CHAPTER 15 PLANT growth and development
Channel: ISHTA’s BIO ACADEMY
Date: 2022-12-11

Growth is one of the most conspicuous events in any living organism. It is an
irreversible increase expressed in parameters such as size, area, length, height,
volume, cell number etc. It conspicuously involves increased protoplasmic material.
In plants, meristems are the sites of growth. Root and shoot apical meristems
sometimes alongwith intercalary meristem, contribute to the elongation growth of
known that not only the duration of light period but that the duration of
dark period is also of equal importance. Hence, it can be said that flowering
in certain plants depends not only on a combination of light and dark
exposures but also their relative durations. This response of plants to
periods of day/night is termed photoperiodism. It is also interesting to
note that while shoot apices modify themselves into flowering apices prior
to flowering, they (i.e., shoot apices of plants) by themselves cannot percieve
photoperiods. The site of perception of light/dark duration are the leaves.
It has been hypothesised that there is a hormonal substance(s) that is
responsible for flowering. This hormonal substance migrates from leaves
to shoot apices for inducing flowering only when the plants are exposed
to the necessary inductive photoperiod.
15.6 VERNALISATION
There are plants for which flowering is either quantitatively or qualitatively
dependent on exposure to low temperature. This phenomenon is termed
vernalisation. It prevents precocious reproductive development late in
the growing season, and enables the plant to have sufficient time to reach
maturity. Vernalisation refers specially to the promotion of flowering by a
period of low temperature. Some important food plants, wheat, barley,
rye have two kinds of varieties: winter and spring varieties. The ‘spring’
variety are normally planted in the spring and come to flower and produce
grain before the end of the growing season. Winter varieties, however, if
planted in spring would normally fail to flower or produce mature grain
within a span of a flowering season. Hence, they are planted in autumn.
They germinate, and over winter come out as small seedlings, resume
growth in the spring, and are harvested usually around mid-summer.
Another example of vernalisation is seen in biennial plants. Biennials
are monocarpic plants that normally flower and die in the second season.
Sugarbeet, cabbages, carrots are some of the common biennials.
Subjecting the growing of a biennial plant to a cold treatment stimulates
a subsequent photoperiodic flowering response.
2015-16(19/01/2015)
PLANT GROWTH AND DEVELOPMENT 253
plant axes. Growth is indeterminate in higher plants. Following cell division in root
and shoot apical meristem cells, the growth could be arithmetic or geometrical.
Growth may not be and generally is not sustained at a high rate throughout the
life of cell/tissue/organ/organism. One can define three principle phases of growth
– the lag, the log and the senescent phase. When a cell loses the capacity to divide,
it leads to differentiation. Differentiation results in development of structures that
is commensurate with the function the cells finally has to perform. General principles
for differentiation for cell, tissues and organs are similar. A differentiated cell may
dedifferentiate and then redifferentiate. Since differentiation in plants is open, the
development could also be flexible, i.e., the development is the sum of growth and
differentiation. Plant exhibit plasticity in development.
Plant growth and development are under the control of both intrinsic and
extrinsic factors. Intercellular intrinsic factors are the chemical substances, called
plant growth regulators (PGR). There are diverse groups of PGRs in plants,
principally belonging to five groups: auxins, gibberellins, cytokinins, abscisic acid
and ethylene. These PGRs are synthesised in various parts of the plant; they control
different differentiation and developmental events. Any PGR has diverse
physiological effects on plants. Diverse PGRs also manifest similar effects. PGRs
may act synergistically or antagonistically. Plant growth and development is also
affected by light, temperature, nutrition, oxygen status, gravity and such external
factors.
Flowering in some plants is induced only when exposed to certain duration of
photoperiod. Depending on the nature of photoperiod requirements, the plants
are called short day plants, long day plants and day-neutral plants. Certain plants
also need to be exposed to low temperature so as to hasten flowering later in life.
This treatement is known as vernalisation.