Factors Affecting Plant Growth

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...

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

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

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

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-)

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

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

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

Tropisms cont`d Gravitropism Thigmotropisms 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

Negative geotropism Fig 10.20

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

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

Effects of gibberellins Fig 10.16

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

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!

Functions of ethylene Fig 10.18

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