The effect of defoliation intensity and history on photosynthesis, growth and carbon reserves of two conifers with contrasting leaf lifespans and growth habits

The effects of partial defoliation on photosynthesis, whole-seedling carbon allocation, partitioning and growth were studied for two species with contrasting foliar traits. Field-grown seedlings of deciduous Japanese larch ( Larix leptolepis ) and evergreen red pine ( Pinus resinosa ) were defoliated by hand in early summer for 2 consecutive years. In the first year (1990), seedlings were defoliated by removing the distal 0, 25, 50 or 75% of each needle. In the second year (1991), seedlings were defoliated either 0 or 50%, regardless of previous defoliation treatments. Defoliation had little effect on photosynthesis and starch concentration in whole seedlings of either species in the first year. In the second year, photosynthesis increased in both species in response to the 1991 defoliation treatment, and in red pine also increased in response to the 1990 defoliation treatment. Further, in 1991 both larch and pine had decreased whole-seedling total non-structural carbohydrate concentrations in all seedlings that were defoliated at least once over the 2-yr period. This decrease was noted mostly in the starch component of the non-structural carbohydrates, and was similar in both species. In 1991, biomass was similarly decreased in both species in response to 1991 defoliation. Both species showed overcompensation in total and component biomass in seedlings defoliated by 25% in 1990. Overall, the results do not support the widely held belief that evergreen trees are substantially more affected than deciduous trees by defoliation.     

Regrowth of spring canola (Brassica napus) after defoliation

Aims

Regrowth of dual-purpose canola after grazing is important for commercial success and the aim of this research was to investigate the effects of defoliation on the development, growth, photosynthesis and allocation of carbohydrates.

Methods

We conducted two pot experiments in which defoliation was conducted at multiple intensities with scissors. Experiment 1 determined changes in flowering date due to defoliation while Experiment 2 investigated the effects of defoliation on growth, photosynthesis and allocation of carbohydrates in canola.

Results

Time to the appearance of the first flower was delayed by up to 9 days after the removal of all leaves at the start of stem elongation (GS30), and up to 19 days if the elongating bud was also removed. Stem growth rate decreased by 56–86 % due to defoliation and tap roots did not increase in mass when plants were completely defoliated. Leaf area continued to expand at the same rate as in un-defoliated plants. The new leaf area established per gram of regrowth biomass over 20 days was 158 cm².g^-1 for the complete defoliation treatments compared with 27 cm².g^?1 for the half-defoliated treatment and 13 cm².g^?1 for the un-defoliated treatment. Despite a reduction in total biomass of up to 60 %, the proportion of dry matter partitioned to the leaves was 18 % for all treatments within 20 days after defoliation. Total non-structural carbohydrate levels were reduced rapidly in the stem by day two (predominately sucrose) and the tap root by day four (predominately starch) after defoliation and did not recover to match un-defoliated plant levels within 20 days. Residual leaves on defoliated plants maintained photosynthetic rate compared with the same leaf cohorts on un-defoliated plants in which photosynthetic rate decreased to 39 % by day 12.

Conclusions

The rapid recovery of leaf area in defoliated canola was facilitated by the sustained high photosynthetic rate in remaining leaves, rapid mobilisation of stored sugars (stem) and starch (root), and a cessation of root and stem growth.     

Dry Matter Assimilation and Partitioning in Pepper Cultivars Differing in Susceptibility to Stress-induced Bud and Flower Abscission

Stress-induced abscission of buds and flowers is a constraintto the production of peppers (Capsicum annuum L.). Susceptibilityof pepper cultivars to stress-induced abscission may be relatedto assimilate levels and partitioning. Growth analysis experimentswere conducted in the field to determine whether cultivars differingin susceptibility to stress-induced abscission showed correspondingdifferences in growth rates and dry matter partitioning whensubjected to low light stress. With the stress susceptible cultivar'Shamrock', reduction in net assimilation rate (NAR) and relativegrowth rate (RGR) was greater under low light stress than withthe more tolerant cultivar 'Ace'. Compared to 'Ace', 'Shamrock'partitioned a smaller proportion of dry matter into reproductiveorgans and a larger proportion into expanded leaves. Cultivarsdid not differ in the amount of dry matter partitioned to youngleaves or in the extent to which accumulation of dry matterby young leaves was reduced by low light stress. Other experimentsconducted in growth chambers and greenhouse examined the effectof removal of leaves less than 3 cm long on high temperatureand low light stress-induced bud and flower abscission. Forboth 'Ace' and 'Shamrock', abscission was not altered by removalof these leaves. Preferential partitioning of assimilate toyoung leaves does not appear to be involved in stress-inducedabscission in these pepper cultivars.Copyright 1994, 1999 AcademicPress Pepper (Capsicum annuum L.), abscission, low light stress, high temperature stress, assimilate partitioning, assimilation/growth rate, growth analysis, defoliation     

Phytohormone changes during storage root growth in Beta species

The physiological and morphological factors necessary for efficient accumulation of sucrose in sugar beet (Beta vulgaris L.) are considered in relation to potential uses of plant growth regulators to modify the anatomy of storage roots so as to increase sucrose content and yield. The percentage of sucrose in root fresh and dry matter is closely related to root structure. Sugar beet, mangold and chard are three sub-species of Beta vulgaris that differ considerably in their anatomy, assimilate partitioning, sucrose concentration and root dry matter yield. The concentrations of indole-3-acetic acid (IAA), abscisic acid (ABA) and cytokinins were measured during the growth of the storage root in each of these cultivars. Correlations were found between the phytohormone levels and the formation of secondary cambia and their subsequent cell division and expansion activity.     

Effect of partial defoliation during the vegetative phase on subsequent growth and grain yield of maize

Maize was grown in two densities, 2–47 or 4–94 plants m^-2, and the following treatments imposed: untreated, plants partly defoliated 51 days after sowing, and alternate plants in a row partly defoliated 44 days after sowing. Plants flowered about 82 days after sowing. Leaf area was decreased by 60–64% by defoliation on day 51. Defoliation resulted in decreases in grain yield and grain number of 6–17%, though when alternate plants were defoliated in the higher density there was a substantial decrease in yield and number of grains in defoliated plants, which was largely offset by an increase in adjacent intact plants. When plants were defoliated on day 51 subsequent growth in leaf area was similar to, and that in leaf weight nearly as large as that in untreated plants, while increase in stem weight was substantially less than in untreated plants. By the time of flowering untreated and defoliated plots differed by c. 30% in leaf area. Increments of dry matter after flowering differed by c. 15% between untreated and defoliated plots. The fraction of these increments which entered the grain was c. 90% in both untreated and defoliated plots. When alternate plants in the row were partly defoliated on day 44 their subsequent increase in leaf area was probably 5–16% less than that of the adjacent intact plants. Increments of dry matter after flowering of plots with alternate plants defoliated were 93–95 % of those of untreated plots; leaf efficiency after flowering was slightly greater than in untreated plots. The fraction of the dry matter increment after flowering which entered the grain was c. 88 % in both intact and defoliated plants of the small density, but was 94% in intact plants and 86% in defoliated plants of the large density.     

Seasonal growth of tree lupins (Lupinus arboreus) and the effect of defoliation on leaf production

The growth of tree lupins was investigated in two experiments. In the first, two ages of plant, 4-wk-old seedlings and 1-year-old plants, were transplanted into a ryegrass sward in an upland environment. Growth, in terms of leaf production, branching and stem elongation, was measured over two successive growing seasons. Plant dry matter and nutrient contents were determined at the beginning and end of each growing season. In the first summer, the rate of production of new leaves on the main stem of seedling plants averaged 1.8 leaves per wk and main stem length increased from 5 to 67 cm. On older plants, where floral apices had been initiated on main and primary stems, there was a 3–10 fold increase in secondary branch length. In the second season, there was no effect of plant age on rates of leaf appearance or stem extension; dry matter production was higher than in the first season. In the second experiment, the effect of removal of 0%, 50% or 100% of fully expanded leaves on the subsequent growth of 23-wk-old plants was investigated. During the 7-wk growth period, defoliation promoted the rate of production of mature leaves, and area and dry weight of new laminae were slightly higher in defoliated plants. Defoliation did not affect the concentrations of N, P or K in the new laminae, but P and K concentrations in petioles of defoliated plants were significantly higher than those in intact plants. The results from the experiments are discussed in relation to the potential use of tree lupins as nurse species and biomass crops in hill and upland environments of the UK.     

WHOLE-PLANT RESPONSES OF BRASSICA CAMPESTRIS (CRUCIFERAE) TO ALTERED SINK-SOURCE RELATIONS

Plants may respond to developmentally or environmentally induced changes in the relationship between source (assimilate exporting) and sink (assimilate importing) organs through a number of regulatory mechanisms. The purpose of this work was to describe responses in leaf physiology, organ biomass partitioning, and reproduction of Brassica campestris L. to altered sink-source relations. Partial defoliation early in ontogeny increased sink-source ratio after 1 day and was accompanied by a significant increase in area-based photosynthetic capacity. Assimilate was preferentially partitioned to new leaf tissue at the expense of stem and reproductive tissue so that sink-source ratio and photosynthesis returned to control levels within a week. Bud excision at flower initiation decreased sink-source ratio; plants in this treatment responded through altered biomass partitioning, but there were no detected changes in leaf physiology. Plants that were hand-pollinated had decreased specific leaf weight, and within a week, sink-source ratio increased. Plants that were pollinated and then completely defoliated produced 40% as many seeds as plants that were not defoliated, presumably through stem and/ or fruit photosynthesis, and at the expense of flower production. Together these results illustrate that regulation of sink-source relations can occur through complex responses at several levels of organization over a wide range of time scales.     

Effects of defoliation on growth and yield in groundnut (Arachis hypogea), cowpeas (Vigna unguiculata), soyabean (Glycine max) and green gram (Vigna aurens)

Defoliation reduced the dry weight of stems, pods, grains and size of individual grains in all four of the legume crops studied and the dry weight of flowering inflorescence stalk in the case of cowpeas and green gram only. The adverse effect of defoliation was more pronounced when defoliation was complete than when half of the number of leaves were removed. The greatest reduction in grain yield occurred when the plants were defoliated during the early podding stage, the percentage reduction being 59.7, 79.0, 86.4 and 95.3 in groundnut, cowpeas, soyabeans and green gram respectively when completely defoliated at this stage and 43.3, 14.0, 42.4 and 46.1 respectively when only half defoliated. The results show that assimilates produced by the leaves during the early stages of growth are used in the growth of stems and leaves, but the assimilates produced during the reproductive stage are used mainly for the growth of the pods. In groundnut, pod number and grain weight were positively correlated with stem weight. It appears that defoliation reduced pod number by depressing the growth of stems and this in turn reduced the number of flowering nodes. The reasons for the differences between the crops in their response to the defoliation treatments and the practical implications of the findings in relation to pest and disease control and plucking of leaves for human consumption are discussed.     

Growth and biomass production in potato grown in the hot tropics as influenced by paclobutrazol

Two similar field trials were carried out during 2003 in a hot tropical region of eastern Ethiopia to investigate the effect of leaf and soil applied paclobutrazol on the growth, dry matter production and assimilate partitioning in potato. A month after planting paclobutrazol was applied as a foliar spray or soil drench at rates of 0, 2, 3, and 4 kg a.i. paclobutrazol ha^–1. Plants were sampled during treatment application and subsequently 2, 4, 6 and 8 weeks after treatment application. The data was analyzed using standard growth analyses techniques. None of the growth parameters studied was affected by the method of paclobutrazol application. Paclobutrazol decreased leaf area index, crop growth rate, and total biomass production, and increased specific leaf weight, tuber growth rate, net assimilation rate, and partitioning coefficient of potato. At all harvesting stages, paclobutrazol reduced the partitioning of assimilate to the leaves, stems, and roots and stolons and increased allocation to the tubers. Although paclobutrazol decreased the total biomass production it improved tuber yield by partitioning more assimilates to the tubers. Paclobutrazol improved the productivity of potato under tropical conditions by redirecting assimilate allocation to the tubers.     

Defoliation of Brassica napus increases severity of blackleg caused by Leptosphaeria maculans: implications for dual-purpose cropping

Canola (Brassica napus) crops for grazing and grain (dual-purpose) production provide an economic break-crop alternative for dual-purpose cereals in Australian mixed farming systems. Infection by Leptosphaeria maculans is the most prevalent disease in Australian canola crops with airborne inoculum released throughout the autumn and winter when crops are grazed. Glasshouse and field experiments were conducted to investigate the effect of mechanical defoliation (simulated grazing) on disease severity at plant maturity. In glasshouse experiments, stem canker severity increased from 4% to 24% in severely defoliated plants, but light defoliation had no effect compared with undefoliated control plants. Disease severity was increased with defoliation in all field experiments. Defoliation increased crown canker severity from 22.6% to 39.3% at Wagga Wagga and from 3.0% to 7.1% at Canberra and lodging from 9.6% to 11.9% at Naracoorte in the same set of cultivars assessed at each site. The increase in disease severity with defoliation was less in canola lines with moderate to high levels of stem canker resistance. Plants defoliated before stem elongation tended to develop less disease than those defoliated during the reproductive phase of plant growth. These findings suggest that the impact of grazing on L. maculans infection of canola crops can be minimised by sowing cultivars with a high level of stem canker resistance and grazing during the vegetative stage of plant growth prior to stem elongation. Further research is required to determine whether these management strategies are applicable in canola crops defoliated by grazing animals.     

The effects of thinning and gypsy moth defoliation on wood volume growth in oaks

Stem dissection and dendroecological methods were used to examine the effects of thinning and defoliation by gypsy moth (Lymantria dispar L.) on wood volume increment in oaks (Quercus rubra L., Q. alba L., Q. prinus L.). A model was developed to evaluate radial volume increment growth at three time periods: before defoliation, during defoliation and after defoliation, as a function of species, defoliation intensity and crown position. Volume increment during these same time periods was also compared at different stem locations. Trees were defoliated for two consecutive years and results indicated that volume loss was greater during the second year of defoliation with complete recovery taking 2–3 years after defoliation. Oaks in thinned stands had similar reductions in annual volume increment during defoliation as those in the unthinned stand. Annual volume increment demonstrated a decreasing trend from stump to base of the live crown and volume increment of the lowest log (from stump height to 1.37 m), was always higher than upper log sections, even during defoliation. Both earlywood and latewood increments were reduced during defoliation; however, latewood reductions were distributed along entire stems while earlywood reductions were greater on upper stem sections within the crown.     

Fast replenishment of initial carbon stores after defoliation by the pine processionary moth and its relationship to the re-growth ability of trees

Defoliation by herbivores may alter the source:sink balance of trees leading to transient decreases in carbon (C) stores. When C stores are replenished concurrently with re-growth both processes may compete, store formation proceeding at the expenses of growth. However, the interactions between both processes are not fully understood. We investigated the effects of defoliation by the pine processionary moth (PPM, Thaumetopoea pityocampa Dennis and Schiff.) on the non-structural carbohydrate (NSC) and nitrogen (N) stores and the growth of Pinus nigra Arnold trees. Short-term effects were evaluated immediately after a PPM outbreak and at the end of the first growing season in trees suffering a range of defoliation damage. Long-term effects were explored by a 17-year-long PPM defoliation experiment, with 11?years of repeated defoliation treatments followed by 6?years of recovery. Defoliation by PPM was followed by transient NSC decreases, but trees were able to exceed initial NSC pools and compensate growth in just one growing season. Such recovery was linked to increased foliage N. Repeated severe defoliations decreased growth and survival of trees in the long-term, but trees increased starch allocation to stems. Defoliation led to an accumulation of C storage compounds in P. nigra trees irrespective of their ability to re-grow. In trees included in the short-term experiment, the accumulation of stores proceeded concurrently with re-growth. However, the repeated severe defoliations included in our long-term experiment impaired the growth of trees, surplus C being accumulated as stores. These results indicate that, growth declines in pines defoliated by PPM are not due to C (source) limitation but may respond to the reduced sink strength of growing meristems due to defoliation, and thus, a decrease in C allocation to growth.     

Removal of the stem terminal and application of auxin change carbohydrates in Pinus banksiana cuttings during propagation

This study determined how surgical removal of the stem terminal, with indole-3-butyric acid (IBA) treatment, influenced concentrations and partitioning of carbohydrates in Pinus banksiana Lamb, cuttings during propagation. Seedlings and cuttings that originated from 90-day-old stock plants were untreated or treated by removing the stem terminal, followed by application of IBA to the severed apical or basal (cuttings only) stem. Fresh and dry weights of the basal 1-cm stems of cuttings were determined daily for the first 10 days of propagation (i.e., before roots were visible). In addition, basal 1-cm stems, upper (ca 9-cm) stems and needles of seedlings and cuttings were analyzed for sucrose, soluble reducing sugar and total non-structural carbohydrate. Net concentrations of each carbohydrate in cuttings were obtained by subtracting corresponding concentrations for similarly treated seedlings, yielding data directly related to only the physiology of rooting. Data for cuttings indicated that presence of the stem terminal combined with applied IBA positively influenced rooting through processes that increased basal stem fresh and dry weights before root emergence. Removal of the stem terminal influenced accumulation of net total carbohydrate in cuttings, but the major effect was on carbohydrate partitioning. Either type of IBA treatment after removal of the stem terminal usually resulted in different net carbohydrate concentrations in each tissue source of cuttings, compared with only removal of the terminal. Neither basal nor apical IBA treatment of cuttings without stem terminals yielded results for carbohydrate accumulation and partitioning like those obtained with intact cuttings. Removal of the stem terminal, even if followed by IBA treatment, may have lessened rooting potential of cuttings because it resulted in greater reducing sugarstarch concentration ratios in basal stems compared with those in intact cuttings.     

Experimental manipulation of patterns of resource allocation in the growth cycle and reproduction of Smyrnium olusatrum L.

Plants of the monocarpic (normally biennial) Smyrnium olusatrum (Umbel]iferae) were grown in pots in soil at a high or low nutrient regime. Some plants receiving full nutrients were grown in a heated glasshouse with 16 h days. The remainder were grown without supplementary lighting or heat and included control plants and others which received surgical treatment after ten months growth: deradication (removal of half of the root stock); defoliation; deradication and defoliation. The distribution of plant biomass and of phosphorus were analyzed at the time of seed set.
Patterns of allocation of dry matter and phosphorus were quite different and were significantly altered by treatments, which produced a range of allocation to reproductive structures ranging from 21 to 74% of total phosphorus and 12 to 35% of dry matter.
Distribution patterns of total phosphorus are discussed in terms of the potential demands being made by alternative structures and functions over the life cycle of the plants.     

Grain Amaranths Are Defoliation Tolerant Crop Species Capable of Utilizing Stem and Root Carbohydrate Reserves to Sustain Vegetative and Reproductive Growth after Leaf Loss

Tolerance to defoliation can be defined as the degree to which productivity is affected by photosynthetic area reduction. This trait was studied in grain amaranth (Amaranthus cruentus and A. hypochondriacus), which are considered to be a highly defoliation-tolerant species. The physiological and biochemical responses to increasing levels of mechanical leaf removal up to total defoliation were quantified. Tolerance appeared to be dependent on various factors: ( i) amount of lost tissue; (ii) mechanics of leaf tissue removal; (iii) environment, and (iv) species tested. Thus, grain amaranth was found to be a highly tolerant species under green-house conditions when leaf tissue loss was performed by gradual perforation. However, tolerance was compromised under similar conditions when defoliation was done by gradual cutting of the leaf. Also tolerance in completely defoliated plants tended to decrease under field conditions, where differences between A. cruentus and A. hypochondriacus were observed. All non-structural carbohydrate (NSC) levels were reduced in stems and roots of totally defoliated amaranths one day after treatment. Such depletion probably provided the carbon (C) resources needed to sustain the early recovery process in the absence of photosynthetic capacity. This was corroborated by shading of intact plants, which produced the same rapid and drastic reduction of NSC levels in these tissues. These results emphasize the role of stored NSCs, particularly starch, in buffering the impact of severe defoliation in amaranth. The fall in sucrose synthase and cell wall invertase activity observed in stems and roots soon after defoliation was consistent with their predicted shift from sink to source tissues. It is concluded that mobilization of C stores in stems and roots, is a physiologically important trait underlying tolerance to defoliation in grain amaranth.     

Modifications of Translocation Rate in Studies of Apple Leaf Efficiency

M.7 apple rootstocks were used during the peak period of shootextension for comparisons of dry-matter production per unitleaf area between intact plants and others which had been partiallydefoliated. Dry-matter increment per unit leaf area over a 16-dayinterval was some 70 per cent higher in partially defoliatedplants than in controls. ¹⁴CO₂ was supplied to designated leaves of comparable age andposition. Sample discs were taken from the ‘fed’leaves at intervals up to 9 days from supplying ¹⁴CO₂. Translocationrates were estimated by comparison with leaves on a third setof plants whose petioles were steamed to prevent translocationimmediately on removal of the ¹⁴CO₂ feeding chambers. Translocationrates in partially defoliated plants were enhanced some 30 percent compared with controls. It is suggested that features of the plant outside the studiedleaves may have contributed to the overall efficiency of assimilateproduction and utilization. Malus sylvestris L., apple, dry matter production, leaf efficiency, defoliation, translocation, assimilate distribution, sorbitol, sucrose     

小麦不同品种光合速率和~(14)C同化物分配对源库比改变的响应

选用千粒重大小不同的小麦品种,研究了去除顶端两个小穗对两类品种（大粒品种和小粒品种）穗部性状、籽粒平均灌浆速率、单穗平均增重速率、植株光合速率及１４Ｃ同化物运输分配的影响。试验结果表明,去除顶端两个小穗后,两类品种的籽粒平均灌浆速率和单穗平均增重速率（分别表征籽粒库容活性和穗粒库容活性）相应提高,穗粒重表现为补偿性增长。两类品种比较,小粒品种的增长幅度大于大粒品种。穗粒库容活性增强使得小粒品种灌浆中后期的植株光合速率提高,使两类品种分配到籽粒中的１４Ｃ同化物比例增加。从而表明,无论是植株光合速率还是同化物的运转分配皆受库容活性的调控,调控方式和幅度因品种类型而不同。     

Allocation of carbon to shoots,roots, soil and rhizosphere respiration by barrel medic (Medicago truncatula) before and after defoliation

The allocation of carbon to shoots, roots, soil and rhizosphere respiration in barrel medic (Medicago truncatulaGaertn.) before and after defoliation was determined by growing plants in pots in a labelled atmosphere in a growth cabinet. Plants were grown in a ¹⁴CO₂-labelled atmosphere for 30 days, defoliated and then grown in a ¹³CO₂-labelled atmosphere for 19 days. Allocation of ¹⁴C-labelled C to shoots, roots, soil and rhizosphere respiration was determined before defoliation and the allocation of ¹⁴C and ¹³C was determined for the period after defoliation. Before defoliation, 38.4% of assimilated C was allocated below ground, whereas after defoliation it was 19.9%. Over the entire length of the experiment, the proportion of net assimilated carbon allocated below ground was 30.3%. Of this, 46% was found in the roots, 22% in the soil and 32% was recovered as rhizosphere respiration. There was no net translocation of assimilate from roots to new shoot tissue after defoliation, indicating that all new shoot growth arose from above-ground stores and newly assimilated carbon. The rate of rhizosphere respiration decreased immediately after defoliation, but after 8 days, was at comparable levels to those before defoliation. It was not until 14 days after defoliation that the amount of respiration from newly assimilated C (¹³C) exceeded that of C assimilated before defoliation (¹⁴C). This revised version was published online in June 2006 with corrections to the Cover Date.     

小麦不同品种光合速率和14 C同化物分配对源库比改变的影响

 选用千粒重大小不同的小麦品种,研究了去除顶端两个小穗对两类品种(大粒品种和小粒品种)穗部性状、籽粒平均灌浆速率、单穗平均增重速率、植株光合速率及14C同化物运输分配的影响。试验结果表明,去除顶端两个小穗后,两类品种的籽粒平均灌浆速率和单穗平均增重速率(分别表征籽粒库容活性和穗粒库容活性)相应提高,穗粒重表现为补偿性增长。两类品种比较,小粒品种的增长幅度大于大粒品种。穗粒库容活性增强使得小粒品种灌浆中后期的植株光合速率提高,使两类品种分配到籽粒中的14C同化物比例增加。从而表明,无论是植株光合速率还是同化物的运转分配皆受库容活性的调控,调控方式和幅度因品种类型而不同。     

Rhizosphere interactions, carbon allocation, and nitrogen acquisition of two perennial North American grasses in response to defoliation and elevated atmospheric CO2

Carbon allocation and N acquisition by plants following defoliation may be linked through plant-microbe interactions in the rhizosphere. Plant C allocation patterns and rhizosphere interactions can also be affected by rising atmospheric CO(2) concentrations, which in turn could influence plant and microbial responses to defoliation. We studied two widespread perennial grasses native to rangelands of western North America to test whether (1) defoliation-induced enhancement of rhizodeposition would stimulate rhizosphere N availability and plant N uptake, and (2) defoliation-induced enhancement of rhizodeposition, and associated effects on soil N availability, would increase under elevated CO(2). Both species were grown at ambient (400 μL L(-1)) and elevated (780 μL L(-1)) atmospheric [CO(2)] under water-limiting conditions. Plant, soil and microbial responses were measured 1 and 8 days after a defoliation treatment. Contrary to our hypotheses, we found that defoliation and elevated CO(2) both reduced carbon inputs to the rhizosphere of Bouteloua gracilis (C(4)) and Pascopyrum smithii (C(3)). However, both species also increased N allocation to shoots of defoliated versus non-defoliated plants 8 days after treatment. This response was greatest for P. smithii, and was associated with negative defoliation effects on root biomass and N content and reduced allocation of post-defoliation assimilate to roots. In contrast, B. gracilis increased allocation of post-defoliation assimilate to roots, and did not exhibit defoliation-induced reductions in root biomass or N content. Our findings highlight key differences between these species in how post-defoliation C allocation to roots versus shoots is linked to shoot N yield, but indicate that defoliation-induced enhancement of shoot N concentration and N yield is not mediated by increased C allocation to the rhizosphere.