Response of Induced Dwarf Mutants of Barley to Exogenous Application of
Gibberellic Acid

# Bandarupalli Ramesh and Bateshwar Kumar

Department of Genetics & Plant Breeding (formerly Agricultural Botany),

Ch. Charan Singh University, Meerut - 250 004 (Uttar Pradesh), India.

e-mail: bandarupalli_ramesh@yahoo.com

# Abstract

Response of five induced plant height mutants (dwarfs and semi-dwarfs)
of barley to gibberellic acid (GA~3~) treatment was studied. The
gibberellic acid response index (GRI) was calculated and based on this
value, the mutants have been categorized as sensitive (high or moderate)
or insensitive. The GRI value was highest for dwarf mutant followed by
early maturing mutant with the former being highly sensitive to GA~3~
treatment while the latter was moderately sensitive. On the other hand,
semi-dwarf and chlorina mutants and the control were insensitive to
exogenous application of GA~3~.

**Key words**: GA~3~ treatment, reduced height mutants, barley,
gibberellic acid response index (GRI).

## Introduction

Gibberellins stimulate extensive growth in intact plants. These enable
to overcome genetic dwarfness in some species if that dwarfness is
because of a gene mutation, resulting into blocked gibberellin
production. Lack of gibberellins causes shortening of internodes and
reduces the height of the plant. Exogenous application of gibberellic
acid (GA~3~) fulfils its requirement and the plant achieves normal
height. Gibberellins have little effect when they are applied to plants
of normal height. The most outstanding effect of gibberellins on plants
is the abnormal stem elongation. This is brought about primarily by
increased cell elongation, and secondarily by accelerated cell division.
Under its influence generally dwarf varieties have been changed in to
tall varieties in some species. The small size of genetically dwarf
plants is due to some natural growth inhibitors. The rapid elongation of
the stem that follows the application of gibberellins is attributed to
the neutralization of such inhibitors by gibberellin.

The development of semi-dwarf cultivars has occurred with little
understanding as to the effects of the plant height genes on plant
traits other than straw length. In wheat, traits pleiotropic to or
closely associated with, *Rht1* and *Rht2* genes include gibberellic
acid insensitivity, cell size and cell number, root weight, coleoptile
length, leaf size, grain yield, yield components, biomass, harvest
index, protein content, and disease reaction (Gale and Youssefian 1985).
In any mutation breeding programme, reduced height mutants are one of
the most frequently occurring class of mutations. However, there are not
many studies dealing with the response of dwarf mutants to GA~3~
treatments. Hence, the present report on the response of induced dwarf
mutants of barley to exogenous application of gibberellic acid.

## 

## Materials and Methods

A laboratory experiment involving five induced dwarf mutants of barley
along with parental control was conducted at the department of
Agricultural Botany, Ch. Charan Singh University, Meerut during 2002-03.
The M~6~ seeds of the five true breeding barley mutants viz. dwarf,
semi-dwarf, early maturing, semi-dwarf with early maturity, and chlorina
types (isolated from gamma irradiated populations of barley cultivar 'K
169') were used. Response of reduced height mutants to GA~3~ was
estimated following Yamada (1990). For the GA~3~ response test, seeds
were germinated in petri-dishes for 2 days at 20^0^C, then transferred
to seed trays containing a mixture of vermiculite, soil and sand in the
ratio of 2:2:1 by volume. The seedlings were grown in trays in a growth
chamber under controlled light and temperature conditions. After 5 days
of growth at 20/15 ^0^C (day/night) for 16/8 hours, the seedlings were
supplied with 10 ppm GA~3~ through irrigation water every other day.
Seedling length and leaf sheath measurements were recorded on 20 days
old seedlings.

## Results and Discussion

The data recorded on seedling length and the length of first leaf sheath
of 20 days old seedlings of both GA~3~ treated and untreated mutants and
control are presented in table 1. The gibberellic acid response index
(GRI) was calculated and based on this value, the mutants have been
categorized as sensitive (high or moderate) or insensitive. The GRI
value was highest for dwarf mutant (141.81) followed by early maturing
mutant (113.18). The dwarf mutant was highly sensitive to GA~3~
treatment while the early maturing mutant was moderately sensitive. A
moderate response was also recorded for the semi-dwarf mutant with early
maturity while the other two mutants of the present study and the
parental control were insensitive to GA~3~ treatment (Table 1).

**Table 1**. Seedling and leaf sheath measurements (mean S E), GA~3~
response index (GRI), and GA reaction in gibberellic acid treated
seedlings of reduced height mutants in barley.

  ---------------- --------- --------- ------------ ------------- ---------- --------- ------------
  Control/Mutant   Length of           GRI (%)      Reaction to   Seedling             GRI (%)
                   first                            GA~3~         length in            
                   leaf                                                                
                   sheath in                                                           

                   GA~3~     Un-                                  GA~3~      Un-       
                   treated   treated                              treated    treated   

  'K 169' control  18.25 ±   17.85 ±   102.34       Insensitive   23.16 ±    22.45 ±   103.16
                   0.34      0.21                                 0.32       0.34      

  Dwarf            11.70 ±   8.25 ±    141.81\*\*   Highly        13.66 ±    10.32 ±   132.36\*\*
                   0.66      0.21                   sensitive     0.16       0.74      

  Semi-dwarf       13.33 ±   13.10 ±   101.75       Insensitive   18.20 ±    17.84 ±   102.01
                   0.67      0.48                                 0.46       0.58      

  Semi-dwarf w/    13.05 ±   12.37 ±   105.49       Mildly        17.42 ±    16.32 ±   106.74
  early maturity   0.48      0.50                   sensitive     0.36       0.51      

  Early maturing   15.28 ±   13.50 ±   113.18       Sensitive     20.50 ±    18.65 ±   109.91
                   0.48      0.45                                 0.77       0.46      

  Chlorina         12.00 ±   11.65 ±   103.00       Insensitive   16.12 ±    16.12 ±   105.91
                   0.28      0.29                                 0.36       0.32      
  ---------------- --------- --------- ------------ ------------- ---------- --------- ------------

In wheat, out of 133 Norin varieties tested by Yamada (1991), 103 were
GA-insensitive and 30 GA-responsive with all the 6 breeding lines being
GA-insensitive. On the other hand, out of the 16 landraces tested, 10
were GA-insensitive and 6 GA-responsive. One of the possible mechanisms
of GA~3~ action appears to be the promotion of changes in RNA synthesis
during cell elongation. Apparently, GA~3~ action in all target tissues
may be via its effect on the transcriptional process. GA~3~ may be
enhancing the rate of synthesis of all classes of RNA, or it may be
inducing some of the specific enzymes as in cereal aleurone.

The many effects of gibberellins suggest that they have more than one
important primary site of action. Thus far, research with hormone
receptors neither verifies nor denies that idea. Even a single effect
such as enhanced stem elongation on whole plants results from at least
three contributing events -- (i) cell division is stimulated in the
shoot apex, (ii) gibberellins sometime promote cell growth because they
increase hydrolysis of starch, fructans, and sucrose into glucose and
fructose molecules, and (iii) gibberellins sometime increase wall
plasticity (Salisbury and Ross 2001).

The positive association between GA-insensitivity and height reduction
has been shown in wheat for both *Gai~1~* with *Rht~1~* and *Gai~2~*
with *Rht~2~* in crosses involving Norin 10 derivative semi-dwarfs (Gale
and Marshall 1973, Hu 1974) and for *Gai~3~* with *Rht~3\ ~*in 'Tom
Thumb' type dwarf (Gale et al. 1975, Fick and Qualset 1975). Whether the
nature of this association is pleiotropic, i.e. the *Gai* and *Rht*
genes are, in fact, the same, or due to linked loci, as suggested by
Konzak et al. (1973), it is clear that the chromosomal location of
*Gai~1,~ Gai~2,~*and *Gai~3~* are the same as those of *Rht~1,~
Rht~2\ ~*and *Rht~3~*. The exact nature of the relationship between
gibberellin insensitivity and height reduction is of practical
importance to wheat and barley breeders who are using different sources
of dwarfism.

## References

Fick, G.N. and C.O. Qualset, 1975: Genetic control of endosperm amylase
activity and gibberellic acid responses in standard height and short
strawed wheats. Proc. National Acad. Sci. U S A 72, 892-895.

Gale, M.D. and G. A. Marshall, 1973: Dwarf wheats and gibberellins.
Pages 513-519 in Proc. 4^th^ Wheat Genetics Symp.

Gale, M.D., C.N. Law, G.A. Marshall and A.J. Worland, 1975: The genetic
control of gibberellic acid insensitivity and coleoptile length in a
dwarf wheat. Heredity 34, 393-399.

Hu, M.I., 1974: Genetic analysis of semi-dwarfing and insensitivity to
gibberellin GA~3~, in hexaploid wheat. Ph D thesis, Washington State
University, USA.

Konzak, C.F., Sadam Mir and E. Donaldson, 1973: Inheritance and linkage
in durum wheats of semi-dwarfing genes with low response to gibberellin
GA~3~. Pages 29-40 in Proc. Symp. 'Genetics and Breeding of Durum
Wheats' (Bari, 1973)

Morris, R., J.W. Schmidt and V.A. Johnson, 1972: Chromosomal location of
a dwarfing gene in Tom Thumb wheat derivative by monosomic analysis.
Crop Sci. 12, 247-249.

Salisbury, F.B. and C.W. Ross, 2001: Plant Physiology. CBS Publishers &
Distributors, New Delhi (India).

Yamada, T., 1990: Classification of GA response, *Rht* genes and culm
length in Japanese varieties and land races of wheat. Euphytica 50,
221-239.