PI-005: Chapter 3 - Plant Morphology

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In the present version of this chapter, contact information, references to specific organizations within the CFIA, and references to other documents or policies may not be current. This information will be updated at the time of the next revision of this chapter. Please contact the CFIA for any questions or further information.

Table of Contents

3.1 Objective

Plant taxonomy and plant identification are important tools in seed potato certification. Varietal integrity and purity are the foundations of the certification program. It is important to know the specific parts of the plant, so that comparisons of structure can be made to discern between similar yet different cultivars. This chapter provides information which will allow inspectors to use visual skills to separate and accurately identify potato varieties. Ultimately, inspectors should use this chapter to identify varietal mixes in a crop submitted for certification.

3.2 Definitions

Apical:
Usually refers to terminal leaflets or the eye, bud, or rose end of tubers;
Axil:
The upper angle between a branch or leaf and the stem from which it grows;
Axillary bud:
Located at the juncture of the stem and leaflet which may give rise to secondary branches;
Bud:
Sprout or shoot consisting of rudimentary foliage or floral leaves;
Cultivar:
A variety of potatoes;
Eyebrow:
The ridge over the eye of a potato;
Foliage:
Masses of leaves and stems that make up a plant;
Haulm:
Plant stems or stalks; the vines of potato plants;
Inflorescence:
The complete flower, including bud, pedicel and peduncle;
Internode:
The portion of the stem between the nodes or branches of the stem;
Leafstalk:
The petiole, midrib, central vein, or supporting stalk of a leaf;
Node:
The slightly enlarged part of a stem where buds are formed and where leaves, branches and eyes originate;
Petiole:
The stalk connecting a leaf or leaflets to a stem;
PLRV:
Potato Leaf Roll Virus;
PSTVd:
Potato Spindle Tuber Viroid;
Pubescent:
Covered with soft hairs;
PVX:
Potato Virus X;
PVY:
Potato Virus Y;
Senescence:
To die prematurely as a result of stress from environmental conditions or disease, or to die as a result of maturity (old age). Senescence may be a gradual process or sudden occurrence;
Stolon:
The underground stem of a plant, the end of which may form a tuber;
Tuber:
The short fleshy underground stem bearing buds or eyes;
Variety:
Seed potatoes that:
  1. are distinguished by common morphological, physiological, cytological, chemical or other common characteristics, and
  2. retain their distinguishing characteristics when reproduced;
Wings:
Protruding ridges running along the stem.

3.3 Taxonomic Identification Methods

The following illustration of a potato plant (see Figure 3-1) shows all of the major plant parts which may be used in variety identification.

Varieties (cultivars) may be identified by the following five specific characteristics, which are in turn discussed in detail throughout this chapter:

  • Foliage
  • Inflorescence
  • Date of maturity
  • Tuber
  • Sprout
Figure 3-1. Illustration of a Potato Plant
Potato plant - Description follows.
Description of Figure 3-1

This image is a diagram of a potato plant. It has a moderate amount of leaves, and at the bottom of the diagram it demonstrates the roots with the tubers forming at the ends of them. The diagram outlines the different parts of a potato plant from the top to the bottom; the flower, leaves, stem, stipule, bud (eye), tuber, the bud or apical end, stolon, seed piece, stem end, and the root.

3.4 Foliage

Environmental factors such as soil conditions, fertility and weather will affect foliage characteristics development similarly in most potato varieties. It should be noted, however, that some varieties are more susceptible to extreme conditions.

Disease present in plants can also alter foliage characteristics (e.g. leaf-roll (PLRV), mosaic (PVY, PVX) and spindle tuber viroid (PSTVd)). Knowledge of the effects of environment and disease on the foliage is advantageous when distinguishing potato varieties.

Foliage characteristics useful in variety identification are:

  • Habit and general appearance;
  • Leaves;  
  • Stems.

3.4.1 Habit and General Appearance

Habit descriptions useful in variety identification are:

  • Tall or low;
  • Upright or spreading;
  • Stiff and erect or drooping;
  • Open or compact;
  • Bushy or straggling;
  • Branching or non-branching;
  • If branching, high-or low-branching;
  • Robust and strong-growing or small and weak.

Varieties are grouped according to growth habit which can assist in variety identification. Early maturing varieties are normally low-growing, spreading, or bushy, whereas main crop varieties which mature later frequently exhibit a medium to tall, upright growth habit. As plants approach maturity and foliage begins to decline, these characteristics are less useful in identification, because varieties lose their distinctive habit. Plant habit, in conjunction with the obvious foliage and floral characteristics, form an image of a variety which can be sufficient for its identification. Foreign cultivars or rogues can usually be identified by their contrasting appearance with the plants of the main crop. Tall-growing rogues in a low-set crop are easily detected. On the other hand, a low-set rogue in a tall-growing crop is often overlooked. Minor, or less obvious foliage characteristics are often of great value in confirming varietal identification.

3.4.2 Leaves

A typical potato leaf is composed of two to four pairs of primary leaflets arranged on the mid-rib with a terminal leaflet on the end (see Figure 3-2). Between the primary leaflets are smaller ones, called secondary leaflets, which are often placed irregularly along the mid-rib. Tiny inconspicuous rudimentary or tertiary leaflets can also be evident along the mid-rib. The leaf is attached to the plant by a petiole.

Depending on the variety, the leaf may:

  • be long or short;
  • be rigid, drooping or spreading;
  • be flat or with side leaflets arched or drooping; or
  • have a distinct petiole angle between leaf and stem which occasionally can be used as a distinguishing feature.

Arrangement and number of leaflets whether primary, secondary or rudimentary, can also vary according to variety. Leaflets may:

  • be light, dark or grey-green in color;
  • be hairy, smooth or wrinkled;
  • be waxy, dull or glossy;
  • be large or small;
  • be broad or narrow;
  • be flat or somewhat folded; or
  • have equal or unequal lobes.

Additionally:

  • The first primary leaflets may be pointed, over-lapping the terminal leaflet, or set at nearly right angles to the leaf midrib;
  • The leaflet stalks may be long or short and may or may not bear stipules; or
  • The terminal leaflet may be heart-shaped and drooping or rigid.
Figure 3-2. The Compound Potato Leaf

The following image is an illustration of a compound potato leaf, which diagrams its different components.

Compound potato leaf. Description follows
Description of Figure 3-2

This image above is a diagram of a compound potato leaf. It is an illustration of a stock with a moderate amount of leaves. It outlines the different components of a compound potato leaf; the terminal leaflet, the first primary leaflets, the secondary leaflet, the midrib, the rudimentary leaf, the petiole, and the primary leaflets.

Figure 3-3. Open and closed leaf structures

The following image is an illustration of open and closed leaf structures.

Open Leaf Structure

Open leaf structure

Closed Leaf Structure

Closed leaf structure
Description of Figure 3-3

This image is of two stocks of potato leaves which demonstrate open and closed leaf structures. The image on the left demonstrates an open leaf structure; it has a moderate amount of leaflets; the midrib and the leaflet stalks that the leaflets are originating from are clearly visible. However, in contrast to the image on the right it appears sparse. The image on the right demonstrates a closed leaf structure; the midrib and the leaflet stalks are not visible, and the stock is overcrowded and very full.

General openness or closeness of the leaf is a valuable feature in variety identification (see Figure 3-3). Whether a leaf is open or closed depends on the:

  • distance between leaflets,
  • length of the leaflet stalks, and  
  • size and number of primary and secondary leaflets.

The most distinguishing feature of the leaf midrib is the presence or absence of pigment (blue-purple to red-purple). The intensity and location of the pigmented area are identification characteristics. In some instances the pigment along the leaf midrib extends to the leaflet midribs.

3.4.3 Stems

Identification criteria for stems include:

  • Amount and location of pigment;
  • Thickness;
  • Hardness;
  • Branching or non-branching habit;
  • Stem emergence;
  • Internode length (between leaves);
  • Node size (swollen nodes are typical of a few varieties);
  • Wing structure either wavy or straight.

Wings are protruding ridges running along the stem. Their characteristic of being wavy or straight are often very useful in confirming variety identification (see Figure 3-4). In most potato variety descriptions, mention is only made to wing structure when it is wavy. In all other cases it is straight or nearly so. Incidentally, most varieties have hollow internodes.

Figure 3-4.  Potato Stems Showing Types of Wings

The following image is an illustration of three potato stems which demonstrate different types of wings.

Potato stems showing types of wings. Description follows.

Wing characteristics are not greatly changed by differences in the environment.

Description of Figure 3-4

This image is of three different kinds of potato stems labelled A, B, and C. Stem A demonstrates a straight, double wing. Stem B demonstrates a waved, double wing. Finally, Stem C demonstrates a slightly waved, double wing. Particularly, stem B has a dentate wing while stem C is more undulate.

3.5 Inflorescence

The term inflorescence includes the complete pedicel (stalk which holds individual flowers), bud and peduncle (stalk that holds flower clusters) as illustrated in Figure 3-5.

Figure 3-5. Potato Inflorescence

The following image is an illustration of a potato infloresence, which diagrams its different components.

Potato inflorescence. Description follows.
Description of Figure 3-5

This image is a diagram of a potato inflorescence. The diagram outlines the different components of the potato inflorescence; the pedicel, the bud, and the peduncle. The image shows  five  pedicels branching from the  peduncle. Three of the  pedicels on the left side have flowers which have opened; the other two  pedicels have only buds on the ends.

The shade of colour or pigment is seldom uniform from the flower centre to petal tips. In determining flower colour, centre and tips should be ignored, although the overall floral pattern expressed can be an indicator itself. Pigmented flowers, depending on variety, can have variable amounts of white on the tips, yet on some varieties, the tips are merely a lighter pigmented shade with no white. Potato flowers may be:

  • White,
  • Purple, and
  • Faint blue.

3.5.1 Colour

White Flowers: There are many variations in the shades of white flowers which can be described as creamy white, milky white, or white with green tips. Some white flowers may have tinges of colour on one or both sides.

Purple Flowers: Identification of the shade of purple in potato flowers often presents some difficulty. Purples may be predominantly red or blue, or any intermediate shade. All pigmented flowers fade after exposure to sun, and red-purple flowers may fade and appear blue-purple. In determining shades of purple, flowers should be examined shortly after full expansion.

Blue Flowers: No potato varieties have pure blue flowers, but there are some whose flowers appear to be nearer to pale blue than any other colour.

3.5.2 Minor Floral Characteristics

The following minor floral characteristics can be a useful aid in confirming or distinguishing varietal identity (see Figure 3-6):

  • Size and shape;
  • Profusion of blossoms;
  • Development and colour of anthers;
  • Length and colour of style;
  • Length of sepal tip;
  • Hairiness (pubescence) and colour of buds;
  • Colour of bud bases (pedicel);
  • Size, frequency and persistency of buds;
  • Length and thickness of flower stalks (peduncle);
  • Colour, hairiness, and leafiness of lower stalks;
  • Natural berry production (true seed).

The peduncle has a cork or abscission ring, where the flower breaks off the stalk as the flowers mature. This can be pigmented which often helps to establish variety identity. Inflorescence stalks can differ between varieties in pigment which may range from green to various shades of purple or blue, and can be uniform or speckled.

Figure 3-6. Potato Flower Cross Sectional View

The following image is an illustration of a potato flower at a cross sectional view, which diagrams its different components.

Potato flower cross sectional view. Description follows.
Description of Figure 3-6

This image is a diagram of a potato flower at a cross sectional view. It outlines the different components of the flower; the stigma, style, petal, ovary, sepals, pore, anther, filament, and ovules.

3.6 Maturity

Potato varieties differ widely in their maturities ranging from 70 days after planting to over 120 days. A plant is considered to be mature when the foliage begins to senesce and the lower leaves start to yellow. Usually at this time the stem can easily be detached from the underground stolon. Varietal maturity is determined by reference to a standard variety of each maturity class. Five maturity groups have been adopted:

  • Very early;
  • Early;
  • Mid-season;
  • Late;
  • Very late.

Foreign cultivars are most difficult to identify when rogues mature at the same time as the variety in which they occur. An early maturing foreign variety in a late crop is easily identified by early flowering or indications of senescence. A late rogue in an early variety crop can be detected when the haulm of the earlier variety goes down and those maturing later are still lush and green.

3.7 Tubers

Tuber characteristics usually cannot be used independently to identify a variety alone. They are, however, good for confirming identification when used in conjunction with foliage or sprout characteristics. The end of the tuber which joins the stolon is called the stem end; the other end where the eyes are clustered is known as the  bud end. Tuber characteristics useful in variety confirmation are:

  • Shape;
  • Eye distribution and depth;
  • Skin colour, texture, and flesh colour.

3.7.1 Shape

Not all tubers from the same plant will be identical and may varyslightly in shape, particularly if they are small or immature. As tubers mature they are more apt to be true to type. The predominant tuber shape of a variety can only be judged by examining a number of mature tubers. There are several tuber shapes (see Figure 3-7), but the four most common shape classifications include:

  • Round: approximately spherical and may be flattened;
  • Oval: greater length than breadth and may be oval or nearly oblong with greatest width near the middle or tapering slightly towards the bud end;
  • Long: elongated or if slightly tapered towards the stem end, they are pear-shaped;
  • Oblong: greatest width near the middle, with minimal tapering very near the ends.
Figure 3-7. Tuber Shape

The following image is an illustration of a tuber shape, demonstrating a tuber growing from the root plant, and the possible shapes of tubers beside it.

Tuber shape. Description follows.
Description of Figure 3-7

This image is a diagram of tuber shapes. On the left side of the image is a tuber that is attached to the root which it had grown from and has been diagrammed with the components bud end, eyes, and stem end outlined. On the right side of the image are different shapes that the tuber could be and are labelled cylindrical, elliptical, oblong, oval, ovoid, or round.

3.7.2 Eye Distribution and Depth

Eye distribution and depth, eyebrow prominence and colour of scale leaves (if noticeable - see Figure 3-8) may provide useful confirmatory identification. For example, deep eyes with eyebrows that are long and prominent are commonly features found in round-shaped tuber varieties. Most plant breeders select varieties with shallow eyes to reduce the waste associated with peeling.

3.7.3 Tuber Skin Colour, Texture and Flesh Colour

Tuber skin colour, while insufficient in itself for positive identification, may be useful when accompanied with other features of the plant. Variations in skin colour may be:

  • Red, pink;
  • Blue, purple;
  • White;
  • Yellow.

The tubers may be uniform or parti-coloured. In parti-coloured tubers, the colour may be patchy or confined to the eye regions. In parti-coloured varieties, noting the pigment location can help to differentiate between varieties which have similar tubers. Skin texture may also vary; it may be smooth, russet or feathered.

All tubers green when exposed to light, but some varieties are characterized by the production of additional pigment which may be blue or purple. Some varieties show conspicuous white dots that are apparent when the skin is removed from green tubers.

Flesh colours which may be used to distinguish between varieties include:

  • White;
  • Creamy white;
  • Creamy yellow;
  • Yellow.

3.8 Sprouts

Tubers that are exposed to diffuse light produce sprouts characteristics of potato varieties. If sprouting takes place in full daylight, the colours become too intense and difficult to distinguish.

Features of sprouts (see Figure 3-8) used in identifying tuber varieties include:

  • Shape;
  • Colour;
  • Pubescence;
  • Number of rootlets.
Figure 3-8. Distinguishing Characteristics of a Sprout

The following image is an illustration of the distinguishing characteristics of a sprout, which diagrams its different components.

Distinguishing characteristics of a sprout. Description follows.
Description of Figure 3-8

This image is a diagram of the distinguishing characteristics of a sprout. It outlines the  main components such as: the terminal leaf scales, the lateral leaf scale, the base of the lateral leaf scale, the papillae, the root protuberances, and the tuber which it had grown from. It also outlines the three main sections of the sprout; top, middle, and base.

3.8.1 Shape

Three distinct parts of the sprout may be recognized, each of which may vary in shape and size according to variety:

  • The base - narrow or bulbous, long or short.
  • The middle portion - long or short and of variable thickness.
  • The tip - long or short, thick or narrow and may remain tightly closed or tend to unfold at an early stage.

3.8.2 Colour

Sprout colour corresponds with the colour (if any) of the other underground parts (i.e. stolon, tuber, scale leaves in the eyes, and the underground stem).

Sprout colour may be:

  • Faint pink - the sprout is white or greenish white with green tips, but with a slight pink colour.
  • Pink - varying from pink to carmine.
  • Blue or blue-purple - beginning at sprout base and tip, and generally spreading so that the whole sprout becomes so coloured.

3.8.3 Pubescence

The frequency and distribution of hairs on the sprout are useful features in identification. Some varieties are markedly pubescent, while others produce almost glabrous sprouts. Hairs are not always evenly distributed over the sprout surface; they may be concentrated on the sprout tip, base or centre.

3.8.4 Rootlets

Rootlets are small protuberances at the sprout base. In some varieties they are numerous; in others, less frequent. Rootlets are usually conspicuous by a concentration of colour around them, their tips generally remaining white, however, in some varieties, the tips are coloured.

Potato variety descriptions are available through the CFIA web site.

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