Plant Responses

Plant Responses

• explain why plants need to respond to their environment in terms of the need to avoid predation and abiotic stress

Like animals, plants must also need to respond to external stimuli. This is important to:

• Avoid predation.
• Avoid abiotic (non-living) stress.
• Maximise photosynthesis.
• Obtain more light, water and minerals.
• Ensure germination in suitable conditions/pollination.
• Seed set/seed dispersal.
• define the term tropism

Tropism – a directional growth response in which the direction of the response is determined by the direction of the external stimulus. Tropisms may be positive (a growth response towards the stimulus) or negative (a growth response away from the stimulus).

They include

• Phototropism (light) – shoots grow towards light – they are positively phototrophic.
• Geotropism (gravity) – roots grow towards the pull of gravity.
• Chemotropism (chemicals) – on a flower, pollen tubes grow down the style, attracted by chemicals, towards the ovary where fertilisation can take place.
• Thigmotropism (touch) – shoots of climbing plants, such as ivy, wind around other plants or solid structures and gain support.
• explain how plant responses to environmental changes are co-ordinated by hormones, with reference to responding to changes in light direction

Hormones, also referred to as plant growth regulators, coordinate plant responses to environmental stimuli. Like animal hormones, plant hormones are chemical messengers that can be transported away from their site of manufacture, by active transport, diffusion and mass flow in the phloem sap or in xylem vessels, to act at target cells or tissues of the plant. They bind to receptors on the plasma membrane. Specific hormones have specific shapes, which can only bind to specific receptors with complementary shapes on the membranes of particular cells. This specific binding makes sure that the complementary shapes on the membranes of particular cells.

The cell wall around a plant cell limits the cell’s ability to divide and expand. Therefore, growth in plants happens where there are groups of immature cells that are still capable of dividing – these places are called meristems.

• evaluate the experimental evidence for the role of auxins in the control of apical dominance and gibberellin in the control of stem elongation

Apical Dominance:

Apical dominance – when a growing apical bud at the tip of the shoot inhibits growth of lateral buds further down the shoot. So if you break the shoot tip (the source of auxin) off a plant, the plant starts to grow side branches from lateral buds that were previously dormant.

Auxin is constantly made by cells at the tip of the shoot. It is then transported downwards, from cell-to-cell. This auxin accumulates in the nodes between the lateral buds. Somehow, its presence here inhibits their activity. Two simple experiments provide evidence for this mechanism:

1. If we cut the tip off two shoots and apply IAA (synthetic auxin) to one of them, the one with IAA will continue to show apical dominance and the side shoots will not grow. The one without IAA will branch out sideways.

If a growing shoot is tipped upside down, apical dominance is prevented and the lateral buds start to grow out sideways. This can be explained by the fact that auxin is not transported upwards against gravity, but only downwards. So in the upside-down shoot, the auxin produced in the apical meristem does not reach the lateral buds and therefore cannot affect them

Gibberellin and Stem Elongation:

Gibberellin – a group of plant hormones that stimulate cell elongation, germination and flowering.

In Japan, a plant disease called Bakanae is caused by a fungus and makes rice grow very tall. Attempts to isolate the fungal compounds involved identified a family of compounds called gibberellins. One of these was gibberellic acid (GA₃). Scientists began applying GA₃ to dwarf varieties of plants (e.g. maize, peas), which made these plants grow taller. These results seem to suggest that gibberellic acid is responsible for plant stem growth, but such a conclusion is too hasty.

Scientists compared GA₁ concentrations of tall pea plants (homozygous for the dominant Le allele), and dwarf pea plants (homozygous for the recessive le allele), which were otherwise genetically identical. They found that plants with higher GA₁ concentrations were taller. However, to show that GA₁ directly causes stem growth, the researches needed to know how GA₁ is formed. They worked out that the Le allele was responsible for producing the enzyme that converted GA₂₀ to GA₁.

They also chose a pea plant with a mutation that blocks gibberellin production between ent-Kaurene and GA₁₂-aldehyde. Those plants produce no gibberellin and only grow to about 1cm in height. However, if you graft a shoot onto a homozygous le plant (which cannot convert GA₂₀ to GA₁), it grows tall. The shoot has no GA₂₀ of its own, but it has the enzyme to convert GA₂₀ to GA₁ – this confirmed that GA₁ caused stem elongation. Dwarf varieties of plants lack the dominant allele for an enzyme needed for synthesis of gibberellins.

Further studies have shown that gibberellins cause growth in the internodes by stimulating cell elongation (by loosening cell walls) and (by stimulating production of a protein that controls the cell cycle). Internodes of dwarf peas have fewer cells and shorter cells than those of tall plants, and mitosis in the intercalary meristems of deep-water rice plants increases with gibberellin treatment.

• describe how plant hormones are used commercially