Shoot Resistance to Water Flow in Cotton

Studies using excised cotton (Gossypium hirsutum L.) plants,attached to a free water source and undergoing transpirationcycles, were conducted at intervals over a 2 year period inorder to quantify shoot resistance components of cotton canopies.Leaf water potential was found to be a linear function of transpirationrate at rates above 0.1 mm h^–1, so shoot resistance wasevaluated as the slope of this function. The value of 4.8 104h (0.48 MPa h mm^–1) total shoot resistance was consistentfor 1.10 m tall, well irrigated, fruit-bearing cotton plants.Further tests, with pre-wrapped and exposed leaves, revealedthat total shoot resistance was comprised of an axial component(40%) and a leaf component (60%). The total shoot resistanceof 0.48 MPa h mm^–1 is likely to be relevant for modellingcotton water relations when LWP is evaluated on exposed, topof the canopy leaves, such as in the ‘big leaf’type models. Key words: Leaf water potential, axial resistance, leaf resistance     

Biophysical Interactions of Membrane Anionic Phospholipids with pH, Calcium and Auxins

A series of phospholipid monolayers—phosphatidylinositol,phosphatidylserine and phosphatidyl 4, 5-bisphosphate, possessingan increasingly negative headgroup potential, were spread andcompressed over aqueous sub-phases and, under varying conditions,assayed for changes in surface parameters. These included surfacetension and degree of molecular compression expressed by surfacepressure/molecular area isotherms. Experimentation employingWilhelmy-Du Nouy tensio-metry and the Langmuir-trough compressionprocedure indicated that with increasing electrostatic Ca²⁺cross-linking, this being a direct function of Ca²⁺ concentrationin the aqueous sub-phase, there was a concomitant increase insurface tension, i.e. monolayer condensation. This effect wasgreater with the more negatively charged phospholipids and wasmore pronounced at pH 7–0 than at pH 4–5. Applicationof the auxins indoleacetic acid, indolebutyric acid and naphthaleneaceticacid (which presumably act as proton donors which may dislodgecross-linking Ca²⁺ ) causes a significant decrease in surfacetension when applied at physiological Ca²⁺ concentrations. Asevidenced by surface pressure/molecular area isotherms, Ca²⁺increases monolayer rigidification and decreases phosphatidyl4, 5-bisphosphate molecular area. The auxins tested possessan opposite and de-ngidifying effect as indicated by increaseof the phospholipid’s molecular area and by the observedconsiderable lowering of monolayer collapse pressures. The surfactiveeffectivity of the auxins increased with decreasing sub-phaseCa²⁺ concentration in the 10^–4 to 10^–6mol dm^–3range. These data collectively suggest that surface-associatedbiophysical changes should be taken into account during interpretationof transmembrane hormonal signal transduction. Key words: Membrane phospholipids, monolayers, surface properties     

Flower development in the passion fruit Passiflora edulis requires a photoperiod‐induced systemic graft‐transmissible signal

Different organisms use gradual seasonal changes in photoperiod to correctly time diverse developmental processes, such as transition to flowering in plants. Florigen is a systemic signal formed in leaves exposed to specific environmental cues, mainly photoperiodic, and capable of triggering flower induction in several species. Here we show that in Passiflora edulis, a perennial climbing vine, flower initiation occurs throughout the year; however, without long photoperiods, flower primordia show arrested growth and differentiation at an early stage. Our results support the existence of a positive, systemic, graft‐transmissible signal, produced in mature leaves under LDs, that is required for normal flower development beyond sepal formation. Our results also suggest that Gibberellin acts to inhibit flower development. We provide evidence for genetic variation in the response to short photoperiods. A genotype capable of forming developed flowers under short photoperiods produces a positive graft transmissible signal allowing normal flower development under short days in a cultivar which normally aborts flower development under these conditions. We believe these findings contribute towards discovering the chemical nature of this interesting mobile signal involved in flower development.     

A Monolayer Model Study of Surface Tension-Associated Parameters of Membrane Phospholipids: Effect of Unsaturation of Fatty Acyl Tails

Degree of unsaturation of sn-2 located fatty acyl side-chainsin typical membrane phospholipids has a marked effect on surfaceten si on-associated parameters of monolayers of these lipidsover aqueous sub-phases. For a fixed area monolayer in a completelyexpanded state, an increase in the number of cis double bondswas found to cause a concomitant increase in surface tension.For the same fatty acids incorporated into phosphatidylcholinemolecules, surface pressure/molecular area isotherms show thatsn-2 linoleoyl monolayers manifest markedly higher surface pressuresthan sn-2 oleoyl ones in the fully compressed state. Furthermore,the isotherms and monolayer collapse points indicate a greaterrigidity of the oleoyl species monolayer. The strong correlationof these effects with molecular radius, rotational diffusionconstant and total molecular peripheral properties of the monolayermay be determined by rotational micro viscosity experiencedby the molecules. This in turn is determined by molecular radiusthrough the total molecular peripheral length. It is suggestedthat a contributing factor to membrane bioregulation may besuch changes in surface tension-associated parameters arisingfrom the structure of the unsaturated fatty acyl phospholipidside-chains. In a biological membrane the surface tension would,therefore, be an average of headgroup effects and of degreeof fatty acyl side-chain unsaturation. Key words: Fatty acyl unsaturation, membrane, phosphatidylcholine