Factors Affecting Plant Growth

1. Factors Affecting Plant Growth

2. External Factors - Light • Terrestrial plants use pigments to capture light • Chlorophyll • Absorbs light in the red and blue areas of the spectrum • Carotinoids • Absorb in the blue-green spectrum • Quantity of light depends on • Geographical area • Competition from taller plants • Cloud cover, shading, time of day etc...

3. External Factors – Light cont’d • Photoperiod • The number of daylight hours • Triggers flowering and dormancy in plants • Short-day plants • Flower and reproduce when the photoperiods are shortening ie, late summer • E.g. chrysanthemum • Long-day plants • Flower and reproduce when the photoperiods are lengthening i.e. spring. • E.g. spinach

4. External Factors - Nutrients • Soil Nutrients • Requirements vary between species • Inorganic nutrients are released by the weathering of rocks or the breakdown of dead organic matter by bacteria and fungi • Absence can cause death or serious weakening

5. External Factors – Nutrients cont’d • Macronutrients: • 9 nutrients required in relatively large quantities • Carbon, oxygen, hydrogen make up 95% of the dry mass of plants • Other 5%: nitrogen, potassium, phosphorus, calcium, magnesium, sulfur • Micronutrients: • 8 nutrients needed in much smaller amounts • Iron, chlorine, boron, manganese, zinc, copper, molybdenum, and nickel

6. External Factors- Nitrogen • Most needed macronutrient • Important part of protein, high protein plants need a lot of nitrogen • Dead organic material • Bacteria and fungi breakdown organic nitrogen compounds into a form plants can use – nitrate ions (NO3-)

7. External Factors- Nitrogen cont`d • Atmosphere – Nitrogen Gas • Nitrogen-fixing bacteria convert nitrogen gas into a usable form • Bacteria live freely in the soil or form a symbiotic relationship with plants • Symbiotic bacteria - Grow within plant root cells and cause bulges called nodules • Occurs most often in legumes (peas, beans, clover, and alfalfa) • Bacteria supply the legumes with nitrates in exchange for carbohydrates from the plants • Most nitrates produced are not used by the plant but are instead added to the soil

8. Internal Factors - Overview • Growth controlled by • Tropisms • growth responses toward or away from unidirectional stimuli • Due to differential growth; positive response if toward stimulus, negative if away • Allow plants to alter their growth so they can grow into more favorable conditions • Hormones • Internal chemical regulators • 5 main hormones

9. Tropisms • Phototropism • Plants detect blue-light in special receptors called phototropins

10. Tropisms cont`d • Gravitropism • Plant will grow so that it stays oriented relative to the source of gravity (the earth). • Dependent on the presence of starch-filled plastids (amyloplasts) in specialized cells. • Starch is heavy, plastids sink to the lower end of the cell, allows plant to identify up and down! • Thigmotropisms • Response to touch • vines

11. Negative geotropism • Fig 10.20

12. Hormones- Auxins • Stimulate cell division and elongation in apical buds • Inhibit these same processes in lateral buds • Influence fruit ripening and dropping of fruit • Synthetic Auxin • Can be used as a herbicide • Triggers production of fruit from unpollinated flowers • E.g. tomatoes and cucumbers • Increases root production in cuttings

13. Hormones - Gibberellins • Promote cell division and elongation in plant stems • Can cause bolting • Sudden growth of stem just before flowering • Raises flower above leafy structures • Synthetic Gibberellins • Cause fruit to enlarge (are often used with grape crops, etc.) • Herbicide

14. Effects of gibberellins • Fig 10.16

15. Hormones - Cytokinins • Stimulates cell division • High levels in endosperm and young fruit • Stimulates the growth of lateral buds by blocking apical dominance • Wider not taller! • Synthetic Cytokinins • Used to produce clones in tissue cultures • Can be sprayed on cut flowers to make them last longer

16. Hormones - Ethylene Gas • Key in fruit development • Causes • Increased rate of ripening • Sweetness of fruit • Sugar content • Colour change • Tissue softener • Can be applied externally to fruit to ripen faster • Also why 1 bad apple spoils the bunch!

17. Functions of ethylene • Fig 10.18

18. Hormones - Abscisic Acid • Stress Hormone • Growth regulator, acts as an inhibitor • Slows down and stops growth • Promotes the closure of stomata • Induces seed and bud dormancy • Helps resist water stress