Water flows in the parasitic association Rhinanthus minor/Hordeum vulgare.
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Using the facultative root hemiparasite Rhinanthus minor and its host Hordeum vulgare several aspects of water relations have been measured in this parasitic association. Extraction of xylem sap by the parasite from the host's roots is facilitated by con siderably higher transpiration per leaf area in the parasite than in the host and by the fact that stomata of attached Rhinanthus were open all day and night despite extremely high ABA concentrations in the leaves. By comparison, another root hemiparasite, Melampyrum arvense, parasitizing various grasses in the field, showed normal diurnal stomatal behaviour. The abnormal behaviour of Rhinanthus stomata was not due to anatomical reasons as closure could be induced by applying high external ABA concentrations. Remarkable differences have been detected between the hydraulic conductance of barley seminal roots showing relatively low values and that of Rhinanthus seminal roots showing very high values. The latter could be related to the observed high ABA concentrations in these roots. Whole plant water uptake, transpirational losses, growth-dependent deposition, and the flows of water within the plants have been measured in singly growing Rhinanthus and Hordeum plants and in the parasitic association between the two. Water uptake, deposition and transpiration in Rhinanthus were dramatically increased after attachment to the barley host; most of the water used by the parasite was extracted as xylem sap from the host, thereby scavenging 20% of the total water taken up by the host's roots. This water uptake by the parasitized host, however, due to a parasite-induced reduction in the host's growth, was decreased by 22% as compared to non-parasitized barley. The overall changes in growth-related water deposition in the host and parasite pointed to decreased shoot growth and relatively favoured root growth in the host and to strongly favoured shoot growth in the parasite. These changes in the host became more severe, when more than one Rhinanthus was parasitizing one barley plant. (+info)
Vessel contents of leaves after excision: a test of the Scholander assumption.
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When petioles of transpiring leaves are cut in the air, according to the 'Scholander assumption', the vessels cut open should fill with air as the water is drained away by tissue rehydration and/or continued transpiration. The distribution of air-filled vessels versus distance from the cut surface should match the distribution of lengths of 'open vessels', i.e. vessels cut open when the leaf is excised. A paint perfusion method was used to estimate the length distribution of open vessels and this was compared with the observed distribution of embolisms by the cryo-SEM method. In the cryo-SEM method, petioles are frozen in liquid nitrogen soon after the petiole is cut. The petioles are then cut at different distances from the original cut surface while frozen and examined in a cryo-SEM facility, where it is easy to distinguish vessels filled with air from those filled with ice. The Scholander assumption was also confirmed by a hydraulic method, which avoided possible freezing artefacts. In petioles of sunflower (Helianthus annuus L) the distribution of embolized vessels agrees with expectations. This is in contrast to a previous study on sunflower where cryo-SEM results did not agree with expectations. The reasons for this disagreement are suggested, but further study is required for a full elucidation. (+info)
Sodium influx and accumulation in Arabidopsis.
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Arabidopsis is frequently used as a genetic model in plant salt tolerance studies, however, its physiological responses to salinity remain poorly characterized. This study presents a characterization of initial Na+ entry and the effects of Ca2+ on plant growth and net Na+ accumulation in saline conditions. Unidirectional Na+ influx was measured carefully using very short influx times in roots of 12-d-old seedlings. Influx showed three components with distinct sensitivities to Ca2+, diethylpyrocarbonate, and osmotic pretreatment. Pharmacological agents and known mutants were used to test the contribution of different transport pathways to Na+ uptake. Influx was stimulated by 4-aminobutyric acid and glutamic acid; was inhibited by flufenamate, quinine, and cGMP; and was insensitive to modulators of K+ and Ca2+ channels. Influx did not differ from wild type in akt1 and hkt1 insertional mutants. These data suggested that influx was mediated by several different types of nonselective cation channels. Na+ accumulation in plants grown in 50 mM NaCl was strongly reduced by increasing Ca2+ activity (from 0.05-3.0 mM), and plant survival was improved. However, plant biomass was not affected by shoot Na+ concentration, suggesting that in Arabidopsis Na+ toxicity is not dependent on shoot Na+ accumulation. These data suggest that Arabidopsis is a good model for investigation of Na+ transport, but may be of limited utility as a model for the study of Na+ toxicity. (+info)
Physiological and antioxidant responses of cotton and spurred anoda under interference and mild drought.
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The influence of plant interference and a mild drought on gas exchange and oxidative stress was investigated using potted plants of two cotton species (Gossypium hirsutum L. cv. Delta Pine 5415, and Gossypium barbadense L. cv. Pima S-7) and spurred anoda (Anoda cristata L. Schlecht.) of the Malvaceae. Without interference, cotton and spurred anoda had similar net photosynthesis (Pnet) but different pigment profiles. Stomatal conductance (gs) and transpiration rate (E) were greater in spurred anoda than cotton. Net photosynthesis and biomass in cotton were reduced more by spurred anoda interference than by intraspecific interference. With interference, the xanthophyll cycle conversion state and alpha-tocopherol levels increased in cotton, but remained unchanged in spurred anoda. Catalase, ascorbate peroxidase (APX) and glutathione reductase (GR) activities were not influenced by plant interference. Without interference, spurred anoda had lower APX, and similar catalase and GR activities compared with cotton. Mild drought increased APX activity more than 40% in cotton, and 26% in spurred anoda. Upon drought recovery, drought-induced APX activity was still higher in cotton, and GR activity was higher in previously drought-stressed cotton and spurred anoda plants compared with well-watered plants. The greater impact of spurred anoda interference than intraspecific interference on cotton biomass is due mainly to reduced carbon gain in cotton. (+info)
Root pressurization affects growth-induced water potentials and growth in dehydrated maize leaves.
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Profiles of water potential (Psi w) were measured from the soil to the tips of growing leaves of maize (Zea mays L.) when pressure (P) was applied to the soil/root system. At moderately low soil Psi w, leaf elongation was somewhat inhibited, large tensions existed in the xylem, and Psi w were slightly lower in the elongating leaf tissues than in the xylem, i.e. a growth-induced Psi w was present but small. With P, the tension was relieved, enlarging the difference in Psi w between the xylem and the elongating tissues, i.e. enlarging the growth-induced Psi w, which is critical for growth. Guttation occurred, confirming the high Psi w of the xylem, and the mature leaf tissue rehydrated. Water uptake increased and met the requirements of transpiration. Leaf elongation recovered to control rates. Under more severe conditions at lower soil Psi w, P induced only a brief elongation and the growth-induced Psi w responded only slightly. Guttation did not occur, water flow did not meet the requirements of transpiration, and the mature leaf tissues did not rehydrate. A rewatering experiment indicated that a low conductance existed in the severely dehydrated soil, which limited water delivery to the root and shoot. Therefore, the initial growth inhibition appeared to be hydraulic because the enlargement of the growth-induced Psi w by P together with rehydration of the mature leaf tissue were essential for growth recovery. In more severe conditions, P was ineffective because the soil could not supply water at the required rate, and metabolic factors began to contribute to the inhibition. (+info)
Vascularization, high-volume solution flow, and localized roles for enzymes of sucrose metabolism during tumorigenesis by Agrobacterium tumefaciens.
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Vascular differentiation and epidermal disruption are associated with establishment of tumors induced by Agrobacterium tumefaciens. Here, we address the relationship of these processes to the redirection of nutrient-bearing water flow and carbohydrate delivery for tumor growth within the castor bean (Ricinus communis) host. Treatment with aminoethoxyvinyl-glycine showed that vascular differentiation and epidermal disruption were central to ethylene-dependent tumor establishment. CO2 release paralleled tumor growth, but water flow increased dramatically during the first 3 weeks. However, tumor water loss contributed little to water flow to host shoots. Tumor water loss was followed by accumulation of the osmoprotectants, sucrose (Suc) and proline, in the tumor periphery, shifting hexose-to-Suc balance in favor of sugar signals for maturation and desiccation tolerance. Concurrent activities and sites of action for enzymes of Suc metabolism changed: Vacuolar invertase predominated during initial import of Suc into the symplastic continuum, corresponding to hexose concentrations in expanding tumors. Later, Suc synthase (SuSy) and cell wall invertase rose in the tumor periphery to modulate both Suc accumulation and descending turgor for import by metabolization. Sites of abscisic acid immunolocalization correlated with both central vacuolar invertase and peripheral cell wall invertase. Vascular roles were indicated by SuSy immunolocalization in xylem parenchyma for inorganic nutrient uptake and in phloem, where resolution allowed SuSy identification in sieve elements and companion cells, which has widespread implications for SuSy function in transport. Together, data indicate key roles for ethylene-dependent vascularization and cuticular disruption in the redirection of water flow and carbohydrate transport for successful tumor establishment. (+info)
Effect of P-deficiency on photoassimilate partitioning and rhythmic changes in fruit and stem diameter of tomato (Lycopersicon esculentum) during fruit growth.
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Tomato (Lycopersicon esculentum) plants were grown in liquid culture inside the greenhouse of Hiroshima University, Japan. At the first fruiting stage, P was withdrawn from the rooting medium for a period of 19 d and its effect was studied on photosynthesis, stomatal conductance, transpiration, partitioning of 13C and 15N, P contents of various organs, and changes in stem and fruit diameter of the plant in order to identify the mechanism of resource management on the part of the plant at low P. Compared to the control, P-deficiency treatment decreased biomass growth of all organs except the roots. The treatment also depressed leaf photosynthesis, stomatal conductance and diameter of fruit and stem after a lag period of about 1 week. The stem diameter of the plant shrank during daytime and expanded during the night; the adverse effect of P-deficiency on stem diameter change was more evident during the night than the day. The circadian rhythm in fluctuations of diameter was less manifested in the fruit compared with the stem. P-deficiency induced daytime shrinkage and reduced night expansion of fruit. However, within the plant, P-deficiency encouraged partitioning of 13C, 15N and P into the fruit at the cost of autotrophic organs such as leaves and the upper parts of the stem. The results were discussed in the light of a plausible effect of P-deficiency on water relations of the plant. It is concluded that, in spite of the preference in partitioning of C and N received within the plant parts, assimilate flow into the fruit is limited at low-P compared with the control, owing to the restriction in fruit expansion. (+info)
Global analysis of river systems: from Earth system controls to Anthropocene syndromes.
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Continental aquatic systems from rivers to the coastal zone are considered within two perspectives: (i) as a major link between the atmosphere, pedosphere, biosphere and oceans within the Earth system with its Holocene dynamics, and (ii) as water and aquatic biota resources progressively used and transformed by humans. Human pressures have now reached a state where the continental aquatic systems can no longer be considered as being controlled by only Earth system processes, thus defining a new era, the Anthropocene. Riverine changes, now observed at the global scale, are described through a first set of syndromes (flood regulation, fragmentation, sediment imbalance, neo-arheism, salinization, chemical contamination, acidification, eutrophication and microbial contamination) with their related causes and symptoms. These syndromes have direct influences on water uses, either positive or negative. They also modify some Earth system key functions such as sediment, water, nutrient and carbon balances, greenhouse gas emissions and aquatic biodiversity. Evolution of river syndromes over the past 2000 years is complex: it depends upon the stages of regional human development and on natural conditions, as illustrated here for the chemical contamination syndrome. River damming, eutrophication and generalized decrease of river flow due to irrigation are some of the other global features of river changes. Future management of river systems should also consider these long-term impacts on the Earth system. (+info)