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

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

10-11

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

10-14

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