Application of methyl jasmonate reduces growth but increases chemical defence and resistance against Hylobius abietis in Scots pine seedlings

Scots pine (Pinus sylvestris L., Pinaceae) produces a terpenoid resin which consists of monoterpenes and resin acids that offer protection against herbivores and pathogen attacks. Methyl jasmonate (MJ) is a potential plant elicitor which induces a wide range of chemical and anatomical defence reactions in conifers and might be used to increase resistance against biotic damage. Different amounts of MJ (control, 10 mm , and 100 mm ) were applied to Scots pine to examine the vigour, physiology, herbivory performance, and induction of secondary compound production in needles, bark, and xylem of 2‐year‐old Scots pine seedlings. Growth decreased significantly in both MJ treated plants, and photosynthesis decreased in the 100 mm MJ treated plants, when compared to 10 mm MJ or control plants. The large pine weevil (Hylobius abietis L.) (Coleoptera: Curculionidae) gnawed a significantly smaller area of stem bark in the 100 mm treated plants than in the control or 10 mm treated plants. The 100 mm MJ treatment increased the resin acid concentration in the needles and xylem but not in the bark. Furthermore, both MJ treatments increased the number of resin ducts in newly developing xylem. The changes in plant growth and chemical parameters after the MJ treatments indicate shifts in carbon allocation, but MJ also affects plant physiology and xylem development. Terpenoid resin production was tissue‐specific, but generally increased after MJ treatments, which means that this compound may offer potential protection of conifers against herbivores.     

Quantitative comparison of chemical, biological and mechanical induction of secondary compounds in Pinus pinaster seedlings

Chemical elicitors and mechanical treatments simulating real insect herbivory have been increasingly used to study induced defensive responses in woody plants. However, simultaneous quantitative comparisons of plant chemical defences elicited by real and simulated herbivory have received little attention. In this paper we compared the effects of real herbivory, simulated herbivory using two chemical elicitors, and mechanical damage treatments on the quantitative secondary chemistry of Pinus pinaster juveniles (namely on non-volatile resin in the stem and total phenolics in the needles). The real herbivory involved phloem wounding by Hylobius abietis and defoliation by Brachyderes lusitanicus (two pine weevils); the chemical elicitors to simulate herbivory induction were 40 mM methyl jasmonate (MJ) and 20 μM benzothiadiazole (BTH); and the mechanical treatments included phloem wounding and needle clipping. We also performed an additional experiment for assessing at what extent insect extracts could increase plant responses over mechanical damage. Chemical induction with MJ, mechanical wounding and real phloem herbivory by H. abietis all produced quantitatively similar results, increasing the concentration of resin in the stem and total phenolics in the needles by equivalent magnitudes. Exogenous application of BTH increased the concentration of phenolic compounds in pine needles, but did not show the same effect on stem resin. Contrastingly, we did not find significant changes in the concentration of resin in the stem or phenolics in the needles after needle clipping and B. lusitanicus feeding. Mechanical damage followed by the application of extracts from the insects B. lusitanicus and H. abietis on the injured tissues did not increase the responses in comparison to mechanical damage alone. The fact that strong induced responses elicited by phloem wounding insects are equally elicited by phloem injuries suggests that defences in pine trees are raised with low specificity regarding biotic enemies. Results from this paper support future methodological approaches using chemical elicitors and mechanical damage as simulated herbivory treatments for the experimental induction of conifer defences.     

Bidirectional exchange of amino compounds between phloem and xylem during long-distance transport in Norway spruce trees (Picea abies [L.] Karst)

14C-Gln, (14)C-Asp, (15)N-Gln, and (15)N-Asp were fed to cut tips of 2- or 3-year-old needles of spruce twigs, still attached to the tree. After incubation, distribution of the radiolabel and (15)N enrichment was studied in needles, bark and wood tissues of girdled twigs and untreated controls. Analysis of the twig tissues showed that between 22% and 26% of the total amount of the tracers applied had been taken up. Since export out of the application segment and distribution between needles, bark and wood was comparable for (14)C and (15)N tracer, it was concluded that, mainly the amino compounds that had been fed were subject to long- distance transport within the plant and supplied the new sprout with nitrogen. Asp was exported to a greater extent to developing needles compared with Gln. This difference in export between the two amino compounds applied may be explained by the different pool sizes of Gln and Asp in xylem and phloem or differences in xylem and phloem loading. Girdling of the stem showed that the transport of reduced nitrogen compounds from older needle generations to current-year needles proceeded in both xylem and phloem. In addition, an intensive bidirectional exchange of Gln and Asp between xylem and phloem was observed during long-distance transport.     

Visualization of lateral water transport pathways in soybean by a time of flight-secondary ion mass spectrometry cryo-system

Water movement between cells in a plant body is the basic phenomenon of plant solute transport; however, it has not been well documented due to limitations in observational techniques. This paper reports a visualization technique to observe water movement among plant cells in different tissues using a time of flight-secondary ion mass spectrometry (Tof-SIMS) cryo-system. The specific purpose of this study is to examine the route of water supply from xylem to stem tissues. The maximum resolution of Tof-SIMS imaging was 1.8 μm (defined as the three pixel step length), which allowed detection of water movement at the cellular level. Deuterium-labelled water was found in xylem vessels in the stem 2.5 min after the uptake of labelled water by soybean plants. The water moved from the xylem to the phloem, cambium, and cortex tissues within 30-60 min after water absorption. Deuterium ion counts in the phloem complex were slightly higher than those in the cortex and cambium tissue seen in enlarged images of stem cell tissue during high transpiration. However, deuterium ion counts in the phloem were lower than those in the cambium at night with no evaporative demand. These results indicate that the stem tissues do not receive water directly from the xylem, but rather from the phloem, during high evaporative demand. In contrast, xylem water would be directly supplied to the growing sink during the night without evaporative demand.     

Floral-induced and constitutive defense against florivory: a comparison of chemical traits in 12 herb species

Little is known about how plants protect flowers—their reproductive organs—against florivory. Additionally, the induced floral defense system has been examined in only a few species. We tested the inducibility of putative floral defenses and investigated the relationship between natural florivory and the floral defenses of 12 naturally growing plant species. The relationships between florivory and four chemical traits (nitrogen, phosphorus, total phenolics, and condensed tannins) were investigated in 12 plant species. We also studied whether flowers induce changes in chemical defenses in response to artificial damage in 10 plant species. A higher concentration of floral nitrogen was associated with a decreasing frequency of florivore attacks. Among the four traits of the 10 plant species studied, no trait changed in response to the artificial damage. We suggest that induced defense systems may not be advantageous for flowers, although it is also possible that these species simply do not use induced defense in any of their plant parts.     

Changes in plant chemical defenses and nutritional quality as a function of ontogeny in <Emphasis Type="Italic">Plantago lanceolata</Emphasis> (Plantaginaceae)

Numerous empirical studies have examined ontogenetic trajectories in plant defenses but only a few have explored the potential mechanisms underlying those patterns. Furthermore, most documented ontogenetic trajectories in plant defenses have generally concentrated on aboveground tissues; thus, our knowledge regarding whole plant trends in plant defenses throughout development or potential allocation constraints between growth and defenses is limited. Here, we document changes in plant biomass, nutritional quality and chemical defenses for below- and aboveground tissues across seven age classes of Plantago lanceolata (Plantaginaceae) to evaluate: (1) partial and whole plant ontogenetic trajectories in constitutive chemical defenses and nutritional quality, and (2) the role of resource allocation constraints, namely root:shoot (R:S) ratios, in explaining whole plant investment in chemical defenses over time. Overall investment in iridoid glycosides (IGs) significantly increased, while water and nitrogen concentrations in shoot tissues decreased with plant age. Significant variation in IG content between shoot and root tissues across development was observed: allocation of IGs into root tissues linearly increased from younger to older plants, while non-linear shifts in allocation of IGs during ontogeny were observed for shoot tissues. Finally, R:S ratios only weakly explained overall allocation of resources into defenses, with young stages showing a positive relationship, while older stages showed a negative relationship between R:S ratios and IG concentrations. Ontogenetic trajectories in plant quality and defenses within and among plant tissues can strongly influence insect herbivores’ performance and/or predation risk; thus, they are likely to play a significant role in mediating species interactions.     

Alterations of the resin canal system of <Emphasis Type="Italic">Pinus pinaster</Emphasis> seedlings after fertilization of a healthy and of a <Emphasis Type="Italic">Hylobius abietis</Emphasis> attacked stand

Changes in resource availability and biotic and abiotic stress may alter the defensive mechanisms of pine trees. The effect of fertilisation on the resin canal structure of Pinus pinaster seedlings established in two trials in NW Spain, one attacked by Hylobius abietis and the other non-attacked, was studied. The leaders of 50 plants were destructively sampled and the resin canal density, the canal area and its relative conductive area in the phloem and xylem were assessed. Experimentally increased nutrient availability significantly decreased resin canal density in the phloem of the seedlings in the two analysed trials, where unfertilised seedlings presented up to 30% more resin canal density than the fertilised seedlings (mean value ± SEM = 0.32 ± 0.02 resin canals mm⁻² in the fertilised plants versus 0.45 ± 0.04 resin canals mm⁻² in the control plants). Fertilisation had no effect on the resin canal system in the xylem, but significantly increased tracheid size. Significant differences of resin canals among sites were observed mainly in the xylem; the resin canal density was 1.7-fold greater in the attacked site than in the non-attacked site. The similar structure of phloem resin canals in both sites supports that phloem resin canals are constitutive mechanisms of defence in P. pinaster, whereas xylem resin canals would be constitutive mechanisms but also inducible mechanisms of resistance following the attack of pine weevils or bark beetles. XM and LS equally contributed to this paper.     

Costs of Defense and a Test of the Carbon-Nutrient Balance and Growth-Differentiation Balance Hypotheses for Two Co-Occurring Classes of Plant Defense

One of the goals of chemical ecology is to assess costs of plant defenses. Intraspecific trade-offs between growth and defense are traditionally viewed in the context of the carbon-nutrient balance hypothesis (CNBH) and the growth-differentiation balance hypothesis (GDBH). Broadly, these hypotheses suggest that growth is limited by deficiencies in carbon or nitrogen while rates of photosynthesis remain unchanged, and the subsequent reduced growth results in the more abundant resource being invested in increased defense (mass-balance based allocation). The GDBH further predicts trade-offs in growth and defense should only be observed when resources are abundant. Most support for these hypotheses comes from work with phenolics. We examined trade-offs related to production of two classes of defenses, saponins (triterpenoids) and flavans (phenolics), in Pentaclethra macroloba (Fabaceae), an abundant tree in Costa Rican wet forests. We quantified physiological costs of plant defenses by measuring photosynthetic parameters (which are often assumed to be stable) in addition to biomass. Pentaclethra macroloba were grown in full sunlight or shade under three levels of nitrogen alone or with conspecific neighbors that could potentially alter nutrient availability via competition or facilitation. Biomass and photosynthesis were not affected by nitrogen or competition for seedlings in full sunlight, but they responded positively to nitrogen in shade-grown plants. The trade-off predicted by the GDBH between growth and metabolite production was only present between flavans and biomass in sun-grown plants (abundant resource conditions). Support was also only partial for the CNBH as flavans declined with nitrogen but saponins increased. This suggests saponin production should be considered in terms of detailed biosynthetic pathway models while phenolic production fits mass-balance based allocation models (such as the CNBH). Contrary to expectations based on the two defense hypotheses, trade-offs were found between defenses and photosynthesis, indicating that studies of plant defenses should include direct measures of physiological responses.     

Methyl jasmonate-induced ethylene production is responsible for conifer phloem defense responses and reprogramming of stem cambial zone for traumatic resin duct formation

Conifer stem pest resistance includes constitutive defenses that discourage invasion and inducible defenses, including phenolic and terpenoid resin synthesis. Recently, methyl jasmonate (MJ) was shown to induce conifer resin and phenolic defenses; however, it is not known if MJ is the direct effector or if there is a downstream signal. Exogenous applications of MJ, methyl salicylate, and ethylene were used to assess inducible defense signaling mechanisms in conifer stems. MJ and ethylene but not methyl salicylate caused enhanced phenolic synthesis in polyphenolic parenchyma cells, early sclereid lignification, and reprogramming of the cambial zone to form traumatic resin ducts in Pseudotsuga menziesii and Sequoiadendron giganteum. Similar responses in internodes above and below treated internodes indicate transport of a signal giving a systemic response. Studies focusing on P. menziesii showed MJ induced ethylene production earlier and 77-fold higher than wounding. Ethylene production was also induced in internodes above the MJ-treated internode. Pretreatment of P. menziesii stems with the ethylene response inhibitor 1-methylcyclopropene inhibited MJ and wound responses. Wounding increased 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase protein, but MJ treatment produced a higher and more rapid ACC oxidase increase. ACC oxidase was most abundant in ray parenchyma cells, followed by cambial zone cells and resin duct epithelia. The data show these MJ-induced defense responses are mediated by ethylene. The cambial zone xylem mother cells are reprogrammed to differentiate into resin-secreting epithelial cells by an MJ-induced ethylene burst, whereas polyphenolic parenchyma cells are activated to increase polyphenol production. The results also indicate a central role of ray parenchyma in ethylene-induced defense.     

Detection and characterization of nitrogen circulation through the sieve tubes and xylem vessels of rice plants

Nitrogen movement through the xylem vessels and sieve tubes in rice plants was studied using xylem and phloem sap analysis in combination with stable and radioactive nitrogen isotope techniques.More than 90% of nitrogen was translocated in the sieve tubes of rice plants as amino acids. When 15N (99.6 atom%) was applied as a nitrate to the root, 15N first appeared in phloem sap of the leaf sheath within 10 minutes and increased to 37 atom% excess 5 hours after the experiment had started. In long-term experiments, 63% of nitrogen in the phloem sap of the leaf sheath and 15% in that of the uppermost internode came from nitrogen absorbed within the last 24 hours and 50 hours, respectively.To obtain information about the more rapid circulation of nitrogen in the plant, radioactive 13N was used as a tracer. A positron-emitting tracer imaging system was used to show that 13N was transferred to the leaf sheath within 8 minutes of its application to the roots. Analysis of the xylem sap of the leaf sheath showed that when the nitrate was applied to the roots, most of the nitrogen in the xylem was transported as a nitrate.These data showed that phloem and xylem sap analysis together with the stable and radioactive nitrogen techniques provide a good method for the detection of nitrogen cycles in plants.     

Allocation of nitrogen to an inducible defense and seed production in Nicotiana attenuata

Resource-based tradeoffs in the allocation of a limiting resource are commonly invoked to explain negative correlations between growth and defense in plants, but critical examinations of these tradeoffs are lacking. To rigorously quantify tradeoffs in a common currency, we grew Nicotiana attenuata plants in individual hydroponic chambers, induced nicotine production by treating roots with methyl jasmonate (MJ) and standardized leaf puncturing, and used ¹⁵N to determine whether nitrogen-based tradeoffs among nicotine production, growth, and seed production could be detected. Plants were treated with a range of MJ quantities (5, 45 or 250 μg plant^?1) to effect a physiologically realistic range of changes in endogenous jasmonic acid levels and increases in nicotine production and accumulation; MJ treatments were applied to the roots to target JA-induced nicotine production, since nicotine biosynthesis is restricted to the roots. Leaf puncturing and 5 μg MJ treatments increased de novo nicotine synthesis and whole-plant (WP) nicotine pools by 93 and 66%, while 250 μg MJ treatments increased these values 3.1 and 2.5-fold. At these high rates of nicotine production, plants incorporated 5.7% of current nitrogen uptake and 6.0% of their WP nitrogen pools into nicotine. The ¹⁵N-labeled nicotine pools were stable or increased for the duration of vegetative growth, indicating that the N-nicotine was not metabolized and re-used for growth. Plants with elevated nicotine production grew more slowly and the differences in plant biomass gain between MJ-treated plants and controls were linearly related to the differences in nicotine accumulation. Despite the reductions in rosette-stage growth associated with nicotine production, estimates of lifetime fitness (cumulative lifetime seed production, mass/seed, seed viability) were not affected by any treatment. Only two treatments (leaf puncturing and 250 μg MJ) increased the allocations of ¹⁵N acquired at the time of induction to seed production. On average, plants used only 14.9% of their WP nitrogen pool for seed production, indicating that either the nitrogen requirements for seed production or the reproductive effort of these hydroponically-grown plants are low. To determine if seed production is strongly influenced by the amount of vegetative biomass attained before reproduction, the experiment was repeated with plants that had 44% of their leaf area (or 29% of their WP biomass) removed before MJ treatments with a removal technique that minimized the nicotine response. MJ treatments of these plants dramatically increased nicotine production and accumulation, but these plants also suffered no measurable fitness consequences from either the leaf removal or MJ treatments. We conclude that when N. attenuata plants are grown in these individual hydroponic chambers, their allocation to reproduction is sufficiently buffered to obscure the large increases in nitrogen allocations to an inducible defense. To determine whether soil-grown plants are similarly buffered, we grew two genotypes of plants in the high-nutrient soil from a 1-year-old burn in a piñyon-juniper forest (the plants' natural habitat) and in low-nutrient soil from an adjacent unburned area, and induced nicotine production in half of the plants with a 500 μg root MJ treatment. Plants grown in burned soils had an estimated lifetime fitness that was on average 2.8-fold greater than that of plants grown in unburned soils. MJ treatment reduced fitness estimates by 43% and 71% in the burned and unburned soils, respectively. We conclude that while hydroponic culture allows one to rigorously quantitate nitrogen allocation to growth, reproduction and defense, the allocation patterns of plants grown in hydroponic culture differ from those of plants grown in soil. Under hydroponic conditions, plants have low reproductive allocations and reproductive-defense tradeoffs are not detected. Reproductive-defense tradeoffs are readily discernible in soil-grown plants, but under these growing conditions, the nitrogen-basis for the tradeoff is difficult to quantify.     

Xylem and Phloem Transport of Mineral Nutrients from Solanum tuberosum Roots

The xylem and phloem transport of mineral elements from stemnodal roots to the stem and stolon of growing potato (Solanumtuberosum L. cv. ‘Russet Burbank’) plants was investigated.Adventitious roots, originating from below-ground nodes of thestem of potato seedlings, were exposed to solutions of SrCI₂or MnSO₄. Relative elemental concentrations were measured inthe conductive tissues using energy dispersive X-ray analysis.After a 5 h daylight uptake period, Sr (a Ca-transport analogue)levels were elevated in the stem xylem tissue, but Sr did notincrease in the stem phloem, nor was it present in either ofthe conductive tissues of stolons located 1–2 nodes abovethe treated roots. In contrast, elevated levels of Cl, S, andMn were found in stolon xylem and phloem tissue during the sameperiod. The absence of Sr in the stolon after 5 h suggests thatno xylem flow into the stolon occurred during the uptake periodand, furthermore, phloem flow is responsible for the transportof the Cl, S, and Mn into the stolon. Elevated levels of thesemobile nutrients in the xylem of the stolon were attributedto xylem-to-phloem transfer in the stem or leaves, transportto the stolon in the phloem, and phloem-to-xylem transfer inthe stolon. During a 19 h uptake period, some Sr was observedin the phloem tissue of the stem, demonstrating slow exchangeof Sr with sieve elements or proximal phloem parenchyma andcompanion cells. Key words: Calcium, manganese, X-ray analysis     

Glutamine Transfer from Xylem to Phloem and Translocation to Developing Leaves of Populus deltoides

The distribution of ¹⁴C from xylem-borne [¹⁴C]glutamine, the major nitrogen compound moving in xylem sap of cottonwood (Populus deltoides Bartr. ex Marsh), was followed in rapidly growing shoots with a combination of autoradiographic, microautoradiographic, and radioassay techniques. Autoradiography and ¹⁴C analyses of tissues showed that xylem-borne glutamine did not move with the transpiration stream into mature leaves. Instead, most of it was transferred from xylem to phloem in the upper stem and then translocated to young developing tissues. Microautoradiography showed that metaxylem parenchyma, secondary xylem parenchyma, and rays were the major areas of uptake from xylem vessels in the stem. Accumulation in phloem (high ¹⁴C concentrations in sieve tubes) took place in internodes subtending recently mature leaves. Little ¹⁴C from xylem-borne glutamine was found in phloem of mature leaves, which indicates restricted retransport of glutamine that did enter the leaf. In the primary tissues of the upper stem, most ¹⁴C was found in the phloem. Cottonwood stems have an efficient uptake and transfer system that enhances glutamine movement to developing tissues of the upper stem.     

Transport Exchange of Carbon,Nitrogen and Water in the Context of Whole Plant Growth and Functioning — Case History of a Nodulated Annual Legume

The economy of carbon, nitrogen and water during growth of nodulated, nitrogen-fixing plants of white lupin (Lupinus albus L.) was studied by measuring C, N and H₂O content of plant parts, concentrations of C and N in bleeding sap of xylem and phloem, transpirational losses of whole shoots and shoot parts, and daily exchanges of CO₂ between shoot and root parts and the surrounding atmosphere. Relationships were studied between water use and dry matter accumulation of shoot and fruits, and between net photosynthesis rate and leaf area, transpiration rate and nitrogen fixation. Conversion efficiencies were computed for utilization of net photosynthate for nitrogen fixation and for production of dry matter and protein in seeds. Partitioning of the plant's intake of C, N and H₂O was described in terms of growth, transpiration, and respiration of plant parts. An empirically-based model was developed to describe transport exchanges in xylem and phloem for a 10-day interval of growth. The model depicted quantitatively the mixtures of xylem and phloem streams which matched precisely the recorded amounts of C, N and H₂O assimilated, absorbed or consumed by the various parts of the plant. The model provided information on phloem translocation of carbon and nitrogen to roots from shoots, the cycling of carbon and nitrogen through leaves, the relationship between transpiration and nitrogen partitioning to shoot organs through the xylem, the relative amount of the plant's water budget committed to phloem translocation, and the significance of xylem to phloem transfer of nitrogen in stems as a means of supplying nitrogen to apical regions of the shoot.     

Effects of nutrient contents and defense compounds on herbivory in reproductive organs and leaves of Iris gracilipes

Optimal defense theory (ODT) states that the plant tissue with the highest value to fitness will receive the most protection compared with other plant parts. ODT can be applied to the differences in defenses among floral organs, although most studies have concentrated on the comparison between leaves and flowers. Using Iris gracilipes, we investigated whether ODT is supported when primary and accessory floral organs and leaves are distinguished. We found that anthers and perianths tended to be attacked more severely than ovaries and leaves in the bud and flower stages and that anthers contained the highest nitrogen and phosphorus concentrations. Although ovaries were also found to contain high nitrogen and phosphorus concentrations, they were less severely attacked by herbivores than anthers, perhaps because ovaries contained the highest condensed tannins concentrations among the floral organs except for perianths in the flower stage. Thus, noting that the number of ovules is very much smaller than that of pollen grains, we concluded that ovaries are the most intensively protected, consistent with the prediction of ODT as applied to floral organs. ODT is applicable to the difference in defense allocation among floral organs.     

Terpenoids are transported in the xylem sap of Norway spruce

Norway spruce is a conifer storing large amounts of terpenoids in resin ducts of various tissues. Parts of the terpenoids stored in needles can be emitted together with de novo synthesized terpenoids. Since previous studies provided hints on xylem transported terpenoids as a third emission source, we tested if terpenoids are transported in xylem sap of Norway spruce. We further aimed at understanding if they might contribute to terpenoid emission from needles. We determined terpenoid content and composition in xylem sap, needles, bark, wood and roots of field grown trees, as well as terpenoid emissions from needles. We found considerable amounts of terpenoids—mainly oxygenated compounds—in xylem sap. The terpenoid concentration in xylem sap was relatively low compared with the content in other tissues, where terpenoids are stored in resin ducts. Importantly, the terpenoid composition in the xylem sap greatly differed from the composition in wood, bark or roots, suggesting that an internal transport of terpenoids takes place at the sites of xylem loading. Four terpenoids were identified in xylem sap and emissions, but not within needle tissue, suggesting that these compounds are likely derived from xylem sap. Our work gives hints that plant internal transport of terpenoids exists within conifers; studies on their functions should be a focus of future research.     

Experimental evidence for diel δ15N‐patterns in different tissues,xylem and phloem saps of castor bean (Ricinus communis L.)

Nitrogen isotope signatures in plants might give insights in the metabolism and allocation of nitrogen. To obtain a deeper understanding of the modifications of the nitrogen isotope signatures, we determined δ¹⁵N in transport saps and in different fractions of leaves, axes and roots during a diel course along the plant axis. The most significant diel variations were observed in xylem and phloem saps where δ¹⁵N was significantly higher during the day compared with during the night. However in xylem saps, this was observed only in the canopy, but not at the hypocotyl positions. In the canopy, δ¹⁵N was correlated fairly well between phloem and xylem saps. These variations in δ¹⁵N in transport saps can be attributed to nitrate reduction in leaves during the photoperiod as well as to ¹⁵N‐enriched glutamine acting as transport form of N. δ¹⁵N of the water soluble fraction of roots and leaves partially affected δ¹⁵N of phloem and xylems saps. δ¹⁵N patterns are likely the result of a complex set of interactions and N‐fluxes between plant organs. Furthermore, the natural nitrogen isotope abundance in plant tissue is not constant during the diel course – a fact that needs to be taken into account when sampling for isotopic studies.     

Cytokinins in the Phloem Sap of White Lupin (Lupinus albus L.)

Cytokinin-like activity in samples of xylem and phloem sap collected from field-grown plants of white lupin (Lupinus albus L.) over a period of 9 to 24 weeks after sowing was measured using the soybean hypocotyl callus bioassay following paper chromatographic separation. The phloem sap was collected from shallow incisions made at the base of the stem, the base of the inflorescence (e.g. stem top), the petioles, and the base and tip of the fruit. Xylem sap was collected as root exudate from the stump of plants severed a few centimeters above ground level. Concentration of cytokinin-like substances was highest in phloem sap collected from the base of the inflorescence and showed an increase over the entire sampling period (from week 10 [61 nanogram zeatin equivalents] to week 24 [407 nanogram zeatin equivalents]). Concentrations in the xylem sap and in the other phloem saps were generally lower. Relatively high concentrations of cytokinin-like substances in petiole phloem sap (70 to 130 nanogram zeatin equivalents per milliliter) coincided in time with high concentrations in sap from the base of the inflorescence (see above). Concentrations in sap (phloem or xylem) from the base of the stem were very much lower. This finding is consistent with movement of cytokinins from leaves into the developing inflorescence and fruit, rather than direct input to the fruit from xylem sap. However, an earlier movement of cytokinins from roots into leaves via the xylem cannot be ruled out. Sap collected at an 18-week harvest was additionally separated by sequential C₁₈ reversed-phase high performance liquid chromatography → NH₂ normal phase high performance liquid chromatography, bioassayed, and then analyzed by electron impact gas chromatography-mass spectrometry. Identification of zeatin riboside and dihydrozeatin as two of the major cytokinins in combined sap samples was accomplished by gas chromatography-mass spectrometry-selected ion monitoring.     

Reproductive water supply is prioritized during drought in tomato

Reproductive success largely defines the fitness of plant species. Understanding how heat and drought affect plant reproduction is thus key to predicting future plant fitness under rising global temperatures. Recent work suggests reproductive tissues are highly vulnerable to water stress in perennial plants where reproductive sacrifice could preserve plant survival. However, most crop species are annuals where such a strategy would theoretically reduce fitness. We examined the reproductive strategy of tomato (Solanum lycopersicum var. Rheinlands Ruhm) to determine whether water supply to fruits is prioritized above vegetative tissues during drought. Using optical methods, we mapped xylem cavitation and tissue shrinkage in vegetative and reproductive organs during dehydration to determine the priority of water flow under acute water stress. Stems and peduncles of tomato showed significantly greater xylem cavitation resistance than leaves. This maintenance of intact water supply enabled tomato fruit to continue to expand during acute water stress, utilizing xylem water made available by tissue collapse and early cavitation of leaves. Here, tomato plants prioritize water supply to reproductive tissues, maintaining fruit development under drought conditions. These results emphasize the critical role of water transport in shaping life history and suggest a broad relevance of hydraulic prioritization in plant ecology.