Crop Physiology Point Wise Notes For Competitive exam

Crop Physiology

Growth is irreversible increase in size or weight.

• The main principle in agronomy is harvesting as much solar energy as possible and convening solar energy into chemical energy by photosynthesis.

• On the leaf surface, a thin layer of undisturbed air (boundary layer) offers resistance to the flow of CO2 which is called boundary layer resistance.

• The resistance offered by stomata is called stomatal resistance.

• Resistance offered by mesophyll cells is called mesophyll resistance.

• In plants, the molecule absorbing radiant energy in the visible range is referred to as a pigment.

Chlorophyll is green, carotenoids red or yellow and phytochrome blue.

• Plants also contain pigments such as anthocyanins and flavones.

• Only chlorophylls and carotenoids function in photosynthesis.

• Solar radiation contains small units called photons.

• By photolysis, water is broken down into hydrogen and nascent oxygen.

• Hydrogen ions released during break down of water are used for reducing NADP to NADPH which releases energy that is utilized for oxidative phosphorylation i.e., conversion of ADP to ATP.

• Reduction of carbon dioxide is also called dark reaction as light is not necessary.

Dark reaction does not mean that this reaction occurs only in dark.

• With the energy supplied by ATP (formed during light reactions), CO2 combines with hydrogen (supplied by NADPH) and forms carbohydrates.

• Plants are classified into C3, C4 and CAM based on method of reduction of CO2

C4 minor millets, even under good management do not out yield C3 rice.

• Photosynthetic advantage of C4 plants compared with C3 plants at the level of carboxylation is reduced at the whole leaf level. It is due to combination of stomatal and mesophyll resistance at the leaf level and by mutual shading and periods of low light at plant community level. As a result, no consistent advantage of the C4 pathway is evident in crop growth rates and crop yields of C4 plants.

• In C4 plants, as PEP enzyme has high affinity and activity with CO2, for known size of stomatal aperture, more CO2 is fixed for the same amount of water lost.

• C4 plants use considerably less water per unit of additional dry weight than calvin cycle (C3) plants.

Respiration is a reverse process of photosynthesis.

• Respiration supplies the necessary energy for the execution of various biological and chemical reactions in plants.

• Growth respiration provides energy and products for the synthesis of structural and storage compounds.

• Growth respiration does not depend on temperature.

• Maintenance respiration provides energy for the entire plant for routine works such as maintenance of membranes, proteins, cellular organization and for ion uptake.

Growth respiration is necessary for the normal growth.

• Maintenance respiration is considered as a wasteful process especially under higher temperature.

• Maintenance respiration depends on temperature.

• Low light and high temperature during grain filling period is considered as unfavorable for high yields.

• Respiratory mechanism common to all types of plants (C3 and C4) is called dark reaction since it occurs regardless of light.

Photorespiration occurs only during high light and oxygen availability.

• Under high light intensity, high levels of energy and oxygen is available in the leaves.

• The enzyme in photorespiration is the same ribulose 1,5 bisphosphate carboxylase which fixes CO2.

• As the enzyme RUBISCO also accepts oxygen, it is also called as ribulose 1,5-bisphosphate oxygenase.

• Photorespiration is high in C3 plants which is considered as wasteful.

• Total amount of photosynthates produced due to the process of photosynthesis is called gross photosynthesis.

• Net photosynthesis is the amount of carbohydrates added in the plant by photosynthesis less the amount of carbohydrates spent in respiration.

• Amount of net photosynthesis depends on light intensity and carbon dioxide concentration.

• The light intensity at which photosynthesis and respiration are equal is called light compensation point.

• The light intensity at which there is no further increase in the rate of photosynthesis is called light saturation point.

• Light saturation is high in C4 plants compared to C3 plants.

• Concentration of carbon dioxide at which respiration and photosynthesis are equal is called carbon dioxide compensation point.

• Carbon dioxide compensation point is lower in C4 plants than C3 plants.

• Carbohydrates are usually translocated as sucrose and to some extent as starch.

Dormancy may be due to

1) Improper development of embryo

2) High concentration of growth inhibiting substances

3) Hard and impermeable seed coat

Positive photoblastic seeds require light for inducing germination.

Negative photoblastic seeds do not germinate when exposed to light.

Non-photoblastic seeds germinate either in light or dark.

• Seeds of many cultivated crops are non-photoblastic.

• Flowering plants are classified into monocotyledons and dicotyledons.

• Cereals and millets belong to monocotyledons.

• When seed is sown in soil, it absorbs water twice its weight.

• Embryo secretes gibberellin which in turn induces the secretion of hydrolytic enzymes by aleurone layer in cereals.

Hydrolytic enzymes break down complex food materials like starches and proteins into simple sugars and amino acids respectively.

Fats in the cotyledons of oil seeds and in the embryos of cereal seeds are split by lipases into fatty acids and glycerol.

Radical is the first organ to emerge from the seed in all cases.

• Radical is followed by plumule of young shoot.

• In dicotyledonous plants such as groundnut cotyledons emerge from the soil and function as first leaves.

• In groundnut cotyledons are brought out of the soil by curved elongation of hypocotyls.

• Type of germination where cotyledons emerge out of the soil is called epigeal germination.

• In monocotyledons plants, cotyledons remain inside the soil. The plumule grows upward by the elongation of epicotyls. This type of germination is called hypogeal germination since the cotyledon is below or inside the soil.

Dicotyledons also exhibit hypogeal germination when sown deep.

• The coleoptiles of grasses emerge from the soil as pale tube like structure that encloses the first true leaf.

• In monocotyledonous plants, radical elongates and constitutes primary root.

Primary root usually grows vertically downwards while secondary roots that appear during the seedling stage grow horizontally for a few centimeters before turning downwards.

• High order laterals spread randomly and these roots are called seminal roots.

• At the end of seedling stage, fresh roots develop from lower nodes of stem called nodal roots or adventitious roots.

Seminal roots penetrate downwards to the greatest depth.

Adventitious roots constitute a major portion of total root mass and become important for the rest of the life of the plants, while the seminal roots decay gradually.

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