Parasitism In Exocarpus bidwillii Hook. f.

Transactions of the Royal Society of New Zealand : Botany, Volume 2, Issue 8, 24 August 1963, Page 109

Parasitism In Exocarpus bidwillii Hook. f.

B. A. Fineran

By

Botany Department, University of Canterbury

[.Received by the Editor, April 26, 1962.]

Abstract

Exocarpus bidwillii Hook. f. is a root semiparasite endemic to the South Island of New Zealand. It attacks a variety of plants, but only roots of woody plants serve as functional hosts. The mature haustorium is characterised by its longevity, secondary growth of the body, and by the possession of a complex branched sucker embedded in host xylem. In this feature it differs from other described santalaceous haustoria.

Introduction

Parasitism among the higher plants has long been recognized, several families being known which possess parasitic members. The first report of parasitism in the Santalaceae was by Mitten (1847) for Thesium linophyllum, whose roots he found attached to those of other plants by means of haustoria. During the latter half of the nineteenth and the early decades of the present century several other genera and species have been investigated, notable workers being Solms-Laubach (1867-68), Kusano (1902), Barber (1906, 1907), Benson (1910), Herbert (1924-25), Moss (1926), and Rao (1942). Comparatively few studies have been undertaken during the last twenty years.

The existence of parasitism in the New Zealand representatives of the Santalaceae, Exocarpus bidwillii Hook. f. and Mida salicifolia A. Gunn., has been in some doubt, however. Both Gheeseman (1925) and Allan (1961) recognize parasitism in the family but fail to indicate whether the local species exhibit the feature, while Aiken (1957) went so far as to say that no parasitic members were known for New Zealand. Laing and Blackwell mention that Mida salicifolia and Euphrasia cuneata are believed to be root parasites but do not elaborate further. This lack of definite information is somewhat surprising in view of the reasonably well known parasitic habit of other members of the family such as Santalum album and species of Osyris and Thesium. Such was the state of knowledge until Philipson (1959) definitely established the occurrence of parasitism in the local species. From his preliminary investigations developed a more detailed study of

the parasitism of Exocarpus bidwillii (Fineran, 1961) which will be published elsewhere.* This paper summarises some of the observations and conclusions.

Habit and Mode of Parasitism

Exocarpus bidwillii (Plate I, Fig. 1) is a small woody plant endemic to the mountains of the South Island. In habit the plant is xeromorphic with longitudinally grooved photosynthetic stems which bear leaves in the form of small triangular black scales (Plate I, Fig 2). Reproduction is by seeds and by layering of the stems. It usually occurs on rocky outcrops or sites where the soil is thin. Here the associated vegetation is dominated by woody plants which form the chief hosts of the parasite, the following species having been recorded with haustoria on their roots.f

Podocarpus nivalis Hook, f.

Nothofagus solandri var. cliffortioides (Hook, f.) Poole

Leptospermum scoparium J. R. et G. Forst

Corokia cotoneaster Raoul

Dracophyllum uniflorum Hook, f

Gaultheria rupestris (Linn, f.) D. Don

G. depressa Hook. f.

Cyathodes frazeri (A. Gunn.) Allan

Cyathodes colensoi Hook. f.

Pentachondra pumila (J. R. et G. Forst.) R. Br. Helichrysum selago (Hook, f.) Benth. and Hook, f

In contrast to several other santalaceous parasites E. bidwillii has not been found to develop mature haustoria within roots of monocotyledons. Self parasitism does, however, occur: an Exocarpus plant will attack other Exocarpus plants and occasionally even its own roots. Young haustoria may be found attached to various dead plant remains, such as leaves present in the surface layers of the soil, but these contacts do not persist, the haustorium usually ceasing growth early in development.

Exocarpus roots are typically associated with a dense mass of other plant roots. But the direction in which the parasite root grows is not obviously determined by the host root, contacts appearing to result mainly from chance encounters. The parasite’s root system is extensive rather than intensive, consisting of a few main roots which branch into a number of subsidiaries ultimately terminating in the distal roots of primary growth. Compared with associated plants, the branching of Exocarpus roots is relatively sparse, a feature which is thought to be related to a parasitic mode of life, since an extensive little-branched system may allow a large number of host plants to be attacked from a wider area. Structurally, young and old Exocarpus roots are similar to corresponding ones of non-parasitic dicotyledons. Root hairs occur on the seedling and on roots of older plants as well as on young haustoria.

General Characteristics of Haustoria

The Exocarpus plant acquires the parasitic habit at an early stage in its growth, the first haustorium appearing on the primary root of the seedling. From then on haustoria are continually produced throughout the life of the parasite. Thus all stages of haustorial development occur on a root system; some haustoria are small (o.6mm in body diameter) having recently made contact with a host root, while others are larger (3—6 mm in body diameter) and well developed. These are usually the mature haustoria considered to be fully functional and responsible for the bulk of parasitic absorption. As they have a brownish appearance matching that of the soil, mature haustoria are less conspicuous than younger ones and hence are easily overlooked.

For the purposes of description, the haustorium is divided into two main regions: the body and the sucker. The body is that rounded portion which joins the parasite to the host root, while the sucker is that part of the haustorium embedded within the host and connected with the body.

Haustoria vary in form between a cone and a hemisphere, the shape depending on the stage of development. The broad end of the body, forming the apex, is appressed to the surface of the host. Haustoria are borne both laterally and terminally on the parasite root. They develop superficially as lateral outgrowths on roots of primary growth (Plate I, fig. 3). Thus they are fundamentally different from terminal organs such as the root tip itself. At maturity some haustoria assume secondarily a terminal position due to abortion of the distal part of the parent root; all stages of atrophy can be traced. The difference between haustoria and lateral roots becomes even more evident when one compares their respective developments. While lateral roots originate from stelar tissue, haustoria arise mainly from the cortex; lateral roots rupture the cortex, haustoria do not.

Usually haustoria are initiated in an acropetal sequence (Plate I, fig. 3) behind the region of elongation—i.e., in a zone where the tissue is little differentiated. Young haustoria do not seem to develop on old woody Exocarpus roots. Of all .the haustoria formed on a root, not all reach maturity. The unsuccessful ones are ultimately sloughed off by the secondary tissues. The greatest concentration therefore occurs on mature roots of primary growth where up to fifteen attached and unattached haustoria may be found on an inch of root. By the time the root has become old and woody, only one or two haustoria may remain. The mature haustorium is an organ of complex structure (Text-fig. 1). It is characterised by secondary tissue in the body and by a complex branched sucker embedded in the host xylem. Both body and sucker possess structural features which are distinctive. The body is divisible into two parts: a central mass called the axial region, which is surrounded by a parenchymatous region known as the cortex. Essentially, the sucker represents an extension of axial tissue within the host.

The cortex of the mature haustorium is little differentiated, exhibiting few notable features apart from densely staining tanniniferous substances within the cells. During primary development, however, a conspicuous layer of collapsed tissue forms midway in the cortex, extending from apex to neck, and completely encircling the inner cortical tissue (Plate I, fig. 4). This collapsed layer appears to be caused through differences in growth between cortical and axial regions; axial tissue continues to grow after the outer cortical tissues have matured. They collapse as the central tissues expand into them and continue to collapse until the axial and inner cortical tissues begin to mature, after which time few further cells are added. Ultimately, the primary cortex, together with the collapsed layer, is sloughed off during secondary growth of the body. However, this may occur

late in development, for collapsed layers of tissue are still discernible in old haustoria where suckers and bodies are well developed. Collapsed layers are characteristic features of other santalaceous haustoria, for example Thesium pratense (Solms-Laubach, 1867-68), Santalum album (Barber, 1906; Rao, 1942). But here collapsed layers are retained in the mature organ in contrast to fully mature organs of E. bidwillii.

The axial region internal to the cortex is remarkable for the complexity of its structure and disposition of tissue components. In outline the region resembles an inverted conical flask, the basal end abutting the parent root and the neck towards the sucker (Text-fig. 1). This shape is also a characteristic feature of comparable regions in other santalaceous haustoria, a notable example being Santalum album (Barber, 1906, 1907). In E. bidwillii the expanded portion of the axial region is referred to as the vascular core, a term originally used by Benson (1910) for a similar region in the haustorium of E. cupressiformis.

Histologically, parenchyma and tracheary elements form the tissue of the axial region. Usually the vascular elements occur closely packed with interspersed parenchyma. In most sections, however, the tissue appears irregular and discontinuous (Text-fig. 1) since the plane of sections tends to run obliquely to the course of the vascular tissue because of the flashed shape of the axial region, described earlier. In the core, the vascular tissue consists of elements which form an interlocking branching network, the amount of branching increasing from the neck to the widest part of the core, then decreasing as the tissue tapers down towards the sucker. In this lower part of the axial region the tracheary elements are dispersed, appearing as a number of distinct tracts which pass into the sucker (Text-fig. 1). These tracts, and those of the sucker itself, are composed of short, narrow, scalariformly-reticulately thickened vessel members with simple perforation plates. While the elements of the vascular core possess similar wall thickening, they differ in being shorter, angular and more irregular in form, and have as their most distinguishing feature spherical granules within the cells. Granules such as these are not known elsewhere in the plant. Below the widest part of the vascular core granules occur with decreasing frequency towards the lower portion of the axial region where they disappear entirely from the tracheary tissue.

Granules apparently similar to those of E. bidwillii have been reported by Benson (1910) from vascular core tissue in haustoria of the Australian E. cupressiformis. Elsewhere in the Santalaceae they have not been recorded, but Heinricher (1895), quoted by Benson, refers to probably comparable granules in haustoria of Lathraea (Rhinanthaceae). In E. cupressiformis Benson suggested the granule-containing vascular elements played a dual role—namely, the conduction of products of both xylem and phloem. For these elements she proposed the term phloeotracheide. This name has here been retained as a convenient descriptive term for granule-bearing vascular elements in E. bidwillii also. However this does not mean that Benson’s hypothesis of their dual function has been accepted, as non-nucleated cytoplasmic lining, the main basis of Benson’s hypothesis of their phloem-like function, was not observed on close examination. There also remains some doubt as to the exact morphological nature of these xylem elements. Thus it is not clear, from a study of their end walls, whether they should be interpreted as vessel elements or tracheids.

The sucker of the haustorium grows out from below the body, ramifying along the host root (Text-fig. 2). Beneath the body it penetrates deeply into the xylem (Text-fig. 1), but as its branches spread out beyond the body, they become less deeply embedded towards the tips. This smaller degree of embedding of the distal tips of old sucker branches is shown by the young sucker branch below the body in Text-fig. 1. In transverse section sucker branches are usually narrowly wedgeshaped.

The fact that the sucker of E. bidwillii is branched deeply embedded distinguishes it from that of other described santalaceous species, where the organ is usually figured as a simple peg- or disc-shaped structure seldom deeply embedded in host wood. However, in some respects the sucker does show a superficial similarity to that of some loranthaceous parasites, in particular that illustrated by Engler and Krause (1935) of Loranthus europaeus.

Unlike the body, the sucker is mainly parenchymatous. Vessels are abundant but are widely separated by parenchyma. The vessels of the sucker are usually little branched small tube-like structures ramifying amongst the sucker parenchyma before finally abutting on a host vessel. This contact is usually between the end wall of the parasite vessel and the lateral wall of the host vessel, the lumen between the two appearing to become continuous, in some instances.* The interrelationship between the host and the haustorium is very intimate, judging by the interaction between host and sucker tissue and by their growth relative to each other. Although a sharp line clearly delineates host from parasite tissue, as seen in transverse section of an infected root, the tissues meet intimately as if they belonged to the same plant. No sign of wound tissue or any other antagonistic reaction is visible in most attacked roots. Nor is there evidence of disintegrated host cells or digestive material to indicate that embedding has resulted from digestion of host xylem, as described by some workers for certain parasites. Rather, the sucker becomes embedded by concomitant radial growth of its tissue with that of the surrounding host xylem. Hence sucker branches become progressively embedded as growth continues. In host roots showing growth rings, for instance, Text-fig. 1, it is possible to see that the sucker is embedded in several years’ increments of wood. Cases of more than six years’ growth are known. Meanwhile, as sucker branches become embedded, their distal tips also grow out along the host root in the zone of its cambium, thus extending the sucker system within the host.

Discussion and Conclusions

The study of E. bidwillii has revealed many features not previously known concerning the parasitic habit of .the plant and the structure and development of its haustorium. There are general similarities to other santalaceous parasites but a few characters of the haustorium stand out. The branched, deeply embedded nature of the sucker is a distinctive feature. Another, but shared also with

E. cupressiformis, is the presence of spherical granules in the phloeotracheides of the vascular core. A general discussion of the parasitic relationships of E. bidwillii will now be presented, followed by comparisons with other santalaceous parasites.

Santalaceous parasites generally attack a wide variety of host plants, a feature probably characteristic of the family. In this respect E. bidwillii is typical, since it has been shown to attack at least eleven species. When the parasitism was first reported, however (Philipson, 1959), Helichrysum selago was the only known host. Mono-specific host connections have also been mentioned for other santalaceous parasites, for example Leptomeria spinosa (Herbert, 1924-25), but the present work suggests that further investigations may well reveal a wider range of hosts.

The host plants of E. bidwillii listed in a previous section include only those which were determined by actually tracing .the attacked root to the parent plant. In the process of digging for haustoria, many roots bearing mature haustoria are severed, so that it becomes practically impossible to identify the plant to which they belong. The list presented therefore may represent only a very small proportion of the total number of available hosts. In some instances, however, root fragments can be identified by comparing their morphology and anatomy with that of roots taken from known plants. This indirect method proved particularly useful for checking the identity of roots of large host plants, which often grow at some distance from the parasite.

Parasitism in E. bidwillii is revealed ,by the occurrence of haustoria connecting its roots to those of other plants; otherwise the plant exhibits few features which might suggest parasitism. The photosynthetic stems with their reduced leaves are more suggestive of a xerophyte than a parasite which has undergone reduction of parts with dependent nutrition. The root system is perhaps the only portion of the plant which shows a modification of structure possibly associated with a parasitic mode of life. E. bidwillii also displays little indication of being a parasite in its relationship to the adjacent vegetation. There is seldom any obvious association with host plants, the parasite usually occurring with apparent random distribution in the community. This seeming randomness is no doubt attributable to the parasite’s wide ranging roots meeting similarly wide ranging roots of host plants. Occasionally, however, a more direct association can be detected. In such instances clumps of Exocarpus grow around the base of the parent tree or bush. But even here the uninitiated observer would not suspect parasitism on cursory examination. The appearance of the surrounding vegetation also does not help one to detect the presence of a parasite. Even when root systems of plants are exposed, there are seldom signs of extensive damage. Some roots show a constriction or, rarely, complete atrophy distal to the haustorium’s point of attachment, but the number of roots affected in this manner is small compared with the total number of haustorial contacts.

Haustoria of E. bidwillii are found fastened to a variety of roots. Small and large roots are both readily attacked, but in order to serve as proper host roots for parasitic attachments they must have a long life span which enables the haustorium to become permanent. Only roots with developing or mature secondary tissues fulfil this condition, while roots of herbaceous plants and ephemeral roots of woody plants are unsuitable as host contacts (PI. I, fig. 4). Similarly dead plant fragments which are occasionally parasitised do not form lasting connections. On foreign bodies, non-woody and short lived roots the haustorium usually ceases growth either before or soon after penetration at a stage when its tissues show little differentiation. Later these unsuccessful haustoria shrivel away to be finally sloughed from the parasite’s roots by the formation of cork.

Because young haustoria may attach themselves indiscriminately to almost any object, their presence on a root does not always mean that the attacked plant will serve as host for the mature organ. Only woody haustoria with deeply embedded suckers permit this conclusion. Since the parasite continually produces a large number of haustoria, a sufficient proportion will encounter suitable roots and reach fully functional maturity, though many meet unfavourable objects. Presumably new host contacts also have to be made to meet the demands of the growing plant and to replace mature haustoria which may have ceased to function.

The degree of dependence of E. bidwillii on parasitism is not yet precisely known. Detailed physiological experiments could not be carried out in the time available for the investigation on account of the slow growth of the plant and its haustoria. Nevertheless, some general idea of its demands can be deduced from morphological features and from observations on transplants into cultivation.

In the seedling, haustoria are present but are still rather undifferentiated, hence at this stage they probably contribute little to absorption. Root hairs are abundant, and through them presumably absorption takes place in the normal manner. However, as the parasite grows and haustoria becomes more developed, a greater proportion of the necessary nutrients will be contributed by parasitism. No doubt, structures involved in normal absorption might then be expected to be reduced. For example, some workers have attributed the reduction of root hairs in adult roots of semiparasites to the increasing importance of parasitism compared with normal absorption from the soil. This hypothesis does not seem to apply to E. bidwillii however, for root hairs are equally as abundant on roots of mature plants as on seedlings—a condition which suggests that normal absorption is still important in the mature plant. If this is the case, the question arises whether E. bidwillii is an obligate parasite or a facultative one in which parasitism is an accessory means of nutrition. Transplants shed some light on this matter. Exocarpus plants removed with a sod containing a host plant usually survive. One specimen transplanted with a small bush of Corokia cotoneaster still flourishes after two years in cultivation. On the other hand, plants taken without a host usually die after a few months. This suggests that normal absorption is insufficient to meet the plant’s demands in the absence of a host. From these results it seems that E. bidwillii is an obligate parasite depending on host contacts in addition to normal absorption for its nutrients from the soil.

An indication of Exocarpus’s nutritional demands could probably be better obtained by raising plants from seed and observing their response in the presence or absence of a host, as some workers have done for a few other santalaceous parasites. This was not possible with E. bidwillii as all attempts to germinate seeds were unsuccessful. The precise nature of the nutrients absorbed by the haustorium is similarly undetermined. But from the structural interrelationship of sucker and host tissues it is fairly conclusive that they are substances transported by host xylem elements. The direct tracheary contact with host vessels and the large surface area of contact with host xylem compared with phloem support this. Phloem elements are not detectable in the body or sucker, and together with the small surface area of host phloem contact, this suggests that phloem products are of little importance to the parasite. The well-developed photosynthetic tissue of E. bidwillii also supports this assumption.

For a parasitic association to be successful, the parasite should cause as little disturbance as possible to its host. In E. bidwillii this appears to be the case: the tissue of the sucker becomes almost an integral part of the host, not greatly disturbing the growth of the root, and root systems of attacked plants are seldom damaged—both indications of a well balanced parasitic relationship.

After this general discussion of the parasitism of E. bidwillii , the structure of its haustorium will now be compared in more detail with that of other santalaceous haustoria.

The haustorium of E. bidwillii, as already noted, shows similarities to and differences from other described santalaceous haustoria. It is a,t the primary stage of growth that the haustorium exhibits the closest similarity to other haustoria, for example in species of Santalum, Comandra and Scleropyrum. There is also a body differentiated into an outer parenchymatous cortical region together with a central flask-shaped axial region of mainly tracheary tissue. A wedge-shaped mass of tissue also extends from the body into the host as the sucker. However, even at this stage haustoria of E. bidwillii differ from the above examples by the absence of a pith extending through the axial tissue and by the structure of the vascular core tracheary elements. In these respects E. bidwillii shows a stronger resemblance to the haustorium of E. cupressiformis. Here the upper part of the vascular core similarly comprises a compact mass of phloeotracheides with no pith. The only pith-like tissue in both these haustoria is a central parenchymatous core situated below the widest part of the vascular core and enclosed by the tracts of vascular tissue converging towards the sucker (Textfig. 1).

E. bidwillii differs also from Santalum album, Comandra palida and Osyris alba by the absence of compound haustoria with successive superimposed development of axial tissue. A type of compound haustorium does, however, exist in E. bidwillii when two or more axial regions develop within the one body mass. The haustorium of E. bidwillii is not a short lived organ with little secondary tissue like most santalaceous haustoria mentioned so far, but is an organ characterised by its longevity, woody structure and branched embedded sucker. The only other haustorium apparently similar in longevity and secondary growth of the body is that of Buckleya quadriala, as described by Kusano (1902). Howthis haustorium possesses a comparatively simple wedge-shaped sucker, typical of most santalaceous haustoria and in this feature, therefore, differs from E. bidwillii. The branched, deeply embedded nature of the sucker is thus the most distinctive feature of the haustorium of E. bidwillii.

In development the haustorium exhibits features which are distinct from other organs, particularly the root. It differs in early development from lateral roots by being exogenous. Epidermal and cortical cells are involved in its formation in contrast to the lateral roots which develop mainly from stelar tissue and are therefore endogenous in origin. For this reason no rupture of root tissue occurs where the haustorium arises from the parent root. While haustoria are only formed by roots of primary growth, lateral roots may be formed also by old woody roots. Usually, when the haustorium first appears, the tissues of the root are not yet mature. The method of primary tissue differentiation differs also, though only to a small degree, from that in the root. In the haustorium the central tissues of the body continue to grow after the outer cortical tissues have matured, a process leading to formation of the collapsed layer which is absent in the root. The tissue derivatives of the haustorial cambium and of the parent root also differ. Unlike the root, the body cambium produces centrifugally not phloem but secondary cortex. This forms most of the protective tissue of the body, as the periderm is only weakly developed compared with that of the root.

Lastly, some previous interpretations of parasitism in the Santalaceae will be mentioned which appear inconsistent with the findings of this study. It is considered that the structure of some santalaceous haustoria may have been misinterpreted, since the organ previously described seems to be only a stage of development and not, as implied, the mature organ. This applies to those haus-

toria where penetration has been said to be superficial. For example, in the first account of the haustorium of E. bidwillii (Philipson, 1959) the sucker was found to make contact with host phloem, while the contact with the xylem was not observed. Because of this, E. bidwillii was said to resemble E. spartea as described by Herbert (1924-25) rather than E. cupressiformis and E. aphylla as described by Benson (1910) and Rao (1942) respectively. Quite clearly then, the organ in the first description by Philipson was merely a young haustorium in the act of penetration. A further examination of other species where penetration is said to be superficial, for instance some of those described by Herbert (1924-25), probably would also show that they too are only stages of development. The instances of misinterpretation of structure have probably stemmed from insufficient examination of the root system and haustoria of the whole plant before the mature organs were recognized. As young haustoria are more numerous and conspicuous than older ones, it is not surprising that young stages are more often described.

The assumption that most santalaceous haustoria are short lived organs which develop little secondary tissue in spite of a cambium seems also inconsistent with the above findings. Since most santalaceous parasites are woody plants, and since their nutrition appears at least in part dependent upon parasitism, it is difficult to see how this demand on the host could be maintained by organs which fail to develop an extensive, intimate and lasting association with the host.

Acknowledgments

The writer is indebted to Professor W. R. Philipson for guidance and constructive criticism throughout this study and to Dr Margaret Mayer for assistance with the correction of this paper.

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B. A. Fineran, M.Sc., Botany Department, University of Canterbury, Christchurch.

* This is being published in “ Phytomorphology ” in six parts. The first paper has already appeared. (Fineran, B. A., 1962. Studies on the Root Parasitism of Exocarpus bidwillii Hook. f. I—Ecology and root structure of the parasite. Phytomorphology 12: 339-355.) Since the present paper was submitted a note has also appeared in “ Nature ”. (Fineran, B. A., 1963. Root Parasitism in Santalaceae. Nature 197: 95.)

f Mature haustoria have also recently been found on roots of Hebe brachysiphon Summerhayes.

* Scalariform perforation plates, which appear imperforate, have recently been found on the end wall of some parasite vessel elements penetrating host elements.