The Role of the Mesocotyl in Sodium Exclusion from the Shoot of Zea mays L. (cv. Pioneer 3906)

Drew, M. C. and Lauchli, A. 1986. The role of the mesocotylin sodium exclusion from the shoot of Zea mays L. (cv. Pioneer3906).—J. exp. Bot. 38: 409–418. The mesocotyl, located between the root and shoot, can stronglyaccumulate Na⁺ from the ascending transpiration stream, therebypotentially acting as a sink to protect the shoot from excessNa⁺. To determine the quantitative importance of the mesocotylas a Na⁺ sink, we grew plants with either short (9·0mm) or long(21 mm) mesocotyls, the latter resembling the sizefound in field-grown plants. At 13 d, plants were transferredfrom Na ⁺ -free nutrient solution to a ²²Na⁺ labelled solutionin which the concentration of NaCl was (mol m^–3) 1·0,10 or 100. The concentration of Na⁺ accumulated in the mesocotylin 24 h (g^–1 fr. wt.) exceeded that in the roots thatwere directly exposed to the nutrient solution. The amountsof ²²Na⁺ retained in the long mesocotyl were about double thatin the short ones and increased with time of exposure and NaClconcentration. At 1·0 and 10 mol m^–3 NaCl, theamounts of ²²Na⁺ retained in the mesocotyl were 6–19%of those reaching the shoot in 24 h, but with 100 mol m^–3NaCl, a damaging concentration for maize, this declined to 3–8%.The mesocotyl, even as a fully elongated structure is, therefore,unlikely to provide an appreciable alternative sink for Na⁺when NaCl reaches injurious concentrations. Key words: Ion transport, potassium, roots, salinity     

Amino acid increases in fruit infested by fruit flies of the family Tephritidae

Solanum mauritianum Scop, (wild tobacco) fruit is the major host of the fruit fly Dacus cacuminatus (Hering), and is a major source of food for the brown pigeon Macropygia phasianella (Temminck) in eastern Queensland. Amino acid analyses were undertaken on fruit fly infested and uninfested S. mauritianum fruits. Infested fruits contained approximately twice the level of protein and essential amino acids compared to uninfested fruit. This increase is probably due to the plant adding additional amino acids to infested tissue and the accompanying growth of bacteria established in the fruit during oviposition. The infested fruit would provide a valuable source of protein during the pigeon breeding season.     

Sodium Exclusion from the Shoots by Roots of Zea mays (cv. LG 11) and its Breakdown with Oxygen Deficiency

The effects of sodium chloride salinity and root oxygen deficiency(anoxia) were studied in 11-12d old maize plants (Zea mays L.cv. LG 11) in nutrient solution culture. Transport of ²²Na bythe roots to the shoot in 24 h was markedly increased by anoxiawhen the external concentration of NaCl was in the range 0·1-10·9mol m^–3. Anoxia severely inhibited uptake of ⁴²K by rootsand its transport to the shoot, so that the ratio of Na⁺/K⁺moving into the shoot was increased by a factor of approximately10. When the external concentration of NaCl was increased to2.4 mol m^–3, the roots showed much less ability to excludeNa⁺ under aerobic conditions, and anoxia caused no further increasein the movement of Na⁺ to the shoot. It is concluded that atthe higher concentration the ability of the roots to excludeNa⁺, presumably through an active mechanism in the xylem parenchymacells or in the root cortex and transporting Na⁺ to the outersolution, is saturated by excessive inward diffusion of Na⁺.The ratio of Na⁺/K⁺ transported to the shoot increased by afactor of 600 when the concentration of NaCl was increased from2·4 mol m^–3 to 40 mol m^–3 and roots weremade anoxic. Such imbalances in the supply of cations to theshoot, particularly when roots are oxygen-deficient, may contributeto salinity damage. Key words: Anaerobic, Anoxic, Oxygen deficiency, Roots, Salinity, Salt stress, Sodium chloride, Zea mays     

Analysis of Time-Varying Biological Data Using Rainflow Cycle Counting

A wide range of biological investigations lead to time-history data. The characterization of such data can be difficult particularly in the presence of signal noise or superimposed signals. Several methods are described which can be brought to bear including FFT, thresholding, peak counting, and range counting. However, each of these approaches has significant disadvantages. In this paper we describe a novel method, known as rainflow cycle counting, for characterizing time varying biological time-history data in terms of spiking or oscillation amplitude and frequency. Rainflow counting is a straightforward algorithm for identifying complete cycles in the data and determining their amplitudes. The approach is simple, reliable, easily lends itself to automation, and robust even in the presence of superimposed signals or background noise. After describing the method, its use and behavior are demonstrated on three sample histories of intracellular calcium concentration in chondrocytes exposed to fluid shear stress. The method is also applied to a more challenging data set that has had an artificial random error included. The results demonstrate that the rainflow counting algorithm identifies signal oscillations and appropriately determines their amplitudes even when superimposed or distorted by background noise. These attractive properties make rainflow counting a powerful approach for quantifying and characterizing biological time histories.     

Bridging the morphological and biological species concepts: studies on the Bactrocera dorsalis (Hendel) complex (Diptera: Tephritidae: Dacinae) in South-east Asia

Defining species accurately is a critical need in fundamental disciplines such as ecology and evolutionary biology and in applied arenas such as pest management. The validity of species designations depends on agreement of different methods of species diagnosis for unique biological species. The Bactrocera dorsalis complex of fruit flies provide an excellent opportunity for such a test of the congruence of different techniques (e.g. morphological, molecular, host-plant based, chemotaxonomy) used for species diagnosis. The complex contains a large number of closely-related species, is distributed over a wide geographical range in South-east Asia and considerable information has been compiled on some species. In the present study, the morphological and biological species boundaries were compared using new data from morphometric analyses of reproductive and body parts, together with a review of data on morphology, chemistry of male pheromones that are important in courtship and mating, molecular analyses, and endemic rainforest host plants. For the populations studied ( Bactrocera carambolae , Bactrocera dorsalis , Bactrocera occipitalis , Bactrocera papayae , Bactrocera philippinensis , Bactrocera kandiensis and Bactrocera invadens ) there appears to be significant congruence between the morphological and biological species boundaries.  © 2008 The Linnean Society of London, Biological Journal of the Linnean Society , 2008, 93 , 217–226.     

Sensing soil oxygen

Abstract. Under natural conditions where gaseous exchange between soil and atmosphere is restricted by excess water, the concentration of O₂ in the rooting zone can become very low while reduced ions and organic compounds that are potentially phytoxic may accumulate. Mechanisms by which shoots and roots detect, and adjust to, this O₂-deficient environment are reviewed. Injury to roots and their inability to function because of insufficient O₂ is communicated to the shoot in a variety of ways, so that it adjusts physiologically. Roots may acclimate metabolically to a gradual fall in O₂ supply, so that they either improve their tolerance of anoxia, or partially avoid O₂-deficiency by structural changes that aid internal transfer of O₂ to the roots from the shoot. Molecular mechanisms regulating such metabolic changes, including environmental cues, are discussed.     

Phosphate Uptake and Transport by Roots and Stolons of Intact White Clover (Trifolium repens L.)

The extent to which phosphate can be absorbed directly fromthe outer medium by stolon internodes and contribute to thetotal accumulation of phosphate by intact plants of white clover(Trifolium repens L. cv. Blanca) was assessed in hydroponicexperiments in a controlled environment room. The uptake ofphosphate by intact roots or stolons was measured by sealinga segment (6-0 mm long) across a flow-cell in which ³²P-labellednutrient solution was circulated for 24 h, the rest of the rootsystem receiving unlabelled nutrient solution. The rate of uptakeof phosphate (µmol g^–1 d^–1 dry wt. basis)by roots was more than 300 times that by intact stolons. Pretreatmentof stolons by gentle abrasion to remove cuticle, so as to simulatethe condition of stolons in the field, increased the uptakeof phosphate 7-fold compared with that of intact stolons. However,the potential of stolons to contribute to the P status of whitedover in the field was calculated to be small (5%). When an incision was made through the hypodermal layer of stolons,the rate of phosphate uptake greatly increased, attaining 71%of that by root segments. This increase, which was greater athigher phosphate concentrations, indicates that the suberi.zedhypodermis constitutes a major barrier to the influx of phosphatein the stolon. After withholding phosphate for different time intervals, thesubsequent rate of phosphate uptake by roots was increased 2-3-foldafter 2 d phosphate deprivation and 3-4-fold after 6 d or 13d phosphate deprivation. A higher proportion of absorbed phosphatewas transported to shoots in phosphate-deprived plants. After1 d of uptake following restoration of the phosphate supply,the concentrations of labelled phosphate in shoots were greaterthan in control plants, although the concentrations of labelin roots was less. However, the rate of uptake of phosphateby stolons, following deprivation, was not significantly increased.These results suggest that the mechanism regulating the enhancedrate of phosphate loading into the xylem, initiated by a periodof phosphate deprivation, is specific to roots and is not inducedin stolons. The results are discussed in relation to the growth and acquisitionof phosphate by white clover in the field. Key words: Nutrient deficiency, phosphate, stolons, transport (ions), Trifolium repens