allocation changes between growth and storage. To aid in understanding paddlefish survival and year‐class strengths, these relationships also need to be linked to inter‐annual differences in early rearing environments for age‐0 and age‐1 fish.     

No evidence for evolutionarily decreased tolerance and increased fitness in invasive Chromolaena odorata: implications for invasiveness and biological control

Tolerance, the degree to which plant fitness is affected by herbivory, is associated with invasiveness and biological control of introduced plant species. It is important to know the evolutionary changes in tolerance of invasive species after introduction in order to understand the mechanisms of biological invasions and assess the feasibility of biological control. While many studies have explored the evolutionary changes in resistance of invasive species, little has been done to address tolerance. We hypothesized that compared with plants from native populations, plants from invasive populations may increase growth and decrease tolerance to herbivory in response to enemy release in introduced ranges. To test this hypothesis, we compared the differences in growth and tolerance to simulated herbivory between plants from invasive and native populations of Chromolaena odorata, a noxious invader of the tropics and subtropics, at two nutrient levels. Surprisingly, flower number, total biomass (except at high nutrient), and relative increase in height were not significantly different between ranges. Also, plants from invasive populations did not decrease tolerance to herbivory at both nutrient levels. The invader from both ranges compensated fully in reproduction after 50?% of total leaf area had been damaged, and achieved substantial regrowth after complete shoot damage. This strong tolerance to damage was associated with increased resource allocation to reproductive structures and with mobilization of storage reserves in roots. The innately strong tolerance may facilitate invasion success of C. odorata and decrease the efficacy of leaf-feeding biocontrol agents. Our study highlights the need for further research on biogeographical differences in tolerance and their role in the invasiveness of exotic plants and biological control.     

Root starch storage and allocation patterns in seeder and resprouter seedlings of two Cape Erica (Ericaceae) species

Post-fire sprouting of dormant buds in resprouter plants is facilitated by stored carbohydrate reserves, with starch being the critical reserve. Starch is mainly stored in xylem parenchyma ray tissue of woody underground organs, such as burls, lignotubers, and roots. We carried out a comparative analysis of the pattern of starch storage and the proportion of parenchymatic ray tissue in the upper root or cotyledonary region of seedlings from seeder and resprouter forms within two Cape Erica (Ericaceae) species: E. coccinea L. and E. calycina L., which were raised in the greenhouse under controlled irrigation. We also explored the root-to-shoot allocation patterns of seeder and resprouter seedlings in these two species. Resprouter seedlings of both species showed higher relative amounts of upper-root starch and upper-root storage tissue as well as a higher root-to-shoot allocation than their seeder counterparts. Pronounced swelling of the upper root region suggests ontogenetic development of a lignotuber in the resprouter forms of the two Erica species. The distinct allocation of starch in roots seems to be genetically determined and would account for the apparent differences in the root-to-shoot allocation patterns between both regeneration forms from the early seedling stage.     

How do nutrients drive growth?

The relationship between plant nutrient concentration and relative growth rate (RGR) was studied under non-steady state conditions using data from a new N interruption experiment with young lettuce plants grown hydroponically in the glasshouse. RGRs estimated from the fit of a versatile growth model were shown to decline curvilinearly with plant N concentration as N deficiency increased. Similar curvilinear relationships were also derived when the same model was used to reanalyse data for N, P and K interruption treatments from other experiments previously published in the literature. These results clearly indicate that the rate of remobilisation of nutrient reserves varies with the nutrient status of the plant. This contrasts with the linear relationships observed where the changes in plant N concentration occurred solely as a response to increasing plant age, or when plants were grown under steady state conditions with constant relative nutrient addition rates. These differences in the pattern of response provide strong evidence to support the hypothesis that the form of the relationship between RGR and plant nutrient concentration can vary depending upon whether a plant's external supplies or internal reserves of a particular nutrient are more limiting.     

Effects of apical damage on plant growth and male and female reproductive investments in <Emphasis Type="Italic">Ambrosia artemisiifolia</Emphasis>, a wind-pollinated plant

In wind-pollinated plants, apical damage may decrease male fitness by reducing height-dependent pollen dispersal distance, but may not affect female fitness because plant height is not always correlated with female fitness. We hypothesized that Ambrosia artemisiifolia responds to apical damage by (1) restoring plant height through compensatory growth from lateral buds, and/or (2) increasing the sex allocation to female function to compensate for the loss of male fitness. We tested these hypotheses by comparing a group of experimental removal of the apical meristem with three control groups and by field surveys on apically damaged plants. Experimental apical damage suppressed main stem growth, but promoted vertical secondary growth from lateral buds. These responses resulted in compensation of stem height in the apically damaged plants to the same height as one of three control groups. The numbers of male and female flowers and male racemes did not differ between damaged and undamaged plants, indicating that apically damaged plants did not change their sex allocation. Therefore, our results support our first hypothesis. The results of a field survey of naturalized populations also supported the first hypothesis in that plant height and the number of male racemes did not change in plants with apical damage. Consequently, our results suggest that A. artemisiifolia has a high ability of fitness compensation after apical damage by restoring height and male function. This ability may contribute to its invasiveness in disturbed habitats.     

The Utilization of Carbohydrate and Nitrogen Reserves by Taxus During Its Spring Growth Period

The spring growth and the utilization of carbohydrate and nitrogen reserves in this growth was studied in Taxus media cv. Hicksii plants 0, 2, 4 and 6 weeks after the plants started growing in the spring. The effect of nitrogen applied the previous season on the storage and utilization of the carbohydrate and nitrogen reserves during spring growth was determined. The plants were separated into buds (all new growth), stems, needles (those produced the previous season) and roots and analyzed for changes in total nitrogen, basic and non-basic amino acids, total available carbohydrate, sugars, hemicelluloses, organic acids and chlorophyll. The bulk of the soluble nitrogen reserves were stored as arginine in the stems and old needles. With the onset of spring growth, arginine nitrogen was converted to other amino acids which accumulated in the new growth (buds). The roots, stems and needles of plants grown under high nitrogen levels always contained more total nitrogen than those grown under low nitrogen levels. The bulk of the carbohydrate reserves were stored as hemicelluloses. The plants grown under high nitrogen levels utilized the bulk of the carbohydrate reserves from the roots and smaller amounts from the stems and old needles, while plants grown under low nitrogen levels used only the reserves in the roots. In the low nitrogen plants, carbohydrates accumulated in the needles and stems. Both the carbohydrate and nitrogen reserves were important in the dry weight increase due to spring growth. However, the nitrogen reserves were the limiting factor and the high nitrogen plants grew twice as much, produced more chlorophyll, and utilized more nitrogen and carbohydrate reserve in spring growth than low nitrogen plants. The additional chlorophyll allowed the production of more carbohydrates and these additional carbohydrates were used in increased growth rates, while in the low nitrogen plants the carbohydrate produced was less and accumulated within the plant.     

Herbivore-induced shifts in carbon and nitrogen allocation in red oak seedlings

* A dual-isotope, microcosm experiment was conducted with Quercus rubra (red oak) seedlings to test the hypothesis that foliar herbivory would increase belowground carbon allocation (BCA), carbon (C) rhizodeposition and nitrogen (N) uptake. Plant BCA links soil ecosystems to aboveground processes and can be affected by insect herbivores, though the extent of herbivore influences on BCA is not well understood in woody plants. * Microcosms containing 2-yr-old Q. rubra seedlings and soil collected from the Coweeta Hydrologic Laboratory (NC, USA) were subjected to herbivory or left as undamaged controls. All microcosms were then injected with 15N-glycine and pulsed with 13CO2. * Contrary to our hypothesis, herbivore damage reduced BCA to fine roots by 63% and correspondingly increased allocation of new C to foliage. However, 13C recoveries in soil pools were similar between treatments, suggesting that exudation of C from roots is an actively regulated component of BCA. Herbivore damage also reduced N allocation to fine roots by 39%, apparently in favor of storage in taproot and stem tissues. * Oak seedlings respond to moderate insect herbivore damage with a complex suite of allocation shifts that may simultaneously increase foliar C, maintain C rhizodeposition and N assimilation, and shift N resources to storage.     

Interspecific and intraspecific variation in tree seedling survival: effects of allocation to roots versus carbohydrate reserves

We examined interspecific and intraspecific variation in tree seedling survival as a function of allocation to carbohydrate reserves and structural root biomass. We predicted that allocation to carbohydrate reserves would vary as a function of the phenology of shoot growth, because of a hypothesized tradeoff between aboveground growth and carbohydrate storage. Intraspecific variation in levels of carbohydrate reserves was induced through experimental defoliation of naturally occurring, 2-year-old seedlings of four northeastern tree species –Acer rubrum, A. saccharum, Quercus rubra, and Prunus serotina– with shoot growth strategies that ranged from highly determinate to indeterminate. Allocation to root structural biomass varied among species and as a function of light, but did not respond to the defoliation treatments. Allocation to carbohydrate reserves varied among species, and the two species with the most determinate shoot growth patterns had the highest total mass of carbohydrate reserves, but not the highest concentrations. Both the total mass and concentrations of carbohydrate reserves were significantly reduced by defoliation. Seedling survival during the year following the defoliation treatments did not vary among species, but did vary dramatically in response to defoliation. In general, there was an approximately linear relationship between carbohydrate reserves and subsequent survival, but no clear relationship between allocation to root structural biomass and subsequent survival. Because of the disproportionate amounts of reserves stored in roots, we would have erroneously concluded that allocation to roots was significantly and positively related to seedling survival if we had failed to distinguish between reserves and structural biomass in roots. Received: 14 December 1999 / Accepted: 2 June 1999     

Adaptive significance of nitrogen storage in Bistorta bistortoides,an alpine herb

Summary Studies were conducted to examine the importance of nitrogen storage to seasonal aboveground growth in the alpine herb Bistorta bistortoides. Stored reserves accounted for 60% of the total nitrogen allocated to the shoot during the growing season. The stored nitrogen was equally partitioned between preformed buds of the shoot and the roots/rhizome. Reliance on stored N was similar in populations of a 105-day growing season site and of a 75-day growing season site. Contrary to our initial hypothesis, stored nitrogen reserves were not used to extend the growing season of this species into the late-spring when soils are still cold, and saturated with snow-melt water. The time at which stored nitrogen was used to initiate shoot growth coincided with the time of root initiation, rapid soil warming, and near maximum soil concentrations of NO _inf3 ^sup– and NH _inf4 ^sup+ . Thus, nitrogen demand and soil nitrogen supply were both high at the same time. The importance of nitrogen storage in this species appeared to be in satisfying the high demand of simultaneous vegetative and reproductive growth during the early-growing season after soils thawed. The initiation of rapid leaf and inflorescence growth occurred in mid-June in both sites. The maximum pool size of shoot nitrogen (maximum nitrogen demand) occurred only 12 days later in the long season site, and 28 days later in the short season site. The early-season utilization of nitrogen stores allows plants of this species to initiate reproductive allocation at the same time vegetative tissues are exhibiting maximal growth rates. By releasing vegetative and reproductive growth from competition for nitrogen, seeds could mature early in the alpine growing season, before the frost probability sharply increases in mid-August.     

Influence of summer sowing dates, N fertilization and irrigation on autumn VSP accumulation and dynamics of spring regrowth in alfalfa (Medicago sativa L.).

Herbage yield of alfalfa (Medicago sativa L.) depends on forage management or environmental conditions that change C and N resource acquisition, and endogenous plants factors such as root organic reserves and number of active meristems. The aim of this work is to study the influence of two sowing dates in summer (12 July or 9 August), N fertilization (0 or 100 kg ha(-1)) and/or irrigation applied during the first year of alfalfa establishment on (i) the accumulation of N organic reserves (soluble proteins and more specifically vegetative storage protein) in taproots during autumn, (ii) the number of crown axillary meristems present at the end of winter and (iii) the dynamics of spring shoot growth. Delaying the sowing date for one month reduced root growth and root N storage, especially vegetative storage proteins (VSP) during autumn. Irrespective of sowing dates, N fertilization did not affect root biomass, number of crown buds, total root N, root soluble protein or VSP concentrations. By contrast, water deficiency during alfalfa establishment in the early summer reduced both root growth and N reserve accumulation. When spring growth resumed, there is a significant linear relationship between leaf area development and soluble protein and VSP concentrations in taproots, and also the number of crown buds. The results showed that an early sowing date and adequate water status during the summer allowed alfalfa plants to accumulate N reserves by increasing taproot mass and soluble protein concentrations, especially VSPs. This resulted in rapid shoot regrowth rates the following spring.     

Hypocotyl of seedlings of the large-seeded species Araucaria angustifolia: an important underground sink of the seed reserves

Araucaria angustifolia exhibits cryptogeal germination, where the root–hypocotyl axis emerges first and penetrates into the soil. In Araucaria bidwillii, the whole process of transferring reserves from the seed to the seedling takes place before shoot emergence, and there is a major storage of these reserves in the underground hypocotyl, which assumes a tuberous form. In A. angustifolia, the shoot emerges before seed reserves are depleted. Though it does not grow like a tuber, the hypocotyl of A. angustifolia grows thicker than the adjacent taproot during initial growth, and we hypothesize that it may act as a major sink for seed reserves during this stage. The study tests this hypothesis by evaluating changes in the mass of different plant parts during initial growth. Four harvests were conducted during a ~6-month period to compare the dry mass of different fractions (attached seed, seedling, its shoot and root and the hypocotyl) of seedlings growing under darkness and high light. While seed reserves were still being depleted, the hypocotyl mass showed an initial increase and then a reduction. This was more abrupt when light was available. After seed mass had stabilized, the mass of the hypocotyl continued to decrease in the dark-grown seedlings, but showed a second increase in the light-grown ones. Results confirm the hypothesis that the hypocotyl represents a major sink for the seed reserves of A. angustifolia, acting as an underground storage structure for the growing seedling. Its reserves seem to be important for sustaining initial shoot growth and might also act as a storage sink for photosynthates.