Carbon and Nitrogen Yield, and N2 Fixation in White Clover Plants Receiving Simulated Continuous Defoliation in Controlled Environments

Single plants of white clover grown in controlled environments,and dependent for nitrogen on N, fixation, were defoliated at1 or 2 d intervals to 3, 2 and 1 expanded leaves per stolon(Expt 1), and to 1,0.5 (1 leaf on every alternate stolon) and0 expanded leaves per stolon (Expt 2), for 43–50 days Plants adapted to severe defoliation by developing much smallerleaves with a slightly reduced specific leaf area, more stolons,a smaller proportion of weight in leaf, root and nodules anda greater proportion of weight in stolons. The daily yield (materialremoved by defoliation) of d. wt and nitrogen generally decreasedwith severity of defoliation, as did the residual plant weight.However, the ‘efficiency’ of yield (daily yield/residualweight x 100) of dry matter and nitrogen was greater in themost severely defoliated treatments, attaining a maximum of5–6 % All plants adapted to the imposed defoliation regimes, howeversevere, with the result that even plants maintained withoutany fully expanded leaves invested a similar fraction of theirmetabolic resources in shoot and root as less severely defoliatedplants, and continued to grow and fix N₂, albeit at a very reducedrate of 1–2 mg Nd^–11. The energetic cost of N₂ fixation(acetylene reduction) remained constant in all treatments at31 mole CO₂ mole C₂H₄^–1, but there was some evidence thatrate of N₂ fixation per unit of nodule weight declined in themost harshly defoliated treatment. Trifolium repens, white clover, continous defolation, growth, N₂ fixation     

The Effect of Lenient Defoliation on the Nitrogen Economy of White Clover: The Contribution of Mineral Nitrogen to Plant Nitrogen Accumulation During Regrowth

Single plants of white clover (Trifolium repens L.) were grownfrom stolon cuttings rooted in sand. All plants were inoculatedwith Rhizobium trifolii, and for 14 weeks received nutrientsolution containing 0.5 mg N each week, as either ammonium ornitrate. Plants were then leniently defoliated or were leftintact and a ¹⁵N-labelled N source was applied at intervalsof 4 d to replace the unlabelled N. Lement defoliation removedfully expanded leaves only; the remaining immature leaves accountedfor 39–44% of the total. At harvests over the following21 d, leaf numbers were counted and dry matter (DM), N contentsand ¹⁵N enrichments of individual plant organs were determined. Rates of leaf emergence and expansion were accelerated in defoliatedplants; numbers of young leaves were similar in defoliated andintact plants. Total DM and N content were less in defoliatedthan intact plants and were not affected by form of N supplied.DM of young leaves, growing points and stolons and N contentof young leaves were, however, greater when ammonium ratherthan nitrate N was supplied. Rates of increase in the contentof plant total N were 8.2 ± 1.36 mg N d^-1 and 10.2±1.82 mg N d^-1 in defoliated and intact plants respectively.The increases were predominantly due to N₂ fixation, since recoveryof ¹⁵N showed that less than 1% of the increment in plant totalN was assimilated mineral N. Nevertheless, the contributionof mineral N to plant total N was 50% more in defoliated thanin intact plants; higher amounts of mineral N were found particularlyin young leaves and growing points. Partitioning of mineralN to nodulated roots increased over time and was greater whenammonium rather than nitrate N was present. White clover, Trifolium repens L. cv. S184, lenient defoliation, N accumulation, N₂ fixation     

Senescence and decomposition of white clover stolons in grazed upland grass/clover swards

The fate of marked sections of stolons of white clover (Trifolium repens) over a 50-week period from May 1987 was followed in grazed grass/clover swards maintained at 5-cm sward surface height with and without N fertiliser. There was little effect of N treatment on the pattern of survival of stolon sections. The proportion of live stolons recovered decreased during the experiment, and in May 1988 on average only 29% of the marked sections remained alive. At all harvests only a small percentage of stolon sections showed signs of senescence; the maximum percentage, on average 20% of those marked, occurred in autumn, 15–20 weeks after marking. Following this period, i.e. in late autumn/winter, the most rapid increase in percentage of decomposed stolons was measured. Over 50% of stolon sections were buried within the 5-week period following marking and nearly all were buried after 20 weeks; generally a much smaller proportion of stolon tips was buried. Nutrient concentrations of N, P and K fell to their lowest levels in autumn, before increasing in the following spring. Results are discussed in relation to the cycling of nutrients via stolon senescence.     

The Effect of Defoliation on the Nitrogen Economy of White Clover: Regrowth and the Remobilization of Plant Organic Nitrogen

Single plants of white clover (Trifolium repens) were establishedfrom stolon cuttings rooted in acid-washed silver sand. Allplants were inoculated with Rhizobium trifolii, and receivednutrient solution containing 0·5 mg ¹⁵N as either ammoniumor nitrate weekly for 12 weeks (i.e. 6 mg ¹⁵N in total). Plantswere then leniently defoliated or left intact, and the labelledN supply was replaced with unlabelled N. Lenient defoliationremoved fully expanded leaves only, leaving immature leaveswhich accounted for 50–55% of the total; growing pointnumbers were not reduced. Nodules, leaves and growing pointswere counted over the following 21 d period, and d. wts, N contents,and ¹⁵N enrichments of individual plant organs were determined. Defoliated plants had fewer nodules, but numbers of growingpoints were unaffected by defoliation. The rates of both leafemergence and expansion were accelerated in defoliated plants;in consequence the number of young leaves remained less thanin intact plants until day 21. Total dry matter (DM) and N accumulationwere less in defoliated plants, and a greater proportion oftotal plant DM was invested in roots. About 97 % of plant totalN was derived from fixed atmospheric N, but there was incompletemixing of fixed and mineral N within the plant. Relatively moremineral N was incorporated into roots, whereas there was relativelymore fixed N in nodules. There was isotopic evidence that Nwas remobilized from root and stolon tissue for leaf regrowthafter defoliation; approximately 2 % of plant N turned overdaily in the 7-d period after defoliation, and this contributedabout 50% of the N increment in leaf tissue. White clover, Trifolium repens L. cv. SI84, lenient defoliation, N economy, regrowth, N remobilization     

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     

Effects of flooding and drought on stomatal activity, transpiration, photosynthesis, water potential and water channel activity in strawberry stolons and leaves

Transpiration, xylem water potential and water channel activity were studied in developing stolons and leaves of strawberry (Fragaria × ananassa Duch.) subjected to drought or flooding, together with morphological studies of their stomata and other surface structures. Stolons had 0.12 stomata mm^–2 and a transpiration rate of 0.6 mmol H₂O m^–2 s^–1, while the leaves had 300 stomata mm^–2 and a transpiration rate of 5.6 mmol H₂O m^–2 s^–1. Midday water potentials of stolons were always less negative than in leaves enabling nutrient ion and water transport via or to the strawberry stolons. Drought stress, but not flooding, decreased stolon and leaf water potential from –0.7 to –1 MPa and from –1 to –2 MPa, respectively, with a concomitant reduction in stomatal conductance from 75 to 30 mmol H₂O m^–2 s^–1. However, leaf water potentials remained unchanged after flooding. Similarly, membrane vesicles derived from stolons of flooded strawberry plants showed no change in water channel activity. In these stolons, turgor may be preserved by maintaining root pressure, an electrochemical and ion gradient and xylem differentiation, assuming water channels remain open. By contrast, water channel activity was reduced in stolons of drought stressed strawberry plants. In every case, the effect of flooding on water relations of strawberry stolons and leaves was less pronounced than that of drought which cannot be explained by increased ABA. Stomatal closure under drought could be attributed to increased delivery of ABA from roots to the leaves. However, stomata closed more rapidly in leaves of flooded strawberry despite ABA delivery from the roots in the xylem to the leaves being strongly depressed. This stomatal closure under flooding may be due to release of stress ethylene. In the relative absence of stomata from the stolons, cellular (apoplastic) water transport in strawberry stolons was primarily driven by water channel activity with a gradient from the tip of the stolon to the base, concomitant with xylem differentiation and decreased water transport potential from the stolon tip to its base. Reduced water potential in the stolons under drought are discussed with respect to reduced putative water channel activity.     

Yield and persistency of contrasting white clover populations grown in pure swards and in mixed swards with S.23 perennial ryegrass

Ten contrasting white clover populations were grown as spaced plants, in pure clover swards and in mixed swards with S.23 perennial ryegrass. Four of the populations were also tested for tolerance of low temperatures. In the establishment year, the autumn yields of populations were correlated with leaf size. However, during the severe winter which followed, populations with large leaves, particularly those of Mediterranean origin, suffered extensive stolon kill. This winter damage reduced the spring yields and total annual yields of populations with large leaves, so that leaf size and total annual yield were not correlated in the year after sowing. Stolon kill was positively correlated with autumn growth activity as measured by leaflet size and stolon internode length in October. Stolon kill during winter was correlated with cold tolerance of naturally-hardened stolon apices, assessed in artificial cold tests.     

The fate of nitrogen from winter-frozen rapeseed leaves: mineralization,fluxes to the environment and uptake by rapeseed crop in spring

For environmental purposes, very early sowing of winter rapeseed may reduce winter nitrate leaching thanks to the high N uptake capacities of rapeseed in autumn. However, freezing could lead to high losses of leaf nitrogen, amounting to more than 100 kg N ha^-1 (Dejoux et al., 1999). Here we investigated the agronomic and environmental consequences of the decomposition of fallen leaves, based on field and laboratory studies with ¹⁵N labeled leaves (C:N=9). The potential kinetics of decomposition of leaves were measured by incubation in the laboratory. In the field, all leaves were removed at beginning of winter and replaced by labeled leaves, artificially frozen at −15°C , which were laid on the soil surface. Compared on a thermal time basis, decomposition proceeded as quickly in the field as in the incubations and was complete after 116 normalized days at 15 °C. The proportion of ¹⁵N derived from labeled leaves, absorbed again by the rape plants, was 28% at flowering and 24% at harvest. This high N recovery is assumed to result from the synchronization of leaves decomposition and active N absorption by rape in spring. Leaf N mineralization did not increase soil N mineral content at flowering or at harvest, but we observed a 40% loss of ¹⁵N. As no leaching was simulated, this loss was supposed to be gaseous. Such a high percentage could be explained by the fact that the decomposing leaves lay on the soil surface, and by climatic conditions conducive to such emissions. For environmental purposes, the quantity and nature of these gaseous N emissions have to be studied for other climatic conditions and types of leaves. As a proportion of N is reabsorbed, N fertilizer application rates could be reduced accordingly. This revised version was published online in August 2006 with corrections to the Cover Date.     

Long-term Partitioning, Storage and Remobilization of 14C Assimilated by Trifolium repens (cv. Blanca)

The fourth fully expanded leaf on the main stolon of white cloverplants was exposed to ¹⁴CO₂. Thereafter, quantitative and fractionalanalysis of the partitioning, storage and remobilization afterdefoliation of the ¹⁴C labelled assimilate was sequentiallyconducted over a 2- to 3-week period. In undefoliated plants, most ¹⁴C reached its final destinationwithin 24 h of feeding. Forty percent of assimilated ¹⁴C waslost through respiration, while the rest was exported, predominantlyto meristems, but also to roots, stolons and leaves. The ¹⁴Cinitially translocated to meristems was subsequently recoveredin stolon and leaf tissue as the plants matured. Approximately 10% of assimilated ¹⁴C was invested into long-termstorage in roots and stolons. These reserves were remobilizedafter both partial and total defoliation, and a portion of theremobilized ¹⁴C was incorporated into new growth, Partly defoliatedplants regrew more rapidly than totally defoliated plants, butmore ¹⁴C reserve depletion took place in the totally defoliatedtreatment. Reserve depletion took place from both stolons androots, but stolon reserves were preferentially utilized. Bothhigh and low molecular weight storage compounds were involved. Trifolium repens, white clover, assimilate partitioning, storage, remobilization, defoliation     

Causal analysis of the diversity of medusae in East China Sea

Based on the maritime data collected from 23°30′–33°00′ N and 118°30′–128°00′ E of the East China Sea (ECS) in four seasons during 1997–2000, the dynamics of medusae diversity and their causes were analyzed. A total of 103 medusae species were identified, and these species mainly distributed in the southern and northern offshore areas of the ECS. Species diversity index (H′) of medusae was higher in the south than those in the north, higher in summer and winter than in spring and autumn, and higher in offshore than in the nearshore areas. The species number was closely correlated with H′ value, whereas the abundance of species had no significant relationship with the diversity index. The lower H′ value of the nearshore in spring and autumn resulted from the aggregation of Muggiaea atlantica in the south nearshore and Diphyes chamissonis in the north nearshore. In addition, water temperature, followed by salinity, is the main environmental factor influencing the distribution of species diversity. The H′ value was related to the water temperature at the 10 m layer in winter and spring, and it was associated with the surface water temperature in summer and with the 10 m-salinity-layer in autumn. In spring and summer, the isoline distribution of H′ value reflected the direction of the Taiwan Warm Current and the variation of the water masses in the ECS. In winter, the isoline of the H′ value indicated the incursion of Kuroshio current. In conclusion, the H′ isoline is an good indicator for water masses in ECS. __________ Translated from Biodiversity Science, 2006, 14(6): 508–516 [译自:生物多样性]     

Transgenic zoysiagrass (Zoysia japonica) plants obtained by Agrobacterium-mediated transformation

Zoysiagrass (Zoysia japonica Steud.) is an important turfgrass that spreads by stolons and rhizomes. By exploring the potential of direct shoot formation from stolons, we developed a straightforward and efficient transformation protocol without callus induction and propagation. Sterilized stolon nodes were infected and co-cultivated with Agrobacterium tumefaciens harboring pCAMBIA vectors. Hygromycin phosphotransferase gene (hph) was used as the selectable marker and hygromycin was used as the selection agent. Both green and albino shoots were directly regenerated from the infected stolon nodes 4–5 weeks after hygromycin selection. Greenhouse-grown plants were obtained 10–12 weeks after Agrobacterium-mediated transformation. Based on the number of transgenic plants obtained and the number of stolon nodes infected, a transformation frequency of 6.8% was achieved. Stable integration of the transgenes in the plant genome was demonstrated by PCR and Southern blot hybridization analyses. Expression of the transgenes was confirmed by RT-PCR analysis and GUS staining. The new transformation system opens up new opportunities for the functional characterization of genes and promoters and the development of novel germplasm in zoysiagrass.     

Contribution from Stolons and Roots to Estimates of the Total Amount of N2Fixed by White Clover (Trifolium repensL.)

Growth and N-accumulation rates in leaves, stolons and rootsof individual white clover plants were studied in three experimentsusing two methods. In a growth chamber experiment, the relativedifferences between tissues were found to be almost constantfor a wide range of clover plant sizes. The stolon dry matter(DM) production was 56% and the root DM production 40% of theDM production in leaves. The N yield of stolons was 30% whileN yield in roots was 34% of N yield in leaves. The effect ofN application on these relations was investigated in a glasshouseexperiment. Application of N reduced the root:shoot N ratiofrom 0.50 to 0.28, whereas the stolon+root:leaf N ratio (i.e.for abovevs.below cutting-height tissues) was only reduced from0.97 to 0.80. In a field trial with two contrasting N regimes,growth and N accumulation were measured on individual cloverplants. Dinitrogen fixation was estimated by¹⁵N isotope dilutionbased on analysis of leaves-only or by including stolons. Usingleaves-only did not affect the calculation of percentage ofclover N derived from N₂fixation (% Ndfa) since the¹⁵N enrichmentwas found to be uniform in all parts of the clover. A correctionfactor of 1.7 to account for N in below cutting-height tissueis suggested when N₂fixation in white clover is estimated byharvesting the leaves only.Copyright 1997 Annals of Botany Company Leaves; N accumulation; N₂fixation; ¹⁵N isotope dilution; pastures; roots; root/shoot ratio; stolons; Trifolium repensL.; white clover     

Seasonal pattern of accumulation and effects of low temperatures on storage compounds in Trifolium repens

The seasonal pattern of concentrations of nitrogen, starch and vegetative storage protein (VSP) in stolons of Trifolium repens L. grown in the field were studied. Two different genotypes, cv. Aran and cv. Rivendel, differing in their morphology (stolon thickness and branching rate) but with similar growth rates, were used. Maximum concentrations of starch were found in summer whereas hydrolysis of starch took place throughout winter, suggesting that C storage is more important for winter survival than for promotion of early spring growth. On the other hand, VSP and nitrogen accumulated in autumn and early winter and then decreased when growth was resumed during early spring. For both cultivars, an inverse relationship was found between VSP concentration in stolons and mean air temperature, suggesting that VSP accumulation may be triggered by low temperature. Further experiments with plants grown under different regimes of temperature and daylength, suggested that VSP synthesis is stimulated by low root temperatures, with a slight synergistic effect of short daylength.
The effects of root temperature on growth, N₂ fixation, NH₄⁺ uptake and N allocation within Trifolium repens L., were studied under controlled conditions. The shoot growth rate was greatly reduced when root temperatures were lowered from 12 to 6°C, while the rate of stolon growth was less affected. Low root temperatures inhibited N2 fixation more than it did NH₄⁺ uptake, but the relative allocation of N to stolons was increased. Lowering root temperature also increased the accumulation of VSP in stolons. These results are discussed in terms of the mechanism associated with low temperature stimulation of VSP accumulation and its coupling with changes in the source/sink relations for allocation of N, between growth and storage.     

Effect of temperature, irradiance and photoperiod on morphological characters of seedlings of contrasting white clover populations

Seedlings of 10 contrasting white clover populations differing in leaf size and origin were grown in a glasshouse in spring and autumn, and in controlled environments in which temperature (10°, 15°, 20°C), photoperiod ( 8 , 16-h) and irradiance ( 13, 47 , 57 Wm^-2) were varied. There were large effects of these environmental variables on a range of morphological characters. Population differences were closely related to classification of the populations by leaf size. Within the large leaf types there was also an effect of origin. In particular, Mediterranean types differed from those of northern origin. Rate of leaf appearance increased with temperature but was less affected by photoperiod. Population differences were greater at lower temperatures. Petiole length was more sensitive to temperature than was leaflet length, particularly in the range 10° to 15°C. Similarly, petiole length increased appreciably with photoperiod extension; smaller leaf types responded more than large leaf types with the result that there were no population differences in the 16-h photoperiod. Population differences in the other environments were closely related to leaf size classification. The onset of stolon production was delayed by lower temperatures and by short photoperiods but stolon number at the sixth leaf stage of development was not closely related to this character in the temperature environments since stolon numbers increased at lower temperatures. In low temperature and short day conditions the large leaf types including those of Mediterranean origin showed a desirable combination of long petioles and large leaves compared with the smaller leaf types. However, these differences between the two groups were eliminated for petiole length, and much reduced for leaflet length, by the greater response to temperature and photoperiod of the smaller leaf types. The implications of these responses for seasonal production are discussed. There were large effects of irradiance on the lengths of leaflets and petioles with longer ones at higher irradiance levels. There were also faster rates of leaf appearance at higher light levels. Stolon production was faster at 57 Wm^-2 than at 47 Wm^-2 while at 13 Wm^-2 no stolons were produced. After an extended period in the lowest light environment there was some plant mortality.     

Effects of simulated herbivory on photosynthesis and N resorption efficiency in <Emphasis Type="Italic">Quercus</Emphasis><Emphasis Type="Italic">pyrenaica</Emphasis> Willd. saplings

We examined the effects of simulated folivory by caterpillars on photosynthetic parameters and nitrogen (N) resorption efficiency in Quercus pyrenaica saplings. We analyzed the differences between intact leaves in control plants, punched leaves in damaged plants, and intact leaves in damaged plants. We then established two levels of simulated folivory: low (≈13% of the leaf area of one main branch removed per plant) and high (≈26% of the leaf area of one main branch removed per plant) treatments. No differences were found in net assimilation rate and conductance between either leaf type or treatment during the most favourable period for photosynthesis. However, the N content was lower in punched than in intact leaves, and as a result PNUE was higher in damaged leaves from treated trees. In leaf-litter samples, N mass was significantly higher in punched than in intact leaves in treated plants, and LMA was significantly higher in damaged than in intact leaves of both the treated and control plants. Consequently, N resorption efficiency was around 15% lower in damaged leaves as compared with intact leaves from treated and control plants. Mechanical injury to leaves not only triggered no compensatory photosynthetic response to compensate a lower carbon uptake due to leaf area loss, but also affected the resorption process that characterizes leaf senescence.     

Annual changes in the concentration of minerals and organic compounds of Quercus suber leaves

Mature leaves are the primary source of sugars, which give rise to many secondary metabolites required for plant survival under adverse conditions. In order to study the interaction of field‐grown cork oak (Quercus suber L.) with the environment, we investigated the seasonal variation of minerals and organic metabolites in the leaves, using inductively coupled plasma atomic emission spectrometry, elemental analysis and nuclear magnetic resonance spectrometry. Statistical analysis showed that the data strongly correlated with seasonal climate and were divided in three groups corresponding to: (1) spring‐early summer, (2) summer and (3) autumn‐winter. The concentration of N, P, K and leaf ash content were highest in spring (recently formed leaves), reached the minimum during the hot and dry summer and increased slightly during the rainy period of autumn‐winter. Conversely, Na, Mg and Ca concentrations were lowest in spring‐early summer and increased during summer and autumn‐winter, the Ca concentration increasing five‐fold. Two cyclitol derivatives, quinic acid and quercitol were the major organic metabolites of the leaves. Their concentration along the season followed opposite trends. While quinic acid predominated during spring‐early summer, when it contributed 12% to the leaf osmotic potential, quercitol was predominant during autumn‐winter, when its contribution to leaf osmotic potential was about 10%. This different preponderance of the two compounds is expressed by the quercitol/quinic acid ratio, which can be as low as 0.2 in early summer and as high as 9 in winter. Sucrose and glucose concentrations also increased during autumn‐winter. Evidence for the quercitol protective role in plants during stress is discussed, and on the basis of structural similarity, it is suggested that quinic acid could have an identical importance, with a protective role against heat and high irradiance. It is concluded that the marked changes in Q. suber leaf composition throughout the year could have important implications in the plant capacity to endure climatic stress.     

Effects of aluminium and pH on growth and potassium uptake by three ectomycorrhizal fungi in liquid culture

Clonal plants of white clover (Trifolium repens L.) were grown in a controlled environment with either low or high rates of applied nitrate-N (providing, notionally, insufficient or sufficient N for unrestricted growth), or in the absence of applied N. Plants receiving no nitrate-N were inoculated with Rhizobia and fixed their own N₂. All plants were maintained with a maximum of three fully unfolded leaves per apex (lenient defoliation) until day 68 when half of the plants were severely defoliated. The export and translocation of carbohydrates from the first fully unfolded main stolon leaf was measured three days later using ¹⁴C.Reduced carbon translocation to stolon tissue and roots, and increased translocation to young branches, occurred following severe defoliation in all three nitrogen treatments. However, N-deficient plants showed large reductions in total export of carbohydrates (44 vs. 17% of ¹⁴C assimilated for lenient vs. severe defoliation) whereas N-sufficient plants (either receiving nitrate-N or fixing their own N₂) showed small increases in total export (means of 54% vs. 62% in the respective defoliation treatments). Furthermore, carbohydrate translocation to old branches ceased altogether in severely defoliated, N-deficient plants, but increased in severely defoliated, N-sufficient plants, illustrating that plant responses to multiple-factor stresses may differ greatly from those seen as the result of single-factor stresses. Interactions between nitrogen nutrition and defoliation in total carbohydrate export, and in carbohydrate supply to old branches, could have serious negative effects on the short-term C economy and physiological integration, and hence on the adaptability, of clonal plants growing with a mineral deficiency in the presence of grazing animals.     

Defoliation effects on isoprene emission from Populus deltoides

Isoprene emission from plants is one of the principal ways in which plant processes alter atmospheric chemistry. Despite the importance of this process, few long-term controls over basal emission rates have been identified. Stress-induced changes in carbon allocation within the entire plant, such as those produced by defoliation, have not been examined as potential mechanisms that may control isoprene production and emission. Eastern cottonwood (Populus deltoides) saplings were partially defoliated and physiological and growth responses were measured from undamaged and damaged leaves 7 days following damage. Defoliation reduced isoprene emission from undamaged and damaged leaves on partially defoliated plants. Photosynthetic rates and leaf carbon and nitrogen pools were unaffected by damage. Photosynthetic rate and isoprene emission were highly correlated in undamaged leaves on undamaged plants and damaged leaves on partially defoliated plants. There was no correlation between photosynthetic rate and isoprene emission in undamaged leaves on partially defoliated plants. Isoprene emission was also highly correlated with the number of source leaves on the apical shoot in damage treatments. Increased carbon export from source leaves in response to defoliation may have depleted the amount of carbon available for isoprene synthesis, decreasing isoprene emission. These results suggest that while isoprene emission is controlled at the leaf level in undamaged plants, emission from leaves on damaged plants is controlled by whole-branch allocation patterns. Received: 12 May 1998 / Accepted: 9 November 1998     

Physiological integration in the clonal perennial herb Trifolium repens L.

Summary Translocation of ¹⁴C-labelled carbohydrates between the parent stolon and branches, and among branches, of Trifolium repens plants was investigated in two glasshouse experiments to determine patterns of physiological organisation in this clonal species. Differential defoliation treatments were applied to the parent stolon and/or branches to test the sensitivity of translocation to the short-term carbon needs of defoliated sinks. Strong reciprocal exchange of carbohydrate between the parent stolon and branches was observed, with 18 41% of the ¹⁴C exported from leaves on the parent stolon moving to branches, while branches simulta-neously exported 25% (for old source branches) to 54% (for young source branches) of the ¹⁴C they assimilated to the parent plant, including translocation to other branches. Branch-to-branch translocation occurred both acropetally and basipetally. Parent-to-branch, branch-to-parent and branch-to-branch carbon fluxes all increased in response to defoliation of the sink, at the expense of carbon supply to stolon tissue or roots of the source module. Reduced export to stolon tissue of the parent axis played a major role in facilitating C reallocation from leaves on the parent stolon to defoliated branches. The observed patterns of C allocation and translocation could be adequately explained by accepted source-sink theory, and are consistent with a high degree of intra-plant physiological integration in resource supply and utilisation. This information provides mechanistic explanations for aspects of the growth dynamics and ecological interactions of T. repens in the patchy environment of a grazed pasture.     

Variation in growth of overwintered stolons of contrasting white clover populations in response to temperature, photoperiod and spring environment

The response of overwintered stolons of nine contrasting white clover populations to temperature, photoperiod and natural conditions was studied in six environments during the spring. Rate of leaf appearance, leaflet length, petiole length, stolon internode length, dry matter distribution within the plant and total dry weight were measured on 15 plants of each population/environment combination. Most characters, except leaf size and proportion of dry matter allocated to leaf, responded positively to temperature in the range 10 – 20°C. A positive effect of photoperiod extension was also found for all characters except rate of leaf appearance, internode length and distribution of dry matter to leaf. Population differences in response to environment were found which were related to both leaf size classification and origin. Stolon dry weight was positively correlated with leaf length, petiole length and stolon internode length in most environments. The relationships between the eight characters were often complex and canonical variate analysis provided a convenient way to discriminate between the populations based on all eight characters.