In this article we will discuss about the growth regulators in flowering.
Phytohormones and Flower Initiation:
In recent years considerable information has accumulated to demonstrate that growth regulations change the flowering behaviour of several plant species. Further the effect of growth regulators varies with the plant species, its age, concentration of growth regulators employed as well as temperature, etc. The role of auxins, gibberellins, cytokinins, ethylene, abscisic acid on control of flower initiation is still not clear.
However, like many morphogenetic processes, flower initiation involves interactions between hormones leading to the synthesis of some flowering stimulating compounds. There is ample evidence to believe that photo-inductive conditions for flowering are concerned with the regulation of the levels of the endogenous hormones which in specific concentration cause flower initiation. In the following a brief account of the role of different growth regulators in flowering is given.
Auxins:
In short day plants (SD) like Kalanchoe, Xanthium, Chenopodiumauxin inhibits flower initiation and the effect is pronounced when it is applied during the dark period. If auxin is applied after the photoinductive cycle, it increases floral differentiation. However, in Long day plants (LD) like Calendula, Namesia, etc. exogenous application of auxin inhibits flowering.
But the flowering was promoted in several other LD plant like Silene, when the auxin treatment was given under low-light intensity. Further, it is also reported that auxin-induced effect could be replaced by increasing the number of photoinductive cycles. The application of antiauxin also counteracted the auxin effect.
By far majority of the experiments have been conducted on the auxin-induced inhibitory effects of SD plants. The general assumption is that auxin(s) interferes with the photoperiodic reactions which occur in the leaves and hinder the synthesis of florigen. Most recent studies have indicated the role of auxin during later stages of flower induction in shoot apex.
One of the suggested possibilities is that auxin is metabolized in the new biosynthetic pathways at the apex. That these biosynthetic pathways are triggered by photoperiodism. Promotion of flowering by the auxin has been reported in Citrus, Litchi, Ananas, etc.
Similarly when the seeds of different cereals are soaked in auxin flower initiation is induced. Auxin treatment also increases vernalizing effects in some plant species.
Gibberellins:
Gibberellins have been shown to induce flowering in several plant species. They have been demonstrated to cause flower initiation in LD plants like Beta, Brassica, etc. Different species respond differently to different gibberellins. In some LD plants the increased flowering may be associated with the elongation of internodes caused by GA.
In L—S—D plants glibberellins can substitute for LD. On the contrary SD plants fail to respond to GA. GA3 has been shown to substitute for cold treatment in some Chrysanthemum species. K.K. Nanda and his students have shown in Impatiens, which is an obligate SD plant, that GA3 could completely substitute for the dark period. Similarly promotion of flowering has been induced by GA in Kalanchoe.
From studies conducted on several species, it has been inferred that GA affects the expression of flower stimulus at the shoot apex rather than leaves. For instance, it increases the mitotic activity in the subapical meristem and thus becomes more responsible to photo-inductive conditions.
Cytokinins:
In Chrysanthemum sp. flowering is accelerated by the simultaneous application of benzyladenine and GA5. The former is also able to overcome the inhibitory effect of auxin in flower initiation. Cytokinins have been shown to promote flowering in Wolffia. There is a view thatcytokinins redirect the flow of assimilates essential for the formation of flowers to the shoot species.
Ethylene:
This hormone has been shown to inhibit flowering when applied in inductive short periods. The effect was more pronounced when given to the cotyledons. The effect of ethylene is confined to the leaves. Ethylene could induce flowering in several cereals. Similarly, the effect of ethylene in stimulating flowering in pineapple and some mango varieties has also been demonstrated.
In recent years, H. Y. Mohan Ram and his students at Delhi University have conducted extensive investigations on the hormonal control of sex expression in Cannabis. For instance, the application of gibberellins could induce male flower formation in genetically female plants of Cannabis sativa whereas treatment of male plants with ethephon resulted in the initiation of female flowers.
Later detailed histological studies in the treated plants showed that feminization of the flowers occurred through the production of a transient intersexual condition. It was further demonstrated that the original sex manifested itself when the effects of ethephon were worn out. The application of ABA also favoured femaleness in Cannabis.
The exogenous application of other major growth substances like morphactins, MH and TIBA have been shown to influence the development of sex organs in monoecious, dioecious and hermaphrodite species.
Mohan Ram and his students have shown that apical applications of cobalt chloride to female plants of Cannabis sativa caused drying of shoot tip and formation of axillary branches. The latter bore male and intersexual flowers. Pollen in the changed sex flowers was viable. Then applications of Ag+ induced male flower formation in the female plants.
In general it is assumed that endogenous level or balance of growth hormones plays an important part in sex differentiation. In addition studies are available where comparisons’ in hormone content have been made between plants with genotypically determined sex differences in sex expression; between plants of the same genotype exposed to treatments involving differences in sex expression and within monoecious individuals bearing flowers of either sex in pre-determined location.
In general, it is observed that female flowers and female plants have higher endogenous levels of auxins and ethylene in male flowers and male plants have a higher gibberellin content. The general conclusion is that it is the balance between growth hormones which determines the nature of the sex expression.
Atsmon and his co-workers have shown in cucumber plants that exogenous application of growth regulators affected sex expression. For instance, GA enhanced the male tendency, auxin, ethylene and different growth retardants had the reverse effects. On the contrary, ABA showed opposing effects depending on the treated sex type.
Most of physiological studies on sex expression in cucumbers described the hormonal effects in terms of changes in sex tendency for the whole plant. Further, exogenous application of GA caused a shift in the sexual pattern of monoecious cucumber plants.
In cucumber, it is also observed that shoot tips of monoecious plants had a higher content that those of gynoecious ones and SD conditions caused higher ABA content that LD conditions.
Based on extensive studies in this plant Atsmon and his associates have concluded that the balance of growth substances in the vicinity of the differentiating bud was closely correlated with sexual differentiation.
Young leaves had a highauxin content and removal of young leaves from the stem tip of a monoecious cucumber increased the male tendency. These authors proposed a scheme regarding nodal pattern of sex differentiation based on the balance of ethylene, auxin, GA and ABA.