The following points highlight the eight physiological effects of gibberellins. The effects are: 1. Seed Germination 2. Dormancy of Buds 3. Root Growth 4. Elongation of the Internodes 5. Bolting and Flowering 6. Parthenocarpy 7. Light Inhibited Stem Growth and 8. De novo Synthesis of the Enzyme-α-Amylase.
Physiological Effect # 1. Seed Germination:
Certain light sensitive seeds e.g., lettuce and tobacco show poor germination in dark. Germination starts vigorously if these seeds are exposed to light or red light. This requirement of light is overcome if the seeds are treated with gibberellic acid in dark.
Physiological Effect # 2. Dormancy of Buds:
In temperate regions the buds formed in autumn remain dormant until next spring due to severe colds. This dormancy of buds can be broken by gibberellin treatment. In potatoes also, there is a dormant period after harvest, but the application of gibberellin sprouts the eyes vigorously.
Physiological Effect # 3. Root Growth:
Gibberellins have little or no effect on root growth. At higher concentration in some plants, however, some inhibition of root growth may occur. The initiation of roots is markedly inhibited by gibberellins in isolated cuttings.
Physiological Effect # 4. Elongation of the Internodes:
Most pronounced effect of gibberellins on the plant growth is the elongation of the internodes, so much so that in many plants such as dwarf pea, dwarf maize etc., they overcome the genetic dwarfism. For instance, the light grown dwarf pea plants have short internodes and expanded leaves. But, when treated with gibberellin the internodes elongate markedly and they look like tall plants.
It is considered that in such dwarf plants (i) the gene for producing gibberellin is missing, or (ii) the concentration of the natural inhibitors is higher. When external gibberellin is applied the deficiency of the endogenous gibberellins is made good or the external gibberellin overcomes the effect of natural inhibitors which fall short.
Deepwater rice (Oryza sativa) is another notable example of pronounced effect of gibberellins on elongation of internodes so that its foliage may remain above water in the field. Striking growth rates of as much as 25 cms. per day have been observed in rice plants under field conditions.
(Partial submergence of rice plants is believed to reduce partial pressure of O2 which triggers ethylene biosynthesis in submerged tissues. Ethylene in turn reduces the level of ABA (abscisic acid) which acts as antagonist of GA. Submerged rice tissues thus become more responsive to endogenous GA resulting in marked elongation of internodes).
Physiological Effect # 5. Bolting and Flowering:
In many herbaceous plants the early period of growth shows rosette-habit with short stem and cauline leaves. Under short days the rosette habit is retained while under long days bolting occurs i.e., the stem elongates rapidly and is converted into floral axis bearing flower primordia. This bolting can also be induced in such plants e.g. Rudbeckia speciosa (It is a Long Day Plant) by the application of gibberellin even under non-inductive short days.
In Hyoscyamus niger (also a Long Day Plant) gibberellin treatment causes bolting and flowering under non-inductive short days. While in Long Day Plants the gibberellin treatment usually results in early flowering, its effects are quite variable in Short Day Plants. It may either have no effect, or inhibit, or may activate flowering.
Physiological Effect # 6. Parthenocarpy:
Germination of the pollen grains is stimulated by gibberellins, likewise the growth of the fruit and the formation of parthenocarpic fruits can be induced by gibberellin treatment. In many cases e.g., pome and stone fruits where auxins have failed to induced parthenocarpy the gibberellins have proven to be successful. Seedless and fleshy tomatoes and large sized grapes are produced by gibberellin treatment on commercial scale.
Physiological Effect # 7. Light Inhibited Stem Growth:
It is common observation that the dark grown plants become etiolated and have taller, thinner and pale stems while the light grown plants have shorter, thicker and green stems, and it may be concluded that light has inhibitory effect on stem elongation. Treatment of light grown plants with gibberellin also stimulates the stem growth and they appear to be dark brown. In such cases the protein content of the stem falls while soluble nitrogen content increases probably due to more breakdowns of proteins than their synthesis.
It is considered that the light in some way lowers the level of endogenous gibberellins and inhibits the stem growth.
Physiological Effect # 8. De novo Synthesis of the Enzyme-α-Amylase:
One of the important functions of gibberellins is to cause de novo (i.e., a new) synthesis of the enzyme a- amylase in the aleurone layer surrounding the endosperm of cereal grains during germination. This enzyme brings about hydrolysis of starch to form simple sugars which are then translocated to growing embryo to provide energy source. (Fig. 17.17)