Flooding effects on plants recovering from defoliation in Paspalum dilatatum and Lotus tenuis

Background and Aims

Flooding and grazing are major disturbances that simultaneously affect plant performance in many humid grassland ecosystems. The effects of flooding on plant recovery from defoliation were studied in two species: the grass Paspalum dilatatum, regrowing primarily from current assimilation; and the legume, Lotus tenuis, which can use crown reserves during regrowth.

Methods

Plants of both species were subjected to intense defoliation in combination with 15 d of flooding at 6 cm water depth. Plant recovery was evaluated during a subsequent 30-d growth period under well-watered conditions. Plant responses in tissue porosity, height, tiller or shoot number and biomass of the different organs were assessed.

Key Results

Flooding increased porosity in both P. dilatatum and L. tenuis, as expected in flood-tolerant species. In P. dilatatum, defoliation of flooded plants induced a reduction in plant height, thus encouraging the prostrated-growth response typical of defoliated plants rather than the restoration of contact with atmospheric oxygen, and most tillers remained submerged until the end of the flooding period. In contrast, in L. tenuis, plant height was not reduced when defoliated and flooded, a high proportion of shoots being presented emerging above water (72 %). In consequence, flooding plus defoliation did not depress plant recovery from defoliation in the legume species, which showed high sprouting and use of crown biomass during regrowth, whereas in the grass species it negatively affected plant recovery, achieving 32 % lower biomass than plants subjected to flooding or defoliation as single treatments.

Conclusions

The interactive effect of flooding and defoliation determines a reduction in the regrowth of P. dilatatum that was not detected in L. tenuis. In the legume, the use of crown reserves seems to be a key factor in plant recovery from defoliation under flooding conditions.Key words: Allocation, defoliation, flooding, Lotus tenuis, Paspalum dilatatum, submergence     

Escape from water or remain quiescent? Lotus tenuis changes its strategy depending on depth of submergence

Background and Aims

Two main strategies that allow plants to cope with soil waterlogging or deeper submergence are: (1) escaping by means of upward shoot elongation or (2) remaining quiescent underwater. This study investigates these strategies in Lotus tenuis, a forage legume of increasing importance in areas prone to soil waterlogging, shallow submergence or complete submergence.

Methods

Plants of L. tenuis were subjected for 30 d to well-drained (control), waterlogged (water-saturated soil), partially submerged (6 cm water depth) and completely submerged conditions. Plant responses assessed were tissue porosity, shoot number and length, biomass and utilization of water-soluble carbohydrates (WSCs) and starch in the crown.

Key Results

Lotus tenuis adjusted its strategy depending on the depth of submergence. Root growth of partially submerged plants ceased and carbon allocation prioritized shoot lengthening (32 cm vs. 24·5 cm under other treatments), without depleting carbohydrate reserves to sustain the faster growth. These plants also developed more shoot and root porosity. In contrast, completely submerged plants became quiescent, with no associated biomass accumulation, new shoot production or shoot elongation. In addition, tissue porosity was not enhanced. The survival of completely submerged plants is attributed to consumption of WSCs and starch reserves from crowns (concentrations 50–75 % less than in other treatments).

Conclusions

The forage legume L. tenuis has the flexibility either to escape from partial submergence by elongating its shoot more vigorously to avoid becoming totally submerged or to adopt a non-elongating quiescent strategy when completely immersed that is based on utilizing stored reserves. The possession of these alternative survival strategies helps to explain the success of L. tenuis in environments subjected to unpredictable flooding depths.     

Phosphorus reserves increase grass regrowth after defoliation

Accumulation of P above levels that promote growth, a common plant response called “luxury consumption”, can be considered as a form of reserve to support future growth when the nutrient can subsequently be mobilized. However, the effect of P reserves on regrowth following defoliation has not been demonstrated. We tested the hypothesis that P luxury consumption increases plant tolerance to defoliation. We performed two experiments with four grass species from a continuously grazed temperate grassland in the Flooding Pampa (Argentina). The first experiment, aimed at generating P luxury consumption by fertilization, resulted in one species (Sporobolus indicus) showing luxury consumption. In this way, we were able to obtain plants of S. indicus with similar biomass but contrasting amounts of P reserves. The second experiment evaluated the subsequent regrowth following defoliation on a P-free medium of these plants differing in P reserves. Regrowth was larger for plants that had shown P luxury consumption during a previous period than for plants with lower levels of P reserves. During regrowth these plants showed a clear pattern of P remobilization from the stubble, crown, and root compartments to the regrowing tissue, in addition to a likely reutilization of P present in leaf-growth zones. This work is the first showing that high levels of P reserves can confer tolerance to defoliation by promoting compensatory growth under P deficiency.     

Flooding effects on rapid responses of the invasive plant Alternanthera philoxeroides to defoliation

In experiments under controlled growth conditions it was examined how flooding affected the responses of the invasive plant Alternanthera philoxeroides to defoliation. In drained and flooded conditions, plants were subjected to five defoliation levels: 0, 10, 50, 90% removal of leaf tissue and apex removal (90% leaf tissue plus apical bud removal). Plants were harvested weekly for five weeks. In drained conditions, plant biomasses including total biomass, shoot biomass and root biomass after 50% defoliation rapidly recovered to the control plant level. They were significantly lower for the 90% defoliation and apex removal treatments compared to control plants throughout the experiment. In flooded conditions, total biomass and shoot biomass after 50% defoliation, 90% defoliation, and apex removal treatments could return to control plant levels before the end of the experiment. In 90% defoliation and apex removal treatments root to shoot biomass ratios of both drained and flooded plants were initially much higher than in control plants, but the difference disappeared rapidly. The final biomasses decreased with increased defoliation intensity in drained conditions, but no significant difference was generally found in any of the defoliation treatments in flooded conditions. The rapid re-growth of A. philoxeroides plants after defoliation may partly be responsible for its invasion success. However, defoliation capable of removing 90% of the leaf tissue may be desirable in restricting the growth of this invasive species in drained conditions.     

Nitrogen Reserve Mobilization during Regrowth of Medicago sativa L. (Relationships between Availability and Regrowth Yield)

An experiment was designed to study the role of N and C reserves on regrowth of the shoots following defoliation of forage species. Starch and N accumulation in root and crown tissue of nonnodulated Medicago sativa L. were modified during regrowth by applying different levels of N and different cutting heights. Plants were obtained with similar crown and root dry weights, but having either low starch and high tissue N or high starch and low tissue N. The plants were then submitted to a second defoliation and supplied with optimal N nutrition, and N flow from reserve was quantified using pulse-chase 15N labeling. Maximum yields following the second regrowth were obtained from those plants having a high tissue N, despite their low level of nonstructural carbohydrate. When N in the roots and crown exceeded 5 mg N plant-1 at the beginning of regrowth, about 68% was translocated to regrowing shoots. Highly significant correlations were also found between the amounts of N available in roots and crown at the beginning of regrowth and (a) the amount of N that was mobilized to new tissues, (b) the amount of N taken up during the regrowth period, and (c) the final shoot yield after 24 d of regrowth. No similar correlations were found for plants that varied in their initial starch content of roots and crown. It is suggested that N reserves were used mainly during the first 10 d after defoliation, and that the resulting aerial growth during this period should be sufficient to restore N2 fixation and/or N uptake to levels equal to those prior to defoliation. These data emphasize (a) the importance of root N reserves in initiating and sustaining new shoot growth, and (b) the need for a re-evaluation of the contribution of C reserves to shoot regrowth.     

Impact of defoliation intensities on plant biomass,nutrient uptake and arbuscular mycorrhizal symbiosis in Lotus tenuis growing in a saline‐sodic soil

The impact of different defoliation intensities on the ability of Lotus tenuis plants to regrowth, mobilise nutrients and to associate with native AM fungi and Rhizobium in a saline‐sodic soil was investigated. After 70 days, plants were subjected to 0, 25, 50, 75 and 100% defoliation and shoot regrowth was assessed at the end of subsequent 35 days. Compared to non‐defoliated plants, low or moderate defoliation up to 75% did not affect shoot regrowth. However, 100% treatment affected shoot regrowth and the clipped plants were not able to compensate the growth attained by non‐defoliated plants. Root growth was more affected by defoliation than shoot growth. P and N concentrations in shoots and roots increased with increasing defoliation while Na⁺ concentration in shoots of non‐defoliated and moderately defoliated plants was similar. Non‐defoliated and moderately defoliated plants prevented increases of Na⁺ concentration in shoots through both reducing Na⁺ uptake and Na⁺ transport to shoots by accumulating Na⁺ in roots. At high defoliation, the salinity tolerance mechanism is altered and Na⁺ concentration in shoots was higher than in roots. Reduction in the photosynthetic capacity induced by defoliation neither changed the root length colonised by AM fungi nor arbuscular colonisation but decreased the vesicular colonisation. Spore density did not change, but hyphal density and Rhizobium nodules increased with defoliation. The strategy of the AM symbiont consists in investing most of the C resources to preferentially retain arbuscular colonisation as well as inoculum density in the soil.     

How to quantify plant tolerance to loss of biomass?

In some plant species the whole shoot is occasionally removed, as a result of specialist herbivory, grazing, mowing, or other causes. The plant can adapt to defoliation by allocating more to tolerance and less to growth and defense. Plant tolerance to defoliation (TOL1) is typically measured as the ratio between the average dry weight of a group of damaged plants and a control group of undamaged plants, both measured some time after recovery. We develop a model to clarify what TOL1 actually measures. We advocate keeping regrowth (REG2) and shoot–root ratio, both elements of TOL1, separate in the analysis. Based on a resource trade‐off, exotic Jacobaea vulgaris plants from populations in the USA (no specialist herbivory) are expected to grow faster and be less tolerant than native Dutch populations (with specialist herbivory). Indeed Dutch plants had both a significantly larger fraction biomass in roots and faster regrowth (REG2), while US plants attained the highest weight in the control without defoliation. Using key‐factor analysis, we illustrate how growth rates, regrowth, and shoot–root ratio each contribute to final biomass (plant fitness). Our proposed method gives more insight in the mechanisms that underly plant tolerance against defoliation and how tolerance contributes to fitness.     

Trade-offs between chemical defence and regrowth capacity in Plantago lanceolata

Resistance and tolerance are different strategies of plants to deal with herbivore attack. Since resources are limited and resistance and tolerance serve similar functions for plants, trade-offs between these two strategies have often been postulated. In this study we investigated trade-offs between resistance and one aspect of tolerance, the ability to regrow after defoliation. In order to minimize confounding effects of genetic background and selection history, we used offspring derived from artificial selection lines of ribwort plantain (Plantago lanceolata) that differed in their levels of leaf iridoid glycosides (IGs), allelochemicals that confer resistance to generalist herbivores, to study genetic associations with regrowth ability. We tested whether high-IG plants (1) suffer allocation costs of resistance in terms of reduced shoot and root growth, (2) have reduced regrowth ability (tolerance) after defoliation compared to low-IG plants, and (3) whether such costs are more pronounced under nutrient stress. High-IG plants produced fewer inflorescences and side rosettes than low-IG plants and showed a different biomass allocation pattern, but since neither the vegetative, nor the reproductive biomass differed between the lines, there was no evidence for a cost of IG production in terms of total biomass production under either nutrient condition. High-IG plants also did not suffer a reduced capacity to regrow shoot mass after defoliation. However, after regrowth, root mass of high-IG plants grown under nutrient-poor conditions was significantly lower than that of low-IG plants. This suggests that under these conditions shoot regrowth of high-IG plants comes at a larger expense of root growth than in low-IG plants. We speculate therefore that if there is repeated defoliation, high-IG plants may eventually fail to maintain shoot regrowth capacity and that trade-offs between resistance and tolerance in this system will show up after repeated defoliation events under conditions of low resource availability.     

Regrowth dynamics of <Emphasis Type="Italic">Calamagrostis epigejos</Emphasis> after defoliation as affected by nitrogen availability

Young plants of a rhizomatous grass Calamagrostis epigejos (L.) Roth were grown from seed in nutrient solutions containing nitrogen in concentrations 0.1, 1.0, and 10 mM. After six weeks of cultivation the plants were defoliated and changes in growth parameters and in content of storage compounds were measured in the course of regrowth under highly reduced nitrogen availability. Plants grown at higher nitrogen supply before defoliation had higher amount of all types of nitrogen storage compounds (nitrates, free amino acids, soluble proteins), which was beneficial for their regrowth rate, in spite of lower content of storage saccharides. Amino acids and soluble proteins from roots and stubble bases were the most important sources of storage compounds for regrowth of the shoot. Faster growth of plants with higher N content was mediated by greater leaf area expansion and greater number of leaves. In plants with lower contents of N compounds number of green leaves decreased after defoliation significantly and senescing leaves presumably served as N source for other growing organs. Results suggest that internal N reserves can support regrowth of plants after defoliation even under fluctuating external N availability. Faster regrowth of C. epigejos with more reserves was mediated mainly by changes in plant morphogenesis.     

Partitioning of Nitrogen Derived from N2 Fixation and Reserves in Nodulated Medicago sativa L. During Regrowth

Nodul{macron}ted alfalfa plants were grown hydroponically. Inorder to quantify N₂ fixation and remobilization of N reservesduring regrowth the plants were pulse-chase-labelled with ¹⁵N.Starch and ethanol-soluble sugar contents were analysed to examinechanges associated with those of N compounds. Shoot removalcaused a severe decline in N₂ fixation and starch reserves within6 d after cutting. The tap root was the major storage site formetabolizable carbohydrate compounds used for regrowth; initiallyits starch content decreased and after 14 d started to recoverreaching 50% of the initial value on day 24. Recovery of N₂fixation followed the same pattern as shoot regrowth. Afteran initial decline during the first 10 d following shoot removal,the N₂ fixation, leaf area and shoot dry weight increased sorapidly that their levels on day 24 exceeded initial values.Distribution of ¹⁵N within the plant clearly showed that a significantamount of endogenous nitrogen in the roots was used by regrowingshoots. The greatest use of N reserves (about 80% of N incrementin the regrowing shoot) occurred during the first 10 d and thencompensated for the low N₂ fixation. The distribution of N derivedeither from fixation or from reserves of source organs (taproots and lateral roots) clearly showed that shoots are thestronger sink for nitrogen during regrowth. In non-defoliatedplants, the tap roots and stems were weak sinks for N from reserves.By contrast, relative distribution within the plant of N assimilatedin nodules was unaffected by defoliation treatment. Key words: Medicago sativa L., N₂ fixation, N remobilization, N₂ partitioning, regrowth     

Photosynthesis, N(2) fixation and taproot reserves during the cutting regrowth cycle of alfalfa under elevated CO(2) and temperature

Future climatic conditions, including rising atmospheric CO₂ and temperature may increase photosynthesis and, consequently, plant production. A larger knowledge of legume performance under the predicted growth conditions will be crucial for safeguarding crop management and extending the area under cultivation with these plants in the near future. N₂ fixation is a key process conditioning plant responsiveness to varying growth conditions. Moreover, it is likely to increase under future environments, due to the higher photosynthate availability, as a consequence of the higher growth rate under elevated CO₂. However, as described in the literature, photosynthesis performance is frequently down-regulated (acclimated) under long-term exposure to CO₂, especially when affected by stressful temperature and water availability conditions. As growth responses to elevated CO₂ are dependent on sink-source status, it is generally accepted that down-regulation occurs in situations with insufficient plant C sink capacity. Alfalfa management involves the cutting of shoots, which alters the source-sink relationship and thus the photosynthetic behaviour. As the growth rate decreases at the end of the pre-cut vegetative growth period, nodulated alfalfa plants show photosynthetic down-regulation, but during regrowth following defoliation, acclimation to elevated CO₂ disappears. The shoot harvest also leads to a drop in mineral N uptake and C translocation to the roots, resulting in a reduction in N₂ fixation due to the dependence on photosynthate supply to support nodule function. Therefore, the production of new shoots during the first days following cutting requires the utilization of reduced C and N compounds that have been stored previously in reserve organs. The stored reserves are mediated by phytohormones such as methyl jasmonate and abscisic acid and in situations where water stress reduces shoot production this potentially enables the enhancement of taproot protein levels in nodulated alfalfa, which may lead to these plants being in better condition in the following cut/regrowth cycle. Furthering our knowledge of legume performance under predicted climate change conditions will be crucial for the development of varieties with better adaptation that will achieve greater and more efficient production values. Furthermore, for this purpose it will be necessary to improve existing methodologies and create new ones for phenotype characterization. Such knowledge will provide key information for future plant breeding programs.     

Species-specific trade-offs between regrowth and mycorrhizas in the face of defoliation and phosphorus addition

Arbuscular mycorrhizal fungi (AMF) enhance nutrient provision in exchange for carbon. However, a shift from nutrient to carbon limitation, induced by repeated or intense defoliation, can represent a trade-off between plant regrowth and the maintenance of mycorrhiza. The combined effects of AMF, phosphorus and defoliation on growth of Agropyron elongatum (C₃ grass, low mycorrhizal responsiveness) and Brachiaria brizantha (C₄ grass, high mycorrhizal responsiveness) were explored. Each species was subjected to a manipulative experiment with AMF inoculation (non-inoculated, inoculated), soluble P supply (low, high) and defoliation (non-defoliated, 60% defoliated). In A. elongatum, at low P supply, mycorrhizal plants showed increased growth rates following defoliation without substantial changes in AMF colonization. At high P supply instead, we found a clear trade-off between regrowth and the maintenance of mycorrhiza evidenced by growth depression (biomass and tillers) and lower AMF activity (reduction of arbuscules). In contrast, in B. brizantha, defoliation effects on plant regrowth were independent from AMF at any P supply (no trade-off). This indicates that cost-benefit relationship in defoliated plants is highly context-dependent and may vary with mycorrhizal responsiveness of species. This variation of responses can play a decisive role on plant recovery in pastures and natural grasslands subjected to herbivory.     

Trampling enhances the dominance of graminoids over forbs in flooded grassland mesocosms

Questions: What are the interactive effects of flooding and cattle trampling upon the structural attributes and the floristic composition of a plant community? Do the effects on the plant community persist over an extended recovery period? Location: Flooding Pampa grasslands, Argentina (36°30′ S, 58°30′ W). Methods: We assessed the effects of 40‐d of flooding, trampling and the combination thereof on plant cover and biomass, vertical distribution of foliage and floristic composition in lowland grassland mesocosms. We considered a 120‐d recovery period to evaluate the persistence of flooding and trampling effects on the plant community. Results: Flooding, with or without trampling, increased cover and biomass of the graminoid species, especially marsh grasses, which developed a taller canopy, whereas most of the forb species were negatively affected. This was enhanced by trampling, as the aerial biomass of the dominant legume Lotus tenuis decreased by 90%, while three major forb species disappeared. Trampling under flooding conditions did not reduce the total above‐ground biomass production, as the growth enhancement of graminoids was enough to compensate for the breakdown of the forbs. Below‐ground biomass was lower when both perturbations occurred simultaneously. After 120‐d of recovery, graminoids continued to be dominant while the remaining forbs (including L. tenuis) recovered only partially. Below‐ground biomass recovered fully at the end of the growing season. Conclusions: The combination of flooding and trampling shifts the community co‐dominance of graminoids and forbs towards a persistent dominance of graminoid species. When both perturbations are combined, the above‐ground production of the grassland is unaffected and root biomass is rapidly recovered. However, the loss of the legume L. tenuis deserves attention because this is the unique nitrogen‐fixing species of the ecosystem, which improves the forage quality for livestock production.     

Effect of water availability on tolerance of leaf damage in tall morning glory,Ipomoea purpurea

Resource availability may limit plant tolerance of herbivory. To predict the effect of differential resource availability on plant tolerance, the limiting resource model (LRM) considers which resource limits plant fitness and which resource is mostly affected by herbivore damage. We tested the effect of experimental drought on tolerance of leaf damage in Ipomoea purpurea, which is naturally exposed to both leaf damage and summer drought. To seek mechanistic explanations, we also measured several morphological, allocation and gas exchange traits. In this case, LRM predicts that tolerance would be the same in both water treatments. Plants were assigned to a combination of two water treatments (control and low water) and two damage treatments (50% defoliation and undamaged). Plants showed tolerance of leaf damage, i.e., a similar number of fruits were produced by damaged and undamaged plants, only in control water. Whereas experimental drought affected all plant traits, leaf damage caused plants to show a greater leaf trichome density and reduced shoot biomass, but only in low water. It is suggested that the reduced fitness (number of fruits) of damaged plants in low water was mediated by the differential reduction of shoot biomass, because the number of fruits per shoot biomass was similar in damaged and undamaged plants. Alternative but less likely explanations include the opposing direction of functional responses to drought and defoliation, and resource costs of the damage-induced leaf trichome density. Our results somewhat challenge the LRM predictions, but further research including field experiments is needed to validate some of the preliminary conclusions drawn.     

Interactive effects of defoliation and an AM fungus on plants and soil organisms in experimental legume–grass communities

We established a 13‐week greenhouse experiment based on replicated microcosms to test whether the effects of defoliation on grassland plants and soil organisms depend on plant species composition and the presence of arbuscular mycorrhizal (AM) fungi. The experiment constituted of three treatment factors – plant species composition, inoculation of an AM fungus and defoliation – in a fully factorial design. Plant species composition had three levels: (1) Trifolium repens monoculture (T), (2) Phleum pratense monoculture (P) and (3) mixture of T. repens and P. pratense (T+P), while the AM inoculation and the defoliation treatment had two levels: (1) no inoculation of AM fungi and (2) inoculation of the AM fungus Glomus claroideum BEG31, and (1) no trimming, and (2) trimming of all plant material to 6 cm above the soil surface three times during the experiment, respectively. At the final harvest, AM colonization rate of plant roots differed between the plant species compositions, being on average 45% in T, 33% in T+P and 4% in P. Defoliation did not affect the colonization rate in T but raised the rate from 1% to 7% in P and from 20% to 45% in T+P. Shoot production and standing shoot and root biomass were 48%, 85% and 68% lower, respectively, in defoliated than in non‐defoliated systems, while the AM fungus did not affect shoot production and root mass but reduced harvested shoot mass by 8% in non‐defoliated systems. Of the plant quality attributes, defoliation enhanced the N concentration of harvested shoot biomass by 129% and 96% in P and T+P, respectively, but had no effect in T, while the C concentration of shoot biomass was on average 2.7% lower in defoliated than in non‐defoliated systems. Moreover, defoliation reduced shoot C yield (the combined C content of defoliated and harvested shoot biomass) on average by 47% across all plant species compositions and shoot N yield by 37% in T only. In contrast to defoliation, the AM fungus did not affect shoot N and C concentrations or shoot N yield, but induced 10% lower C yield in non‐defoliated systems and 17% higher C yield in defoliated T. In roots, defoliation led to 56% and 21% higher N concentration in P and T+P, respectively, and 28% higher C concentration in P, while the mycorrhizal fungus lowered root N concentration by 9.7% in defoliated systems and had no effect on root C concentrations. In the soil, the nematode community was dominated by bacterivores and the other trophic groups were found in a few microcosms only. Bacterivores were 45% more abundant in defoliated than in non‐defoliated systems, but were not affected by plant species composition or the AM fungus. Soil inorganic N concentration was significantly increased by defoliation in T+P, while the mycorrhizal fungus reduced NH₄–N concentration by 40% in T. The results show that defoliation had widespread effects in our experimental systems, and while the effects on plant growth were invariably negative and those on bacterivorous nematodes invariably positive, most effects on plant C and N content and soil inorganic N concentration varied depending on the plant species present. In contrast, the effects of defoliation did not depend on the presence of the AM fungus, which suggests that while the relative abundance of legumes and grasses is likely to have a significant role in the response of legume–grass communities to defoliation, the role of AM fungi may be less important. In line with this, the AM fungus had only a few significant effects on plant and soil attributes in our systems and each of them was modified by defoliation and/or plant species composition. This suggests that the effects of AM fungi in legume–grass communities may largely depend on the plant species present and whether the plants are grazed or not.     

Legume defoliation affects rhizosphere decomposers, but not the uptake of organic matter N by a neighbouring grass

Legume–grass interactions have a great influence on grassland primary production and it was recently shown how defoliation of a legume can increase the transfer of fixed N to a neighbouring grass. It has also been shown that defoliation of a plant can increase soil microbial activity and lead to better soil N availability in the rhizosphere of the defoliated plant. We combined these two perspectives and tested whether defoliation of a legume (Lotus corniculatus) can enhance N nutrition of the neighbouring grass (Holcus lanatus) by increasing growth of soil decomposer biota and the availability of soil organic matter N for grass uptake. We grew mixtures of L. corniculatus and H. lanatus in grassland soil that included ¹⁵N-labelled L. corniculatus litter. In half of the systems, we subjected L. corniculatus to a defoliation treatment mimicking insect larvae feeding. At destructive harvests 1, 3, 9 and 30 days after the last defoliation event, we determined how L. corniculatus defoliation affected decomposer microbes, protozoa and nematodes and whether these changes among decomposers created a feedback on the growth and ¹⁵N uptake of the neighbouring H. lanatus. Defoliation reduced the growth and litter-N uptake, but increased shoot N concentration of L. corniculatus. Of the soil variables measured, defoliation doubled the number of bacterial-feeding protozoa, but did not affect the abundance of decomposer microbes and bacterial- and fungal-feeding nematodes. Defoliation did not have statistically significant effects on H. lanatus shoot growth, shoot N concentration or litter-N uptake. Our results demonstrate how defoliation-induced changes in legume ecophysiology can affect the growth of decomposers in soil. However, these effects did not appear to lead to a significant change in the availability of soil organic N to the neighbouring grass. It seems that when positive effects of legume defoliation on grass N nutrition are found in grassland ecosystems, these are more likely to be explained by direct transfer of fixed N rather than changes in the availability of soil organic matter N.     

Dynamics of biomass and N accumulation of alfalfa under three N fertilization rates

The dynamics of biomass and N accumulation following defoliation of alfalfa and the application of N fertilization has rarely been studied under field conditions, particularly in the seeding year. Our objectives were to determine the effect of N fertilization on the dynamics of biomass and N accumulation during the first regrowth of alfalfa in the seeding year, and to determine if a model describing critical N concentration developed for established stands could be used in the seeding year. In two separate experiments conducted in 1992 and 1993, the biomass and N accumulation of alfalfa grown with three N rates (0, 40 and 80 kg N ha^-1) were determined weekly. Maximum shoot growth was reached with 40 kg N ha^-1 in 1992, and maximum shoot growth was not reached with the highest N fertilization rate in 1993. Nitrogen fixation, root N reserves and soil inorganic N uptake when no N was applied were, therefore, not sufficient to ensure non-limiting N conditions, particularly when growth rates were the highest between 14 to 21 d after defoliation. Nitrogen fertilization increased shoot biomass accumulation in the first 21 d of regrowth, biomass partitioning to the shoots and shoot and taproot N concentrations. The model parameters of critical N concentration developed by Lemaire et al. (1985) for established stands of alfalfa were not adequate in the seeding year. The N requirements per unit of shoot biomass produced are greater in the seeding year than on established stands, and this was attributed to a greater proportion of leaves in the seeding year.     

Influence of initial levels of carbohydrates, fructans, nitrogen, and soluble proteins on regrowth of Lolium perenne L. cv. Bravo following defoliation

An experiment was designed to evaluate the role of N and C reserves on regrowth of Lolium perenne cv. Bravo following defoliation. By using two nitrogen fertilization levels together with three photoperiodic conditions, plants with variable contents of water-soluble carbohydrates (43-216 mg g^-1 DW in stubble) and contrasting amounts of nitrogen (7-49 mg g^-1 DW) were obtained. Plants were severely defoliated and regrowth was followed for 28 d under the same environmental conditions. The yield of leaf dry matter at the end of the regrowth period was not related to the initial level of carbohydrate reserves. However, levels of fructan in leaf sheaths and in elongating leaf bases strongly influenced the shoot yield during the first 2 d following defoliation. Fructan exohydrolase activity increased 2-3-fold in sheaths and 3.5-5-fold in elongation leaf bases, suggesting that not only fructans from sheaths but also fructans from immature cells may be used as substrates for growth. In contrast, no direct relationship was found between shoot production and nitrogen or soluble protein accumulation in source organs during early regrowth. A significant correlation existed with the initial amount of soluble proteins in sheaths and in elongating leaf bases after only 6 d of regrowth.     

Coping with herbivory: Photosynthetic capacity and resource allocation in two semiarid Agropyron bunchgrasses

Summary Agropyron desertorum, a grazing-tolerant bunchgrass introduced to the western U.S. from Eurasia, and Agropyron spicatum, a grazing-sensitive bunchgrass native to North America, were examined in the field for photosynthetic capacity, growth, resource allocation, and tiller dynamics. These observations allowed identification of physiological characteristics that may contribute to grazing tolerance in semiarid environments. A uniform matrix of sagebrush, Artemisia tridentata, provided an ecologically relevant competitive environment for both bunch-grass species. Physiological activity, growth, and allocation were also followed during recovery from a severe defoliation treatment and were correlated with tiller dynamics.Potential photosynthetic carbon uptake of both species was dominated by stems and leaf sheaths during June, when maximum uptake rates occurred. For both species, water use efficiency of stems and sheaths was similar to that of leaf blades, but nitrogen investment per photosynthetic surface area was less than in blades. In addition, soluble carbohydrates in stems and sheaths of both species constituted the major labile carbon pools in control plants. Contrary to current theory, these findings suggest that culms from which leaf blades have been removed should be of considerable value to defoliated bunchgrasses, and in the case of partial defoliation could provide important supplies of organic nutrients for regrowth. These interpretations, based on total pool sizes, differ markedly from previous interpretations based on carbohydrate concentrations alone, which suggested that crowns contain large carbohydrate reserves. In this study, crowns of both species contained a minor component of the total plant carbohydrate pool.Following defoliation, A. desertorum plants rapidly reestablished a canopy with 3 to 5 times the photosynthetic surface of A. spicatum plants. This difference was primarily due to the greater number of quickly growing new tillers produced following defoliation. Agropyron spicatum produced few new tillers following defoliation despite adequate moisture, and carbohydrate pools that were equivalent to those in A. desertorum.Leaf blades of regrowing tillers had higher photosynthetic capacity than blades on unclipped plants of both species, but the relative increase, considered on a unit mass, area, or nitrogen basis, was greater for A. desertorum than for A. spicatum. Agropyron desertorum also had lower investment of nitrogen and biomass per unit area of photosynthetic tissues, more tillers and leaves per bunch, and shorter lived stems, all of which can contribute to greater tolerance of partial defoliation.Greater flexibility of resource allocation following defoliation was demonstrated by A. desertorum for both nitrogen and carbohydrates. Relatively more allocation to the shoot system and curtailed root growth in A. desertorum resulted in more rapid approach to the preclipping balance between the root and shoot systems, whereas root growth in A. spicatum continued unabated following defoliation. Nitrogen required for regrowth in both species was apparently supplied by uptake rather than reserve depletion. Carbohydrate pools in the shoot system of both species remained very low following severe defoliation and were approximately equivalent to carbon fixed in one day by photosynthesis of the whole canopy.Dedicated to Drs. Michael Evenari and Konrad Springer     

Compensatory growth response of the legume, Medicago sativa, to defoliation and denodulation

A laboratory study was conducted to determine the effects of defoliation and denodulation on compensatory growth of Medicago sativa (L.). Plants grown hydroponically in clear plastic growth pouches were subjected to 0 and 50% nodule pruning, and 0, 25, 50, and 75% defoliation by clipping trifoliate leaves. An additional experiment was conducted to determine if clipping leaves simulated herbivory by Hypera postica (Gyllenhal) larvae. Previously, we determined that nodule pruning accurately simulated herbivory by Sitona hispidulus (L.) larvae (Quinn & Hall, 1992). Results indicated that denodulation stimulated nodule growth and caused exact compensation in standing and total number of nodules per plant within 15 days and in standing nodule biomass within 22 days of treatment. Denodulation caused a significant reduction (13%) in final shoot biomass, but did not affect significantly final root biomass. Percentage of change in number of trifoliate leaves per plant increased with the level of defoliation. Within 22 days of treatment, total number of trifoliate leaves per plant was similar to controls. However, final standing shoot biomasses were significantly less that controls, indicating undercompensatory growth. Shoot biomasses of the 25-, 50-, and 75%-defoliated plants were 18, 20, and 36% lower than controls, respectively. Nodule biomass per plant was reduced by 24 and 32% in 50- and 75%-defoliated plants, respectively, but was not affected significantly by 25% defoliation. Root biomass was affected by all levels of defoliation. Clipping trifoliate leaves accurately simulated defoliation by H. postica larvae. Our results indicated that partial defoliation affected shoot, root, and nodule biomass of M. sativa, but that partial denodulation only affected shoot biomass.