Transpiration inhibition by stored xylem sap from well-watered maize plants

There is increasing evidence that a chemical signal exists in xylem sap of plants subjected to water deficits which influences physiological responses in plant shoots. An important method of studying this signal is the transpiration response of excised leaves exposed to xylem sap collected from plants. However, Munns et al [Plant, Cell & Environment 16, 867–877] cautioned that transpiration inhibition is observed when xylem sap collected from wheat and barley is stored before determining physiological activity. The objective of the study reported here was to determine if transpiration inhibition develops in maize sap collected from well-watered plants when the sap is stored under various conditions. It was found that storage of maize sap collected from well-watered plants for only 1 d at -20°C resulted in the development of substantial transpiration inhibition in bioassay leaves. Storage of sap at 4°C resulted in the development of the effect after 2 weeks, while storage at ?86°C showed only small transpiration inhibition after 3 weeks. The major source of the transpiration inhibition was the development of a substance in the stored sap that resulted in physical blockage of the transpiration stream in bioassay leaves. However, a small signal component may also have developed in the stored sap. Because of the possibility of ionic activity under freezing conditions at ?20°C, calcium was studied for its potential involvement in the transpiration inhibition. However, the calcium concentrations found to inhibit transpiration were nearly an order of magnitude larger than the calcium concentrations observed in xylem sap.     

Acclimation of plants to light gradients in leaf canopies: evidence for a possible role for cytokinins transported in the transpiration stream.

The mechanism of response of plants to vertical light intensity gradients in leaf canopies was investigated. Since shaded leaves transpire less than leaves in high light, it was hypothesized that cytokinins (CKs) carried by mass transport in the transpiration stream would be distributed over the leaf area of partially shaded plants parallel to the gradient in light intensity. It was also hypothesized that this causes the distribution of leaf growth, leaf N and photosynthetic capacity, and possibly chloroplast acclimation as observed in plants growing in leaf canopies. In a field experiment, the distribution of Ca, N and CKs in a bean leaf canopy of a dense and an open stand supported the concept of a role for CKs in the response of N allocation to the light gradient when a decreasing sensitivity for CKs with increasing leaf age is assumed. Both shading of one leaf of the pair of primary bean leaves and independent reduction of its transpiration rate in a growth cabinet experiment caused lower dry mass, N and Ca per unit leaf area in comparison to the opposite not treated leaf. Shading caused a parallel reduction in CK concentration, which supports the hypothesis, but independent reduction of transpiration rate failed to do the same. Application of benzylaminopurine (BA) counteracted the reduction caused by shade of leaf N, photosynthetic capacity and leaf area growth. The experiments show an important role for the transpiration stream in the response of plants to light gradients. Evidence is presented here that CKs carried in the transpiration stream may be important mediators for the acclimation of plants to leaf canopy density.     

Nitrogen Redistribution during Grain Growth in Wheat (Triticum aestivum L.) : IV. Development of a Quantitative Model of the Translocation of Nitrogen to the Grain

Translocation of nitrogen was measured in wheat (Triticum aestivium L. cv SUN 9E) plants grown without an exogenous supply of nitrogen from the time that the flagleaf began to emerge, and a model of nitrogen translocation was constructed to describe translocation on one day during the linear period of grain growth. Nitrogen for grain development was derived entirely by the redistribution of nitrogen from vegetative organs. Leaves contributed 40%, glumes 23%, stem 23%, and roots 16% of the nitrogen incorporated by the grains on the fifteenth day after anthesis. Less than 50% of the nitrogen exported from leaves was translocated directly to the grain via the phloem, the rest was translocated to the roots and was cycled in the roots and exported to the shoot in the transpiration stream. Nitrogen imported by leaves and glumes via the xylem was not accumulated in these organs but was transferred to the phloem for reexport from the organs. A large proportion (60%) of the nitrogen in the transpiration stream was cycled in the glumes. The glumes were also a major source of nitrogen for grain development. It was considered likely that this organ always plays an important role in nitrogen metabolism in wheat.     

Absorption and Distribution of Radioactive Phosphorus and Calcium in the Bean Plant

When ³²P and ⁴⁵Ca were supplied simultaneously to bean plantsgrowing in culture solution under controlled conditions thatfavoured rapid transpiration it was found that after one hour7.9 per cent of the ³²P taken up had penetrated into the stem,while the ⁴⁵Ca present was still in the root. There was a similardelay in the movement of ⁴⁵Ca from stem to leaves. Althoughmovement in the transpiration stream may be one important mechanismfor translocation, it is apparent that the distribution of theseelements is greatly affected by selective exchange reactionsand differential absorption in transit.     

Mannose metabolism in corn and its impact on leaf metabolites, photosynthetic gas exchange, and chlorophyll fluorescence

When intact corn leaves were provided millimolar concentrations of d-mannose through the transpiration stream photosynthesis was inhibited; 5.7 millimolar resulted in a 50% inhibition of the carbon exchange rate. This inhibition was partially reversible by the addition of orthophosphate to the feeding solution. Mannose metabolism by corn leaves was limited in that it did not act as a resource for sucrose or starch synthesis. Mannose 6-phosphate accumulated in the leaf tissues and was slowly metabolized by a pathway involving mannose 1-phosphate. Correlated with the mannose-6-phosphate accumulation were decreases in ATP, orthophosphate, sucrose, and phosphoenolpyruvate and increases in starch and maltose. When provided in the transpiration stream mannose had access to both mesophyll and bundle sheath cells. Mannose feeding led to oscillations in steady state chlorophyll fluorescence emission (680 nanometers) and an elimination of the Kautsky effect during fluorescence induction. Pyridoxal 5-phosphate and 2,4-dinitrophenol were found to be inhibitors of CO₂ exchange when provided in the transpiration stream of intact corn leaves. However, Pyridoxal 5-phosphate induced a quenching of steady state fluorescence while 2,4-dinitrophenol led to an increase in fluorescence emission.     

Water translocation in Kalanchoë daigremontiana during periods of drought

Abstract. Kalanchoë daigremontiana strongly reduced daily water loss within 6 d of drought using CAM to restrict transpiration and net CO₂ uptake to the dark period.
Water translocation from old to young leaves of the plant was an additional mechanism which reduced the negative effects of drought on the water relations of young leaves. Excision of old leaves after 7–9 d of drought resulted in a decrease in the water content of young leaves. This was observed despite a decrease in transpirational water loss from young leaves. Water content in young leaves increased slightly in plants with all their leaves in place.
The dry weight of young leaves clearly increased during the experimental period when old leaves were present, but it remained relatively constant in plants without old leaves. Obviously, in addition to water, solutes were transported from old to young leaves of the plant via the phloem. Xylem tension was higher in young compared to old leaves; thus, water translocation could have occurred via xylem elements.
Since transport of organic matter in the phloem is also linked to water flow, phloem transport additionally may contribute effectively to the balance of the water budget in young leaves.     

Distribution and Volatilization of Organic Compounds Following Uptake by Hybrid Poplar Trees

Hybrid poplar trees were exposed to eleven organic compounds in hydroponic systems. The eleven contaminants were common pollutants with a wide range of physio-chemical properties such as the octanol-water partition coefficient, Henry's constant, vapor pressure, and molecular weight. Contaminants, ¹⁴C-labeled, were introduced into the root zone, and contaminant transport and fate were examined. Aqueous concentrations were monitored throughout each experiment as was vapor phase concentrations in the air stream passing over the leaves. At experiment conclusion, plant tissues were oxidized to determine ¹⁴C concentrations. The uptake, distribution, and volatilization of these contaminants varied greatly among the 11 contaminants in the study. Uptake and translocation of the contaminants ranged from < 0.3% (of the applied ¹⁴C-labeled compound) for 1,2,4-trichlorobenzene to 20% for benzene. Volatile compounds were volatilized from the leaves. Volatilization in the transpiration stream was related to the vapor pressure of the compound. The fate and transport mechanisms investigated in this study provide valuable insight into the potential fate of contaminants in full-scale phytoremediation.     

Nitrate Reductase Activity in Maize (Zea mays L.) Leaves: II. Regulation by Nitrate Flux at Low Leaf Water Potential

Experiments were conducted to determine whether the nitrate flux to the leaves or the nitrate content of the leaves regulated the nitrate reductase activity (NRA) in leaves of intact maize (Zea mays L.) seedlings having low water potentials (ψ_w) when other environmental and endogenous factors were constant. In seedlings that were desiccated slowly, the nitrate flux, leaf nitrate content, and NRA decreased as ψ_w decreased. The decrease in nitrate flux was caused by a decrease in both the rate of transpiration and the rate of nitrate delivery to the transpiration stream. Upon rewatering, the recovery in NRA was correlated with the nitrate flux but not the leaf nitrate content.     

Antitranspirant Activity in Xylem Sap of Maize Plants

Xylem sap from unwatered maize plants was collected and testedfor antitranspirant activity. Two assays were used. These werea transpiration assay with detached wheat leaves and a stomatalbio-assay involving the direct microscopic observation of epidermisof Commelina communis. The reduction in transpiration of detached wheat leaves promotedby xylem sap could be duplicated almost exactly by the applicationof solutions of ABA of equivalent concentration to that foundin the xylem sap. Removal of virtually all the ABA from thexylem sap, using an immunoaffinity column, removed virtuallyall the antitranspirant activity in both assays. These results are discussed in the context of other resultswhich suggest the presence of as-yet unidentified inhibitorsin the xylem sap of unwatered plants. We suggest that with maize plants at least, stomatal responsesto soil drying can be entirely explained by enhanced concentrationof ABA in the xylem stream. Key words: Antitranspirant activity, ABA, ABA bio-assay, xylem sap     

Diurnal pattern of acetaldehyde emission by flooded poplar trees

The emission of the tropospheric trace gas acetaldehyde was determined in leaves of 4-month-old poplar trees ( Populus tremula × P. alba ) grown under controlled environmental conditions in a greenhouse. Using a dynamic cuvette system together with a high sensitivity laser-based photoacoustic detection unit, rates of acetaldehyde emission were measured with the high time resolution of about 15 min. Submergence of the roots resulted in the emission of acetaldehyde by the leaves. The emission increased linearly before reaching more or less steady-state values (ca 350 nmol m⁻² min⁻¹; ca 470 ng g⁻¹ dry weight min⁻¹) after approximately 6 h. Prolonged flooding of poplar trees resulted in a clear diurnal rhythm of acetaldehyde emission. The emission rates decreased when the light was switched off in the evening and peaked in the morning after the light was turned on again. This pattern significantly correlated with diurnal rhythms of stomatal conductance, photosynthesis, transpiration and with the concentrations of ethanol, the assumed precursor of acetaldehyde, in the xylem sap of flooded poplar trees. It may be concluded that under conditions of diminished stomatal conductance, acetaldehyde emission declines because its diffusive flux is reduced. Alternatively, reduced transpiration may decrease ethanol transport from the roots to the shoots and appreciable amounts of the acetaldehyde precursor ethanol are lacking in the leaves. The present results support the view that acetaldehyde emitted by the leaves of plants is derived from ethanol produced by alcoholic fermentation in submerged roots and transported to the leaves with the transpiration stream.     

Water potential components in growing citrus fruits

Growing navel orange fruits (Citrus sinensis) 5.4 to 5.7 centimeters in diameter were used as a model system to determine the effects of transpiration and carbohydrate translocation on water and osmotic potentials in fruit tissues. Evidence supported the hypothesis that osmotic potential in the vesicles would be affected little by changes in transpiration or carbohydrate translocation because the vesicles are anatomically isolated from the transpiration stream and are at the end of the carbohydrate translocation pathway. In the mesocarp tissue, which contains a vascular network, osmotic potential decreased during the daytime when environmental conditions favored transpiration and increased at night. Exocarp water potential followed a similar pattern. Girdling of the stem above the fruits 5 days before sampling caused an increase of osmotic potential in the mesocarp but had no effect on exocarp water potential. Neither diurnal changes in transpiration nor girdling of the stem affected the osmotic potential of the vesicles.     

Cytokinin activity in Lupinus albus

The distribution and metabolism of {8-¹⁴C}zeatin incorporated into the transpiration stream of fruiting white lupin plants ( Lupinus albus L.) has been studied. The distribution pattern of ¹⁴C in the different aerial organs suggests that the amount of cytokinin being incorporated into any one organ may have been a function of its transpiration rate. Once in these organs, particularly the leaves, zeatin was rapidly metabolised and or utilised. This resulted in the formation of a number of labelled compounds that did not give a response with the soybean callus bioassay. Substances co-eluting with zeatin glucoside and ribosylzeatin appeared to be the principal biologically active metabolites. From the present evidence it can be concluded that the leaf and side shoots received a major proportion of the applied labelled cytokinin. However, the presence of a small amount of radioactivity co-eluting with zeatin and ribosylzeatin in the fruits indicates that the high levels of cytokinins normally associated with these organs need not necessarily all have been synthesised in situ.     

Assimilate Transport in the Xylem Sap of Pedunculate Oak (Quercus robur) Saplings

Abstract: The rates of photosynthesis and transpiration, as well as the concentrations of organic compounds (total soluble non-protein N compounds [TSNN], soluble carbohydrates), in the xylem sap were determined during two growth seasons in one-year-old Quercus robur saplings. From the data, the total C gain of the leaves, by both photosynthesis and the transpiration stream, was calculated. Large amounts of C were allocated to the leaves by the transpiration stream; depending on the time of day and the environmental conditions the portion of C originating from xylem transport amounted to 8 to 91% of total C delivery to the leaves. Particularly under conditions of reduced photosynthesis, e.g., during midday depression of photosynthesis, a high percentage of the total C delivery was provided to the leaves by the transpiration stream (83 to 91 %). Apparently, attack by phloem-feeding aphids lowered the assimilate transport from roots to shoots; as a consequence the portion of C available to the leaves from xylem transport amounted to only 12 to 16 %. The most abundant organic compounds transported in the xylem sap were sugars (sucrose, glucose, fructose) with concentrations of ca. 50 to 500 μmol C ml^-1, whereas C from N compounds was of minor significance (3 to 20 μmol ml^-1 C). The results indicate a significant cycling of C in the plants because the daily transport of C with the transpiration stream exceeded the daily photosynthetic CO₂ fixation in several cases. This cycling pool of C may sustain delivery of photosynthate to heterotrophic tissues, independent of short time fluctuations in photosynthetic CO₂ fixation.     

Uptake and translocation of griseofulvin by wheat seedlings

Summary 1. A roughly quantitative technique for studying uptake and translocation of the antibiotic griseofulvin by wheat plants has been devised. Wheat plants were grown in nutrient solutions containing griseofulvin and translocation measured by bioassay of the griseofulvin appearing in the guttation drops induced by transfer to a humid atmosphere.2. Griseofulvin was phytotoxic at concentrations of 5 µg/ml and above, the first symptoms observed being stunting and swelling of the roots.3. The concentration of griseofulvin in the guttation drops was directly related to the concentration in the nutrient solution; there was evidence of griseofulvin accumulation in the leaves, the concentration in the guttation drops being frequently higher than that in the nutrient solution.4. Atmospheric conditions favouring transpiration increased uptake and translocation of griseofulvin.5. Uptake and translocation of griseofulvin was inhibited by inclusion of respiratory enzyme inhibitors in the nutrient solution.     

Declining hydraulic efficiency as transpiring leaves desiccate: two types of response

The conductance of transpiring leaves to liquid water (Kleaf) was measured across a range of steady-state leaf water potentials (Psileaf). Manipulating the transpiration rate in excised leaves enabled us to vary Psileaf in the range -0.1 MPa to less than -1.5 MPa while using a flowmeter to monitor the transpiration stream. Employing this technique to measure how desiccation affects Kleaf in 19 species, including lycophytes, ferns, gymnosperms and angiosperms, we found two characteristic responses. Three of the six angiosperm species sampled maintained a steady maximum Kleaf while Psileaf remained above -1.2 MPa, although desiccation of leaves beyond this point resulted in a rapid decline in Kleaf. In all other species measured, declining Psileaf led to a proportional decrease in Kleaf, such that midday Psileaf of unstressed plants in the field was sufficient to depress Kleaf by an average of 37%. It was found that maximum Kleaf was strongly correlated with maximum CO2 assimilation rate, while Kleaf = 0 occurred at a Psileaf slightly less negative than at leaf turgor loss. A strong linear correlation across species between Psileaf at turgor loss and Psileaf at Kleaf = 0 raises the possibility that declining Kleaf was related to declining cell turgor in the leaf prior to the onset of vein cavitation. The vulnerability of leaves rehydrating after desiccation was compared with vulnerability of leaves during steady-state evaporation, and differences between methods suggest that in many cases vein cavitation occurs only as Kleaf approaches zero.     

Leaf teeth, transpiration and the retrieval of apoplastic solutes in balsam poplar

The rapid flow of the transpiration stream through major veins to leaf teeth was followed in leaves of Populus balsamifera L., using the tracer sulphorhodamine G (SR), which probes for cells with H⁺-extrusion pumps. The tracer accumulated quickly in the hydathodes of the teeth. It was shown by freeze-substitution and anhydrous processing that SR was taken up by phloem parenchyma and epithem cells of the hydathode. When ¹⁴C-labelled aspartate was fed to the leaves in the transpiration stream, it also was taken up most strongly by the same phloem parenchyma and epithem cells. It is proposed that one function of the hydathodes in leaf teeth is the retrieval of solutes from the transpiration stream.     

Abscisic Acid is not the only stomatal inhibitor in the transpiration stream of wheat plants

Xylem sap was collected from the transpiration stream of wheat (Triticum aestivum L.) plants and assayed for the presence of an inhibitor of transpiration using leaves detached from well-watered plants. Transpiration of detached leaves was reduced by nearly 60% by sap collected from plants in drying soil, and to a lesser extent (about 25%) by sap from plants in well-watered soil. As the soil dried the abscisic acid (ABA) concentration in the sap increased by about 50 times to 5 × 10⁻⁸ molar. However, the ABA in the sap did not cause its inhibitory activity. Synthetic ABA of one hundred times this concentration was needed to reduce transpiration rates of detached leaves to the same extent. Furthermore, inhibitory activity of the sap was retained after its passage through an immunoaffinity column to remove ABA. Xylem sap was also collected by applying pressure to the roots of plants whose leaf water status was kept high as the soil dried. Sap collected from these plants reduced transpiration to a lesser extent than sap from nonpressurised plants. This suggests that the inhibitory activity was triggered partly by leaf water deficit and partly by root water deficit.     

Effects of Kinetin on Transpiration Rate and Abscisic Acid Content of Water Stressed Young Wheat Plants

Effects of kinetin on transpiration rate and abscisic acid content were determined. Leaves from 9-day-old wheat plants (Triticum aestivum L. cv. Weibull's Starke II) were used. —Transpiration rate decreased in excised leaves put in water, but it was maintained at a higher rate when kinetin was supplied. When excised leaves were water stressed by air-drying for 1 h, addition of kinetin resulted in a considerable stimulation of transpiration rate. The effect reached its maximum after 15 h and this level remained relatively unchanged for at least 10 h. Intact seedlings which were stressed before leaf excision, showed only a slight stimulation of kinetin on transpiration rate. — Abscisic acid content slowly increased up to three-fold in 2 days in excised leaves put in water. In excised and water-stressed leaves the abscisic acid content was reduced during the first 24 h and then increased. As the leaves were fully turgid, the increase could not have been caused by water stress. However, both in stressed and unstressed leaves kinetin addition reduced the increase in abscisic acid content. — It is suggested that the stimulation by kinetin on transpiration rate in excised and water stressed leaves was mainly due to the combined effect of (1) a reduction in the activity of endogenous cytokinins, (2) kinetin acting as a ‘substitute’ for the inactivated cytokinins but exerting a stronger effect on transpiration than the endogenous cytokinins, and (3) the ‘extra’ reduction in abscisic acid content caused by the kinetin treatment. Furthermore, the results indicate that changes in cytokinins might be partly responsible for the aftereffect on transpiration.     

Phloem Loading in Coleus blumei in the Absence of Carrier-Mediated Uptake of Export Sugar from the Apoplast

Phloem loading in Coleus blumei Benth. leaves cannot be explained by carrier-mediated transport of export sugar from the apoplast into the sieve element-companion cell complex, the mechanism by which sucrose is thought to load in other species that have been studied in detail. Uptake profiles of the export sugars sucrose, raffinose, and stachyose into leaf discs were composed of two components, one saturable and the other not. Saturable (carrier-mediated) uptake of all three sugars was almost completely eliminated by the inhibitor p-chloromercuribenzenesulfonic acid (PCMBS). However, when PCMBS was introduced by transpiration into mature leaves it did not prevent accumulation of ¹⁴C-photosynthate in minor veins or translocation of labeled photosynthate from green to nonchlorophyllous regions of the leaf following exposure to ¹⁴CO₂. The efficacy of introducing inhibitor solutions in the transpiration stream was proven by observing saffranin O and calcofluor white movement in the minor veins and leaf apoplast. PCMBS introduced by transpiration completely inhibited phloem loading in tobacco leaves. Phloem loading in C. blumei was also studied in plasmolysis experiments. The carbohydrate content of leaves was lowered by keeping plants in the dark and then increased by exposing them to light. The solute level of intermediary cells increased in the light (phloem loading) in both PCMBS-treated and control tissues. A mechanism of symplastic phloem loading is proposed for species that translocate the raffinose series of oligosaccharides.     

Translocation of radioactive kinetin

Kinetin has generally been thought to be immobile in plants. This was confirmed in the case of laminar applications in this study, but not in regard to petiole, vein, or root applications. Radioactivity from kinetin-8-¹⁴C (Kn^*) moved freely in the vascular system of several types of leaves. This movement was usually distal to the point of application and seemed to occur with the transpiration stream. Basipetal as well as acropetal translocation of radioactive kinetin was achieved in tobacco leaves. The translocated material was extracted from veinal tissue, shown to be radioactive, and to be able to retard senescence. Similar but less decisive results were obtained from agar blocks inserted into the vascular system of leaves receiving Kn^* by petiole uptake.

A bioassay employing disks from primary bean leaves was developed for the qualitative determination of substances like kinetin which possess the ability to retard chlorophyll breakdown and plant senescence. The use of radioactive kinetin provided a refinement in this bioassay because treated non-senescent areas could be correlated with exposed areas on radioautographs made from dried leaf disks.

Root treatments showed that cotton seedlings did not take up Kn^* but that similarly treated tobacco seedlings both absorbed and translocated the isotope readily.