![if !IE]> <![endif]>
Growth occurs by cell division, cellelongation and cell maturation. The first phase is lag phase, the second is log phase and the final phase is steady state phase. The log phase is otherwise known as exponential phase. The three phases are collectively called Grand period of growth. Plant exhibits plasticity in development. Plant growth and development are controlled by both internal and external factors. The internal factors are chemical substances called Plant Growth Regulators (PGRs). The hormones are classified into five groups: Auxins, gibberellins, cytokinins, abscisic acid and ethylene. These PGRs are synthesized in various parts of the plant. PGRs may act synergistically or antagonistically. The external factors affecting growth includes water, nutrition, temperature, oxygen and light. Mechanism of flowering is controlled by light period (photoperiodism) and temperature (vernalization). The physiological changes on flowering with effect from relative length of light and darkness (photoperiodism) are called photoperiodism. A bluish biliprotein responsible for the perception of light in photophysiological process (induction and inhibition of flowering) is called Phytochrome. Besides photoperiod certain plants require a low temperature in the earlier stages for flowering. Many biennial and perennial plants are induced to flower by low temperature (0oC to 5oC). This process is called vernalization and the reversal effect of vernalization is called devernalization. The condition of a seed when it fails to germinate even in suitable environmental condition is called seed dormancy. Thus, dormancy can be overcome by following methods such as scarification, impaction, stratification, alternating temperatures and light. Senescence refers to all collective, progressive and deteriorative processes which ultimately lead to complete loss of organization and function. Senescence is of four types and they are overall, top, deciduous and progressive. Senescence is controlled by plant’s own genetic programme. Death of the plant or its parts consequent to senescence is called Programmed Cell Death (PCD). The final stage of senescence is abscission. Abscission is a physiological process of shedding of organs from the parent plant body. The study of functioning of plants under adverse environmental conditions is called stress physiology. The environmental stress may broadly be divided into biotic and abiotic stress. The adverse effect on plants caused by other living organisms such as viruses, bacteria, fungi, parasites, insects, weeds is competing plants are called biotic stress. Abiotic stress may occur due to an atmospheric condition or soil condition.
Copyright © 2018-2023 BrainKart.com; All Rights Reserved. Developed by Therithal info, Chennai.