Effects of temperature on the development of a bean plant.

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Effects of temperature on the development of an organism

Abstract

Beans require a suitable temperature to germinate, one effect that temperature has is to regulate the rate at which enzymes digest their substrate. Temperature affects the kinetic energy at which particles collide. All enzymes have an optimum temperature at which the rate of reaction is at its fastest. If the temperature becomes too high the enzyme can become denatured preventing it from binding to its substrate. The results show that there is a significant correlation between temperature and the rate at which germination proceeds. Also the difference between the lines of best fit for broad beans grown at 20°C is close to that of beans grown at 25°C. The results do clearly show that there is a correlation between the temperature of the environment and the rate of germination.

Introduction

For a seed to successfully germinate there are several environmental factors which need to be met. These include:

  • An adequate supply of water
  • A suitable temperature
  • An appropriate partial pressure of oxygen
  • A suitable supply of light

Water and Seed Germination

Water is the crucial step in the activation of germination; it is responsible for activating the enzymes that stimulate the growth of the embryo. Once the enzymes have been activated the immature seed begins to consume the nutrients inside the endosperm. Beans take in water via a process known as osmosis. Osmosis occurs when a water molecule moves from an area of high water potential to one of lower potential. Water enters into the testa through the micropyle1.

Bewley, J.D. (1997). Seed germination and dormancy. Plant Cell 9, 1055-1066.

Beans Awakening from the Dormant Phase

When enough water has entered through the micropyle certain chemical processes begin. Before water has entered the seed it is said to be in a dormant phase. The embryo remains dormant due to the inhibitory effect of the hormone abscisic acid and the seed coat is dense and tough enough to prevent the penetration of gas and the inhibition of other enzymes2. When water enters the testa the seed expand and the seed coat swells and splits. One other action of the penetration of water is the activation of a hormone called Gibberellic acid. Gibberellic acid counteracts the effects of abscisic acid rescuing the seed from dormancy. Once the effects of abscisic acid have been reversed the digestive enzyme alpha- amylase is activated. This causes the breakdown of starch stored in the endosperm, into sugar that can be used for respiration to produce ATP. This energy can then be used for cell division and growth of the new seedling.

The barley seed shown in figure 1 is a typical monocot. It has a seed coat (fused to fruit coat), a large endosperm area filled with starch, and an embryo (or germ). The seed takes up water from its environment in a process termed imbibition. The movement of water through the embryo moves the hormone Gibberellic acid from the embryo to the aleurone layer of the endosperm. The aleurone layer contains a high level of protein (some 30% of the kernels protein in Barley). The movement of water into the aleurone layer causes the activation of hydrolysis enzymes. These enzymes degrade the stored proteins into amino acids3. The gibberellic acid activates the gene responsible for encoding the enzyme amylase. The amino acids that are produced through hydrolysis are used to create the amylase enzyme during translation3. The amylase enzyme is exported from the Golgi apparatus and excreted from the cell (exocytosis) into the interior of the endosperm3. Amylase catalyses the breakdown of maltose into glucose molecules which can used into cellular respiration to produce energy3.

A similar mechanism exists in the beans of dicots. Unlike monocots, dicots have no aleurone layer, and most of the starch is present in the embryo as apposed to the endosperm. Unlike gibberellic acid the activating signal is via phytochrome. The phytochrome pigment exists in two forms Pr and Pfr. Pfr is the activated form when there are sufficient levels of the pigment in the seed when imbibition takes place, the amylase gene is photoactivated. In the presence of red light (660 nm), all the inactive Pr is converted to Pfr. In the presence of far-red (730 nm) light, the far-red light causes all the Pfr to change into Pr4.

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Schematic diagram of the activation of the enzyme amylase and the level of its substrate starch in a germinating seed5.

Temperature Effects

Beans also require a suitable temperature to germinate, one effect that temperature has is to regulate the rate at which enzymes digest their substrate. Temperature affects the kinetic energy at which particles collide. This is particularly important for enzymes, as for them to work they need to come into contact with their substrate in order to digest them into a product. All enzymes have an optimum temperature at which the rate of reaction is at its fastest. ...

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