The Growing of Sugarcane

Chapter 1

THE G R O W I N G OF S U G A R C A N E

1. THE S U G A R C A N E PLANT

The sugarcane plant is a perennial belonging to the grass family (Gramineae). Its cultivated varieties today are mainly derived from the hybridization of the noble cane (Saccharum officinarum) with the cultivated species S. sinense and S. barberi. The other two wild species S. spontaneum and S. robustum of the genus Saccharum are used only by geneticists in breeding work. Morphologically the sugarcane plant consists of three parts: the stem, the roots and the leaves. Detailed anatomical description of the plant parts is not intended as it is beyond the scope of this treatise. (1) The stem Sugarcane is propagated asexually by cuttings (sets, seed cane), each normally containing one or occasionally more buds. The buds on a set may develop into primary stems which, from their basal underground portions, may subsequently produce a number of secondary stems, then the tertiaries, etc., the whole constituting a cane stool (Fig. 1). The stems are composed of joints which, at the basal portion, start very short, then gradually increase in length until a maximum is reached, and then a decrease sets in. Many short joints are therefore present at the top and base sections of a stem. The basal part of a stem thus enables the formation of many ratoon tillers after the upper part is cut for seed pieces or for stalks for milling. The top of the stem is poor in sucrose and rich in various organic acids; consequently it is of little value to the factory. It is very useful for planting purposes, however, as it contains many buds. The height of mature cane stalks varies in the range 2—3 m and the diameter of stalks in the range 2—4 cm, depending on varieties, and internal and external growth factors. Each joint of the cane stalk consists of an elongated part, the internode, and a node where a leaf scar remains after the leaf has dropped. The shape and color of the internodes are usually characteristic of varieties. The basal region of an internode, just above the leaf scar, is the root band (root ring) where the root primordia (root initials) are located. Below the root band is the wax band, a zone covered with a layer of wax in varying density. A narrow section above the root band, called the intercalary meristem (growth ring) is a zone in which the cells remain potentially active. It is through differential growth of this meristem that cane stalks, after lodging, are capable of growing upward to expose their foliage to better light conditions. On alternate sides of each internode is normally located

4

Primary

stalk

Secondary

Tertiary

Ground

stalks

stalks

level

Point o1 a t t a c h m e n t to o r i g i n a l c u t t i n g Fig. 1. The underground portion of a cane stool showing primary, secondary and tertiary stalks. (From Van Dillewijn, 1952).

a bud, surrounded by the root band (sometimes double buds are present). The bud is an embryonic shoot consisting of a miniature stem with small leaves, the outer ones having the form of scales. The stalk is made up structurally of rind, consisting of a few rows of thickened cells. Within the rind is the ground tissue in which the vascular bundles are embedded, comprising the xylem and phloem as the transporting routes for water and nutrients from the roots to the leaves, and for synthetic products from the leaves to the roots. In the parenchymatous cells of the ground tissue the sugar juice is stored. (2) The leaf The leaves in two ranks are attached alternately to the nodes of the stalk. As the stem elongates, the older leaves gradually dry off and are eventually shed. Each leaf consists of an upper flattened portion with a distinct mid-rib (the blade or lamina) and a lower tubular portion, the leaf sheath, which protects a lateral bud. At the junction of the blade and leaf sheath is a membranous out-growth, the ligule. On either side of the blade joint there are two more or less wedge-shaped areas called dewlaps or joint triangles (Fig. 2). In anatomy, the leaf consists of an upper and lower epidermis within whose boundary lie the mesophyll cells with their chloroplasts and the fibrovascular bundles. On the epidermis is a superficial deposit of a fatty substance, the cuticle. Each fibrovascular bundle is surrounded by a ring of parenchyma cells which contain chlorophyll, the so-called chlorophyll-bearing bundle sheath. The bundle itself has a different size and is composed of xylem and phloem. The xylem consists mainly of the protoxylem with adjacent lysigenous cavity (lacuna or air tube) and two large vessels

Fig. 2. Diagrammatic representation of the sugar cane leaf showing its different parts. (From Van Dillewijn, 1 9 5 2 ) .

surrounded by flattened parenchyma cells. The phloem is made up of sieve tubes and companion cells. The xylem is responsible for movement of water and nutrients from the roots to the leaves while the phloem is responsible for the translocation of elaborated foods from the leaves to storage cells and to regions where growth occurs. The leaf is the site of photosynthetic activity from which sucrose is synthesized for the growth of the cane plant. (3) The root When a sugarcane cutting is planted under favorable conditions, the root primordia on the root band develop into the set-roots (temporary roots). Later, from the base of a growing shoot, the shoot-roots (permanent roots) are produced. The set-roots are of temporary nature; they are active only during the germination and growth of the bud. As the shoot-roots become established, the set-roots gradually cease to function, and decay and die, and the task of supplying the cane with water and nutrients is taken over by the shoot-roots. The roots undergo continuous branching as they grow, so that they become finer at the tips.

6 In a well-developed plant cane stool, three types of roots may be present: the superficial roots which constitute the main absorbing system of the plant; the buttress roots which appear to be essentially supporting in function; and the rope system, the main function of which is anchorage. The latter two share the burden of absorbing water and nutrients from deeper soil layers. The absorption of soil water and nutrients is through the root hairs, developed a short distance behind the root tip, which are unicellular tubular outgrowths of the epidermis. They are generally short-lived, but as the old ones die, fresh ones are continually produced near the apex as long as it is actively growing. Structurally a young cane root consists of an epidermis below which is the exodermis, followed by the cortex and the vascular cylinder. In the vascular cylinder, xylem and phloem strands alternate with each other and occur in longitudinal rows. (4) The inflorescence When a cane plant has reached a certain stage of development, its growing point may change from the vegetative to the reproductive phase. The growing point ceases forming leaf primordia and starts the production of a flower primordium and an inflorescence is eventually formed. It is an open-branched panicle consisting of numerous spikelets arranged in pairs (one being sessile and the other stalked) from the axes which are all supported by a straight main axis.

2. THE GERMINATION, TILLERING, A N D GROWTH OF S U G A R C A N E

( 1) Germination Sugarcane is generally propagated by cuttings of the stalk containing one or more buds. After planting, germination begins with the development of organs already present in the buds. A bud is an embryonic shoot consisting of a miniature stem with small leaves having the form of scales. The outermost scale forms into two halves overlapping each other as a hood, with a germ pore exposed at the top. The scales are only membranous when the bud is dormant. They become thick and fibrous and form into leaf sheaths as the bud is germinating. As the membranous scales of the naked bud are vulnerable to the penetration of herbicides, a cutting planted without any cover of soil would be very susceptible to injury by chemicals. During germination a bud becomes more tolerant as the growing point is under the protection of scales that become thicker and more fibrous. The maximal tolerance occurs when the bud has developed into a spike-like sprout prior to the expansion of leaves, because the growing point is now seated deep within layers of leaf-sheath. Normally the bud remains dormant as long as the stalk is under favorable growing conditions. This is associated with a phenomena called top dominance, known to be governed by the growth regulating substances in the stalk. During germination of the bud the root primordia on the cutting also develop into set-roots and function as such until the young shoot has produced its own roots.

7 Germination of the bud is the transition from the dormant into the active stage, which is preceded by changes in the food constituents and by the activity of enzymes and growth regulating substances in the cutting. Maximum germination and shoot vigor will result when both internal and external factors are optimal. Since the position of the bud on the stalk corresponds to its age, which increases from the top towards the base, a germination gradient is found along the stalk. Cuttings with younger buds from the top section generally germinate more readily than those with older buds from the lower section. Thus the top cuttings are more favored for propagating purposes. Also the planting material from well-developed and well-nourished cane give significantly better germination than those from poorly grown cane. Other internal factors such as different varieties and geographic origin may also influence the germination of cuttings. Among the external factors governing the germination process, soil temperature is one of the most important. Verret (1927) in Hawaii found that 20°C is too cold and 44°C too warm for good germination. The best temperature seemed to be between 33.9°C and 37.8°C. In Taiwan, cane cuttings are planted in two different seasons of the year. The spring-planting cane, under an average temperature of 20°C, will expand its first leaf from a germinating bud one month or so after planting, while the autumnplanted cane will do so in only about a week, at 35°C. Other important factors like the aeration and moisture in the soil are interrelated with temperature for providing optimal conditions for the germination process. Some artificial pretreatments of cuttings, like soaking with fungicides, insecticides and lime water, are practised to improve germination, because the soil pests which may attack a developing bud can be warded off or the bud can be stimulated to germinate in cold weather. When it is intended to continue cultivating a ratoon crop after the preceding crop is harvested, the stubbles which are composed of many short internodes and buds of the basal stalk are left in the soil. Under similar conditions, the buds develop into the young shoots of the ratoon crop. (2) Tillering Tillering is the underground branching of sugarcane and is characteristic of the grass family. Soon after planting, the buds of a cutting start developing into shoots called mother shoots or primaries. The little stem of these primaries consists of many short internodes, each of which carries a lateral bud. These buds give rise to secondary shoots which in turn may produce tertiary shoots, and so on (Fig. 3). This phenomenon which marks a tillering phase prevails only during the early growth stage. After an appropriate number of tillers are formed, each begins to undergo the elongation phase until maturity. Towards harvesting time, however, only a certain number of tillers will successfully become millable stalks, due to acute competition for nutrients. The trend of tillering and the ultimate number of tillers at harvest (millable stalks) are characteristics of varieties.

8

Fig. 3. Young cane plant showing t w o kinds of roots: set-roots originating from the root primordia of the cutting, and shoot-roots from that of shoots. (From Van Dillewijn, 1 9 5 2 ) .

(3) Growth Growth is not only in terms of elongation of shoots, but it includes the increase in size and weight as well as the dry matter of both aerial and underground parts of sugarcane. The growth of the aerial parts of sugarcane has been thoroughly studied by many workers and a long list of publications dealing with this subject can be found from the literature. In contrast, the study of root development has long been neglected due to the particular difficulties associated with this kind of research. However, as far as our interest, in this treatise, is concerned, the growth of roots will be emphatically reviewed and not shoots. From an exact knowledge of the development and distribution of roots in the soil, the relationships between herbicide effect and plant response can be better perceived. (a) Set-roots and shoot-roots Both types of roots originate from primordia and are therefore fundamentally identical. The set-roots originate from primordia located on the cutting which is a

9 section of a more or less fully-grown stem while the shoot-roots form primordia of the young shoots. Since the root initials of the cutting are older than those of the shoot, the difference between the two kinds is essentially a matter of age. Similar differences occur between the roots originating from successive nodes of the young shoot. While the roots developing from the lower nodes of the shoot are thick and vigorous, the roots from the higher nodes tend to become thinner, resembling much more the set-roots (Fig. 3). The number of set-roots produced during the early developing stage of a cutting, varies widely in different varieties; the average figure per node being: tropical cane 28, Indian cane 10 and S. spontaneum (wild cane) 2.3. This depends on two factors: the number of root primordia present at a node and the percentage of these root primordia that actually develop into roots. This varies not only from variety to variety but also from stool to stool within a variety. Often the position and age of the nodes affect the production of the set-roots. Other external factors such as artificial pruning (injury by cultivating implements) and natural injury by insects and diseases may stimulate the extra germination of dormant root primordia in reserve under normal conditions. Though both the set-roots and the shoot-roots are structurally and fundamentally identical and their potential longevity should thus be the same, the former is in fact short-lived due to competition from the latter. As observed in India and Hawaii, the set-roots grow first soon after planting the cutting, their total amount reaching a maximum of no more than 1 g in dry weight in one month and gradually diminishing, whereas the permanent shoot-roots which grow later soon pick up the momentum of growth and increase it sharply, and toward the third month may be more than 140 g in dry weight. Obviously during the first month the germinating plant is furnished with nutrients almost entirely from the set-roots. In the second month there is a transition period during which the burden of supplying nutrients shifts from the set-roots to the shoot-roots. At the end of the third month and thereafter, the burden of supplying nutrients rests almost entirely on the shoot-roots. However, without competition from shoot-roots, the set-roots will grow fast and assume the same function of supplying nutrients independently and live as long as the shoot-roots. This can be demonstrated by removing a bud at the lower node of a twoeyed cutting and then planting it vertically with the lower node in soil. Only set-roots will be developed in the soil while the upper aerial bud will develop into a shoot. No shoot-roots will be produced as long as the primordia are exposed to air. The shootroots as a whole, although permanent in nature, are in fact undergoing continuous regeneration of new individuals to replace the old ones that die and decay, through the entire growing season of the cane plant. (b) Root development As cited by Van Dillewijn (1952), in Mauritius three types of roots, i.e. superficial absorbing roots, buttress roots and rope systems, are observed to develop from a cane stool. In other countries, though morphological distinguishing of such roots may not be possible, their development and function are not changed (Fig. 4). The root primordia at the very base of the young shoot are much larger than those born later. These large root initials give rise to thick white roots, the buttress roots. They pass

10

θ'

7*

6'

5*

4'

3'

2

2'

3'

4'

5'

6'

7"

8'

Fig. 4. Root system of sugarcane showing different types of roots: s, superficial roots; b, buttress roots; r, rope system. (From Van Dillewijn, 1952).

outward and downward at an angle of 45—60° thus resembling underground stilts. They are often considerably distorted and flattened in various planes, apparently adjusting their growth in passing through dry subsoil. From the nature of their branching and localization in the subsoil which is poor in mineral nutrients, these roots serve as an anchoring structure to withstand stresses, though undoubtedly they do absorb to some degree. The rope system and other deep roots grow more or less vertically downwards and form, along the way, strands which may contain 15—20 roots. Such rope-like strands of roots may not be found in other regions and not even in different varieties. The rope systems can withstand much higher stresses, from 2.5 to 12 kg, and they are capable of very vigorous absorption, particularly since they descend to depths of the soil where moisture exists during extreme drought. Most of the superficial absorbing roots grow out of the later-appearing nodes of the stool. Prolific branching of these roots occurs when the growth in length is completed. The branch-rootlets are covered very densely with root hairs, thus exposing a much

11 larger area for absorption. Under moist conditions, the main superficial system supplies the stool with large quantities of water and most of its mineral substances. During drought, the stool has to rely upon the deep roots for its supply. Under field conditions, the root system of the cane plant is much more complicated as each stalk of a stool produces its own root system. All the individual systems, new and old, interlink with each other and form the root complex in the root zone. (c) Root distribution As cited by Van Dillewijn (1952), workers in Hawaii and Mauritius have contributed much to our knowledge of the distribution of cane roots in soil. By collecting data of the weight of cane roots in successive layers of soil, they showed that the percentage of roots present in the top 8 in. decreased from about 85% at the end of the first month from planting, to 60% at the end of the fourth month, while in the lower strata a reverse trend in root distribution was observed. It was concluded therefore, that water and nutrients, to reach the greatest proportion of roots, should be placed in the uppermost 18 in. of soil as more than 75% of the roots are present there. The distribution of root weight in the various layers of soil is not necessarily proportional to the absorption activity. From another investigation that takes account of both the length and the diameter of roots in vertical and horizontal cross-sections of soil, this relationship is better interpreted. It was shown that the vast majority of fibrous roots comprising the absorbing root-hairs are present in the uppermost foot of soil. They are most numerous between 3—4 ft. away from the plant, whilst within the 1 -ft. circle surrounding the plant only l/8th to l/9th of their total length occurs. Moreover, roughtly 70% of the total surface of root hairs calculated from data is concentrated in the first foot of soil and nearly 90% of this is distributed more than 1 ft. away from the center of the stool. It is obvious that as the cane grows older, the absorbing root-hairs which are always produced from young roots move farther and deeper away from the stool. (d) Ratoon roots The root system of a ratoon crop is more shallow than that of a plant crop, at least as far as the absorbing system is concerned. This is associated with the fact that the shoots of a ratoon plant originate at a higher level than those of a first year plant. The roots of a plant cane remain active for a considerable period after the crop is harvested. However, the developing ratoon crop takes advantage to only a limited extent of the root system inherited from the plant crop by forming new rootlets on the main framework already in existence. The old root system gradually ceases to function and decays, while a completely new root system is formed by the developing shoots of the ratoon crop. It is a process of gradual replacement of the old roots of the plant crop by the new roots of the ratoon shoots, until finally the entire root system is a product of the ratoon shoots. ( e) Factors influencing the growth of roots The root development is influenced by internal and external factors. The difference between one cane variety and another in its type of root system, appears to be an

12 inherent characteristic. Moreover, varietal differences in root development are not constant throughout the growing season. Also the nature and extent of the root system by itself has very little relationship to the yield capacity of a variety. Comparatively little is known about the influence of temperature on root growth. Exceptionally low or high temperatures will definitely impede the root development of sugarcane. With the spring-planted cane in the cold season in Taiwan for example, germination of shoot-roots from the base of young shoots is almost 3—4 weeks slower than in the autumn-planted crop. Aeration undoubtedly plays an important role in influencing the development of cane roots. Like other organs which require oxygen for respiration, they are positively aerotropic. When oxygen is lacking in one place, cane roots are directed to grow towards a region containing more oxygen. Aeration is interrelated with the moisture content of the soil. It is a common fact that the distribution of cane roots is clearly affected by weather conditions: low soil moisture encourages deep rooting, and adequate soil moisture promotes surface rooting. How soil moisture influences root development can be demonstrated by irrigation. Often a variety develops its root system mostly in the upper layers of soil when irrigation is practised. When grown without irrigation the same variety develops a more extensive and deeper root system in order to secure the necessary water. The quality of irrigation water also plays a role, saline water exerting a harmful effect on the root development. The vertical distribution of cane roots is largely controlled by fluctuations in the ground water level. In monsoonal regions it is a common phenomenon that during the dry season root development proceeds to a considerable depth, and that with a rising ground water level during the wet season, the deeper roots die and decay. During the wet season the newly-developed roots no longer pass beneath the ground water level. Other factors such as soil type, soil acidity, fertilizers and cultivation also influence root development in various ways.

3. THE CULTIVATION OF S U G A R C A N E

In order to provide optimal growing conditions and hence to bring about a possibly high yield, a series of field operations specific to local conditions is carried out. Basically these operations are similar in all producing regions. The unique pattern of intensive farming which is the feature of land usage in Taiwan and elsewhere will be emphasized. (1) Planting materials After the field is thoroughly cross-ploughed and harrowed, the seed bed is prepared and made ready for planting sugarcane. The three-eyed top cutting used in Hawaii and the stalk-cutting which utilizes the entire cane stalk is employed also in Java and other regions. In Taiwan, use of the two-eyed top cutting is common. Except for greenhouse experiments, the single-eyed top cutting is rarely used in the field, as germin-

13 ation and growth of the young shoot is comparatively poor due to less food reserves being contained in the shorter cutting. Occasionally for growing a spring crop, the pregerminated plantlets (rayungans) are used for transplanting when there is an inadequate supply of top cuttings from nurseries. This method is based on a natural phenomenon of top dominance governed by indigenous growth regulating substances (I A A) in the cane plant, which inhibit the germination of lateral buds on the stalk as long as the terminal growing point remains active. The growing stalks are topped, fertilized and irrigated copiously. As the source of IAA in terminal buds is removed with topping, the lateral buds on the stalks sooner or later sprout into shoots from the upper sections downwards, and the plantlets, together with the attached nodes, are cut one by one (analogous to pregerminated, single-eyed cuttings) to be transplanted to the field. About 1—2 months are required from topping the stalks to obtaining the plantlets for growing. Sometimes the two-eyed cuttings having developed both shoots and roots by being temporarily planted on side rows or raised in polyethylene bags, are used for replanting gaps in the ratoon fields, or for transplanting entire lots. Instead of planting top cuttings, a ratoon crop is grown by cultivating the stubbles left after harvesting the millable stalks. The young crop plants from such different types of propagating materials will have varied responses to herbicides applied at planting time because they have different shoot/root relations when treated. (2) Pretreatment of seedpieces Pretreatment of the seedpieces is practised generally throughout the cane-producing regions. This is often necessary because the seedpieces, after stimulated germination, need to be protected against rotting when they have been planted. Disinfecting the cut ends of the seedpieces by dipping them in a solution of an organic mercurial provides protection against the invasion by soil-borne organisms that might otherwise destroy the pieces before new shoots can establish themselves. Soaking the seed-pieces for 20 min in hot water at 52°C would result in a rapid development of buds and a precocious growth of young cane stools. Also, soaking in an aqueous solution of lime and magnesium sulphate or in saturated lime water alone, is very useful to stimulate the germination of seedpieces. By treating with the organo-mercurial fungicides, or the benzene hexachloride and chlorodane insecticides which are mixed with fertilizers and applied in furrows, an earlier and more vigorous root and shoot development is expected, in addition to pest control (Van Dillewijn, 1952). In recent years, the synthetic plant growth regulators have been extensively tested for application at the various growth stages of sugarcane to stimulate the germination, growth and yield of this crop. This subject will be reviewed in the final section of this chapter. (3) Planting seedpieces In arid, irrigated districts the seed is planted horizontally in grades for facilitating irrigation. Furrow width varies from 1.83 1.37 m in Florida, Hawaii, Queensland, Cuba, Jamaica, South mechanical planting and cultivation is practised, to 1.25 m and

furrows laid out in m in Louisiana, to Africa, etc. where 1.37 in Taiwan and

14 the Philippines where both manual and mechanical operations are used. Based on planting density, determined by variety, soil type, soil fertility and planting season, the seedpieces in furrows can be arranged in single or double rows at various spacings. The planted seedpieces should be covered with a thin layer of soil for protection from the spray of herbicides. In areas of poor drainage accompanied by salt accumulation, or where the problem of underground insects is serious, as in Taiwan, it is often necessary to plant the cuttings at a 45° angle to the soil surface, exposing the upper buds to the air. When cane is interplanted with rice paddy, such slant planting of seedpieces is also needed. This type of planting obviously poses a difficult problem in the use of herbicides because the exposed buds, before developing into spike-shaped sprouts, are very sensitive to chemicals. The ratooning process begins with harvesting the preceding crop stalks above ground level. The stools (stubbles) left over are then shaved smooth, disced on both sides of the stool rows (off-barring), fertilized and banked up. All these operations can be completed by the combined tools of a tractor, in one run. (4) Planting time Sugarcane is planted throughout the year in the tropics but is mostly limited to the spring and autumn in temperate and sub-tropical regions. The distribution of rainfall is intimately related to the usage of herbicides during planting time. Throughout the entire planting season, from May to October in Hawaii, for instance, rainfall is evenly distributed and the programs of chemical weed control can be expected to give satisfactory results. In the southern part of Taiwan, cane planting in the spring and autumn also follows the alternation of one dry with one wet season of the year. The dry season is from October until the June of the following year, while the rainy season is in July and August, during which more than 80% of the nearly annual 2000 mm rainfall occurs. The cane harvesting and milling in sugar factories is from November to April when the sugar formation of cane stalks culminates in these cold months, having grown about 12 months with the spring-planted and ratoon crops and 18 months with the autumn-planted crop. The autumn-planted crop usually gives a faster and more uniform germination and a higher stalk yield than the spring-planted and ratoon crops, but it suffers from easier attack by the prevalent pests, and the occurrence of a higher proportion of dead stalks. Moreover, a special nursery — which would otherwise be used for raw materials — needs to be reserved for supplying the top cuttings to be used in autumn-planting. The spring-planted crop, on the other hand, can utilize, from the matured and ready-forharvest cane stalks, a few topmost joints for planting seedpieces. However, about a 3 0 - 4 0 % lower cane yield is produced by the spring-planted and ratoon crops due to their shorter growing season. The stubbles (stools) left for ratooning are the basal sections of the stalks of the preceding plant crop. Because of the clonal relationship, the ratoon crop is much dependent on the plant cane for its vigour and its yield. A ratoon crop originating from a spring plant crop 12 months old, usually gives a much higher yield than one

15 originating from an 18-month-old cane. This is because the younger stubbles from the former possess more viable and vigorous buds to develop into new ratoon shoots. Under the conditions in Taiwan, the ratoons generally yield more if the ratooning operation is done in January and February, rather than in other, warmer months of the year. Under non-competitive conditions, the period between the planting and 'close-in' of cane leaves to shade interrow spaces, varies with the row width and the age of growth of this crop in different regions. Effective weed control to maintain this period weed-free is most important. Otherwise the cane plant will be adversely affected by competition, causing a significant reduction in yield. In Hawaii where the cane is grown at its widest 1.37 m row spacing, for 24 months, the close-in of cane leaves will be as late as eight months after planting. In Taiwan this period extends to no more than four to five months. A chemical weed control that depends on the persistence of effective herbicides over this period should therefore be planned to suit local conditions. (5) Intercropping and rotational

cropping

Intensive farming is a feature of agriculture in the industrial and overpopulated Taiwan. For growing a cane crop the land is utilized to such a great extent that, along with the cane itself, a short-season subsidiary crop is interplanted to take advantage of the interrows and, instead of fallowing the fields for a season, as is usual after 2—3 ratoons in succession, such a subsidiary crop is planted in rotation. Fortunately such intensive cropping is limited to the contract growers, with their most fertile fields. It has not been carried out to a significant degree on the large, industry-owned plantations, for fear of possible ill effects on the future productivity of the soil. Rice, peanuts, sweet potato, soybeans, cotton, tobacco and various vegetables have been used as the intercrops and rotational crops. In irrigated areas the planting of rice paddy coincides with the planting of the two cane crops each year. So a rice—cane intercropping system can be either spring-planted or autumn-planted. Firstly the rice is either direct-seeded or transplanted at 25 cm row spacing, each four planted rows alternating with one empty row to be planted with cane about one to two months later. The two-eyed cuttings should be planted at an angle of 45° by exposing one upper bud in air for respiration under the submerged field conditions. The rice is harvested after 4—5 months of growth, leaving the sugarcane alone to grow through the rest of its season. Throughout the growing of the rice the field is impounded with irrigation water, and so cane varieties that tolerate the wet conditions should be used. If an upland crop is used for intercropping, it is drilled on the field ridges at the same time as the cane is planted in furrows. Generally 4—6 months are needed to grow an intercrop. The yield of sugarcane will be more or less influenced by intercropping; but the total income from such a duplicate system will be higher than from cultivating sugarcane alone. This kind of cultivation system, of course, calls for special consideration regarding chemical weed control, as the two crops in co-existence respond differently to the chemicals. After two or more ratoons a field will be cross-ploughed to clear off remaining

16 stubbles, and to idle it for a season, either because of fatigue of the land or buildup of injurious soil factors that have caused decreasing yields of the ratoon crops. During the fallow, a green manure crop is grown and ploughed under for restoring fertility to the soil. Often the ploughing-down of green manures is accompanied by deep tillage on heavy clayey soil to retain good tilth for subsequent cane planting. In recent years in Taiwan the fallow period is mostly utilized for planting one more cash crop instead of growing green manures. Such a rotational system will certainly result in a change of the weed community in a field and may create the problem of residual injury on the susceptible crops arising from herbicides applied for the preceding sugarcane. (6) Tillage, irrigation and

fertilization

In Taiwan, several intertillages by hand-hoeing have traditionally been required to get rid of weeds in the cane fields during the early growth of sugarcane. As the labour shortage has become more and more acute in recent years, mechanical cultivation and chemical control have been adopted. The mechanical operations have been restricted only to the breaking of the field ridges and the banking-up of cane rows for anchoring the cane stand and for irrigation and drainage after the cane plants have grown 5—6 months. The application of pre-emergence herbicides may occasionally cause some effect on early cane growth by residual activity. But the toxic symptoms of plants often disappear with the breaking of the field ridges. It appears that the disturbance of soil by this operation may contribute to the decomposition of the herbicides, and bring their residual activity on the crop plants to an end. Irrigation and fertilization to compensate for insufficient water and nutrients available from the soil are most important for sustaining the vigorous growth of sugarcane. Irrigation and fertilization also play a role in influencing the herbicide activity in the soil and consequently the results of weed control. For herbicides to be effective, irrigation during the planting and ratooning of cane in the dry and cold spring is particularly necessary. The level of nitrogenous fertilization is often related to the intensity of irrigation, the cane variety and the soil types. For thick-stalked varieties, high-level nitrogen application combined with copious irrigation in arid regions is found to result in higher crop yields. The well-nourished cane plants will be capable of tolerating a potent, less selective herbicide. ( 7) Planting in saline soils Planting sugarcane in coastal areas affected by soil-salinity is common to all the producing countries. Saline soils are those whose electric conductivity of the satur_1 ation extract is greater than 4.0 m i 2 / c m and the exchangeable sodium percentage is less than 15%. These soils which often have a pH below 8.5, are commonly called 'white alkali' and contain principally sodium, calcium and magnesium chlorides and sulfate. These compounds make up the white crusts on the soil surface and the salt streaks along irrigation furrows from evaporation. The salinity of soil is often caused by contamination from the rise of the saline ground water table or by the irrigation of saline water.

17 Such salinity-affected soil is not usually suitable for growing cane before certain measures of reclamation have been adopted to bring down the soil conductivity to below 2 n ^ ' V c m , and the soil acidity to pH 7 . 8 - 8 . 3 . The installation of open or tile drainage to lower the ground water table and the desalinization of the upper soil by frequent flushing with fresh water pumped from a deep well or a down-stream river, in combination with soil amendments, have been used up to now for this improvement. In Taiwan, the improved saline land is first planted with rice paddy for several years to wash down salts in the upper soil by continuous irrigation with fresh water. After this desalination has been carried on to some extent, the rice—cane interplanting and then the sole cane planting will be followed when the salts in the soil have gradually decreased to a minimum. During the post stage of cropping, continuous irrigation with fresh water is still necessary to maintain the upper soil free from a re-accumulation of salts. Such an improved field is usually prepared in June/July after having been idled for a few months from harvesting the last crop in the spring. It is then leveed into blocks, irrigated and impounded with fresh water. Depending on the texture of the soil, several weeks are required for the impounded water to percolate, along with the dissolved salts, down to the underground stream. The reponding of water by continuing irrigation is necessary lest the field becomes dry, soil clods form and, as a result, resalination from evaporation occurs. The ponding of water in a field is continued from cane planting in September to the banking-up of cane rows, when the cane is 5—6 months old. After that, flood irrigations every 20—30 days are still needed, until the cane is 10—11 months old. When practising the rice-cane intercropping, the paddy rice at 25 cm row spacing is either direct-seeded or transplanted in July, each four rows alternating with one empty row which is to be planted with cane later in October. This system of intercropping has the advantage of removing more salts by the continual ponding of fresh water for the summer rice and the easier percolation of salts aided by the downward growth of the rice roots. The rice will be harvested in December to let the cane grow alone to harvest. When planting the cane using the two-eyed cuttings among the rice rows, the lower bud is planted slantwise into the submerged soil for developing the set-roots while exposing the upper bud in air for respiration. About 4—5 weeks later when the aerial buds have sprouted into primary shoots, the protruding pieces are stamped to lie flat in the soil for developing normal shoot-roots from the base of the sprouted shoots. To grow sole cane in saline land, the same slant planting of seed pieces under the submerged soil conditions, followed by the irrigation and ponding of fresh water for desalinization are used, except that the rice paddy is omitted. (8) Use of plant growth regulators for the improvement yield of sugarcane

of sprouting, tillering and

As reviewed by Vlitos (1974), at least four groups of naturally-occurring plant growth regulators: auxins, giberellins, cytokinins and various growth inhibitors, had been isolated from the sugarcane plant during the ten years before 1974, by his

18 colleagues, B.H. Most, H. Cutler, A. Yates and others. These indigenous plant growth regulators in stem apical tissues, which act in opposition to one another, serve as the hormonal factors in governing the germination of lateral buds, cell division, cell elongation and cell maturation and hence the integrated growth and maturity of sugarcane. The on-stalk germination by means of topping the stalks to produce the pre-germinated plantlets (rayungans) for transplanting is an example of breaking the apical dominance imposed on the lateral buds by one type of the indigenous auxins, the indole-3acetic acid (IAA). During the recent decades, a prominent subject in sugarcane agriculture has been how to promote the sucrose storage at maturity and hence the sugar yield of this crop by means of applying synthetic plant growth regulators, i.e. the chemical ripeners. The kinds of compounds which have been tested are 2,4-D, 2,3,6-TBA, MCPA, and dalapon which are also familiar phenoxy-type and chlorinated aliphatic herbicides, and, more recently, gibberellins, Ethrel, CCC, Polaris and others. As reviewed by Wittwer (1971), the physiological and biochemical studies on the effects of these plant régulants on various crop plants have been impressive, with more than 200 articles published. The fruitful results produced could be worthy of commercial application even to sugarcane agriculture if, in the future, the costs of a few prospective synthetic chemical ripeners are no longer prohibitive. As this subject is irrelevant to the scope of this treatise we would like to pass over it and instead review briefly the use of these synthetic compounds for the improvement of the germination and the tillering of sugarcane, since they are applied mostly at the same planting time as the herbicides for weed control, and the actions of both kinds could influence each other. Apart from being mainly used in growth studies to gain more stalk elongation and higher yields, (Tanimoto and Nickell, 1968; Nickell, 1976; Moore and Buren, 1978; Buren et al., 1979), gibberellic acid (GA) and related gibberellins have also been tested in the soaking of sugarcane seedpieces (top cuttings) by a few authors in Taiwan; but the stimulative effect on the germination of buds was too weak to merit further investigations (Chang and Lin, 1962; Shia and Pao, 1963). Similar results with more effect on the shoot growth than on tillering were obtained with CEPA (Ethrel) in the treatment of seedpieces in Jamaica (Eastwood, 1979). In more recent studies on the effect of some synthetic growth régulants in improving the sprouting of ratoon crops, results with meaningful and practical values were reported in India and Taiwan. In the cane-growing belt of Punjab, Northern India, where temperatures during November through December are as low as 24°C at maximum and 4.0°C at mean minimum, the sprouting of ratoons. following the harvesting of the plant crops during this period is very poor, apparently due to the winter coldness. This results in lower shoot population and stalk yield of the ratoon crops. In the first week of December 1975, a plant crop growth in this area was harvested and to its stubbles were applied such growth régulants as IAA, IBA (indole-butyric acid), GA, TIBA (tri-iodo benzoic acid), Ethrel (2-chloroethyl phosphonic acid) and Cycocel (CCC). All the chemicals were found to effect evident stimulation of the sprouting of the stubbles, resulting in an increased shoot population and cane yield by comparison with the control. Among the treatments, Ethrel at 500 ppm, IBA at 100 ppm and TIBA at 50 ppm were the most effective as they had caused 49.58, 46.50 and 43.75 tons cane per

19 hectare, respectively, compared to only 8.33 t/ha given by control at the harvest approximately one year afterwards. It was postulated that the higher concentrations of these applied exogenous auxins had induced synthesis of ethylene in the treated tissues of the cane stubbles, leading to the destruction of the endogenous auxins responsible for bud inhibition (Burg and Burg, 1968; Warner and Leopold, 1969). This resulted in diminishing apical dominance, thus relieving the buds from inhibition, and a higher number of the ratoon sprouts were obtained (Kanwar and Kaur, 1977). In Taiwan, due to the increasing costs and the shortage of farm labour, yearly production of sugarcane has been relying heavily upon the ratoon crops in recent decades. Though a ratoon crop with usually 12 months of growing season gives a cane yield that is about 30—40% lower than that given by an autumn-planting crop with 18 months of age, both crops have almost the same tonnage of cane per hectare per month. Ratooning is more advantageous than new planting because at least the seedpieces are saved. However, the number of successive ratoons harvested following a plant crop, has become shortened from 3—4 to no more than 2 in recent years due to the evident build-up of injurious insect, weed and disease problems under consecutive ratooning. How to improve the ratoon yield is becoming an urgent task of the agronomists in Taiwan. Even before 1970, attempts were made to tackle this problem by means of growth régulants and a series of field and pot experiments, principally with CCC on the various growth stages of ratoon cane, were carried out (Peng and Twu, 1978). This growth régulant, which is an aqueous solution containing 500 g/1 of 2-chloroethyltrimethylammonium dichloride (or chlorocholine chloride) was first field-tested in application on the young foliage of the cane varieties F 156 and F 160 planted in early November 1969. At 2 and 4 kg ai/ha and in single, double and triple foliar applications to the cane from the age of one month, this compound inhibited the shoot growth of both varieties, complying with its function as a growth retardant used in Europe for wheat. About two months after planting, the treated plants were an average of 2 cm shorter than the untreated plants and gradually recovered to normal in a few months. There was no significant difference in the retarding effect of the dosages or the number of repeated applications, and the subsequent tillering of plants was not affected at all. Also with a bath of this chemical in 1/500th concentration for the preplânting treatment of cane cuttings, no stimulation of bud germination was indicated. Definite results were obtained from a replicated field trial at three locations (plantations) by using CCC to treat the ratooning stubbles of the same varieties. The plant crop being harvested at three different times, early February, late March and early April, 1971, CCC at four rates of 5, 10, 15 and 20 kg ai/ha was applied to the stubble rows for ratooning and was followed 3—5 days later by a usual pre-emergence application of atrazine to control weeds. Observed 1—2 months later, the optimal application rate of CCC appeared to be 10 kg ai/ha and the stools treated as such sprouted 24%, 32.6% and 33.4% significantly more tillers and resulted in 20%, 30% and 34% significant increases in the cane yield of the ratoon crop on respective locations. This compound at all doses did not affect sugar content of the cane stalks at harvest time. The variety F 156 was generally known as a slow-sprouting ratooner; thus its axillary buds

20 on the stools were more responsive to stimulation of CCC at tillering temperatures up to 22.6°C and 24.9°C when ratooned in late March and early April. As the ratoon plants of F 160 could normally sprout in the coldest February, this variety's response to CCC on bud germination could only be observed with the treatments at this time. At one location the weeds caused the unweeded plots to produce about 10% less tillers than all the plots sprayed with atrazine. This emphasizes the importance of weed control for CCC-treated plots, that the stimulated buds, if any, may sprout and grow unimpeded by weeds. In contrast with preceding results in India, Ethrel added in trial at one location did not show any stimulative effect on the ratoon stools. With two more experiments in succession, to determine the steady effect of CCC under the climatic conditions of different crop years, a significant promotion of tillering was found as before to occur only on stubbles ratooned during the cold months between December and February. However, the subsequent crop yields did not exhibit significant differences from the untreated plots. To determine more precisely how the axillary buds on the ratoon stools respond to the growth régulant and how the sprouts grow into stalks at different temperatures, an experiment was carried out in a growth chamber partially in the open air. On April 4, 1977, individual cane stools of the varieties F 156 and F 160 were dug out, given a dip in 0.1% CCC bath and replanted in clay pots. The treated pots were then placed in two growth chambers with the temperature constantly adjusted to 14°C for one and 18°C for the other. After growing for nearly ten weeks, the sprouted plants in the growth chambers were removed to the open air for another 13 weeks, with temperatures recorded from 26°C to 29°C, until harvesting on Oct. 1. Another group of treated pots were placed in the open air throughout the growing season with temperatures recorded from 22°C in early April, to 29°C in the summer. For each temperature group, an equal number of untreated pots was placed alongside for comparison. The weekly observations made on the total length of tillers per stool (the numbers of tillers per pot times the average plant height) for each temperature group and for each variety are depicted in Figs. 5 and 6. The observations made for the fresh weight of the cane stalks and for their sugar contents after five months of growth are shown in Table 1.1. From Figs. 5 and 6 and Table 1.1, it is seen that the cane stools of both varieties placed at 18°C in the growth chamber after being treated by a dipping in 0.1% CCC bath, are as inhibited in the ten weeks to sprout by the coldness as are the untreated controls. Any significant stimulation in the sprouting and growth of ratoon shoots by this chemical is not observed until the stools are removed outside to the open air, and grow at temperatures from 26°C to 29°C for another 13 weeks. The resulting increases in the stalk weight of the treated stools at harvest are 26.7% with F 160 and 100% with F 156, based on 300 and 100 g per stool respectively given by the two untreated varieties. The treated cane stools with their first part of growth at 14°C in the growth chamber, or with their entire growing time between 22°C and 29°C in the open air, do not show much promotion in their growth of ratoon plants, due to being too cold and too warm. Moreover, the percentage of sugar content of the harvested stalks in first growing under 18°C is apparently not affected by the growth regulator, except that the lowest temperature of 14°C has evidently retarded the sugar formation

21 F156 200 150

14 °C

2 6 ° - 29°C

— ccc treated --- c o n t r o l

100(cm

50 0

tf)

ο ο -C 200 m ^_ Ο

150

-C

100

Len

en

α ο

50 0

t—

200ι 150

22°-29°C

100 50 0

10 No. of w e e k s

15

20

Fig. 5 . A ratoon stool o f F 156 treated b y dipping in 0.1% CCC solution and placed at 18°C in a growth chamber for ten weeks shows a much larger growth rate o f shoots w h e n removed t o the o p e n air at 2 6 - 2 9 ° C , as a result o f stimulated bud sprouting. Whereas the treated stool shows little stimulation under t o o l o w or t o o high temperatures.

in the cane stalks from both the treated and the control stools of both varieties. With further trials, it was confirmed that a good pre-emergence control of weeds in the fields is necessary for an effective stimulation of the sprouting of ratoon shoots by the application of CCC on stubble rows at the optimal 10 kg ai/ha. It was also shown that this growth regulator and such herbicides as diuron or atrazine, currently in use, could be applied separately or in mixture without any antagonistic effect. It was further noted that a relatively high level of nitrogenous fertilization caused the ratoon stools treated by CCC to result in retarded growth of ratoon shoots. Presumably the over-fertilized plants have been so softened as to be rendered susceptible to the growth regulator. It was postulated from the apical dominance shown by the small tuber corms of purple nutsedge (Ranade and Burns, 1925; Smith and Fick, 1937), that the axillary buds on compressed internodes of a cane stool should exhibit the same phenomenon because they do not sprout uniformly. During ratooning in the coldest month of February, the delay in sprouting from a cane stool becomes even more evident than at other times, owing to suppression at the low temperature. The application of this growth régulant to bring about an early and uniform sprouting therefore appears to be a remedy for ratooning under the inadequate conditions of a dry and cold season.

22

Even during warmer times its effect is detectable as shown by spraying its 0.1% solution on topped cane stalks in September (26°C), from which about a 30% on-stalk germination of the buds was obtained after the first week. Apart from its main antigibberellin effect, widely utilized as a growth retardant in wheat, the anti-auxin aspect of CCC has been applied to azaleas to promote the early initiation and development of flower buds, relieved from inhibition by apical dominance (Anonymous, 1966). Such an effect will, to a large measure, depend on the penetration and retention of the chemical when used for promoting the ratoon sprouts of sugarcane. Thus the buds with tender outer structures on younger internodes would be more responsive to this chemical, and its greater retention by an absorbent soil, surrounding underground stools, would result in a higher stimulation of sprouting. Since the harvesting and ratooning of sugarcane is generally carried on from December through April in Taiwan and elsewhere in the subtropical regions, a ratoon crop is grown for only about 12 months. Low temperatures from 16°C to 19°C during ratooning in December through February would delay the sprouting of the crop by a month or so. With the application of CCC, the sprouting of stubbles would be accelerated and about a 30% increase of tillers would be obtained in the first month which, under favorable conditions, would contribute to a higher cane yield on account of gaining an adequate growth for forming into millable stalks. However, as the final yield of a cane crop is associated with other factors, uncontrolled during the long growing

380 300 395 385 158 140 210 100

treated untreated treated untreated treated untreated treated untreated treated untreated

18°C in phyto tron for 10 weeks + 2 6 - 2 9 ° C in open air for 13 weeks

2 2 - 2 9 ° C in o p e n air through all season

14°C in phytotron for 10 weeks + 2 6 - 2 9 ° C in open air for 13 weeks

18°C in phytotron for 10 weeks + 2 6 - 2 9 ° C in open air for 13 weeks

2 2 - 2 9 ° C in open air through all season

* denotes significance at 0.05 level of probability. ** denotes significance at 0.01 level of probability. NS denotes non-significance.

F test LSD (0.05) (0.01)

F 156

3.41** 142.5 191.6

285 270

300 250

treated untreated

14°C in phytotron for 10 weeks + 2 6 - 2 9 ° C in open air for 13 weeks

F 160

Weight of fresh stalks harvested (gm/stool)

Temperature through growing season

Variety

Treating with or without 0.1% CCC

0.70

3.55*

0.73 NS

4.12*

3.28*

0.56 NS

Τ test between paired temperature group

18.9 19.6

11.1 9.5

11.6 11.2

19.2 20.0

14.1 13.0

13.5 14.8

Sugar con. of o f stalks (%)

Yields and sugar contents of ratoon stalks harvested 5 months from cane stools treated by 0.1% CCC and placed under different growing temperatures, as compared with untreated plants (average of 4 replicates)

Κ)

1.1

24 season, any promotion in ratoon sprouting by this chemical would not conclusively result in a significant increase in yield. Nonetheless, just like the preplanting treatments of seedpieces under inadequate conditions, such CCC treatment can at least be employed as a precautious remedy for ratooning during the cold months to get a possible improvement in sprouting and tillering, regardless of whether or not the crop yield may be benefited. More recently, Yang and co-workers (1980) of this Institute reported similar results with IBA and Ethrel. After the ratooning of F 156 and F 160 at three different times, i.e. in the late days of December, February and April of the 1979/80 grinding season, they applied these regulators to the stubble rows for ratooning. They found that both IBA at 500 ppm and Ethrel at 1,000 ppm caused all the stubbles to produce an average of 20% more tillers than the check plots, observed after one month from application. Compared with yields of the ratoon crops at harvest, the IBA treatment on F 156 ratooned in the cold months of December and February, and on F 160 ratooned in warmer April, gave 19%, 12% and 13% non-significant increases, respectively, over the controls. Much smaller improvements were found with Ethrel on the two varieties. The activity of IBA with regard to the varieties and the ratooning temperature appears to be similar to that of CCC as described just before.