Respiration and Phloem Translocation in the Roots of Chickpea (Cicer arietinum)

Root growth in chickpea (Cicer arietinum) has been studied fromthe early vegetative phase to the reproductive stage in orderto elucidate its growth and maintenance respiration and to quantifythe translocation of assimilates from shoot to root. A carbonbalance has been drawn for this purpose using the growth andrespiration data. The increase in the sieve tube cross-sectionalarea was also followed simultaneously. Plants growing in a nutrient culture medium were studied todetermine the relative growth rate (RGR) 5–60 d aftergermination. RGR declined from 113 to 41 mg d^–1 g^–1during the measurement period. Simultaneous with the RGR analysis,respiration rate was also measured using an oxygen electrode.The respiration rate declined as the plants aged and a drasticreduction was recorded following anthesis. The relationshipbetween RGR and respiration rate was used to extrapolate themaintenance respiration (m) and growth respiration (1/Y_EG).The respiration quotient (r.q.) of the roots was 1.2 and theQ₁₀ in the range 20–25 °C was 2·2. A carbon balance for the roots was constructed by subtractingthe carbon lost during respiration from that gained during growth.The roots were found to respire no less than 80% of the carbontranslocated. The increase in the cross-sectional area composed of sieve tubeswas measured near the root-shoot junction as the plants grew.Chickpea has storied sieve plates which simplifies these measurements.Their cross-sectional area increased during growth mainly becauseof an increase in sieve tube number. The diameter of individualsieve tubes remained constant. Specific mass transfer (SMT) values for seive tubes into theroots have been computed during various stages of growth. SMTvalues were relatively constant before anthesis (approx. 6·5g h^–1 cm^–2), but decreased following anthesis. Wedid not evaluate possible retranslocation from roots: any suchretranslocation would have the effect of increasing our SMTvalues. Chickpea, Cicer arietinum, legume, root, respiration, phloem, translocation, carbon balance, specific mass transfer, sieve-tube dimensions     

Sap Ascent in Vascular Plants: Challengers to the Cohesion Theory Ignore the Significance of Immature Xylem and the Recycling of Munch Water

In recent years the cohesion theory has been attacked on thegrounds that direct measurements made with the pressure probeindicate that sap tensions are much less (maximum tension approx.0.7 MPa) than indicated by parallel measurements made with themore conventional methods: osmotic methods, pressure bomb, orpsychrometer. It has also been claimed that other direct methodsdo not support the cohesion theory. Thus a re-examination usingthe Renner technique indicated sap tensions of approx. 2.5 MPa.Also an independent method based on mercury penetrometry providesevidence that sap tensions of at least 2.0 MPa can be demonstrateddirectly implying, that serious limitations arise from the pressureprobe method itself. Without tensions exceeding 2.0 MPa mangroveswould be unable to extract fresh water for transpiration fromseawater. It is suggested that the pressure probe is susceptibleto bias because it investigates the least mature xylem conduitswhile they are still under varying degrees of turgor pressureand only partially interconnected with the main xylem system.This supposition is supported by claims that the xylem sap sampledby the probe contains significant concentrations of solutes.Additionally water, supplied by reverse osmosis from the sievetubes (‘Münch water’), is continually beingliberated in the vicinity of the outermost xylem vessels hydratingthem to an atypical degree which can explain several of thediscrepancies claimed. These results, which are supported bythe work of others, demonstrate that the challenges to the cohesiontheory for the ascent of sap are ill-founded. The release ofwater from the phloem can explain not only some discrepanciesclaimed by the cohesion challengers, but also explain the refillingof cavitated xylem conduits: a hitherto unsuspected role forthe phloem transport system. Cohesion theory; sap ascent; cavitation; pressure probe; xylem transport; vessel development; recycled water; reverse osmosis     

An Ideal Viscous Flow Porometer

A new design of viscous flow porometer is described which isbased on the pronciple of air suction into a syringe which thenmeasures the volume of air atmosphere pressure. This porometerallows accurate readings to be made on the 0-100 scale on thesyringe barrel when the piston has returned following its release.Each reading takes only seconds to make so that data can becollected for statistical analysis before stomata change inaperture and with minimal interference with the leaf envirnionment.Calibiration procedures are described: examples ofthe new porometersand also silicone rubber imprints support the validity of measurementsmade by the new porometer.     

The Acoustic Detection of Cavitation in Fern Sporangia

Cavitation of water in the annular cells of fern sporangia wasmonitored simultaneously by acoustic detection and visual analysisusing a light microscope. The number of cavitable annular cells was found to be 21.3 ±1.7 (n=40). The number and timing of ultrasonic acoustic emissions(UAE) received from a fully hydrated sporangium allowed to dehydrateon the acoustic probe closely approximated the average numberof cavitable annular cells. The number and timing of audibleacoustic emissions (AAE) was found to be lower than the numberof UAE. AAE were associated with cavitation-induced forwardmovements of the sporangium rather than individual cavitationevents within each annular cell. The results are discussed in view of an hypothesis proposedby Ritman and Milburn (1988) which describes the relationshipbetween the frequency of acoustic emission and the size of theresonating element. Key words: Cavitation, acoustic detection, sporangia     

Emergence of temperate pasture grases from different sowing depths: importance of seed weight, coleoptiele plus mesocotyl length and shoot strength

Inter-specific relationships between mean seed weight, coleoptile + mesocotyl (sub-coleoptile internode) length and width, shoot strength and emergence from different sowing depths were examined for timothy (Phleum pratense L., mean seed weight 0.33-0.48 mg depending on seed line), cocksfoot (Dactylis glomerata L., 0.65-0.78 mg), perennial ryegrass (Lolium perenne L., 1.71-2.19 mg), tall fescue (Festuca arundinucea Schreb., 1.73–2.60 mg), annual ryegrass (Lolium multiflorum Lam., 5.10-5.20 mg) and prairie grass (Bromus willdenowü Kunth., 10.5–12.2 mg). Across species at 10,30 and 60 mm sowing depths in the field and 10, 15 and 30 mm sowing depths under controlled environment conditions, there was a significant (P < 0.05) positive correlation between emergence % and mean seed weight. Across species at 10–30 mm sowing depth under controlled environment conditions, emergence % was not significantly correlated with coleoptile + mesocotyl length but there were significant positive correlations between emergence % and coleoptile and mesocotyl width: shoot strength increased with increased coleoptile width across species. For seed lines of timothy of different mean seed weight (0.21-0.81 mg), emergence %, coleoptile + mesocotyl length and coleoptile and mesocotyl width increased with increased seed weight at 10 and 15 mm sowing depth. Shoot strength increased with increased coleoptile width for timothy. For emerged and non-emerged cocksfoot and timothy seedlings regardless of seed weight, mean coleoptile + mesocotyl length was > 10 mm at 10 mm sowing depth. It is concluded that at 10–30 mm sowing depth, increased emergence % with increased seed weight across species is not due to increased coleoptile + mesocotyl length. It is proposed that increased emergence % with increased seed weight across species at 10–30 mm sowing depths and across seed lines of timothy at 10 mm sowing depth is primarily due to increased coleoptile and mesocotyl width resulting in increased shoot strength and hence an increased ability to penetrate the substrate.     

Acoustic Emissions from Plants: Ultrasonic and Audible Compared

Audible acoustic emissions (AAE) and ultrasonic acoustic emissions(UAE), produced by stem segments during dehydration in air,have been recorded and compared. We hypothesize that cavitationof xylem sap generally results in the production of a broadband acoustic emission (AE) with a lower cut-off frequency determinedby the dimensions of the resonating element. The larger a conduit'sdimensions, the lower is the frequency of its major resonance.Thus the vessels, the largest conduits, can be expected to produceboth AAE and UAE. Fibres and small cavitating elements suchas small tracheids are expected on the other hand to produceonly UAE. Most work utilized Acacia tissues but work was extended to otherplant tissues from a range of species with differing anatomicalcharacteristics. Evidence supporting our hypothesis shows thatAAE and UAE did not coincide in different tissue types or dependsimply upon the degree of dehydration. AAE were detected fromtissue with intact major conduits (vessels) but not in similartissue in which these major conduits had been severed, whereasUAE were detected from both types of tissue. In general, ourhypothesis that larger conduits produced lower frequency signalsand smaller units at the ultrasonic frequencies was supported.We are forced to conclude that some UAE are generated by eventsother than cavitating vessels or fibres. Possible interpretationof our data is discussed in terms of the size of the cavitatingconduits but including differential signal absorption withinthe tissue. Key words: Cavitation, acoustic emission, ultrasound, plants, water stress     

Phloem sap exudation in Ricinus communis: elastic responses and anatomical implications

Abstract. Ricinus communis plants have an unusually high capacity to exude considerable quantities of phloem sap from bark incisions. We have used Ricinus as an experimental system to study different aspects of sap exudation. Dimensional changes in the bark, monitored by a displacement transducer, showed that pressure release in the sieve tubes was accompanied by elastic shrinkage. The rate of exudation was also controlled by the degree of pressurization and elastic properties of the sieve tubes. A displacement transducer was used to measure the elastic modulus (ɛ) of phloem samples by immersing them in a range of different osmotica. The cells had a low elastic modulus (ɛ= 1.62 ± 0.41 MPa at full turgor). ɛ of phloem tissue in massage pretreated bark, from which exudation was enhanced, was not significantly different from that of unmassaged bark in contrast with the suggestion of Lee (1981). However, anatomical studies showed that massage pretreatment has a stimulating influence on the cambial cell division, which increased the phloem tissue cross-section up to 160%. The newly-formed sieve tubes were 'spliced' into existing ones in the unmassaged zone to re-establish vascular continuity. Plants with a greater capacity to exude phloem sap from a given stem location had a greater cross-sectional area of sieve tubes in the vicinity.
The significance of observations with respect to other sap exudation phenomenon is discussed. The importance of the present work in understanding the technique of palm tapping, on which the palm sugar industry depends, is also considered.