Solutions-Class-12-Biology-Chapter-7-Plant Growth and Mineral Nutrition-Maharashtra Board

(c)  Gibberellins

(c) Xylem

(b) leaf fall and dormancy

(b) Ethylene

(b) Definite photoperiod

(b) dark/night

(d) Soybean

(c) absorbed from soil

(b) biosynthesis of 3-IAA

(b) death of tissue

(d) denitrification

(d) 16

The three phases of growth in plants :

• Phase of cell division,
• Phase of cell enlargement
• Phase of cell maturation.

IAA = Indole Acetic Acid.

In plants the growth is indeterminate and takes place throughout the life at specific

regions having meristems.

 Abscissic acid is the plant stress hormone

Anaerobic bacteria can convert nitrates of soil back into nitrogen gas. That process

performed by denitrifying bacteria is denitrification.

Nitrobacter bacteria responsible for conversion of nitrite to nitrate.

Gibberellins promotes bolting i.e. elongation of internodes just prior to flowering in plants those with rosette habit like beet, cabbage.

The response of plant to the influence of low temperature on flowering in plants is called vernalization.

The response of plant to the influence of light for initiation of flowering is known as photoperiodism.

There are three phases of growth and the total time required for all phases to occur is called grand period of growth.

• It is a process of maturation of cells derived from apical meristems.
• Differentiation is a permanent change in structure and function of cells that leads to its maturation.
• Cell undergoes major anatomical and physiological change during differentiation process.
• In hydrophytic plants parenchyma cells develop large schizogenous cavities which help them in aeration, buoyancy and mechanical support.

• The cells produced by dedifferentiation once again lose the capacity to divide and mature to perform specific function. This is called
• The cells mature to perform specific function.
• Interfascicular cambium is formed by process of dedifferentiation loses its capacity to divide.
• Secondary xylem and secondary phloem are formed from dedifferentiated cambium present in the vascular bundle.

(a) Nitrogen :

• Role : Constituent of proteins as amino acids, nucleic acids, vitamins, hormones, coenzymes, ATP and chlorophyll molecule.
• Deficiency symptoms : stunted growth and chlorosis.

(b) Phosphorus :

• Role : Constituent of cell membrane, certain proteins, nucleic acids and nucleotides, required for all phosphorylation reactions.
• Deficiency symptoms : Poor growth, leaves dull green

(c) Potassium :

• Role : Determination of anion – cation balance in cell, necessary for protein synthesis, involved in formation of cell membrane, opening and closing of stomata, activates enzymes, helps in maintenance of turgidity of cells.
• Deficiency symptom : Yellow edges in leaves, premature death.

• The plants which flower when the day length or light period is shorter than the critical photoperiod are called short day plants or SDP.
• SDPs usually flower during winter and late summer.
• Examples — Dahlia, Aster, Tobacco, Chrysanthemum, Soybean (Glycine max) and Cocklebur (Xanthium).

A low temperature or chilling treatment that induces early flowering in plants is known as vernalization.

Significance :

• Due to chilling treatment crops can be produced earlier.
• Crops can be grown in areas where they do not grow naturally.

When growth occurs in plants three distinct phases of growth are noticed.

• The initial phase of cell production occurs when meristematic cells divide and new cells are formed.
• During the cell expansion phase, freshly produced cells elongate, and there occurs cell enlargement with turgidity.
• During the cell maturation phase, cells differentiate.
• The growth rate differs in these three phases when compared.
• Growth rate is sluggish in the initial phase or lag phase, but increases and reaches a maximum in the log phase or exponential phase.
• During the stationary phase of maturation, the growth rate decreases and stabilises.
• When this changing rate of growth is plotted against time duration in a graph a sigmoid or S-shaped growth curve is obtained.

Effect of light duration on flowering of plants is known as photoperiodism.

Depending on photoperiodic response, plants are categorised into three types — Short day plants, long day plants and day neutral plants.

(1) Short day plants :

• Plants that flower under short day length conditions are called short day plants.
• Plants such as Dahlia, Xanthium, Soybean, Aster, Tobacco and Chrysanthemum are short day plants or SDP.
• Short day plants require along uninterrupted dark period for flowering, Therefore, they are also called long night plants.

(2) Long day plants :

• Plants that flower only when they are exposed to light period longer than their critical photoperiod are called long day plants or LDP.
• Long day plants require a short dark or night period for flowering. Hence, they are also called short night plants.
• Plants such as radish, spinach, wheat, poppy, cabbage, pea, sugar beet, etc. are long day plants.

(3) Day neutral plants :

• Plants in which the floweri.ng is not affected by the day length period are called day neutral plants or DNP or photoneutral plants.
• Plants such as cucumber, sunflower, cotton, balsam, maize, tomato, etc. are day neutral plants.

• Nitrogen fixation is the process of converting atmospheric nitrogen into nitrogenous salts and making it accessible to plants for uptake.
• Biological nitrogen fixation occurs when live organisms participate in the nitrogen fixation process.
• The process is mostly carried out by prokaryotic organisms, which are various types of bacteria found in soil.Nitrogen fixing organisms are known as diazotrophs or nitrogen fixers, and they fix around 70% of the nitrogen.
• Nitrogen fixers are either free-living bacteria or symbiotic bacteria linked with higher plants, such as Rhizobium.
• The cyanobacteria have specialized cells heterocysts which help in process of nitrogen fixation.
• Nitrogen fixation is high energy requiring process and 16 ATP molecules are needed for fixation of one molecule of nitrogen to ammonia.
• Soil bacteria like Nitrosomonas, Nitrosococcus convert ammonia to nitrate and the Nitrobacter convert nitrite to nitrate. This is known as nitrification, biological oxidation.
• These bacteria are chemoautotrophic and utilize these processes for their metabolism.
• Fabaceae plants like pea, bean have root nodules which harbour symbiotic bacterium Rhizobium which fixes nitrogen. It is host specific, soil bacteritun, Nitrogen is made available to host plant.