In this essay we will discuss about Plants. After reading this essay you will learn about: 1. Subject-Matter on Plants 2. Seed Germination in Plants 3. Flowering Process.
Essay on Plants Contents:
- Subject-Matter on Plants
- Seed Germination in Plants
- Flowering Process in Plants
Contents
Essay on Plants
Essay # 1. Subject-Matter on Plants:
A plant experiences some complicated changes during the repeating cycle of development. This cycle generally starts with germination of seed and continues with the passage of a juvenile phase of growth and the graduation into maturity, followed by a progress into a state of senescence.
With maturity the organism is capable of shifting from vegetative to reproductive phase of growth, with initiation and development of flowers, fruits and again the production of a new generation of seeds. Emergence of radicle from seed coat is called germination. In case of seeds in which plumule first emerges, this event should also be considered as germination.
During germination of seeds, dormant embryo contained in it starts growing and developing into a new plant which resembles its parent. Some seeds start germinating as soon as water is available, while some other seeds require some periods of suspended growth even when favourable conditions are available. This period is called the dormancy period of seeds.
Different kinds of seeds remain viable for different durations of time. Many difficulties are encountered in keeping the seeds in viable condition for maximum length of time, as well as to enable the seeds to germinate when desired. Seeds possessing dormancy may be artificially made to germinate either by physical or chemical treatments.
The flowering process involves a complete change of the products of developing meristems, which instead of giving vegetative parts like leaves, internodes, etc. form the highly specialized structure of flowers and associated appendages.
This redirection of growth to the induction of reproductive state in leaves, the initiation of floral meristems, the morphological development of flowers, or anthesis itself, is due to some yet uncharacterized chemical substances of hormonal nature synthesized in the maturing leaves.
The triggering of this reproductive developments is usually controlled by different environmental variables such as temperature, light and photoperiods. These are variables which change with seasons, hence permitting a programming of the reproductive activities on a seasonal basis. Photoperiodic conditions are usually perceived by the leaves.
Photoperiodism is a response to the timing of light and darkness. In some species the appropriate photoperiodic schedule need only be given for a few days in order for flowering to occur, even though the plants are subsequently maintained under photoperiods unfavourable for flowering.
Such persistent after-effects of photoperiodic treatment are the results of photoperiodic induction. The modern concept of the mechanism of photoperiodic induction postulates that two different types of stimulatory factors or hormones take part in flowering process.
These two hormones together are called the florigen complex which consists of a gibberellin like hormone and the other an auxin-like, yet unidentified hormone, called anthesin. The photo stimulation requires the involvement of some pigment present in the leaves. This pigment is called phytochrome which can exist in two inter-convertible fed and far-red light absorbing forms.
It is conceived that light or red light (R-form) converts photochromic to another form (FR-form) and the latter when formed in suitable quantity induces certain dark reactions which result in the production of flowering hormone. The concentration of this hormone formed is dependent on the duration of both light and dark periods.
Thus during day far-red form is accumulated in the plant which is inhibitory to flowering in short day plants and stimulatory in long day plants. During dark period far-red form is slowly converted into red- form again which is stimulatory to flowering in short day plants but inhibitory to long day plants.
It is now known that this pigment is responsible for controlling other growth phenomena like germination, dormancy, etc. besides flowering.
According to the photoperiodic requirement plants can be classified into three major classes:
(i) The short day plants (which flower only when the day length is less than a certain critical duration, e.g., Chrysanthemum, Xanthium, Cosmos, tobacco, etc.),
(ii) The long day plants (which flower only when the photoperiod is higher than a critical duration, e.g., Hyoscyamus, wheat, potato, spinach, etc.) and
(iii) The indeterminate or day neutral plants (which have indefinite requirement: of photoperiod and flower in all the seasons, e.g., tomato, cotton, pea, etc.).
Like light, temperature also controls flowering in certain species especially in winter varieties. The pro-motive effect of low temperature on flowering is termed verbalization.
The low temperature effect may be obtained in some species when the moistened seeds are chilled or in others when the growing plants are chilled. Some species respond to chilling at either stage. The fact that high temperature treatment could nullify the verbalization effect establishes the devernalisation process, a physiologically significant event.
When vernalised grains are exposed to 35°C for one day the vernalisation effect is erased. It has been postulated that during vernalisation treatment certain chemicals of hormonal nature are produced in the meristematic cells which develop the capacity of differentiation of floral organs. This hypothetical hormone is called vemalin by certain authors which is perhaps closely related to gibberellins.
Essay # 2. Seed Germination in Plants:
Three important events must occur during germination of seeds:
(i) Imbibition and absorption of water,
(ii) Initiation of cell division in the meristematic region of embryo, and
(iii) Enlargement of embryonic cells.
(a) Methods of Studying Germination of Seeds:
Experiment:
There are several suitable methods for studying germination of seeds. Healthy, viable and non-dormant seeds of Oryza, Phaseolus, Vigna, cotton, gram and pea are surface sterilized with 1 % mercuric chloride for 1 to 2 minutes. The seeds were then washed well with water.
The seeds may be allowed to germinate on:
(i) Moist filter paper on a petridish,
(ii) On moist cotton,
(iii) On moist sand,
(iv) On commercial vermiculite (germination medium), or
(v) On net-wire kept on vessel containing water. These are kept in normal laboratory conditions.
Results:
Germination behaviour of different types of seeds on different media are observed and recorded. The number of seeds germinated per day may also be noted.
Discussion:
The above methods are generally used for germination experiments and in many physiological experiments with seedlings. Some methods are suitable for some experiment while others are not. For simple germination study, moist filter paper or cotton serves best.
The method is especially satisfactory when seeds are required in their earlier stages of germination (for enzyme study). For studying seedling growth moist sand or vermiculite media are suitable. To study the root growth of the seedling the net-wire may be conveniently used.
The seedling can be removed easily from such a germinator by carefully cutting the meshes of net. It should be remembered that all the media be kept moist at all times and that these be kept within a favourable range of temperatures.
N.B. (i) The rate of germination, if slow, can be hastened by artificial, physical and chemical treatments (variable for different seeds) like red light, dark, temperature (high or low), KNO3, thiourea, gibberellin, etc.
(ii) Germination capacity or germ inability of seeds can be calculated from the above experiment. It is the percentage of seeds capable of germination under favourable conditions. It is determined at a certain specified time after sowing, called the final or ultimate germination.
(iii) Germination speed, rate or energy:
This is calculated from the time taken by the seeds to germinate and can be expressed in the following ways:
(a) As the proportion of seeds germinated by certain time after sowing.
(b) As the time required to reach 50% of germination capacity or of final germination.
(c) As a special figure which takes into account the time taken by each seed to germinate, e.g., coefficient of velocity (Kozlowski, 1926) which expresses the mean germination of a sample by integrating the germination times of all individual seeds:
(b) Methods of Testing Viability of Seeds:
It is often of great importance to know the viability of different seed samples. The percentage of viability is usually determined directly by germination tests with small samples.
Many kinds of seeds are perfectly viable but may be very slow in germination or may be dormant. For such seeds an indirect and more rapid method of testing viability would be desirable. Many such methods have been used with varying degrees of success.
Some methods are summarized below:
(i) Respiration test:
Respiration is an indication of life but such tests are complicated by the fact that dormant or resting seeds show very low respiration and micro-organisms growing on seeds may show respiration when seeds themselves are all dead.
(ii) Electrical conductance method:
Some authors have observed that if seeds are soaked in distilled water and the electrical conductance of the bathing solution is then tested, the increase in conductance is roughly proportional to the percentage of dead seeds. This increase in conductivity is due to leaching of metabolites from dead seeds which become pervious owing to increasing permeability.
(iii) Potassium permanganate method:
Using weak solution of KMn04 it has been observed that the decolourisation of this solution is increased as the proportion of the dead seeds in the sample increases. It is a well-known fact that dead cells become freely permeable to their contained solutes which leach out into the bathing solution, whereas leaching from living cells is very much less.
This increased permeability on death undoubtedly accounts for the increased content of electrolytes (as in b) and reducing substances in the bathing solution resulting in decolourisation of KMnO4 solution.
(iv) Indigo-carmine method:
A still simpler method of testing of increased permeability of the dead seeds involves soaking for a few hours in certain aniline dyes, such as indigo carmine which will stain dead cells but not living ones. For applying this and similar tests it may be necessary to remove non-living seed coats which show low permeability.
(v) Embryo culture method:
A method that has proved successful in testing the viability of seeds having prolonged dormancy involves removal of the embryo from its cotyledon or endosperm and placing the naked embryo on granulated peat moss or on sterilized nutrient agar medium (White’s nutrient medium). By this method viability may be tested within seven to ten days. The nonviable seeds fail to germinate.
(vi) Tetrazolium test:
A common and generally adopted method of testing viability of seeds involves soaking of seeds in 1 to 2% tetrazolium chloride solution (2, 3, 5-triphenyltetrazolium chloride). The living and viable seeds take bright red colouration (more intense in embryo) while the nonviable seeds do not.
Tetrazolium chloride solution is colourless but forms carmine red formazan upon reduction.
The salt is thus an oxidation- reduction indicator and the development of the non-diffusible red colour in a specific tissue is presumably an indication of the presence of active respiratory processes in which hydrogen ions (due to dehydrogenase activity in respiration) are transferred to the tetrazolium chloride causing its reduction.
This process is of general value in physiological experiments as well as an index of germ-inability of seeds.
(c) Effect of Pre-soaking in Water on Germination of Seeds:
Experiment:
Perfectly viable seeds of (i) pea, (ii) bean and (iii) rice (125 each) are soaked in distilled water in three petridishes. A lot of 25 seeds from each petridish is taken out at intervals of 0, 8, 24, 48 and 72 hours and allowed to germinate in dry petridishes. The number of seeds germinated after three days in each case is recorded.
Results:
The number of seeds germinated after different periods of pre-soaking in water is tabulated in each case and percentage is calculated.
Discussion:
Absorption of water being the first prerequisite of germination, it is expected that pre-soaking of seeds in distilled water should accelerate germination.
Within limit, this is correct but the effects depend to a great extent on duration of pre-soaking, soaking condition, size of seeds and other properties of seeds. Excessive pre-soaking frequency results in decrease of germination percentage due to anaerobic condition of the cellular environment.
(d) Effect of Temperature on Germination of Seeds:
Experiment:
Perfectly viable healthy seeds of rice, wheat and cucumber are taken for this experiment. Twenty-five seeds of each type are sown in well moistened sand contained in three petridishes.
Petridishes are then placed at a temperature of 10°C. ± 2 (refrigerator), 25°C. ± 2 (at room) and 40°G. ± 2 (incubator). Petridishes are kept under dark condition. The percentage of germination is noted every alternate day for a period of 10 days.
Results:
The rate of germination for each type of seed at a particular temperature is graphically plotted.
Discussion:
The rate of germination for each type of seed is expected to be maximum at 40°C., optimum at 25°C and minimum at 10°G. In the range of temperature within which certain seeds germinate there is usually an optimal temperature below and above which germination is delayed but not prevented.
The optimal temperature may be taken to be that at which the highest percentage of germination is attained in the shortest time. There may be three temperatures where germination may be found to be minimum, optimum and maximum (Sachs 1860, introduced the concept of three cardinal points for these temperatures).
(e) Effect of Light on Germination of Seeds:
Experiment:
Perfectly viable healthy seeds of Ruellia tuberosa, Mirabilis jalapa and Datura stramonium are taken for this experiment. Twenty-five seeds of each are soaked in water for an hour and placed on moistened filter paper in petridishes.
Each type of seeds is placed in three petridishes and is kept in continuous light and continuous dark. The third serves as control (12 hours light and 12 hours dark). The rate of germination is observed from time to time for a period of one week.
Results:
Rate of germination of seeds under light, dark and control conditions is calculated and graphically plotted.
Discussion:
Light influences germination of many seeds.
Seeds may be divided into three categories, Viz.:
(i) Negatively photoblastic seeds which germinate only in dark,
(ii) Positively photoblastic seeds which germinate only in light, and
(iii) Photo insensitive seeds which are indifferent to the presence or absence of light in their germination responses.
In the present experiment Mirabilis jalapa falls to the category (i) because its germination is favoured by dark only, Ruellia tuberosa falls to the category (ii) because its germination is favoured by light only, while the germination of Datura stramonium is indifferent to either light or dark and belongs to the category (iii).
(f) Effect of Carbon Dioxide on Germination of Seeds:
Experiment:
Using the same type of apparatus and procedure employed in the effect of CO2 concentration on germination of suitable seeds is observed.
The rice seeds are first soaked in water before starting the experiment and then placed in petridishes under bell jars containing different concentration of CO2 (different concentrations are made by allowing CO2 to enter the bell jar for different durations).
One petridish containing such seeds is placed under bell jar containing normal CO2 which serves as control. The percentage of germination is observed every alternate day for a period of seven days.
Results:
The rate of germination in different concentrations of CO2 is noted and graphically plotted.
Discussion:
Most seeds fail to germinate if the CO2 tension is greatly increased. The response of germination of seeds towards CO2 concentration is variable from species to species. Concentration of CO2 in excess of 10% is in most cases inhibitory to germination, Inhibition of germination of seeds is due to the toxic effect of CO2 on respiration.
N.B. The effect of oxygen concentration may also be studied provided arrangement for O2 supply is available. The effect of O2 is usually pro-motive within a particular limit.
(g) To Study the Phenomenon of seed Dormancy:
Experiment:
Freshly harvested seeds of Xanthium, Nicotiana and Aman variety of rice (e.g., Nagra, Kalma, Kiunargore, etc.) are sun-dried for 2-3 days to bring down the moisture level to 11-12 percent. Fifty seeds of each are surface sterilized with mercuric chloride (1% for 1 minute) and allowed to germinate in petridishes containing moistened filter paper.
Petridishes are kept at laboratory temperature of 25-30°C. After seven days the percentage of germination of seeds is recorded in each case.
Results:
Number of seeds germinated in each case is determined and percentage of seeds germinated is calculated.
Discussion:
Many seeds do not germinate when placed under conditions which are normally favourable for germination (adequate water supply, suitable temperature, normal atmospheric composition). These seeds are visible as they can be induced to germinate by various treatments.
Such seeds are said to be dormant. Dormancy is due to various causes such as immaturity of embryo, impermeability of seed coat to water or gases, prevention of embryo development to mechanical causes, special requirements for temperature or light or presence of substances inhibiting germination.
In the present experiment the seeds are found to be very poor in germination (i.e., less than 20 per cent or so) though the external conditions have been favourable. Hence the seeds may be called dormant.
(h) Methods of Breaking Seed Dormancy:
The dormancy of seeds can be erased artificially by various special treatments which may be grouped into two broad categories:
(a) Physical treatment, and
(b) Chemical treatment.
Some treatments are found to be most suitable for some species for breaking dormancy while others are not. The effective dormancy breaking agent is to be found out by trials on seeds of a species having dormancy.
Most widely used treatments are enlisted below:
(I) Physical treatments:
(i) Heat treatment at 40°C to 45°C for different durations.
(ii) Low temperature treatment at 2°C to 8°C. (Seeds pre-soaked for 36 hours) for 12 to 24 hours.
(iii) Alternate heating and cooling for several times.
(iv) Alternate drying and wetting for several times.
(v) Soaked seeds (24 hours) exposed to red light for 1 to 2 hours at 15-25°C.
(vi) By dehiscing of seeds or removal of seed coat.
(II) Chemical treatments:
Inorganic chemicals:
(i) By acid treatment: Dilute solutions of HNO3, HCL or H2SO4 (0.l-0.5%) for different durations in minute.
(ii) By KNO3 (1.3%), NH4NO3(1.3%),H2O2,H3BO4, etc.
(iii) By gasses: By increasing oxygen concentration.
Organic chemicals:
(i) Non-hormonal:
Thiourea, KSCN, Ascorbic acid, etc. (10 to 100 ppm).
(ii) Hormonal:
Gibberellic acid (1 to 100 ppm), Kinetin (1 to 100 ppm). Ethylene (Ethrel solution of 100 to 300 ppm).
Experiment:
The efficiency of the above methods for breaking dormancy of seeds referred in Expt. 7 may be determined by trials.
Discussion:
There are three general types of internal control of dormancy.
(i) Limitations of permeability,
(ii) Limitations by growth substances including inhibitors and their counteracting of pro-motive substances and the activation or inactivation of promoters and inhibitors by temperature or light, and
(iii) Physical limitations on the enlargement of the embryo and its emergence.
It appears that most dormancy breaking treatments can be roughly accounted for on the basis of these three types of controls. For example, scarification treatment (physical treatment) alters the first and the third types of limitations; low temperature and high oxygen treatments facilitate the water and gas entry and affect the inhibitor content which may be present.
Inhibitor accumulation may be a consequence of high temperature experience. Gibberellins, cytokines and nitrates may also modify the effective inhibitor levels in various ways and other chemicals which break dormancy may act either on the effective inhibitor level or on water or gas entry step through physical alterations of seed coat and metabolic alterations of the tissues concerned.
Essay # 3. Flowering Process in Plants:
(a) Demonstration of Photoperiodic Response of Plants towards Flowering:
Experiment:
The following short-day, long-day and day-neutral plants are selected.
(i) Short-day plants (SDP):
Chrysanthemum or Xanthium or Aman variety of rice or Cosmos or Micotiana tabacum.
(ii) Long-day plants (LDP):
Spinach or wheat or potato or radish or winter barley or Hyoscyamus niger, or Nicotiana plumbaginifolia.
(iii) Day-neutral plants (DNP):
Tomato or maize or cucumber. The plants are grown in pots and when the seedlings are grown to a certain height these are given photoperiodic treatment, i.e., half of the plants consisting of all the three types is exposed to long-day (16 hours light and 8 hours dark), and the other half consisting of the same types is exposed to short-day (16 hours dark and 8 hours light).
Long-day treatment may be given by keeping the plants at a distance of 3 feet from a 200 W lamp. The position of the pots is to be changed frequently so that the plants get uniform light. The dark Treatment may be given by keeping the pots in dark room or keeping them under black tent in the field.
The experiment is continued till flowering is achieved in at least some of the plants and photoperiodic type of the plants is determined from such treatments.
Results:
It is observed that the long-day plants flower only where long-day exposure is given but remain in vegetative state in short-day treatment. The reverse result is obtained in case of short-day plants, i.e., plants flower only in short-day condition but not in long-day treatment. In case of day-neutral plants it is observed that they flower in both the conditions.
Discussion:
Short-day plants are those which flower only when exposed to day lengths of less than a certain critical duration (generally less than 12 hours). The long-day plants are those which flower only when exposed to photoperiods greater than a critical duration (generally greater than 12 hours).
It has been found that the flowering is not only dependent on definite light period but also on definite dark period. Consequently, the short-day plants may be considered as long-night plants. The dark period requirement is more important so far the delimitations of plants on the basis of flowering response are concerned.
(b) Demonstration of Photoperiodic Induction of Plants towards Flowering:
Experiment:
Several short-day and several long-day plants are selected and grown as in Expt. 9. A few short-day plants of uniform age of each type are given one photo-inductive cycle (16 hours dark + 8 hours light) and then transferred to long-days.
Again a few long-day plants are given one photo-inductive cycle (16 hours light 8 hours dark) and then transferred to short-days. Now the numbers of photo-inductive cycles is increased by one day up to ten days in case of other plants and are similarly transferred to their opposite photoperiodic conditions.
The flowering is observed in each case. If the plants do not flower even after giving 10 photo-inductive cycles the number of cycles may be increased.
Results:
From the experiment the minimum number of photo-inductive cycles required by each type of plant for flowering is determined.
Discussion:
The relative length and sequence of both light period and dark period influence the flowering of plants. There are two stages in flowering, first the differentiation of floral primordia in the meristematic apex and then the development of mature flowers.
Different stages of the reproductive growth may have their own specific requirements of photo- inductive cycles, i.e., light and dark periods in alternating sequence. Long- day plants naturally flower during spring and summer while the short-day plants flower during winter and autumn.
Plants can be made to flower by artificially giving any desired cycle of light and dark periods. The minimum requirement of this cycle varies from species to species. This is called photoperiodic induction.
(c) To Demonstrate that Leaf is the Locus of Stimulus of Flowering:
Experiment:
Suitable short-day plants of uniform age are selected for this experiment. One lot of four plants is completely defoliated. Leaves of another lot of four plants are defoliated leading only one fully mature leaf at the lower node of each plant. Other lot of four plants is kept as control. All the plants are now given appropriate short-days till flowering. (This experiment can be repeated with any suitable long-day plant.)
Results:
Flowering is noted in all the plants excepting the lot where all the leaves have been defoliated or removed.
Discussion:
For both long-day and short-day plants it is known that the stimulus- of photoperiod is received by the leaves and not by the growing points where the flower appear.
Plant whose leaves have all been removed fail to produce flowers even when given the appropriate conditions which induce flowering in intact plants or plants having at least one leaf.
It is now known that the inter-convertible pigment photochromic is involved in initiating floral stimulus which is mediated by some chemical substances of hormonal nature and this pigment is present in the leaves.
(d) Effect of Red and Far-Red Light on Flowering:
Experiment:
Short-day Xanthium plant is conveniently used for the experiment. Arrangements are made for red and far-red treatments of the plants in the following way. A white bulb (100 W) is covered with double layered red cellophane paper which acts as a red light source and a white bulb covered with blue and red cellophane papers serves as a far-red source.
A set of four plants is kept under red light, a second set under fair red light, a third set first under red light for 6 hours and then transferred to far-red light, and the fourth set is kept under far-red light for 6 hours (critical photoperiod for Xanthium is 8 5 hours) and then transferred to red light.
Results:
Plants under continuous red light do not flower but under continuous far-red light flower. Plant, treated with red light followed by far-red a\M flower while plants treated with far-red light followed by red light do not.
Discussion:
Study of the action spectrum of light flash given during dark period to inhibit flowering in Xanthium reveals that red light (640 to 680 nm, nm=mµ) is most effective in causing inhibition. It is also found that the effect of red light can be reversed or nullified by far-red light (710 to 740 nm).
If far-red flash is again followed by red flash flowering is again inhibited. The quality of the light experienced last by the plants determines the flowering response.
The fact that the red light effects on plants are so often reversible with far-red light and that the extracted pigment (photochromic) shows a spectral shift after exposure to red or far-red light, establishes that there are two forms of photo chrome—a red absorbing form and a far-red absorbing form.
Exposure to red light preferentially activates the red absorbing form and converts it into the far-red absorbing form. Exposure to far-red light has the reverse effect. The manner in which the two forms of this pigment regulate the physiological process of flowering in plants IS not fully known.
(e) Effect of Low Temperature (Vernalisation) on Flowering:
Experiment:
Some wheat seeds are sown in pots during autumn (October and November) and some seeds are sown during spring (March and April). A few seeds are allowed to germinate partially and these are then kept at cold temperature (4 to 5°C) for a period of 12 to 14 weeks during January to February. After this the partially germinated seeds arc sown into pots during spring at normal temperature.
Results:
The plants raised from seeds sown during autumn and raised from the low temperature-treated seeds sown in spring flower during the following summer whereas the plants grown from the un-vernalised seeds sown in spring do not flower in the following summer.
Discussion:
Many species are induced or promoted to flower by low temperature especially many biennial and perennial plants. Less common are species which are caused to flower by high temperatures.
The pro-motive effects of low temperature on flowering are termed vernalisation. By vernalisation treatment of plants or by normal fulfilment of the cold requirement of the plant, certain chemicals of hormonal nature are produced in the meristematic cells which develop the capacity of differentiation of floral organs.
Some authors have postulated that a hormone called vernalin is produced in meristems of shoot apex and embryos by vernklisation treatment. When this hormone is synthesized in suitable amounts, the shoot apex differentiates into floral organs instead of forming vegetative shoots.
(f) Effect of Gibberellin on Flowering of Long-Day Plants:
Experiment:
Moderately matured plants of Lactuca sativa and Micotiana plumbaginifolia are selected for this experiment 250 ml of 100 ppm aqueous solution of Gibberellic acid (GA3) is prepared.
The plants are either sprayed with GA3 solution with the help of holm sprayer or atomiser or GA3 may be applied to the leaves with the help of a brush for 7 days consecutively. A few plants of each are maintained as control. All the plants are kept at normal environment.
Results:
The time of flowering of all the treated and control plants if recorded.
Discussion:
Application of GA3 causes rosette plants to emerge out the inflorescence stalk (bolting) and then to flower.
The stimulation of flowering with GA3 can be assigned to two groups of plants; those that are caused to flower by low temperature and those which flower under long-day condition; In each case where GA3 is effective, the species is rosette type which generally bolts before flowering; species which are long-day plants and are not rosette generally do not respond to GA3.
N:B. Flowering may be controlled by regulating C/N ratio as follows. Very low carbohydrate, high nitrogen—no flowering and no fruiting; moderate carbohydrate, high nitrogen—no flowering, no fruiting; high carbohydrate and moderate nitrogen—abundant flowering and high fruiting; very high carbohydrate and low nitrogen—little or no flowering and fruiting.
The flowering of a few plants can be rather precisely initiated by treatments with ethylene, acetylene, NAA or 2, 4-D and with similar compounds.