Botany
Maize as a member of the grass family, Gramineae has many characters common to other grasses, such as conspicuous nodes in the stem, a single leaf at each node, the leaves in two opposite ranks i.e. distichous, each leaf having a sheath surrounding the stem and an expanded blade connected to the sheath by a blade joint. As in other grasses there is a tendency to form branches at the nodes and adventitious roots at the base of the internodes. The lower branches may take root and develop into stems known as tillers or suckers which resemble the main stem, whereas the others develop as rudimentary or functional ear shoots. Maize is a monoecious plant with its functional staminate flowers borne in the tassels which terminate the stems, and its functional pistillate flowers borne in the ears which terminate all but the basal branches or tillers. The heritable vegetative size of plants varies greatly with variety and regional adaptation. In most grasses the elongated internodes become hollow, but in the members of the tribe maize Maydeae (Tripsaceae) and the sorghum tribe Andropogoneae the stems remain solid.
1.1 .1 . Origin of maize
No wild plant is known from which maize could readily have been derived. This might be accounted for by the assumption that the wild maize plant has become extinct. Teosinte is usually regarded as very closely related to maize; not only because of its morphological resemblance, but also because it can be hybridised readily with maize, the progeny being fertile. Upon crossing, the chromosomes pair normally and crossing over takes place between com and Teosinte chromosomes. Teosinte is also susceptible to such common maize diseases as maize smut and maize rust. The geographic point of origin is generally conceded to be somewhere in the tropics of Central or South America with the latter seeming most probable. This belief is based upon archaeological and ethnological evidence and upon the theory that the birth place of a new species is likely to be found in the region of its greatest variability.
1.2. Development and structure of vegetative parts
1.2.1. The Kernel or 'Seed'
The maize kernel is not merely a seed but a one seeded fruit, in which the seed, consisting of embryo and endosperm and remnants of the seed coat and nucleus is permanently enclosed in the adhering pericarp. Upon shelling, the flower stalk or pedicel commonly remains attached to the base of the kernel.
1.2.2. Pericarp
The pericarp or transformed ovary wall forms the tough outer covering of the kemel, it furnishes protection for the interior parts. At the apical end of the kemel the peri carp bears the silk scar and the basal end it merges into the tissues of pedicel or tip cap as it is often called.
1.2.3. Endosperm
When the pericarp and the adhering remnants of the seed coats and nucellus are removed which can easily be done after soaking the kernels in water, only the endosperm and embryo are left. The endosperm makes up the greater part; the relatively small embryo being situated on one side near its base.
1.2 .4. Embryo
The embryo or young maize plant is embedded near one face of the endosperm at the base of the kernel or caryopsis. It has a central axis which is terminated at the basal end by the primary root and at the other end by the stern tip. The stern comprises five or six short internodes and bears a leaf at each node. The first leaf known as the scutellum is attached to the scutellar node. This is modified as a food storage organ and serves to digest and absorb the endosperm during growth of the embryo and seedling. The second leaf, the coleoptile is attached to the second or coleoptilar node and is modified asa protective covering for the plumule or first bud of the plant which it spearheads through the soil upon germination.
1.2.5. Pedicel
The tissue of the pedicel or flower stalk merges into those of the ovary wall without close demarcation between the two. Upon shelling from the cob, the pedicel with adhering lemma and palea usually remains attached to the kernel.
1.3. Seed germination and seedling development
When the maize kernel is placed under moisture and temperature conditions are favourable for germination, growth activity is quickly resumed by the embryo. After the soil has warmed up, full emergence may take place in about 5 days. Healthy, well preserved seed usually has high viability of 95-1 00%. The chief cause of loss in germination is injury from freezing (in temperate countries). This results from exposure to low temperatures while moisture content is still high. A second cause of reduced viability is infection with seed borne or soil borne organisms. Improper storage conditions which delay drying may cause damage with loss of viability. During germination the organs of the embryo which were formed during the development of the kernel and have remained dormant in the dry seed resume their growth and development. The primary root and its enclosing sheath, the coleorhiza elongates and breaks through the pericarp. The root soon breaks through the end of the coleorhiza. The plumule and its enclosing sheath the coleoptile, begins to elongate and also breaks through the pericarp of the kernel. At first the coleoptile grows faster than the plumule but when it reaches the surface of the soil and is thus exposed to the light it soon ceases to grow and the plumule breaks out through its tip. About the time the tip of the coleoptile reaches the soil surface thefirst crown roots appear immediately above the coleoptile node and later an additional whorl of roots forms at the base of each succeeding 6 to 10 internodes of the stern. These crownroots soon form the major part of the root system of the plant.
1.3. The root system
The root system of maize, as of other grasses consists of two sets of roots:
1.3 .1. Seminal roots
These consist of the radicle or primary root and a variable number of lateral roots which arise adventitiously at the base of the first internode of the stem. The radicle is always present except when killed by some injury, such as, freezing. It may be the only seminal root as is frequently the case in some flint varieties or there may be averages of one to seven lateral seminal roots in various dent maize varieties.
1.3.2. Adventitious roots
These constitute the principal part of the root system after the seedling stage. Any aerial brace roots are also included in this category. The first whorl of 4 or 5 crown roots appears at the base of the second internode about as soon as the tip of the coleoptite reaches the soil surface. A few of the succeeding higher internodes may have about the same number, after which the successive ones have more and larger roots up to a little above the soil surface. Those from about the lower five internodes like the seminal roots grow horizontally for some distance before turning downwards, while those from the higher internodes which appear later in the season grow downward at once. The functional crown roots average 85 per stalk with a total combined length of about 350 feet. These roots are all branched and rebranched so that the total length of the roots is about 6 miles per plant.
1.4. The stem
A typical maize stem consist of about 24 alternating nodes and internodes. About 8 internodes remain very short and under ground forming an inverted cone - shaped basal end of the stem known as the crown. The crown inter-nodes give rise to the adventitious crown-root system and adventitious brace roots may develop at the base of one to several aerial internodes. Under favorable growing conditions, the above ground internodes are distributed over a stemlength of about I 00 inches or more. With its greatest diameter of about 1 1/4 inches near theground level, the stem gradually tapers towards its tip. Each of the lower internodes adjacent to an ear shoot becomes permanently grooved throughout its length as a result of the pressure of the developing shoot during early growth. 1.5. Leaves of the seedling and mature plantUpon gern1ination the leaves already formed in the embryo resume growth and the formation of the leaf is resumed and continues until all the leaves have started and the growing point of the stem begins to form the tassel. The total number of leaves formed varies both within and between varieties. Some varieties have been found to average 17 leaves attached aboveground and 6 under ground. Not all these are functional at any one time, as some of the lower leaves are lost before the upper ones have expanded. The lower leaves tend to be tom loose and destroyed by formation of the crown roots and enlargement of the stem. Under favorable conditions the blades of the surviving leaves of full-grown stalks total about 1400 squareinches in area.The fully formed leaf consists mainly of the blade which is thin except for the midrib. Theblade tapers gradually towards the tip and slightly towards the base, until it abruptly narrowswhere it joins the sheath. Both blade and sheath have parallel veins which are united by cross connections at irregular intervals
1.1.6. Development and structure of the reproductive organs.
Maize being monoecious, bears staminate flowers in the tassels and pistillate flowers on the ear shoot. The main stem terminates in a staminate inflorescence or tassel, as do the basal branches or tillers when present. The branches arising from nodes above the soil surface tenninate in a pistillate int1orescence or ear, but usually all soon degenerate except the upper one or two located about midway on the stalk.
1.6.1. Shedding of pollen
At anthesis, just before pollen is shed, the lodicules swell to several times their former size and pry the lemma and palea apart, making it possible for the anthers to be extended by elongating filaments. Soon the anthers break open near the tip, forming pores through which the pollen escapes. Little pollen is shed until the anthers are shaken by the wind or otherwise disturbed. This tends to insure a high percentage of cross pollination under ordinary field conditions. In each plant the tassel usually sheds some of its pollen before the silks of its ears merge from the husks, but the tassel normally continues to shed for several days after the silks are ready to be pollinated. tassel begins to shed pollen a short distance below the tip of the central axis and pollen shedding progresses both upwards and downwards reaching the tip of the central axis long before it reaches the base. A tassel may shed pollen tor a week or more.
1.6.2. Amount of pollen produced
Pollen grains are produced in large numbers as is the rule with wind pollinated plants. A maize plant can produce about 25,000,000 pollen grains. Thus 25,000 pollen grains are produced for each kernel, on an ordinary ear with 1 000 kernels.
1.7. Reproduction and kernel development
When pollen is shed by the tassels, only that which is intercepted by fresh silks can germinate and where several pollen grains germinate on the silk usually only one functions in fertilisation. Most of the pollen is lost by falling to the ground or is caught by the leaves and accumulates in the leaf axis. In pollination, the pollen is usually caught by the hairs of the silk although it may function when caught on the body of the silk. The silk supplies the pollen with moisture which causes it to germinate, sending out a pollen tube through the germ pore.
1. 7 .1. Fertilisation
When the tip of the pollen tube reaches the micropyle it grows between the protruding cells of nucellus tissue until the embryo sac is reached. On entering the embryo sac the end of thepollen tube ruptures, setting free the two sperms. The nucleus of one sperm fuses with the egg nucleus, forming the zygote within which the chromatin from both sources completely intermingles. The other sperm nucleus fuses with one of the polar nuclear and this fused nucleus in turn fuses with the other polar nucleus, thus establishing the primary endosperm nucleus with 30 chromosomes. This double fertilisation explains how characters of the male parent may show up in the endosperm, as well as, in the embryo and the new plant within which it will develop.
1.8. Making hand pollinations
The com plant is monoecious, with the male flowers in the tassel and the female flowers on the ear shoots. When the male flower is mature the anthers are exserted from the spikelet, andpollen is dispersed through a pore at the tip of the anther. Anther exsertion begins on thecentral spike a short distance below the tip. Each spikelet has two flowers and the anthers areexserted from the upper flower first and then from the lower flower, either later the same dayor the following day.Pollen shed for a tassel may vary from only I or 2 days to more than a week depending upontemperature, humidity, air movement and genotype. Pollen is dispersed by wind currents, as aresult, extensive cross pollination and little self pollination occurs on an individual basis.Insects cause an insignificant amount of pollen dispersal but they can cause contamination incontrolled self or cross pollination. The amount of pollen dispersed from one tassel will varyamong genotypes. Hybrids normally shed more pollen for a longer period compared to inbredlines. The top ear shoot usually is at the sixth or seventh node below the tassel. There is axillary bud at each node below the top ear node, but elongation of the cob and silk emergence occurs at only the upper 2 or 3 nodes. The silks that emerge from the tip of the ear husk are the functional stigmas and there is one silk for each potential kemel. The first silks to emerge usually are from near the basal part of the ear. Complete emergence many occur in only 2 or 3 days under favorable growth conditions or may require 5-7 days with cool temperatures. The silks usually emerge at the top ear node I to 3 days after anther dehiscence has began. Tassel development seems to control development of the ear shoot. This dominance is greatest for genotypes that produce only one ear per plant. Prolific genotypes don't have dominance for the tassel and thus their silks frequently emerge before the tassel begins to shed pollen. Silks become receptive as soon as they emerge from the ear husk. The length of time for receptive silks on the ears is determined by the time required for all silks to emerge from the husk and time a silk remains receptive after emergence. Data indicate that it may require up to 5 to 6 days for all silks toemerge from the ear. Receptivity up to I 0 days after emergence has been reported. Selfing and crossing are essential procedures in breeding crop plants. It is important that the breeder masters these procedures/techniques in order that he may manipulate pollination according to his needs. The exact procedures that he may use to ensure self or crosspollination of specific plants will depend upon the particular species with which he is dealing with; structure of the flowers in the species and the normal manner of pollination. Thus he should know the flowering habit of the crop. Selfing or inbreeding of self-pollinated cropsoffers no particular problems to the breeder, as this follows the normal mode of pollination of these crops. This is the procedure used for crops like wheat, rice, barley, pulses, soybean andgroundnuts. However, the breeder should know something about the extent of natural crosspollinationwithin his breeding material. If it is slight, this natural crossing may be ignored in
this normal breeding procedure; but if natural cross pollination is excessive or if precise results are desired, it may be necessary to protect the flower by bagging to prevent foreign pollen from reaching the stigmas. In the selfing or inbreeding of cross-pollinated species it is essential that tl1e f1ower be bagged or otherwise protected to prevent natural cross pollination. In the cross-pollinated species of grasses which are normally pollinated by wind blown pollen, bagging the heads with parchment or glassine envelopes is a normal practice. Seed set is frequently reduced in the heads enclosed in bags because of excessive heat inside the bags. In maize a bag is placed over the tassel to collect pollen and the shoot is bagged to protect it from
foreign pollen. The pollen collected in the tassel bag is then transferred to the ear shoot.
1.9. Preparation of the female for pollination
The ear shoot (husk tip) must be covered by an ear shoot bag before the silks emerge from the husk tip. Ear shoots may be covered any time during the day, but it frequently is the first operation of the day. The length of ear shoot development from the leaf axil before silks beginto emerge will vary among genotypes and is affected by environmental conditions. The earshoot bag should be placed so that it is firmly nchored between the shoot and the auricle ofthe ear leaf. Sometimes it may be necessary to break off the ear leaf and pull the bag down sothat it is anchored between the shoot and the stalk. The best ear shoots to use in self or crosspollination exercise will have had their silks emerged under the shoot bag for 2 to 3 days. If a covered shoot has silks emerged more than 2 to 3 days before it can be used, it may be necessary to trim the silks to prevent them growing out of the bag and becoming contaminated. On the day preceding intended pollination, the extended/overgrown silks arecut to within 2 em of the husk tip. This procedure will cause the formation of a brush of silks on which the pollen can be spread the next day. If it is necessary to use the ear before all silkshave emerged, it may be convenient to cut the ear shoot to within 2 em of the cob tip, so thatall silks will be available the next day. The shoot bag must be replaced securely after preparation of the ear shoots.A tassel normally produces its greatest volume of pollen in the second and third days ofdehiscence. When the ear is prepared for pollination the tassel earmarked as the source ofpollen is covered by a tassel bag the same day. The tassel should have at least 1 day of antherexsertion and pollen dehiscence.
1.11. Seed production and maintenance - General principles to be borne in mind in the
process of seed production
1.11.1. Agronomic management
Husbandry practices applied in maize seed production fields are in general similar to those used in a commercial grain crop. However, there are some additional requirements peculiar to seed production, namely:Value of good seed is higher than that of grain- Therefore a seed crop warrants greater care and more inputs than a grain crop. It is important to realise the goal of obtaining the maximum number of high quality genetically pure seeds while minimising the risk. Realise that care taken in the selection and preparation of the seed field to obtain the most uniform growing environment possible will greatly facilitate identification of off-type plants in future rouging operations. Careful consideration must be given to the acreage planted, taking into account projected sales, labor availability during critical periods such as detasseling.
1.8 Varieties of Maize-Refer to class notes and add for ASALS
Katumani composite: The variety does well in both high potential and low potential areas. It matures between 3and 4 months eg KAT comp 1 & 2. It can be re-used as seed for up to three seasons, without affecting the yield cos its an OPV not hybrid so no Heterosis /hybrid vigour loss. Other ASALs hybrids that can produce about 12 bags an acre eg H511 and H512: These two varieties do well in high altitude areas with moderate rainfall. They mature between 4 and 5 months. They can produce between 16 and 18 bags an acre. Other varieties that do well in depressed rains are DH01, DH02, DH03, DH04, DH09DH10 and PH4-suitable for coastal areas.