crop growth theory

This presentation deals with various scientific theories that explain the mechanism of plant growth.

1. Crop growth theory

2. Theories Various theories/laws regarding crop growth in relation to growth factor, possibility and scope of CO 2 has been proposed by different scientists.

3. Liebig’s law of minimum Justus Von Liebig proposed law of minimum in 1840 which states that the growth of plants is limited by the plant nutrient present in smaller quantity, all other being in adequate amount. This has been re-stated as barrel concept. A barrel with staves of different lengths cannot contain anything above the height of the shortest stave. similarly, growth can be no greater than allowed by the factor lowest in availability. The level of plant production can be no greater than that allowed by the most limiting of the essential plant growth factors.

4. Barrel concept- the liquid can be filled upto the length of shortest stave only

5. Blackman’s Law of Optima and Limiting Factor Blackman in 1905 proposed law of optima and limiting factor. He stated that when a process is conditioned to its rapidity by a number of separate factors, the rate of the process is limited by the pace of the slowest factor.

6. Mitscherlich’s Law of Diminishing Returns Mitscherlich in 1909 developed an equation relating growth with the supply of plant nutrients. When plants are supplied with adequate amount of all but one limiting element, their growth is proportional to the amount of this one limiting element. Plant growth increases as more of this element is applied but not in direct proportion to the amount of growth factor added. An increase in growth with each successive addition of the limiting elements is progressively smaller.  

7. Mitscherlich expressed this mathematically as : dy / dx = (A-y)C   Where, dy/dx is growth rate A is the maximum possible yield with sufficient level of all growth factors y is the yield obtained at a given level of growth factor x and c is the proportionality constant where x=0 and y=0

8. Mitscherlich’s law in terms of nutrient application

9. Inverse Yield- Nitrogen Law Wilcox (1929) proposed inverse yield nitrogen law which states that the power of growth or yielding ability of any crop plant is inversely proportional to the mean nitrogen content in the dry matter. A crop plant with high mean percentage of nitrogen in dry matter has less dry matter production potential than a crop plant with low percentage of nitrogen.

10. Macy Poverty adjustment Macy in 1936 proposed that relationship exists between sufficiency of a nutrient and its percentage content in the plant. According to him, there is a critical percentage of each nutrient in each kind of plant. Above that point, there is luxury consumption and below that point there is poverty adjustment. This poverty adjustment is proportional to the deficiency until a minimum percentage is reached.

11. Possibility and scope of CO 2 Fertilization Carbon is a constituent of all organic compounds, carbohydrate, protein or fatty acid. Its source in the plant is carbon dioxide which enters in the leaf stomata from the atmosphere through diffusion and then utilized in photosynthesis. Hence, the concentration of carbon dioxide in the atmosphere is important in deciding the crop yields.

12. Carbohydrate enrichment experiments conducted in glass and plastic houses confirmed the above hypothesis. Wittwer (1966) reported spectacular yield increases in a green house from CO 2 enrichment. A similar experiment in a plastic house at IARI, New Delhi on vegetable crops further substantiated the above hypothesis. Similar studies under field conditions have not been possible as regulating CO 2 concentration in open field is not possible.

13. yield of crops is higher in valleys due to high partial pressure of CO 2 in valleys. The reason for higher percentage of CO 2 in plant atmosphere in valleys is low wind velocity than in plains. Increasing CO 2 pressure in the plant atmosphere is beneficial but under controlled conditions only because CO 2 is known to increase temperature also and higher temperature leads to more evaporation and consequently higher chances of crop failure under rainfed conditions while more irrigation requirement under irrigated conditions.

14. Higher temperature also leads to higher respiration and consequently more loss of photosynthates and thus probably no gain in net assimilation rate. Besides above, high temperature may deplete the organic matter in soil much faster. Thus, CO 2 enrichment has to be viewed in totality before arriving at some conclusions.

15. Sources of CO 2 Since, piping gas into an open field may be wasteful as wind will blow it off, we have only two sources : ( 1) Dry ice - The application of dry ice may alter the CO 2 pressure only temporarily because of its high rate of evaporation. (2) Organic matter - This is cheaper and long lasting but a weak source. Delivery of CO 2 to the field crops is still a basic problem and probably this is the reason why field experiments on CO 2 enrichment could not be yet precisely conducted.

16. Scope of CO 2 Fertilization Unless an efficient source of carbon dioxide is available, it is difficult to make use of this knowledge on a field scale i.e. for field crops. As for horticultural crops, plastic structures can be fabricated and used to regulate CO 2 concentration and eventually obtain higher yields.

17. Global Increase in CO 2 Often the agronomists think this as a chance to boost the crop yields but it has its own disadvantages because it simultaneously increases the global temperature. Any further increase in CO 2 level will further increase the temperature and that may result in more agronomic droughts and there will be crop failures, shifting of crop boundaries towards polar regions and loss of plant and animal biodiversity.

18. Agronomic droughts It refers to failure of crops due to insufficient water supply; under very high temperature conditions the evaporation demand will be higher than the water supply. Such conditions which results in failure of crops are considered as agronomic drought. High temperature leads to warming of air. So, air will become lighter and rise up in the form of eddies (circular movement) thus, creating a vacuum. To fill this vacuum, air from surrounding areas would blow to this place leaving it with excess CO 2 .

19. Difference between C 3 & C 4 plants no. C 3 plants C 4 plants 1. The first product of photosynthesis is a three carbon compound formed via the Calvin-Benson pathway. E.g. Wheat, Rice, Barley, Rye and Oat. The first product of photosynthesis is a four carbon compound formed via the Hatch- Slack pathway. E.g. Corn, Sorghum , millets and Sugarcane. 2. Less respond to light intensities. Respond to higher light intensities than C 3 plant. (Double than C 3 ) 3. Translocation rate are slow. Translocation is about twice as fast as in C 3 leaves.

20. no. C 3 plants C 4 plants 4. Less efficient user to carbon dioxide. CO 2 compensation point is 50-150 ppm of CO 2 . More efficient user of carbon dioxide. CO 2 compensation point is 0-10 ppm of CO 2 . 5. Lower photosynthetic efficiency due to high photorespiration. High photosynthetic efficiency due to low photorespiration. 6. Lower Net Assimilation Rate. Higher Net Assimilation Rate. 7. Adversely affected by high temperature Not adversely affected by high temperature.

21. no. C 3 plants C 4 plants 8. Many C 3 plants become unproductive at temperature from 25 ◦ - 35 ◦ c. C 4 plants increase in productivity at these temperatures. 9. Lower water use efficiency in this type. The mean dry matter produced for each 1000 g of water used was 1.54 g. Greater water use efficiency in this type. The mean dry matter produced for each 1000 g of water used was 3.29 g. 10. Growth rates are slower than C 4 plants. 13 gm/day. Growth rates are much greater than C 3 plants. 22 gm/day. 11. C 3 plants suffer an oxygen stress and are adversely affected by the oxygen level of 21% found in nature. C 4 plants do not suffer an oxygen stress.

22. C 3 plants W heat R ice

23. C 4 plants M aize Sugarcane

24. Crassulacean Acid Metabolism (CAM) In this type, plants are characterized by their adaptiveness to arid regions. Xerophyte plants are of this type. e.g., Pineapple, cactus. Photosynthetically Active Radiation (PAR) Light energy within the spectral wavelength of 0.4-0.7 microns or 400-700 nanometers, which is useful in photosynthesis process by plants.  Photosynthesis The process that transforms carbon dioxide (CO 2 ) into food; the basis of all crop yields and the essences of agriculture.

25. CAM plants P ineapple C actus

26. Photorespiration A form of respiration stimulated by light, found in C 3 plants and having no known essential function.   Photoperiod The duration of the light period between sunrise and sunset, including the light period.   Photoperiodism Reproductive response of plants to the relative length of light (Day) and dark periods (Night) in a day.

27. Short Day Plant (SDP) Flower initiation takes place when the days are short (less than 10 hrs.) or when dark period is long. Most of the tropical crops like rice, sorghum, maize etc. are short day plants.   Long Day Plants (LDP) Long day plants require comparatively long days (usually more than 14 hrs.) for flower initiation. They put forth more vegetative growth when days are short. Most of the temperate crops like wheat, barley and oats are long day plants. Day-Neutral Plants (DNP) Plants in this category do not require either long or short dark periods. Photoperiod does not have much influence for basic change for these plants. E. g. Cotton, sunflower, buckwheat, etc.