An Assignment Presentation On Physiology of fruit set, fruit growth, fruit drop, fruit ripening & Parthenocarpy in vegetable crops Course – VSC 504 GROWTH AND DEVELOPMENT OF VEGETABLE CROPS (2+1) Submitted To, Prof. J. R. Vadodaria College of Horticulture, S. D. A. U., S. K. Nagar Submitted By, SUGANI DEVI Reg.No.-04-00772-2010 M.Sc.(Hort.) 2nd sem.
Physiology of fruit set
After pollination, the development of fruits begins which is followed by wilting or abscission of petals, stamens etc. These transitional changes from flower into young fruit are called fruit set. The various stages of fruit set are as follows: 1. Flower fading: After pollination, most flowers close, their corolla or petals rapidly fall and collapse. Such a collapse of different floral parts is sometimes associated with rapid increase in the rate of respiration and ethylene production. 2. Receptivity: The capacity of flower to set a fruit often depends upon the receptivity of the female parts to the pollen. This receptive period may last long for only a few hours or for a week (as in tomato).
In Brussels sprout, it has been found that the receptivity for self pollination is a best on the day a flower opens whereas for cross pollination, it remains best for five days before bud opening to five days after blooming. 3. Growth relations: As soon as successful pollination occurs, the growth in ovary shows abrupt changes and thus pollen often acts as a catalyst for ovary growth at this stage. In cucumber such increase in ovary growth alter pollination is very evident. In several cases, the extent of growth has been found related to the number of pollen grains. The growth shows considerable increase with the use of a large number of pollen grains during pollination.
As such pollen is a rich source of auxins. Now it is a well documented fact. Such auxin plays an important role in fruit set. Growth of ovaries in many species can be induced with auxin treatments and without pollination, particularly in solanaceous and cucurbitaceous fruits. Auxin stimulus for fruit set comes not only from pollens but also from ovaries. In many fruits, pollination stimulates auxin synthesis in ovary. The synthesis of auxin in ovaries increases with the increase in pollen tubes down the style. The effect of auxin in causing fruit set has been found in tomato, egg plant, tobacco, okra and several forms of cucumbers. But, such effects are not found in all cases of plants.
4. Parthenocarpy: Parthenocarpy means formation of fruits without seed (i.e. seedless fruits). Such fruits are of no advantage to plants but may be of great interest and commercial use for human beings. In nature, Parthenocarpy is commonly found in banana, pineapples, melons and figs. Parthenocarpy may occur as – a. Fruit development without pollination as in tomatoes, pumpkins, cucumbers, bananas, pineapples etc. Fruit development due to stimulus received by pollination followed by unsuccessful fertilization. Abortion of embryos before attaining maturity of fruits. Induction of Parthenocarpy has been found by the use of auxin and gibberellins and in some cases by cytokinins and some other substances.
5. Limitation of fruit set: Fruit set affects the productivity of fruit crops. There are three main causes of limitation of fruit set- It may be due to limited pollination. It may be due to limited nutrients. It may be due to abscission of flowers and young fruits. Some plants set only one or few fruits out of several flowers in the inflorescence. However, at present, very little is known about its physiological control mechanisms.
PHYSIOLOGY OF FRUIT GROWTH
The covering surrounding the ovule in angiosperm develops fruit. The nature of covering may be: Only the ovary tissues as in tomato and stone fruits, Ovary plus receptacle tissues as in apple, and The combination of several gynoecia into a single fruit The combination of numerous flowers into a common fruit as in pineapple. It is believed that angiosperms have some advance physiological capabilities which played a major role in the evolution of fruit or angiosperms. They are uniquely capable of mobilizing nutrients into fruits and bringing out changes in quality which makes them effective in the dispersal of seeds through their appeal to various animals.
Some aspects of fruit growth may be considered as follows: 1. Fruit growth rates: The growth of fruit has two important steps, the growth of pericarp, and (ii) the growth of embryo and endosperm. Pericarp, in general, consists of epicarp, mesocarp and endocarp. Each of these components may have distinctive periods of rapid and slow growth. Several fruits have simple sigmoid type of growth pattern, e.g. pea, tomato etc. But some fruits have two periods of growth with a period of slow or suspended growth in between. These show double growth curve as in most stone fruits.
2. Mobilization: Large sized fruits have to import enough amount of food material from other parts of plants. The mechanism by which mobilization of food material occur is poorly understood. However, the major portion comes from nearby photosynthetic leaves. In some cases as in small grains, about 10 per cent of food material is derived from photosynthetic awns and 30 per cent from flower spikes. 3. Fruit size: In several cases,the size of the fruit is found connected with the cell size. So fruit growth may be an expression of a wide variety of events, from development of air spaces to the loading of sugars into the fruit.
4. Role of seeds: It has been observed that seeds regulate many aspects of fruit growth, besides there are seedless fruits which show growth in a manner very similar to seeded fruits. However, there are three types of evidences supporting the regulatory role of seeds in the development of fruits: The removal of the fertilised ovules frequently terminates the growth of the fruits. The geometry of fruits reflects the distribution of seeds in it. The seed containing regions of fruits are usually distinct from parts without seeds. In many cases, the size of fruits has been found strongly correlated with the number of seeds. The distribution of seeds often affects the shape of fruits. Many parthenocarpic fruits are of different shapes than seeded fruits of the same variety. In general, seeds provide strong influences on the growth of fleshy fruits. The influence may be growth promotive or inhibitory
5. Growth substances: Nitsch (1950) suggested that endogenous auxins play an important role in the fruit growth which he confirmed by analysing the auxin contents of growing fruits in strawberry. In some fruits, different types of auxins have been found regulating different growth patterns. There are several evidences and contradictions regarding the universal nature of growth regulating substances in fruits. Seeds of many fruits are a good source of gibberellins, or cytokinins or both. Seeds of tomato contain ferulic acid which causes dormancy in them. Ethylene has also been reported as a growth regulating substance in some fruits.
PHYSIOLOGY OF FRUIT DROP
Fruit drop is an important problem of horticulturists, related with the abscission of both ripe and unripe fruits. In many fruits, abscission occurs shortly after the pollination and fruit set. Fruits resulting from self-pollination are more susceptible to fruit drop than those developed as a result of out cross (cross pollination). Auxin has been suggested associated with abscission of fruits. When auxin content is low, the fruit drop is common. It has been suggested that the auxins that control fruit growth and fruit drop may be of different types. However, the natural drop of fruits relates at least in part to a low endogenous auxin content in fruit.
Two other growth regulators, abscissic acid and ethylene are certainly involved in fruit drop. Both of these increase quantitatively during the period of dehiscence. The use of compounds which increase the ethylene levels in fruits are now successfully used in mechanical harvesting of horticultural crops.
PHYSIOLOGY OF RIPENING
After the period of growth, fruit undergoes some characteristic qualitative changes. These changes are collectively referred to as fruit ripening. Some important events in fruit ripening are as follows: 1. Changes with ripening: The general changes that occur during the process of ripening of fruits are, (i) softening of fruit flesh, (ii) hydrolytic conversion of storage material in the fruit, and (iii) changes in pigments and flavours. SOFTENING is an important change with the ripening of fruits. The major role played in this process is that of cell wall degrading enzymes. But in suashes and avocado, the softening is associated with hydrolysis of cell contents. As such pectolytic enzyme activities induces solubilisation of pectic substances found in middle lamellae. The solubilisation may occur through an increase in methylation of the galacturonic acid or through reduction in the size of a chain of polygalacturonide or both.
HYDROLYTIC CHANGES in the fruit during ripening usually lead to the formation of sugars. Such changes show different rates in different fruits. During ripening of fruits some qualitative change occur such as change in pigmentation, production of flavour material and depletion of astringent substances. The changes in pigments in fruits are normally the loss of chlorophyll and development of carotenoids. These changes in colour may be due to loss of chlorophyll with little or no formation of carotenoids or due to formation of carotenoids.
2. The respiratory climacteric: Kidd and west (1930) observed that in apple fruits, a major change occurs in respiration rates during their ripening. They found lowering of respiration rate in maturing fruits followed by a large increase in respiration during ripening. And after reaching a climacteric peak, the rate of respiration again falls. The period of occurance of climacteric peak in fruits shows variation in different fruits, e.g. just before the fruit is fully ripe as in tomatoes. As regards occurrence of climacteric, fruits may be divided into two types: Climacteric fruits, e.g. tomatoes, guava papaya etc. Non-climacteric fruits, e.g., pepper, cashew grapes.
In non-climacteric fruits, the rate of respiration remains steady during their ripening. Climacteric rise has been found affected by low oxygen and increased concentration of carbon dioxide. Both these factors prevent climacteric rise and thus improve storage quality of fruits. Storage of fruits in polythene bags procure nearly the same effect because plastic can lower oxygen and elevate carbon dioxide around fruits. It has been found that the ability of ethylene to induce climacteric rise is found in both climacteric and non-climacteric fruits.
3.Ripening mechanism: The mechanism of fruit ripening mainly comprises respiratory source of energy, synthesis of new sets of enzymes and action of these enzymes inducing softening, pigmentation, and qualitative changes. A modern concept of ripening of fruits is based on the synthesis of new enzymes required for the ripening process. It was found that with the blocade of production of energy with dinitrophenol, arsenite, of fluoroacetate, the ripening process is terminated and also that ripening can proceed even though the climacteric has been remarkably suppressed. Based on these findings it seems reasonable to believe that respiration provide energy for the synthesis of new enzymes and for the action of these enzymes in bringing about the ripening oranges.
4. Hormonal controls of ripening: Ethylene has been established as a ripening hormone. Massive doses of ethylene can bring about ripening changes in immature fruits. So, it is the hormone which has the most powerful regulatory role in the ripening. In general, ethylene may be bringing about the formation of new types of enzymes in fruits. Hormonal regulation may be involved in the change from a mature fruit resistant to a ripening to one which becomes receptive to ripening signals.
In botany and horticulture, parthenocarpy is the natural or artificially induced production of fruit without fertilization of ovules. The fruit is therefore seedless. Stenospermocarpy may also produce apparently seedless fruit, but the seeds are actually aborted while still small. Parthenocarpy (or stenospermocarpy) occasionally occurs as a mutation in nature, but if it affects every flower, then the plant can no longer sexually reproduce but might be able to propagate by vegetative means. However, parthenocarpy of some fruits on a plant may be of value. Up to 20% of the fruits of wild parsnip are parthenocarpic. The seedless wild parsnip fruit are preferred by certain herbivores. Being able to produce seedless fruit when pollination is unsuccessful may be an advantage to a plant because it provides food for the plant's seed dispersers.
In some plants, pollination or other stimulation is required for parthenocarpy. This is termed stimulativeparthenocarpy. Plants that do not require pollination or other stimulation to produce parthenocarpic fruit have vegetative parthenocarpy. Cucumber is an example of vegetative parthenocarpy, seedless watermelon is an example of stenospermocarpy. Plants moved from one area of the world to an other may not always be accompanied by their pollinating partner and the lack of pollinators has spurred human cultivation of parthenocarpic varieties. Some parthenocarpic varieties have been developed as genetically modified organisms.
Commercial importance Seedlessness is a very desirable trait in edible fruit with hard seeds such as pineapple,banana, orange and grapefruit. Parthenocarpy is also desirable in fruit crops that may be difficult to pollinate or fertilize, such as tomato and summer squash. In dioecious species, such as persimmon, parthenocarpy increases fruit production because staminate trees do not need to be planted to provide pollen. Parthenocarpy is undesirable in nut crops, such as pistachio, where the seed is the edible part. Horticulturists have selected and propagated parthenocarpiccultivars of many plants, including breadfruit and eggplant. Some cucumbers produce seedless fruit if pollinators are excluded.
Strange as it seems, seedless watermelon plants are grown from seeds. The seeds are produced by crossing a diploid parent with a tetraploid parent to produce triploid seeds. When sprayed on flowers, any of the plant hormonesgibberellin, auxin and cytokinin can often stimulate the development of parthenocarpic fruit. This is termed artificial parthenocarpy. Plant hormones are seldom used commercially to produce parthenocarpic fruit. Home gardeners sometimes spray their tomatoes with an auxin to assure fruit production. Some parthenocarpic cultivars are of ancient origin. The oldest known cultivated plant is a parthenocarpic fig first grown at least 11,200 years ago.