Factors affecting plant growth ABT-320 Dr. Rabia Amir Lecture 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...

Light Photoperiod The number of daylight hours Triggers flowering and dormancy in plants 1. Short-day plants Flower and reproduce when the photoperiods are shortening ie, late summer E.g. chrysanthemum 2. Long-day plants Flower and reproduce when the photoperiods are lengthening i.e. spring. E.g. spinach

Short day vs long day

Nutrients Macronutrients: Micronutrients: 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

Macronutrients; 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-)

Nitrogen Atmosphere – Nitrogen Gas 1. Nitrogen-fixing bacteria convert nitrogen gas into a usable form Bacteria live freely in the soil or form a symbiotic relationship with plants 2. 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

Nitrogen Cycle

Internal factors 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

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

Tropism 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!

Negative Geotropism

Thigmotropism Thigmotropism Movement in which a plant moves or grows in response to touch or contact stimuli.

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 un-pollinated flowers E.g. tomatoes and cucumbers Increases root production in cuttings

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

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

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

Functions of Ethylene

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

Thank You !