Suberin lamellae in the hypodermis of maize (Zea mays) roots; development and factors affecting the permeability of hypodermal layers

Abstract The development of suberin lamellae in the hypodermis of Zea mays cv. LG 11 was observed by electron microscopy and the presence of suberin inferred from autoliuorescence and by Sudan black B staining in nodal (adventitious) and primary (seminal) root axes. Suberin lamellae were evident at a distance of 30–50 mm from the tip of roots growing at 20°C and became more prominent with distance from the tip. Both oxygen deficiency and growth at 13°C produced shorter roots in which the hypodermis was suberized closer to the root tip. There were no suberin lamellae in epidermal cells or cortical collenchyma adjacent to the hypodermis. Plasmodesmata were not occluded by the suberin lamellae: there were twice as many of them in the inner tangential hypodermal wall (1,14 μn^?2) as in the junction between the epidermis and hypodermis (0.54 μm^?2). Water uptake by seminal axes (measured by micropotometry) was greater at distances more than 100 mm from the root lip than in the apical zone where the hypodermis was unsuberized. In the more mature zones of roots grown at 13°C rates of water uptake were greater than in roots grown at 20°C even though hypodermal suberization was more marked. Sleeves of epidermal/hypodermal cells (plus some accessory collenchyma) were isolated from the basal 60 mm of nodal axes by enzymatic digestion (drisclase). The roots were either kept totally immersed in culture solution or had the basal 50 mm exposed to moist air above the solution surface. In both treatments the permeabilities to tritiated water and ₈₆Rb were low (circa 10^?5mms^?1) in sleeves isolated from the extreme base. In roots grown totally immersed, however, the permeability of sleeves increased 10 to 50-fold over a distance of 40 mm. In roots exposed to moist air the permeability remained at a low level until the point where the root entered the culture solution and then increased rapidly (> 50-fold in a distance of 8 mm). Growth of roots in oxygen depleted (5% O₂) solutions promoted the development of extensive cortical aerenchymas. These developments were not associated with any reduction in permeability of sleeves isolated from the basal 40 mm of the axis. It was concluded that the presence of suberin lamellae in hypodermal walls does not necessarily indicate low permeability of cells or tissues to water or solutes. The properties of the walls (lamellae?) can be greatly changed by exposure to moist air, perhaps due to increased oxygen availability.     

Short-term Effects of Some Chemicals on Cambial Activity

Aqueous solutions of indol-3yl-acetic acid (IAA), 1-naphthyl-aceticacid (NAA), 2,4-dichlorophenoxyacetic acid (2,4-D), gibberellicacid (GA), 6-furfuryl-aminopurine (FAP), myo-inositol, and sucrosewere applied singly and in mixtures to the apical ends of disbuddedstem segments of willow. After 4 weeks all substances had hadsome effect on differentiation of xylem from cambial derivatives.The production of potential xylem cells, as well as their differentiationwere most markedly enhanced when IAA, GA, and FAP were appliedtogether, although the response was further augmented by additionof inositol or sucrose. The action of the substances when appliedas mixtures was often synergistic. This means that it is difficultto assess the role of different chemicals in xylem productionby extrapolation from experiments involving the applicationof single substances.     

Influence of Environmental Factors on Cell Production and Differentiation in the Early Wood of Picea sitchensis

Three phases are identified in tracheid production by the mainstem of a 12th year forest-grown tree of Picea sitchensis: cellproduction, radial cell expansion and cell wall thickening.For 15 days during formation of early wood daily cell productionwas positively correlated (P=001) with daily solar radiation.Increases in cell production appear related to time-lagged increasesin numbers of cells undergoing expansion and then wall thickening. Radial diameter and wall thickness were measured for each individualcell along the post wall thickened section of radial files oftracheids. The radial diameters of neighbouring cells were positivelycorrelated, groups of 4–5 large cells tending to alternatewith groups of small cells. Furthermore, large cells had thickerwalls than small cells. It is suggested that this distinctivestructure is the result of fluctuations in the environ mentwhich induce changes in the rates of cell production. An increasein cell production increases the length of the queue of cellswaiting to pass successively out of the zones of cell expansionand wall thickening. Consequently cells spend longer in thedevelopment stages, they expand to a greater diameter and theirwalls thicken to a greater thickness than when fewer cells areproduced and the queue is shorter. We suggest that to investigate the influence which weather mayhave on the dynamics of tracheid pro duction it is necessaryto sample developing wood with a frequency twice as great asthat of the frequency of change of the weather variable underconsideration.     

X-ray Microanalysis of Ion Distribution in Abutilon Nectary Hairs

Robards, A. W. and Oates, K. 1986. X-ray microanalysis of iondistribution in Abutilon nectaryhairs—J. exp. Bot. 37:940–946. X-ray microanalysis of frozen-hydrated Abutilon nectary hairsshows that potassium is present at easily detectable levelsin all cells between the phloem and the trichome tip cells.There is little overall variation in the amount of potassiumfrom cell to cell although it was found that: (i) some basalcells accumulate potassium to a significantly higher level thanthe hair cells; and (ii) there is more potassium in the cellstowards the distal end of the trichome. These findings wouldbe consistent with a model for secretion envisaging a trans-membraneloading of nectar into a ‘secretory reticulum’ inall trichome cells. This loading process would be selectiveand would exclude ions. A pronounced increase in the level ofchlorine from the basal cell along the hair was observed. Nocomplementary cation was detected but equivalent changes inthe level of Na would have been below the detection limit ofthe system. Key words: Abutilon, nectary hairs, X-ray microanalysis, ion distribution     

Tension Wood and Eccentric Growth in Crack Willow (Salix fragilis, L.)

Shoots of S. fragilis in their first year of growth were bentthrough 360^° and grown in this position for 11 weeks. Duringthis period the shoots made greater radial growth on the upperside of both top and bottom halves of the loops. Coincidingwith this eccentric radial growth was the consistent productionof gelatinous fibres on the side with the longer radius, orin the region of the longer radius. Under these experimentalconditions both eccentric growth and tension wood productionwere related to the stimulus of gravity and not to tension orcompression forces. The quantitative results obtained clearlyshow a statistically significant degree of eccentric growthof the stem, and also a positive correlation between the maximumeccentricity and the localization of tension wood fibres inboth upper and lower halves of the loops. A 26-year-old horizontal branch of S. fragilis was also examined.It showed little overall eccentricity and no consistent eccentricityin the same growth ring on opposite sides of the branch. Tensionwood was present in the majority of growth rings on both upperand lower sides.     

The Appearance of Differentiating Vascular Cells after Fixation in Different Solutions

The cambium and differentiating xylem elements of Salix fragilisand Fagus sylvatica were most satisfactorily fixed in a 3 percent solution of glutaraldehyde in 0.05 M phosphate buffer and0.4 M sucrose at pH 7.2 with sodium, calcium, and potassiumchlorides added to a total combined molarity of about 0.1 M,so giving a total osmolality in excess of 1,000 milliosmols.As various factors such as the concentration of fixative, natureof buffer system, pH, addition of salts, addition of sucrose,osmolality, and temperature all affect fixation, it is importantto report the exact constituents and conditions of fixationin published work.     

The Application of the Modified Sine Rule to Tension Wood Production and Eccentric Growth in the Stem of Crack Willow (Salix fragilis L.)

One-year-old shoots of S. fragilis were placed on speciallyprepared frames and grown at various angles from the uprightposition for six-and-a-half months. At the end of this periodthe stems were found to have made greater growth on the upperside than the lower, and the majority of the fibres of the upperxylem were of the gelatinous type. The degree of eccentricityand the amount of gelatinous fibres produced were clearly shownto reach a maximum when the stems were displaced by 120°from the upright position. This provides evidence that in stems,as in roots, geotropic responses are not proportional to thesine of the angular displacement from the vertical, but followa modified sine relationship. These results are considered toprovide support for the hypothesis that statolith graviperceptorsmust be present close to the vascular cambium along the lengthof young woody stems. There is no reason to suppose that theperception of gravitational stimulus in stems is in any materialway different from that in roots.     

Effects of Mechanical Impedance on Root Growth in Barley, Hordeum vulgare L.: II. EFFECTS ON CELL DEVELOPMENT IN SEMINAL ROOTS

Anatomical changes induced in the roots of barley plants (Hordeumvulgare cv. Proctor) by an applied pressure of 2 ? 10⁴ Pa (0.2bar) have been examined; previous studies had shown that thelength of roots grown under these conditions was about 50% ofthat of unimpeded roots. Cryomicrotomy/light microscopy andscanning electron microscopy were used. Mechanical impedanceincreased the diameter of roots, this being due largely to anincreased thickness of the cortex; the number of cells in transversesection was increased and the diameter of the outer cells wasgreater though that of the inner ones was reduced.