Introduction

ALIGARH MUSLIM UNIVERSITY

 CHAPTER!
INTRODUCTION
Brassica juncea (L.) Czem & Coss is the member of the family Brassicaceae. It is
chiefly grown in China, India, Pakistan, Canada, France, Poland and Germany. It is an
annual herb attaining the height up to 1 m. The branches are long, erect or patent.
Lower leaves are pinnately divided, stalked, 10-20 cm long, toothed. Upper leaves are
alternate, stalkless or nearly so, not clasping, smaller than lower leaves, lance-shaped
to linear, mostly non-toothed. Flowers are in branched clusters, yellow in color. Fruit
is a narrow pod, round with a beak and net-veined. It is an important oilseed crop
known for its oil content, edible and medicinal uses. It is reported to be used as
anodyne, apertif, diuretic, emetic, rubefacient, and stimulant, folk remedy for arthritis,
foot ache, lumbago and rheumatism (Duke and Wain, 1981; Mishra et al, 2012). The
seed is a warming stimulant with antibiotic effects. The leaves, seeds and the stem of
mustard are edible. These are rich in vitamin A and vitamin K.
The special feature of mustard is phytoremediation. It is cultivated to remove
heavy metals from the soil in hazardous waste sites because it has higher tolerance for
these chemicals and can store the heavy metals. The plant is then harvested and
disposed-off properly. It also prevents erosion of soil from these sites preventing
fiarther contamination. India ranks second in the world with regard to the production
of Brassicas (Afroz et al., 2005) and suppUes nearly 7% of the world's edible oil
(Khan et al., 2002). However, production of rapeseed still remains insufficient to
fulfill the daily requirements of the people (Khan et al, 2002). The insufficient
economic yield can be attributed to various biotic and abiotic stresses among which
salinity has emerged as one of the serious problems limiting productivity of
agricultural crops as well as claiming substantial farmable area (Allakhverdiev et al.
2000; Al-Karaki, 2001; Koca et al, 2007). About 20% of the world's cultivated land
area and 50% of all irrigated lands are affected by salinity (Moud and Maghsoudi.
2008). High salinity level causes ionic imbalance due to excess accumulation of Na
and Cr in the cells thus reducing the uptake of essential mineral nutrients, such as K^.
Ca and Mn thus affecting the normal physiology of plants, both at the cellular as
well as whole plant levels (Bayuelo-Jimenez et al, 2003). The excess amount of Na'
ions in cells cause enzyme inhibition and metabolic dysfunction such as degradation
of photosynthetic pigments (Chaves et al, 2009). Salinity stress causes decrease in
the stomatal conductance (Parida et al, 2004), internal CO2 pressure and stomatal
 Introduction 2

opening that affect gaseous exchange which result in the inhibition of photosynthesis
in the salt affected plants (lyenger and Reddy, 1996). This decrease in photosynthesis
under saline conditions is considered as one of the most important factor responsible
for reduced plant growth and the productivity (Manikandan and Desingh, 2009).
Plants have been classified as glycophytes or halophytes according to their
capacity to grow on high salt medium. Glycophytes are the plants which cannot
tolerate salt stress that include most of the cultivated plants (Sairam and Tyagi, 2004).
The salinity stress has deleterious effects on growth which are associated with: (a)
low osmotic potential of soil solution (water stress), (b) nutritional imbalance, (c)
specific ion effect (salt effect) or (d) a combination of these factors (Marschner,
1995). With the onset and development of salt stress within a plant, all the major
processes such as photosynthesis, protein synthesis and energy and lipid metabolism
are adversely affected. The earliest response is a reduction in the leaf surface
expansion rate followed by its cessation as stress intensifies, but growth resumes
when the stress is relieved (Parida and Das, 2005). Soil salinity causes a lower rate of
photosynthesis by decreasing the chlorophyll content, the activity of rubisco (Soussi
et ai, 1998) and the closure of stomata thereby, decreases partial CO2 pressure
(Bethke and Drew, 1992). Salinity reduces plant productivity first by reducing plant
growth during the phase of osmotic stress and subsequently by inducing leaf
senescence during the phase of toxicity when excessive salt is accumulated in
transpiring leaves (Munns, 2002). All of these cause adverse pleotropic effects on
plants.
Recently, brassinosteroids (BRs) have emerged as a new paradigm in the
category of phytohormones. These are the class of poly-hydroxysteroids that have
been recognized as sixth class of plant hormones. Till now, about 69 BRs have been
isolated from plants (Bajguz, 2010a). Like other plant hormones (auxins, gibberellins,
cytokinins, ethylene and abscisic acid), BRs act at very low concentration to control
numerous processes associated with plant growth and development (Friedrichsen and
Chory, 2001; Bajguz and Hayat, 2009). BRs have been implicated in a wide range of
physiological and molecular responses in plants. They have the ability to cause cell
elongation and cell division in stem, inhibit root growth, promote xylem
differentiation, and abscission of plant organs (Mandava, 1988; Nemhauser et ai,
2004). They have also been noted to control several other process in plants, such as
 Introduction 3

induced synthesis of nucleic acids and of proteins (Khripach et al., 2003), activation
of several enzymes (Hasan et al., 2008), photosynthesis (Hayat et al, 2007a) and
increased fruit set (Fu et al, 2008; Ali et al, 2006). Apart from this, BRs also have
the ability to confer tolerance against osmotic stress (Sairam, 1994; Ahmed et al.
201 la; Vardhini and Rao, 2003), temperature stress (Fariduddin et al, 2011), salinit}
(Hayat et al, 2007b; Ali et al, 2007a), and various heavy metal stress like, cadmium
(Hayat et al, 2007a, 2010a), nickel (Alam et al, 2007; Yusuiet al, 2011), aluminium
(Ali et al, 2008a), copper (Fariduddin et al, 2009a) and nitrosative stress (Hayat et
a/., 2010b).
On being exposed to stressful conditions, plants accumulate an array of
metabolites, particularly amino acids that have traditionally been considered as
precursors as well as constituents of all proteins. Proline, an amino acid, plays a
highly beneficial role in plants, exposed to various stress conditions. Besides acting as
an excellent osmolyte, proline plays three major roles during stress, i.e., as a metal
chelator, an antioxidative defense molecule and a signaling molecule. Stressful
enviroimient results in an overproduction of proline in plants which in turn imparts
stress tolerance by maintaining cell turgor or osmotic balance; stabilizing membranes
thereby preventing electrolyte leakage; and bringing concentrations of reactive
oxygen species (ROS) within normal ranges, thus preventing oxidative burst in plants.
Proline, when supplied exogenously at lower concentrations to plants exposed to
various biotic and abiotic stresses, results in stress mitigation thereby enhancmg
growth and activating other physiological processes of plants. Exogenous proline
maintains the turgidity of the cells and also enhances photosynthesis during the times
of stress. Lower concentrations of proline are known to provide tolerance against the
damaging effects of salinity, drought, irradiance or heavy metal stress (Ashraf and
Foolad, 2007). Moreover, the exogenous proline improves the activity of
antioxidative enzymes viz. CAT, POX, SOD, etc. and also the enzymes of nitrogen
metabolism (Hoque et al, 2007a).
Keeping in view the above recognized roles, assigned to BRs and proline and
the ever increasing salinity stress in soil, the present studies were designed with an
objective to relate the changes in growth, photosynthetic parameters and the level of
antioxidative enzymes, in salinized plants of Brassica juncea (L.) Czem & Coss with
the BRs and proline induced resistance. The hypothesis that is put to trail is that the
 Introduction 4

application of brassinosteroids and proline will ameliorate the toxic effects of salinity
on the growth and yield of the test plant, Brassica juncea (L.) Czem & Coss, which is
widely cultivated throughout the world and is accepted by the local farmers as a cash
crop.
The following objectives were kept in mind while planning the experiments:
1. To establish the tolerant and resistant levels of salinity on mustard.
2. To observe the effect of foliage applied HBL/EBL on mustard plants.
3. To screen out the best concentration of proline for foliar spray to the given
mustard cultivars.
4. To observe the impact of leaf applied HBL/EBL on mustard plants, raised in
the soil amended with three doses of NaCl.
5. To observe the response of mustard plants, raised in the soil treated with three
doses of NaCl, to the leaf applied proline.
6. To observe the cumulative effect of BR and proline on mustard plants, raised
in the soil treated with three doses of NaCl.
7. To select the growth, physiological and biochemical traits showing maximum
response to the treatment that may be designated as a marker to forecast the
growth and crop productivity, or to ensure corrective measures.