Suberized Lamellae Research Articles

The barrier to radial oxygen loss impedes the apoplastic entry of iron into the roots of Urochloa humidicola.

Lack of O2 and high concentrations of iron (Fe) and manganese (Mn) commonly occur in waterlogged soils. The development of a barrier to impede radial O2 loss (ROL) is a key trait improving internal O2 transport and waterlogging tolerance in plants. We evaluated the ability of the barrier to ROL to impede the entry of excess Fe into the roots of the waterlogging-tolerant grass Urochloa humidicola. Plants were grown in aerated or stagnant deoxygenated nutrient solution with 5 µM or 900 µM Fe. Quantitative X-ray microanalysis was used to determine cell-specific Fe concentrations at two positions behind the root apex in relation to ROL and the formation of apoplastic barriers. At a mature zone of the root, Fe was 'excluded' at the exodermis where a suberized lamella was evident, a feature also associated with a strong barrier to ROL. In contrast, the potassium (K) concentration was similar in all root cells, indicating that K uptake was not affected by apoplastic barriers. The hypothesis that the formation of a tight barrier to ROL impedes the apoplastic entry of toxic concentrations of Fe into the mature zones of roots was supported by the significantly higher accumulation of Fe on the outer side of the exodermis.

Ultrastructure and Carbon Isotope Ratios of Leaves in C4 Species of Rhynchospora (Cyperaceae) That Differ in the Location of Kranz Cells

The genus Rhynchospora in the Cyperaceae includes both C3 and C4 species. The C4 species shows two distinct Kranz anatomies: the rhynchosporoid and the fimbristyloid–chlorocyperoid intermediate. This study reports the use of herbarium specimens to obtain the leaf ultrastructure in eight Rhynchospora C4 species in relation to their C4 biochemical subtypes, together with carbon isotope ratios. Despite the use of dried materials, electron microscopic observation revealed the structures of chloroplasts and cell walls. In C4 species with the fimbristyloid–chlorocyperoid intermediate anatomy, Kranz cell chloroplasts had reduced grana and convoluted thylakoids, and suberized lamellae occurred in the mestome sheath cell walls. In C4 species with the rhynchosporoid anatomy, the Kranz cell chloroplasts also had reduced grana and convoluted thylakoids but with less convolution; suberized lamellae occurred in the Kranz cell walls. These data show that the Rhynchospora C4 species have ultrastructural features present ...

Ultrastructure of leaves in C4 Cyperus iria and C3 Carex siderosticta

The ultrastructural aspects ofCyperus iria leaves showing the C4 syndrome and the typical C3 species,Carex siderosticta, in the Cyperaceae family were examined.C. iria exhibited the chlorocyperoid type, showing an unusual Kranz structure with vascular bundles completely surrounded by two bundle sheaths. The cellular components of the inner Kranz bundle sheath cells were similar to those found in the NADP-ME C4 subtype, having centrifugally arranged chloroplasts with greatly reduced grana and numerous starch grains. Their chloroplasts contained convoluted thyla-koids and a weakly-developed peripheral reticulum, although it was extensive mostly in mesophyll cell chloroplasts. The outer mestome bundle sheath layer was sclerenchymatous and generally devoid of organelles, but had unevenly thickened walls. Suberized lamellae were present on its cell walls, and they became polylamellate when traversed by plasmodesmata. Mesophyll cell chloroplasts showed well-stacked grana with small starch grains. InC. siderosticta, vascular bundles were surrounded by the inner mestome sheath and the outer parenchymatous bundle sheath with intercellular spaces. The mestome sheath cells degraded in their early development and remained in a collapsed state, although the suberized lamellae retained polylamellate features. Plastids with a crystalline structure, sometimes membrane-bounded, were found in the epidermal cells. The close interveinal distance was 35–50 μm inC. iria, whereas it was 157–218 μm inC. siderosticta. These ultrastructural characteristics were discussed in relation to their photosynthetic functions.

Ultrastructural aspects of leaves inFestuca ovina andPoa sphondylodes (C-3 Poaceae)

Structural aspects of the leaves of two common festucoids,Festuca ovina andPoa sphondylodes, have been examined employing the electron microscopy. The nature of vascular bundles and of sheaths that surround vascular tissues was discussed in the study. The festucoids exhibited a non-Kranz C-3 anatomy with more than four mesophyll cells separating the bundle sheaths of a leaf blade. Vascular tissues in theseFestuca andPoa leaves were surrounded by a double sheath: an inner distinct mestome sheath (MST) and an outer indistinctive layer of parenchymatous bundle sheath (PBS) cells. The PBS cells were much larger than the MST and had thin walls. The MST cells were relatively small and rectangular inP. sphondylodes and more or less hexangular in transverse sections ofF. ovina. InP. sphondylodes, MST had conspicuously thickened inner tangential walls with asymmetrically uninterrupted suberized lamellae in radial and tangential walls. In most differentiated MST cells, all walls were highly suberized. During suberin deposition, MST cells were quite vacuolated and most of the cytoplasm was present as a thin peripheral layer. However, MST walls inF. ovina revealed very thin suberized lamellae with translucent striations. No chloroplasts were detected inP. sphondylodes, whereas the MST inF. ovina contained small chloroplasts. Plasmodesmata were well developed in the primary pit fields of walls between MST and vascular cells, and between adjacent MST cells. Plasmodesmata were less frequent in the walls between the inner and outer sheath cells. Suberized lamellae were totally absent from the PBS cell walls in all veins. External to the PBS, the mesophyll comprised thin walled cells with abundant intercellular spaces. Peripherally arranged chloroplasts in the mesophyll were numerous and often larger than those of PBS and MST cells. Characteristics associated with C-3 and other ultrastructural features were also discussed in the study.

Ultrastructural aspects of kranz anatomy inDigitaria sanguinalis andSetaria viridis (poaceae)

Structural differentiation of Kranz anatomy has been investigated in leaf cross sections of two C-4 Poaceae:Digitaria sanguinalis andSetaria viridis. The study mainly focused on cellular and interfacial features of bundle sheath (BS) and mesophyll (MS) cells of the C-4 structure. Prominent BS, spaced by only two MS cells apart, were surrounded concentrically by a layer of MS cells. BS cells ofS. viridis had centrifugally arranged relatively large chloroplasts containing much starch, but the chloroplasts had agrana to rudimentary grana. Structural and size dimorphisms, when starch was present, were detected between BS and MS chloroplasts. Loosely arranged MS cells had peripherally displaced smaller chloroplasts containing little to none starch. BS chloroplasts ofD. sanguinalis were similar to those ofS. viridis, but had very little starch and well-developed long agranal stroma lamella. Features of MS cells were similar in both species, but well-defined peripheral reticulum (PR) was easily recognized in MS chloroplasts ofS. viridis. Virtually no PR was developed in BS chloroplasts examined. BS cells contained more mitochondria and microbodies, but no structural dimorphism was noticed. The electron-dense suberized lamella were often observed between BS and MS cells, especially in the primary wall of BS cells. It was most frequently found at the BS and MS cell interfaces and terminated in radial walls of the adjacent BS cells. Prominent pits with plasmodesmata (pd) were seen in the walls of both cells. There also were numerous pd in outer tangential walls of the BS cells. The number of pd ranged from 20 to 60. The pd trasversed a segment of cell wall much thinner than the adjacent wall. The current cellular data have been compared to the ultrastructural features known in leaves of other C-4 plants, especially NADP-ME species.

Effects of Local Variations in Soil Moisture on Hydrophobic Deposits and Dye Diffusion in Corn Roots

Abstract:The effects of local soil water content (SWC) alone on the development of hydrophobic deposits and on diffusivity of root tissues were studied in primary roots of maize seedlings whose top regions were growing at a constant rate in damp soil. Corn primary roots were grown through 30 cm‐profiles of soil with pre‐set dry (8% SWC), or wet (23% SWC), middle layers, and moist (15% SWC) top and bottom layers. When the root tissues in the soil of variable SWC were maturing (approximately 17% of the total root length), the tips of the roots were in damp soil several centimeters away and growing at the rate characteristic of this environment. Histochemistry and fluorescence microscopy on root sections of similar age showed that in these roots local SWC had no effect on hypodermal and endodermal suberization: both had equally developed Casparian bands and suberized lamellae. However, local SWC did alter the hydrophobic deposits in the walls of the epidermis; they increase in dry soil. To study tissue diffusivity, root portions from dry or wet soil were divided in the transverse direction, and half was dipped in sulphorhodamine G (SRG), sectioned, and observed with fluorescence optics. The other half was studied with a cryo‐scanning electron microscope to observe the contents of the cortical intercellular spaces. The rate of SRG diffusion in a portion of root from dry or wet soil was independent of local SWC but was positively correlated with numbers of fluid‐filled intercellular spaces. The implications of these observations for the movements of ions through the root cortex are discussed.

Structure of Hair Roots in Lysinema ciliatumR. Br. and its Implications for their Water Relations

The fine lateral roots of Lysinema ciliatumR. Br., an epacrid from habitats subject to periodic drought in Western Australia, are hair roots resembling those of Ericaceae. The finest (ultimate) hair roots have a cortex consisting only of an endodermis and an exodermis. Both layers have Casparian strips on the radial walls. The exodermis develops to state III very close to the root tip, showing wall thickening and a suberized lamella encircling each cell. In many roots collected after tip-growth had ceased and the tip had fully differentiated this suberized exodermis completely encircled the apex. In older hair roots the epidermis collapses or is sloughed off leaving the suberized exodermis as the outermost layer. The very fine hair roots have a very small stele containing only one xylem tracheid, and phloem consisting of a single sieve element with companion cell. The very small diameter of the single tracheid indicates a high resistance to water flow along the hair roots. This may tend to conserve soil moisture in the region of the hair roots, leading to improved survival and prolonged function of mycorrhizas in the field.

Branch Roots of Zea. V. Structural Features that may Influence Water and Nutrient Transport

AbstractFirst‐order branch roots of field‐grown Zea mays L. were examined by optical and electron microscopy. They were small‐scale versions of nodal roots except for the usual retention of a live epidermis throughout their length. The Casparian strips and suberized lamellae of hypodermis and endodermis developed closer to the root tip than reported for main roots (in the zone 0.5 to 5.5 cm from the tip for the hypodermis, and 0.5 to 4 cm for the endodermis), in branches retaining an apical meristem. The hydrophobic deposits were in place to the distal ends of determinate branches. All hydrophobic deposits were fully formed before the late metaxylem elements were mature. Gaps in the suberized lamellae of both hypodermis and endodermis may permit apoplastic diffusion of solutes through these layers. Pit frequency in the outer tangential walls of the hypodermis and endodermis was 0.3 per 100 μm2, and 0.6 to 0.7 per 100 μm2, respectively, in both branch and main roots. Numbers of plasmodesmata per pit in the branches were 60 and 30 in the hypodermis and endodermis, respectively. Water fluxes from published data were used to calculated the possible flux through plasmodesmata on a symplastic path. Values up to 0.2 pl h−1 for the hypodermis and twice this for the endodermis were obtained.

Sites of entry of water into the symplast of maize roots

Following our publication of a new method of calculating rates of water uptake by roots from measurements of the rate of accumulation on the roots of a marker solute, this paper describes the sites of accumulation of the solute, which indicate the sites where the water entered the symplast. Sulphorhodamine G (SR) was supplied in aeroponic mist culture to large maize plants with fully developed root systems. Root samples were collected after 4 to 8 h of transpiration in the dye-mist from both axes and branches of the main roots, and from non-transpiring (detopped) controls, frozen rapidly, freeze-substituted, and embedded and sectioned by an anhydrous procedure that preserves the SR in place. Whole mounts and sections were examined by bright-field, polarizing and epifluorescence microscopy. Major accumulations of SR were all at the outer surface of the roots, on Epidermal or root hair cell walls, or, in older roots where the epidermal cells were separating or dead, on the outer wall of the hypodermis. On some branch roots, though not on any main axes, the accumulations of SR were conspicuously aligned in the grooves over anticlinal cell walls of the epidermis. Non-transpiring plants showed very slight accumulations. Diffusion of SR into the cell wall apoplast was limited by the suberized lamellae and Casparian bands of the hypodermis, except in some branch roots, where SR diffused throughout the cortical cell walls. In parts of roots where the epidermis and hypodermis had been damaged, SR diffused through cell walls of the cortex from the wound site. These patterns of accumulation show that water enters the symplast of roots at the outermost cell membranes of the root, whether they are epidermal or hypodermal cells. Water enters roots with fully developed hypodermises at high rates. The rote of the hypodermal suberization is to limit solute movement in the wall apoplast. A symplastic path for water throughout the cortex, endodermis and living cells of the stele is suggested.

Structural features of NAD-malic enzyme type C4 Eleocharis: An additional report of C4 acid decarboxylation types of the cyperaceae

The genusEleocharis, a blade-less sedge group, has been very recently recorded to include NAD-malic enzyme type C4 species. The ultrastructural features of culms of two C4 representatives in the genus were examined in relation to the C4 acid decarboxylation type. They possessed non-chlorophyllous mestome sheath cells between mesophyll cells and Kranz cells, and were confirmed biochemically to be NAD-malic enzyme type. The oval or lenticular chloroplasts with well-developed grana are scattered in the Kranz cells with abundant large mitochondria, and do not show such centripetal position as is known in the “classical NAD-malic enzyme type”. The suberized lamellae occur in the mestome sheath cells internally surrounding the Kranz sheath and may contribute to maintaining high CO2 concentration in the Kranz cells. These new structural features of the NAD-malic enzyme type found inEleocharis are added to the structural and functional relationships of the C4 types in the Cyperaceae reported previously

δ13C Values and the Occurrence of Suberized Lamellae in Some Panicum Species.

δ13C values and the occurrence of a suberized lamella were determined for leaves of some Panicum species. Mean δ13C values for C4 subtypes were: NADP-malic enzyme (ME) species, −11·50±0·92%(2 spp.); NAD-ME species with centrifugal chloroplasts in bundle sheath cells (NAD-ME(F) species), −11·95±0·35 (4 spp.); phosphoenolpyruvate carboxykinase (PEP-CK) species, −12·06±0·51 (6 spp. and 1 var.); NAD-ME species with centripetal chloroplasts in bundle sheath cells (NAD-ME(P) species, −1248 + 0·28 (9 spp. and 1 var.). The difference between mean for NAD-ME(F) and that for NAD-ME(P) species was significant at 5 % level. A suberized lamella was present in the bundle sheath cell (BSC) wall of NADP- ME species (1 sp.) and NAD-ME(F) species (2 spp.) and absent from that of NAD-ME(P) species (2 spp.). These results suggest that NAD-ME(F) species and NAD-ME(P) species differ in carbon dioxide leakage from the bundle sheath cells and also confirm the postulation that the occurrence of a suberized lamella may minimize the carbon dioxide leakage from the bundle sheath cells. However, NAD-ME(F) and NAD-ME(P) species have been reported to show the same sharp pattern of postillumination carbon dioxide burst (PIB) within 30 s of darkness. Therefore, the results obtained in this study also suggest that the sharp PIB may be caused not by the absence of suberized lamella but by some other leaf anatomical characteristics and/or biochemical processes.

Suberized bundle sheaths in grasses (Poaceae) of different photosynthetic types I. Anatomy, ultrastructure and histochemistry

We investigated the histochemistry and ultrastructure of the cell walls of mestome sheaths and parenchymatous bundle sheaths of ten species of grasses. The species surveyed included representatives from all the major photosynthetic types: C3-Bromus tectorum, Phalaris arundinacea; C4/NAD-ME-Eragrostis cilianensis, Panicum capillare; C4/NAD-ME/PCK-Bouteloua curtipendula; C4/PCK-Chloris gayana, Sporobolus elongatus; C4/NADP-ME-Echinochloa crus-galli, Setaria glauca, Themeda triandra. All vein orders (designated here as major, minor and transverse) from mature leaves of each species were tested histochemically for lipids and phenols, and the majority of species were also examined with the electron microscope. A suberized lamella was detected ultrastructurally in at least some walls of major vein bundle sheath cells of all species examined. These lamellae were also present in some cells associated with the minor veins of the C3 species and in the minor and transverse veins of the C4/NADP-ME species. Histochemical tests for lipids and phenols consistently failed to differentiate this layer. Based on these tests, none of the vein orders in any species showed evidence of a Casparian band. In all suberized bundle sheaths, the compound middle lamella between cells with suberin lamellae is modified by the presence of phenols. These did not, however, confer resistance to acid digestion to the cell layer, in contrast to cell layers with Casparian bands. Therefore, although the mestome sheath has some features in common with the root endodermis (i.e. cells with a suberized lamella and thick, cellulosic walls which may be further modified), we could find no substantive anatomical or ultrastructural evidence for the presence of a Casparian band in any of the grass leaves investigated. The significance of these observations is discussed in the context of apoplastic permeability of these walls.

Leaf ultrastructure of C4 species possessing different Kranz anatomical types in the cyperaceae

The leaf ultrastructure of NADP-malic enzyme type C4 species possessing different anatomical features in the Cyperaceae was examined: types were the Rhynchosporoid type, a normal Kranz type in which mesophyll cells are adjacent to Kranz cells, and Fimbristyloid and Chlorocyperoid types, unusual Kranz types in which nonchlorophyllous mestome sheath intervenes between the two types of green cells. They show structural characteristics basically similar to the NADP-malic enzyme group of C4 grasses, that is, centrifugally located chloroplasts with reduced grana and no increase of mitochondrial frequency in the Kranz cells. However, the Kranz cell chloroplasts of the Fimbristyloid and Chlorocyperoid types exhibit convoluted thylakoid systems and a trend of extensive development of peripheral reticulum, although those of the Rhynchosporoid type do not possess such particular membrane systems. The suberized lamella, probably a barrier for CO2 diffusion, is present in the Kranz cell walls of the Rhynchosporoid type and in the mestome sheath cell walls of the other two types, and tightly surrounds the Kranz cells (sheaths) that are the sites of the decarboxylation of C4 acids. These ultrastructural features are discussed in relation to C4 photosynthetic function.

C4 acid decarboxylation type in Eragrostis (Poaceae) patterns of variation in chloroplast position, ultrastructure and geographical distribution

Abstract We investigated the activity of C4 acid decarboxylating enzymes, the PCR (‘photosynthetic carbon reduction’, or ‘Kranz’) bundle sheath anatomy and ultrastructure, and the geographical distribution of Australian species of the C4 grass genus Eragrostis. Species had either the even sheath outline and centripetally located PCR cell chloroplasts characteristic of NAD‐malic enzyme (NAD‐ME) species (29 spp.), the uneven sheath outline and centrifugal PCR cell chloroplasts characteristic of PEP carboxykinase (PCK) species (28 spp.), or were intermediate between these types (7 spp.). The suberized lamella was present in PCR cell walls of species with PCK‐like and intermediate anatomy, and absent from those of species with NAD‐ME‐like anatomy. Biochemical determination of C4 type for 11 species, however, revealed only NAD‐ME activity, irrespective of anatomical type; no PCK activity was detected. PCK‐like species arc most numerous in northern, high rainfall, tropical Australia and also predominate in relatively humid coastal and subcoastal areas. NAD‐ME‐like species are numerically and proportionally dominant where rainfall is < 30 cm year−1. Overall, as many species occur in high as in low rainfall areas. Results are discussed in relation to previously established anatomical/ultrastructural/geographical/biochemical correlations and to Infrageneric taxonomy.

Comparative ultrastructure and gas exchange characteristics of the C3–C4 intermediate Neurachne minor S. T. Blake (Poaceae)

Abstract Ultrastructural and physiological characteristics of the C3‐C4 intermediate Neurachne minor S. T. Blake (Poaceae) are compared with those of C3 and C4 relatives, and C3‐C4Panicum milioides Nees ex Trin. N. minor consistently exhibits very low CO2 compensation points (τ: 1.0, usually 0.3–0.6 Pa) yet has C3‐like δ13C values. CO2 assimilation rates (A) respond like those of C3 plants to a decrease in O2 partial pressure (2 × 104–1.9 × 103 Pa) at ambient CO2 levels, but this response is progressively attenuated until negligible at very low CO2. By contrast, other species of the Neurachneae are clearly either C4 (two spp.) or C3 (seven spp.). For plants grown and measured at different photon flux densities (PFDs), τ for N. minor and P. milioides increases from 0.5 to 1.0, and from 1.0 to 2.1 Pa, respectively, as PFD is decreased from 1860 to 460 μmol m−2s−1. In N. minor, the O2 response of τ is either biphasic as in P. milioides, but much diminished and with a higher transition point, or is very C4‐like. As in C4 relatives, inner sheath cells contain numerous chloroplasts. Their walls possess a suberized lamella, which may make them more CO2‐tight than bundle sheath cells of P. milioides, contributing to the almost C4‐like τ characteristics of N. minor. The biochemical basis of C3‐C4 intermediacy is considered.

Permeability of the Suberized Mestome Sheath in Winter Rye

Mestome sheath cells of winter rye (Secale cereale L. cv Puma) deposit suberized lamellae in their secondary cell walls. Histochemical tests including acid digestion and staining with Sudan IV and Chelidonium majus root extract were used to detect the presence of suberin in the primary cell wall. There was no evidence of a Casparian band between adjacent mestome sheath cells. Fluorescent dye techniques were used to trace solute movement through the rye leaf apoplast. Calcofluor white M2R, a fluorescent dye which binds to cell walls as it moves apoplastically, proved to be too limited in its mobility in leaves to test mestome sheath permeability. Trisodium 3-hydroxy-5,8,10 pyrene trisulfonate, a fluorescent dye which is mobile in the apoplast, moved easily up the vascular bundles in the transpiration stream, and diffused outward from the veins to the epidermal cell walls within minutes of reaching a particular level in the leaf. We conclude that the suberized mestome sheath of rye leaves is freely permeable to solutes moving apoplastically through radial primary cell walls.

Ultrastructure of vascular bundles and fundamental parenchyma in relation to movement of photosynthate in leaf sheath of rice.

The ultrastructure of vascular bundles and fundamental parenchyma (parenchyma in fundamental tissue system) in the 8th leaf sheath of rice were examined with a light and an electron microscopes in reference to possible pathways for photosynthate between phloem and fundamental parenchyma. 1. In the phloem of small bundles, the sieve element-companion cell complexes located in the middle and adaxial side of phloem remained without degeneration after sheath elongation (Fig. 1). However, the degeneration of phloem in large bundles is earlier than that in small bundles, and only several sieve element-companion cell complexes that abut on xylem remained without degeneration in the second leaf sheath from the uppermost fully-expanded leaf (Fig. 5). 2. In the boundary of xylem and phloem in small and large bundles, the plasmodesmatal connections were found in each interface between sieve element-companion cell complexes and xylem parenchyma cells, and between parenchyma cells of xylem and phloem (Table 1, Fig. 1). These connections may play a role in transfer pathways to phloem for solutes absorbed from transpiration stream by xylem parenchyma cells. 3. Cell walls in phloem of large and small bundles were densely stained purple with toluidine blue O (Figs. 3 and 4) and the plasmodesmatal connections were rare in the walls between sieve element-companion cell complexs and phloem parenchyma cells (Table 1). The phloem parenchyma cells contained many mitochondria with well-developed cristae and remained without degeneration after sheath elongation (Figs. 1, 2, 6 and 7). These data support the view that sucrose moves in the apoplast between sieve element-companion cell complexes and phloem parenchyma cells. 4. Suberized lamellae occur in all walls of the mestome sheath cells of small and large bundles (Figs. 1, 7, 8 and 9). Aggregates of plasmodesmata were observed in the walls between phloem parenchyma cells and mestome sheath cells (Figs. 1 and 7), between mestome sheath cells and fundamental parenchyma cells (Figs. 8 and 9), and also between fundamental parenchyma cells (Figs. 10 and 13). Judging from these observations, it appears that sucrose moves in the symplast between phloem parenchyma cells and fundamental parenchyma cells. 5, Different types of plastids were contained in the fundamental parenchyma cells of leaf sheath. The plastids in the top of sheath were similar in structure to the chloroplast (Fig.11), whereas those in the base of sheath were the typical amyloplasts having large starch grains and a few thylakoids (Fig. 13). The plastids in the middle of sheath showed an intermediate structure of chloroplast and amyloplast (Fig. 12).

Occurrence of the Suberized Lamella in Leaves of Grasses of Different Photosynthetic Type. II. In Herbarium Material

Dead air-dried leaves have been conventionally prepared for transmission electron microscopy to ascertain if the occurrence of a suberized lamella in cell walls can be detected. Our species sample includes representatives of all known photosynthetic types within the Poaceae (viz. C3, C4 NAD-malic enzyme type, C4 NADP-malic enzyme type and PEP carboxykinase type). Each photosynthetic type exhibits a characteristic pattern of suberized lamella occurrence in mestome sheath and/or 'photosynthetic carbon reduction' (PCR or 'Kranz') cell walls, consistent with that in fixed living material. Plasmodesmatal structure, and even on occasion chloroplast structure, is remarkably well preserved. Leaves from herbarium specimens, therefore, could be used to assign C4 species to their C4 acid decarboxylation type. This has potential application in large-scale systematic surveys for which living material may be difficult to obtain.

In vitro digestion of bundle sheath cells in rumen fluid and its relation to the suberized lamella and C4 photosynthetic type in Panicum species

AbstractDifferences in ease of digestion of bundle sheath cell walls of leaf blades of Panicum spp. are shown to be associated with differences in bundle sheath cell wall structure, including the presence or absence of a suberized lamella. These structural differences are correlated with photo‐synthetic type (viz. C3, intermediate C3/C4, and the C4 types, PCK, NADP‐ME and NAD‐ME), as are mesophyll: bundle sheath area ratios.

δ13 Values of C4 Types in Grasses

δ13 values were determined for leaves of C4 grasses (Poaceae) of different photosynthetic types. For plants grown in the same environment at the same time, mean δ13 values for the three C4 types were: NADP-ME-type, - 11.350 � 0.13 s.e. (11 spp.); PCK-type, - 11.950 � 0.19 s.e. (11 spp.); and NAD-ME-type, -12.70 � 0.21 s.e. (9 spp.). Although there is some overlap between the values for individual species of the three groups, the difference between any two means is highly significant [P(t) < 0.01] and is not due to taxonomic sampling bias at the subfamily level. The differences in means may suggest that C4 types differ in rates of leakage of CO2 and HCO3- from PCR tissue ('photosynthetic carbon reduction' tissue: equivalent to 'Kranz' tissue), and/or, using Farquhar's (Appendix) expression for plant δ values, that C4 types differ in their average intercellular CO2 concentrations (c1). It is also possible that differences between C4 types exist in some other, unknown, leaf fractionation process. Apoplastic CO2 leakage from PCR tissue in NAD-ME-type C4 grasses, which do not possess a PCR 'suberized lamella' as found in NADP-ME- type and PCK-type C4 grasses, may give these species the most negative δ13 values. Expressions for C4 plant δ13 values, and a model for the δ13 values of CO2 and HCO3- in various pools and fluxes in C4 plant leaves, are given.