Plant Environment: Light

1. Plant Environment: Light • Plants convert light energy into chemical energy • Light • photosynthesis • 6H2O+6CO2+light --> C6H12O6 + 6O2 • Photosynthesis provides plant with energy for growth • Effects of light on plants important in horticulture

2. Pigment formation Plant Environment: Light • Growth rates and habits • Light influences... • Flowering and fruiting • Seed germination • Onset of dormancy

3. Plant Environment: Light • Quality (color of light) • Light made up of different colors: R,O,Y,G,B,I,V • 3 characteristics of light important to plants: • Chlorophyll (main pigment that captures light in plants) absorbs red and blue light • Accessory pigments capture different colors • Quantity (amount of light) • Photoperiod (duration of light)

4. Plant Environment: Light • Spacing of plants • Must prevent shading of neighboring plants • Quality and quantity of light influences: • Orientation of plants • Taller plants to north and shorter plants to south to prevent shading • e.g. squash and bean at south end of garden, corn to north • Pruning strategies • Fruit trees often pruned to remove middle branches; admit more light to center of tree • Hedges pruned so that base wider than top to prevent shading

5. Plant Environment: Light • Etiolation of plants • Light necessary for chlorophyll formation in most angiosperms • Quality and quantity of light influences: • Plants moved to location without light become blanched • Blanching applied to several vegetables: cauliflower, celery, scallions • Potatoes should be covered completely to prevent production of chlorophyll; produces green spots • Etiolation = development of plant in absence of light • Whitish stems and leaves, elongated internodes • Etiolation useful for cuttings; etiolated stems produce adventitious roots better (light inhibits rooting hormones)

6. Plant Environment: Light • Etiolation of plants • Quality and quantity of light influences:

7. Plant Environment: Light • Pigment production • Chlorophyll breaks down when exposed to light • Quality and quantity of light influences: • Carotenoids (yellow-red accessory pigments) protect chlorophyll from photodestruction • Normally chlorophyll masks carotenoids; destruction of chlorophyll results in yellowing of leaves (=chlorosis) • Chlorosis also results from diseases and nutrient deficiencies (N, Mg, Fe, and other nutrients used to produce chlorophyll) • Leaves turn color in fall because chlorophyll destruction unmasks carotenoids (tannins present produce browns) • red leaves in some plants indicates high levels of carotenoids

8. Plant Environment: Light • Seed dormancy • Photoblastic = seeds influenced by light • Quality and quantity of light influences: • Positive or negative photoblastic • Many weeds are positive photoblastic; tilling soil brings weeds seeds to surface and breaks dormancy • Potato tubers are positive photoblastic (store in cool dark place) • Some seeds require darkness to germinate (e.g. tomato)

9. Plant Environment: Light • Branching and stem elongation • Red light results in plants with short internodes and vigorous axillary growth • Quality and quantity of light influences: • Fluorescent lights high in red light • Incandescent lights have little red light; plants grown beneath them show etiolation • Plants growing below tall plants get little red light (absorbed by leaves of tall plants) and have internode elongation

10. Plant Environment: Light • Anthocyanin production • High levels of light promotes anthocyanins (red pigment) • Quality and quantity of light influences: • Brightest fall leaves often on trees in full sun • Rain causes leaching of anthocyanins; dry falls produce most vivid colors • Many fruits produce anthocyanins while ripening; apples redder on outside of tree where exposed to more light

11. Plant Environment: Light • Photoperiod = response to changes in day length • Day length often triggers flowering • Effects of photoperiod on plants: • Day-neutral plants: plants that flower regardless of day length (e.g. dandelions, many houseplants) • Long-day plants: plants that flower when exposed to long day length (short nights) • Spinach not planted in midsummer because it will bolt (flower) • Short-day plants: plants that flower as day length shortens (e.g. tomato) • Flowering can be prevented by extending day length with artificial light or by night interruption

12. Plant Environment: Light • Shorter day length promotes formation of storage organs • Effects of photoperiod on plants: • Short days stimulates formation of tubers and bulbs (modified stems) and tuberous roots (modified roots) • Strawberry and spider plant stolons produced in presence of long day length

13. Plant Environment: Light • Sudden change in lighting can cause damage to plants • Annual flowers grown indoors need slow acclimatization to more intense sunlight outside • Acclimation to light • Tropical plants grown outdoors must be acclimatized to low light in offices and homes • Weeping figs particularly sensitive to changes in light; will drop all leaves