Growth and Development of Horticultural Crops (1+1)

Physiological effects of gibberellins

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.
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.
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.
4. Elongation of the Internodes

• Most pronounced effect of gibberellins on the plant growth is the elongation of the internodes, so in 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.


http://image.wistatutor.com/content/plant-growth-movements/gibberellins-dwarf-pea-assay-principle.jpeg

• It is considered that in such dwarf plants
  
(i) the gene for producing gibberellins is missing, or
(ii) the concentration of the natural inhibitors is higher.

• When external gibberellins are applied, the deficiency of the endogenous gibberellins is made good or the external gibberellins overcome the effect of natural inhibitors which fall short.
5. Bolting and Flowering

• In many herbaceous plants the early period of growth show 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 plant e.g. Rudbeckia speciosa (It is a Long Day Plant*) by the application of gibberellins even under non-inductive short days.
• In Hyoscyamus niger (also a Long Day Plant) gibberellins treatment causes bolting and flowering under non-inductive short days. While in Long Day Plants the gibberellins 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.



http://image.wistatutor.com/content/plant-growth-movements/bolting-rosette.jpeg




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 gibberellins 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 gibberellins treatment on commercial scale.
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 gibberellins also stimulates the stem growth and they appear to be dark grown.
• 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.
8. De nova Synthesis of the Enzyme-α-Amylase

• One of the important functions of gibberellins is to cause de novo synthesis of the enzyme α-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.