biology B1 (4)

• Hormones and the control of plant growth
• A tropism is a directional growth response of a plant
• The direction of response is determined by the direction of the external stimulus
• The stimulus light effects the shoots by making them grow towards the light called positive phototropism
• The stimulus gravity affects the shoots by making them grow away from the force of gravity is called negative gravitropism
• The stimulus gravity affects the roots by making them grow towards the force of gravity is called positive gravitropism
• The stimulus moisture affects the roots by making them grow towards the moisture is called positive hydrotropism
• Theses responses aid a plants survival:
• -shoots grow towards light to allow maximum photosynthesis
• -roots grow into the soil to provide anchorage
• -roots grow towards water as it it vital for plant support, photosynthesis and a supply of dissolved mineral ions

Auxins

• The plant growth responses happen due to a plant hormone called auxin
• Auxin is synthesised in cells at the tip of shoots
• Auxin moves away from the tip by diffusion
• Auxin causes the cell walls of the cells in a region behind the tip to become elastic
• These cells fill with water, stretch, elongate and the plant increases in length
• The stimuli of light and gravity cause auxin to be unevenly distributed in the shoot and root
• This causes cells on one side to elongate more than cells on the other side, causing bending in the shoot or root
• Shoot and roots respond differently to auxin:
• -a high concentration of auxin in shoots stimulates cell elongation and hence growth
• -a high concentration of auxin in roots inhibits cell elongation and reduces growth
• Phototropism in shoots:
• For a straight shoot:
• -tip is receptor and auxin is synthesised
• -auxin diffuses equally
• -response area - all cells elongate equally
• For a bend shoot:
• -tip detects unequal light distribution and auxin diffuses sideways away from the light
• -unequal amounts of auxin diffuse lack each side
• -causing cell elongation to occur unequally on each side
• Gravitropism:
• A normal young bean plant is laid on it side
• Auxin is equally spread through tissues resulting in straight shoot and root
• Seedling placed horizontal in dark:
• -gravity causes more auxin to gather on the lower sides of the root and shoot
• -in the shoot the extra auxin increases cell elongation so it curves upwards
• - in the root, raised auxin levels slows down the elongation of the cells
• -so it curves downwards

Human use of plant hormone

• Rooting powder:
• -they are used to encourage root growth in cuttings so you can make many clones from fewer plants
• -it is auxins and other types of plant hormone
• -it may force roots to grow at the expense of the shoots
• -it can help achieve many clones with the same desired characteristics successfully
• Selective weed killer:
• -it is artificial auxins that only effect broad leaf plants
• -it is used to remove weeds which compete with crops and to make lawns look wee-free
• -it works by the broad…