A Comparison of the Boron Requirements of Intact Tomato Plants and Excised Tomato Roots grown in Sterile Culture

A method is described by which boron deficiency in excised tomatoroots was obtained. Lack of boron caused complete inhibitionof growth after three days in sterile culture. The boron concentrationsupporting 50 per cent of the maximum root-growth response issimilar to that giving a 50 per cent growth response with intactplants of the same variety. These results are discussed withregard to the function of boron in plant growth.     

The Biological Activity of Organoboron Compounds in the Promotion of Root Growth

The growth of the bean (Vicia faba var. minor) radicle in nutrientsolution requires the presence of borate. Optimum extensiongrowth, over 41 h, was obtained in the presence of 0.5 µMboric acid. This requirement of borate for root growth couldalso be satisfied by PhB(OH)₂, 2-OCH₃PhB(OH)₂, and 4-OCH₃PhB(OH)₂.These three compounds also complex in vitro with catechol-3,5-disulphonic acid. NaPh₄B and 2, 6-OCH₃)₂PhB(OH)₂ did not formsuch complexes in vitro, but were biologically active as sourcesof boron for radicle growth. This activity of NaPh₄B was absentif roots were grown in solutions which were changed every 8h. The activity of both NaPh₄B and 2,6-(OCH₃)₂PhB(OH)₂ was increasedby using stock aqueous solutions which were not freshly prepared.Theresults are considered to provide support for the hypothesisthat the activity of borate, as an essential plant nutrient,depends upon its ability to form a biologically active complexwith an in vivo cis-diol compound.     

The influence of water deficits on needle conductance, assimilation rate and abscisic acid concentration of seedlings of Pinus radiata D. Don

Abstract Seedlings of Pinus radiata, 10–20 weeks old and hitherto fully watered, responded rapidly when water was withheld. Wilting occurred 9d later, at which time soil matric water potential at dawn (Ψ_m) was –1.06MPa and shoot water potential (Ψ) was –1.9 MPa. Small reductions in Ψ_m elicited large responses in assimilation rate (A) and leaf conductance to water vapour (g). Seedlings appear to be more sensitive to small water deficits than are older Plants of P. radiata. After rewatering, significant increases of A and g occurred within one day, but neither regained the values measured prior to the imposition of a single drying cycle. This residual effect of drought on A, after one or six drying cycles, was partially caused by a decrease in photosynthetic capacity. In plants wilted for the first time, the concentration of abscisic acid (ABA) in the bulk foliage increased 3.4 times as Ψ decreased to –1.77 MPa. In comparison, pretreatment with six drying cycles significantly reduced Ψ to –2.13 MPa (indicating some osmotic adjustment) and induced only a doubling of ABA concentration. However, these differences in Ψ and ABA concentration did not Persist after the plants of all pretreatments had been watered for 7 d, although g of drought-pretreatment Plants remained approximately half that of continuously-watered plants.     

Carbon dioxide blocks the stigma callose response following incompatible pollinations in Brassica

Abstract. Carbon dioxide is known to overcome sporophytic self-incompatibility in Brassica. Elevated CO₂ (30 mmol CO₂ mol^-1 air), supplied via a flowthrough gas system, was shown to block the formation of rejection callose in the surface stigmatic papillae of Brassica campestris var. T15 following self-pollination. Possible mechanisms by which CO₂ may affect callose formation are discussed.     

Do leaves contribute to the abscisic acid present in the xylem sap of ‘draughted’ sunflower plants?

Abstract. The root systems of 30-d-old sunflower plants were treated with polyethylene glycol (PEG; osmotic potential - 1.0MPa) for 2h, causing mild and transient wilting. Ten minutes before this treatment was applied, half the plants were defoliated. At varying times after the imposition of the PEG 'drought stimulus', the plant stems were cut and the sap exudate was collected and analysed for abscisic acid (ABA), using an elisa method. When stems were cut 2.25h after the treatments were applied, the ABA concentration in the sap of the controls did not vary with time: the mean concentration was 10.7 ± 1.0μ, mol m⁻³. However, in the treated plants, the first sample contained 78.1 ± 10.1 μmol m⁻³, decreasing to 13.6 ± 2.8 μmol m⁻³ over 8.75h. Defoliation did not affect the ABA concentration in the sap. When stems were cut at varying times (up to 25h) after treatment, the PEG treatment again caused an immediate increase in the ABA concentration in the sap, from 20 ± 1 to 136 ± 21 μmol m⁻³. However, defoliation reduced this increase, but only in plants sampled 4–25h after treatment. We conclude that, after the PEG treatment to the roots, the initial increase of the ABA content of sap, and its attenuation with time, may be ascribed to synthesis in the roots whereas, thereafter, ABA derived from the leaves makes a major contribution to the ABA found in the xylem sap.     

The Stem as a Temporary Sink before Tuberization in Helianthus tuberosus L.

The growth of individual Helianthus tuberosus (Jerusalem Artichoke)plants, and that of their component parts, was measured overone growing season of 33 weeks. The stem served as a temporarysink for assimilates before tuberization began and during thisperiod constituted up to 65 per cent of the total dry weightof the plant. Greatest tuber growth occurred when the leaveswere senescing rapidly and it was sustained by transfer of drymatter from the stem. The relationship of this growth patternto tuber and stem metabolism and its possible hormonal controlis discussed.     

The Effects of Partially Drying Part of the Root System of Helianthus annuus on the Abscisic Acid Content of the Roots, Xylem Sap and Leaves

Plants of Helianthus annuus were grown in soil in pots suchthat approximately 30% of the root system protruded throughthe base of the pot. After 7 d further growth in aerated nutrientsolution, the attached, protruding roots were air-dried for10–15 min and thereafter surrounded with moist still air,in the dark, for 49 h, whilst the soil was kept at field capacity.The roots of the control plants remained in the nutrient solutionthroughout the experiment. This treatment rapidly reduced the water content of protrudingroots from 20.5 to 17.8 g g^–1 dry mass (DM), which remainedless than that of the control roots for the rest of the experiment.This treatment also reduced root turgor and water potential.The abscisic acid (ABA) concentrations in the protruding roots,xylem sap and leaves of the treated plants increased significantly,compared to values recorded for control plants. In treated roots, the ABA concentration was significantly increased4 h after treatment, with a maximum of 4.4+0.1 nmol g^–1(DM) after 25 h. The ABA concentration in the xylem sap of thetreated plants was significantly greater than in the controls25 h, 30 h, and 49 h after the partial drying of the roots,with a maximum concentration of approximately 970 pmol ABA cm-3at 49 h. Initially, the ABA concentration in the leaves was0.45 nmol g^–1 (DM) which increased significantly to 1.1±0.1 nmol g^–1 at 25 h, to 1.7±0.3 nmol g^–1at 49 h. Leaf conductance was significantly less in plants with air-driedroots than in the controls 8 h after the start of the treatmentand thereafter. The water relations of the leaves of the treatedplants did not differ from those of the control plants. These results confirm previous reports that ABA is rapidly generatedin partially-dried and attached root systems and demonstratesa concomitant large increase in the ABA content of the xylemsap. It is suggested that partial dehydration of some of theroots of Helianthus annuus, increases ABA concentration in thetranspiration stream and decreases leaf conductance in the absenceof changes in leaf water status. As these responses were initiatedin free-growing roots the stimulus is independent of any increasesin soil shear strength that are associated with soil drying. Key words: Soil drying, roots, ABA, leaf conductance, water relations     

The Effects of Cotyledon Excision on Growth and Leaf Senescence in Soya-bean Plants

One effect of the removal of cotyledons of soya-beans 24 h afterimbibition was to reduce subsequent growth, especially in thefirst 10 weeks after planting. The longevity of treated plantswas increased by 30 days and the onset of leaf senescence delayed,as judged by the time-course of leaf chlorophyll and solubleprotein content, and also by the time-course of net photosynthesisand respiration rates. These effects are similar to those reportedto result from partial defoliation of more mature plants.     

The Boron Requirement of Flax Roots Grown in Sterile Culture

Methods are described which eliminate born contamination inthe culture medium. Excised flax roots, grown in sterile sulture, are shown to havea boron requirement which is satisfied by 0.05 p.p.m. boron.Root growth stopped after 48 hours in media lacking boron. Theboron requirement of excised roots is similar to that of wholeflex seedlings. It is concluded that the observed restriction of root growthin media lacking boron is not connected with the movement ofsugars into flax root cells.