Effect of defoliation on concentrations of allelochemicals and soluble sugars in leaves of weeping birch (<Emphasis Type="Italic">Betula pendula</Emphasis> roth)

Effect of strong (75%) and complete (100%) artificial defoliation of weeping birch Betula pendula Roth on the dynamics of soluble sugars and phenols—flavonols, catechins, and tannins in leaves of damaged plants was investigated. Within the first 15 days after strong defoliation of birch, no changes were found in leaf contents of flavonol, catechin, and tannin. The concentration of sugars first increased but, on the 10th day after defoliation, it returned to the normal level. One year after strong defoliation, the lead concentrations of catechins and tannins in damaged trees increased, while the concentrations of flavonols and sugars did not differ from that in leaves of control trees. In two years after strong damage, the increased concentration of tannins was retained, while catechins and sugars remained at the control level. One year after complete (100%) artificial defoliation, the leaf concentrations of flavonols and sugars in damaged plants did not differ from that in control plants, while the leaf concentrations of catechins and tannins exceeded those in control plants. Two years after complete damage, the leaves contained an increased amount of tannins, whereas the amounts of catechins, flavonols, and sugars did not differ from the control levels.     

Growth,chemical responses and herbivory after simulated leaf browsing in <Emphasis Type="Italic">Combretum apiculatum</Emphasis>

Vegetative and chemical responses to simulated leaf browsing during the growth season, and their subsequent effect on herbivory, were studied on Combretum apiculatum Sonder (Combretaceae) in Botswana. Treatments (50% and 100% leaf and shoot apex removal) were performed just before the shoot growth curve levelled out, and responses recorded 3 months later, just before leaf fall. Compared to controls, defoliation treatments, removing apical dominance, reduced growth in tree height and increased shoot mortality, although the production of lateral shoots increased. At the end of the trial, there was no difference in total length of annual shoots between treatment groups. Significant refoliation occurred only after 100% defoliation. Refoliated leaves were smaller and the 100% defoliated trees had a lower final leaf biomass. Total leaf biomass production was, however, equal for all treatment groups. Refoliated leaves contained higher levels of N, lower levels of acid-detergent fibre (ADF) and total phenolics, and showed a trend towards lower levels of condensed tannins, compared to leaves on control trees. Such chemical changes may be due to either carbon stress or to younger physiological age of new leaves. In spite of the observed potential increase in food quality, we found no evidence of increased levels of insect or ungulate herbivory on refoliated leaves, which, at least for insect herbivory, may be explained by the reduction in temporal availability of leaves. We conclude that the single severe defoliation was not detrimental to C. apiculatum in the short-term, although the resource loss and induced compensatory growth may produce negative effects during subsequent growth seasons.     

Delayed induced changes in the biochemical composition of host plant leaves during an insect outbreak

In birch, Betula pubescens, herbivore-induced delayed induced resistance (DIR) of defoliated trees may cause a strong reduction in the potential fecundity of a geometrid folivore Epirrita autumnata. In this study, we examined the biochemical basis of DIR in birch leaves during a natural outbreak of E. autumnata. A set of experimental trees was defoliated at four sites by wild larvae in the peak year of the outbreak, whereas control trees were protected from defoliation by spraying with an insecticide. The biochemical composition of leaves was analysed in the following year and, although the DIR response was weak during this outbreak, causing less than a 20% reduction in the potential fecundity of E. autumnata, some consistent relationships between defoliation, biochemistry and pupal mass of E. autumnata suggested a general biochemical basis for the defoliation-induced responses in birch leaves. Total concentrations of nitrogen, sugars and acetone-insoluble residue (e.g. cell wall polysaccharides, cell-wall-bound phenolics, protein, starch, lignin and hemicellulose) were consistently lower, and total concentrations of phenolics, especially of gallotannins and soluble proanthocyanidins, were higher in the leaves of trees defoliated in the previous year than in those protected from defoliation. The capacity of tannins to precipitate proteins correlated with contents of gallotannins, and was highest in defoliated trees. The pupal mass of E. autumnata showed a strong, positive correlation with concentrations of nitrogen and sugars, and a negative correlation with the acetone-insoluble residue and gallotannins in foliage. Correlations with other measured biochemical traits were weak. The correlation coefficients between biochemical traits and pupal mass consistently had similar signs for both defoliated and insecticide–sprayed trees, suggesting that variation in leaf quality due to defoliation in the previous year was based on similar biochemical traits as variation for other reasons. We suggest that DIR is associated with reduced growth activity of leaves, and may be seen as a delay in the biochemical maturation of leaves in defoliated trees. This explains the high concentration of gallotannins in defoliated trees, a characteristic feature of young leaves. However, the lower content of nitrogen and the higher content of soluble proanthocyanidins in defoliated trees are traits usually characterising mature, not young, leaves, indicating defoliation-induced changes in chemistry in addition to modified leaf age. Our results emphasise the importance of understanding the natural changes in chemistry during leaf maturation when interpreting defoliation-induced changes in leaf biochemistry. Received: 26 January 1998 / Accepted: 10 April 1998     

Leaf Growth in Dactylis glomerate following Defoliation

Defoliation to a height of 2.5 cm considerably reduced the increasein leaf area in young Dactylis glomerata (cocksfoot) plantscompared with that of intact plants, the reduction in the rateof appearance of new leaves being relatively greater than thereduction in expansion of existing leaves. The growth of thoseexpanding leaves which were cut during defoliation accountedfor 94 per cent of the total increase in leaf area during thefirst four days after defoliation. In such a leaf, expandingcells were confined to a basal section which was well belowthe ligule of the enclosing fully expanded leaf. There was a positive relationship between rate of leaf expansionand total soluble carbohydrate content of the stubble when thecarbohydrate content was varied by placing the plants in thedark, but not when it was varied by defoliation and subsequentgrowth. These and other results suggest that the concentrationof soluble carbohydrate in the bases of expanding leaves wasa factor controlling leaf expansion following defoliation, andthat the concentration in any one leaf depended on the photosyntheticcontribution from its exposed portion. When the external nutrient supply was high, removal of the laminaeof fully expanded leaves, which comprised about two-thirds ofthe total leaf area, did not reduce leaf expansion. When thenutrient status was low, these leaves were of primary importance,presumably because of their role as a source of labile nutrients.     

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.     

桑苗非结构性碳水化合物和生长激素对水淹胁迫的响应

桑(Morus alba)具有较强的耐水淹特性, 为了探究水淹胁迫对其非结构性碳水化合物和生长激素的影响, 揭示变化规律, 该研究采取室内模拟水淹实验, 以三年生盆栽桑苗作为研究对象, 设置对照组(CK)、根淹组(GY)、浅淹组(QY)、深淹组(SY)等4个不同水淹胁迫的处理, 定期观测并记录桑苗叶片非结构性碳水化合物(可溶性糖和淀粉)含量、内源生长激素(乙烯、脱落酸、赤霉素)含量变化情况。研究结果表明: (1)水淹胁迫会促进桑苗叶片内的生化反应, 造成叶片可溶性糖含量增加。水淹75天, GY、QY、SY桑苗叶片可溶性糖含量较水淹前分别增加182.18%、170.21%和94.16%, 差异显著, 且显著高于CK。水淹胁迫下桑苗叶片淀粉含量在水淹0-50天无显著变化, 水淹75天, GY、QY、SY桑苗叶片淀粉含量较水淹50天分别增加290.84%、244.65%和130.04%, 差异显著, 且显著高于CK。(2)水淹胁迫下桑苗叶片乙烯和赤霉素含量均显著增加, 水淹75天, GY和SY桑苗乙烯含量较水淹前分别增加62.80%和26.78%, 差异显著; GY、QY和SY桑苗赤霉素含量分别增加27.48%、18.02%和25.04%, 差异显著。随着水淹时间增加, GY和SY桑苗乙烯和赤霉素含量总体均呈增加趋势, QY桑苗乙烯和赤霉素含量先增后减, 但仍高于水淹前。水淹胁迫下水淹各组桑苗叶片脱落酸含量随着水淹深度的增加而增加, 水淹75天, QY和SY桑苗叶片脱落酸含量较水淹前分别增加19.20%和36.16%, 差异显著; GY桑苗脱落酸含量无显著变化。上述研究结果表明桑苗可通过调整体内非结构性碳水化合物(可溶性糖和淀粉)的含量和分配, 同时通过积累乙烯、赤霉素、脱落酸等内源激素以适应水淹环境, 具有较强耐淹能力。     

Condensed tannins and sugars in the diet of chimpanzees (Pan troglodytes schweinfurthii) in the Budongo Forest, Uganda

Fruits, leaves and bark forming part of the diet of chimpanzees were collected and it was noted whether samples were of a kind being eaten or not eaten. Samples were dried and analysed for condensed tannin content and for three sugars, glucose, sucrose and fructose. It was found that chimpanzees did not select foods according to the level of tannins but did so according to the levels of sugars, preferring the higher levels. Fig seeds contained higher tannin levels than fig pulp, and the chimpanzees made oral boli (“wadges”) of fig seeds which they spat out. Two fig species were compared: the one with lower tannin and higher sugar content was preferred. The bark of one tree species often eaten contained high levels of tannins but also contained sugars. Young leaves with lower tannin levels were preferred to mature leaves with higher levels. Chimpanzees appear to be able to tolerate higher tannin levels than three monkey species in this forest, and considerably higher levels than marmosets (Callitrichidae). Received: 20 October 1997 / Accepted: 1 March 1998     

铁皮石斛醇溶性浸出物和六类物质的动态变化研究

为了解不同生长期铁皮石斛(Dendrobium officinale)化学成分的变化,对茎、叶醇溶性浸出物、多酚、黄酮、三萜类、有机酸、氨基酸、糖含量进行了测定。结果表明,从2015年9月至2016年9月,茎的醇溶性浸出物总量及六类物质含量呈现先下降后上升的变化趋势,在春末夏初时达到最低,而叶片中各成分的变化未体现一致性规律。六类物质中以小分子的糖含量最高,其次是三萜类和有机酸。茎的醇溶性浸出物总量与多糖含量呈显著负相关;而叶的相关性不显著。这些为评估铁皮石斛质量和确定采收期提供科学依据。     

外源茉莉酸诱导的青杨生化抗性及其对舞毒蛾幼虫食物利用的影响

【目的】为探讨外源茉莉酸(jasmonic acid, JA)诱导的青杨 Populus cathayana Rehd.抗性对舞毒蛾 Lymantria dispar (L.)幼虫食物利用的影响。【方法】在室内对青杨苗木喷施0.001和0.1 mmol/L两种浓度的茉莉酸,对照喷0.2%的丙酮水溶液,喷施后1, 5和10 d分别采集叶片,分析其初生和次生物质含量的变化。另选喷施0.1 mmol/L茉莉酸的青杨苗木,喷施后1, 5和10 d分别接舞毒蛾2龄幼虫,单头饲养,测定其取食量、体重和排粪量及发育历期,统计分析其食物消化率、转化率和利用率。【结果】两种浓度茉莉酸处理均使青杨叶片中的蛋白质和可溶性糖含量降低,而木质素、单宁、黄酮和酚类物质含量增加。蛋白质和可溶性糖在处理第10天时含量最低,其中高浓度茉莉酸处理较对照分别降低46.5%和49.1%,低浓度处理分别降低30.6%和22.8%。叶片中酚类物质含量在高浓度处理第10 天时增幅最大,较对照增加102%。木质素、黄酮和单宁酸在处理第5天时的含量最高,其中高浓度处理分别较对照增加113%, 75%和57%。用0.1 mmol/L茉莉酸诱导处理后,舞毒蛾2龄幼虫对叶片的消化率、转化率和利用率均有所降低, 取食处理后1, 5和10 d的青杨叶片的食物利用率较对照分别降低了29.4%, 27.6%和28.2%,且幼虫体重降低、发育历期延长。【结论】结果提示外源茉莉酸诱导青杨可对舞毒蛾产生明显的生化抗性,实践中可利用这种生化抗性防治舞毒蛾的危害。     

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.     

Changes during recovery from sodium deficiency in atriplex

Although the concentration of sodium in leaves of Atriplex plants increased rapidly after receiving sodium, no growth response was detectable for about 6 days. It was found that respiration rate increased to its maximum within 3 days. Chlorophyll content also increased from an early stage, whereas the concentrations of sugars and starch did not increase, and ratios of soluble to total nitrogen did not decrease until later.

The respiratory response appears to be specific to sodium as different salts of sodium caused similar responses, and no other univalent cation substituted for sodium. In addition, both growth response and respiration rate tended towards their maxima with the same concentration of applied sodium. The rate of anaerobic CO₂ production increased when sodium was fed to leaves, suggesting that the effect of sodium is in the glycolytic sequence.

     

Long-term effects of defoliation on quaking aspen in relation to genotype and nutrient availability: plant growth,phytochemistry and insect performance

This research tested the long-term effects of defoliation on aspen chemistry and growth in relation to genotype and nutrient availability. We grew saplings of four aspen genotypes in a common garden under two conditions of nutrient availability, and subsequently subjected them to two levels of artificial defoliation. Artificial defoliation suppressed plant growth, and saplings of the four genotypes did not show evidence of genetic variation in tolerance to defoliation. Phenolic glycoside concentrations did not respond to defoliation, but were influenced by genotype and nutrient availability. Condensed tannins responded to defoliation and varied among genotypes. Although defoliation affected condensed tannins, plant quality was not altered in a manner important for gypsy moth performance. Regression analyses suggested that phenolic glycoside concentrations accounted for most of the variation in insect performance. The lack of a strong response important for herbivores was surprising given the severity of the defoliation treatment (nearly 100% of leaf area was removed). In this study, plant genotype was of primary importance, nutrient availability was of secondary importance and long-term induced responses were unimportant as determinants of insect performance.     

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.     

Growth,morphology and leaf characteristics after simulated herbivory in Chinese subtropical evergreen saplings

Growth, morphology and leaf characteristics were assessed in late spring following simulated autumnal defoliation in second-year saplings of three Chinese subtropical evergreen tree species.Castanopsis fargesii showed strong compensatory growth in terms of plant biomass after removal of both 50 and 75% of leaf biomass and slight compensatory growth after 90% defoliation. DefoliatedC. fargesii saplings had more leaves per unit shoot length than non-defoliated saplings. New leaves on defoliated plants were smaller and had higher per area nitrogen content than new leaves on non-defoliated plants.Pinus massoniana andElaeocarpus japonicus showed strong and no compensatory growth, respectively, after 50% defoliation. The strong compensatory growth inP. massoniana andC. fargesii may partly explain why these species predominate in the early and late successional phases of evergreen broad-leaved forests     

Protein, lipid and carbohydrate use during metamophosis in the fire ant, Solenopsis xyloni

Abstract. Concentrations of total sugars, lipids and soluble proteins were measured in prepupae and pupae of a native fire ant, Solenopsis xyloni W. M. wheeler (Hymenoptera: Formicidae). Changes in the concentratons of these reserves were examined over the course of metamorphosis, in response to starvation, and in relation to body size. During metamorphosis, only lipid concentrations changed during the prepuapal period, dropping slightly. During the pupal state, however, approximately 75% of total sugars, 45% of lipids and 40% of soluble proteins were used. Starvation immediately prior to metamorphosis greatly decreased the quantity of carbohydrate detected at the beginning of the prepupal stage. Soluble protein levels were also slightly reduced. In contrast, lipid concentrations in prepupae increased in individuals that had been starved for 2–4 days immediately before metamorphosis. The relationship between body size and amount depended on the type of reserve. Lipid concetrations decreased with increasing size, while carbohydrate levels tended to increase slightly. Overall, soluble protein concentration did not change with size. Gel electrophoresis showed that two major polypeptides account for most of the soluble protein and one on these decreased sharply over the pupal period.     

A seasonal census of phenolics, fibre and alkaloids in foliage of forest trees in Costa Rica: some factors influencing their distribution and relation to host selection by Sphingidae and Saturniidae

The foliage of 80 species common in the Santa Rosa National Park, Costa Rica, has been analysed for content of total phenolics, condensed tannins, acid detergent fibre and water. Wherever possible analyses were performed at three stages in the life cycle of the leaf: young but fully expanded (coinciding with the beginning of the rainy season); middle-aged (two months later); and old (six months later). A comparison of the three age classes showed no significant change in the levels of phenolics or fibre as leaves aged but water content decreased significantly. A comparison of deciduous and evergreen species in the sample showed that the latter group had leaves with a significantly higher fibre content at all three sampling times, most particularly at the beginning of the rainy season, but other measures were not significantly different. Alkaloids were much more common in the foliage of deciduous species and it was observed that their distribution differed significantly from that of total phenolics and condensed tannins. It is suggested that the interaction that occurs between many tannins and alkaloids would be liable to reduce the defence capability of both classes of compounds if they occurred together.
High levels of defoliation occur in the early rainy season (third to tenth weeks) due to larvae of moths of the Sphingidae and Saturniidae. A comparison of investigated tree species that host larvae of these two taxa shows a striking dichotomy. Species that are selected by Sphingidae tend to be relatively deficient in levels of phenolics but are more likely to contain alkaloids, and probably other small toxic molecules. Saturniidae, on the other hand, appear to prefer host-species rich in phenolics but poor in alkaloids.     

Changes in Nucleic Acids and Sugars of Tobacco Leaves during Maturity Stage and Flue-curing

Changes in the contents of nucleic acids and sugars in tobacco leaves during maturity stage and flue-curing were investigated. The content of RNA continued to decrease during both periods and is lower in lower leaves on the stalk. However, a temporary reverse trend was observed after topping. A sharp decrease after topping in ribonuclease activity was followed by an increase in RNA several days later, and thereafter soluble protein also increased temporarily in maturity stage.

Sucrose, glucose and fructose decreased temporarily after topping and then increased gradually in the latter stage of maturity, followed by an abrupt increase in the yellowing stage of flue-curing. Minor sugars, i.e. maltose, ribose, mannose, galactose and xylose were determined quantitatively in tobacco leaves. However, these sugars have not shown any remarkable changes during maturity stage and flue-curing.     

Effects of variable phytochemistry and budbreak phenology on defoliation of aspen during a forest tent caterpillar outbreak

1 The present study assessed the relationship between clonally variable rates of defoliation in trembling aspen (Populus tremuloides Michx.) and two potential resistance traits: defensive chemistry and leaf phenology. 2 In 2001, coincident with a major outbreak of the forest tent caterpillar (Malacosoma disstria Hubner) in the northcentral U.S.A., we monitored defoliation rates, phytochemical composition, and foliar development in 30 clones of trembling aspen. Leaf chemistry was also assessed in re‐flushed leaves and 2 years post‐outbreak. 3 Early in the season, differences in defoliation among clones were substantial but, by mid‐June, all clones were completely defoliated. Leaf nitrogen, condensed tannins, and phenolic glycosides varied among clones but did not relate to defoliation levels. Budbreak phenology differed by 3 weeks among clones and clones that broke bud early or late relative to forest tent caterpillar eclosion experienced reduced rates of defoliation. 4 Defoliation led to increased tannins and slight decreases in phenolic glycoside concentrations in damaged leaf remnants, but to moderately decreased tannins and a six‐fold increase in phenolic glycosides in reflushed leaves. This shift in chemical composition may significantly affect late season herbivores. 5 These results suggest that aspen chemical resistance mechanisms are ineffective during intense episodic eruptions of outbreak folivores such as the forest tent caterpillar. Variable budbreak phenology may lead to differential susceptibility during less intense outbreak years and, at peak forest tent caterpillar population densities, mechanisms affording tolerance are probably more important than chemical defences.     

Impact of defoliation intensity and frequency on N uptake and mobilization in Lolium perenne

The aim of the study was to evaluate the impact of defoliation intensity, defoliation frequency, and interactions with N supply on N uptake, N mobilization from and N allocation to roots, adult leaves, and growing leaves. Plants of Lolium perenne were grown under two contrasted N regimes. Defoliation intensity treatments consisted of a range of percentage leaf area removal (0, 25, 50, 75, or 100%). These treatments were applied in parallel to a set of plants previously undefoliated, and to a second set of plants which had been defoliated several times at a constant height. A (15)N tracer technique was used to quantify N uptake, mobilization, and allocation over a 7 d period. A significant reduction in plant N uptake was observed with the removal of more than 75% of lamina area, but only with high N supply. As defoliation intensity increased, the amount of N taken up and subsequently allocated to growing leaves during the labelling period was maintained at the expense of N allocation to roots and adult leaves. Increasing defoliation intensity increased the relative contribution of roots supplying mobilized N to growing leaves and decreased the relative contribution of adult leaves. Defoliation frequency did not substantially alter N uptake, mobilization, and allocation between roots, adult and growing leaves on a plant basis. However, tiller number per plant was largely increased under repeated defoliation, hence indicating that allocation and mobilization of N to growing leaves, on the basis of individual tillers, was decreased by defoliation frequency.