HRT 100
Dr Art Cameron
Lecture 8
Spring, 2001

Study session - today and tomorrow - 3 pm today and 4 pm tomorrow -

Light and Plant Growth Indoors
 Read Hessayon - page 8,
Pages 228-229

Light - provides the energy for used for photosynthesis- photosynthesis is the process by which plants take this energy from the sun to make carbohydrates - the building blocks for life and growth

Amount of total light - affects photosynthesis - the process whereby plants fix light energy into sugars and carbohydrates.

        show curve describing process of net photosynthesis vs light

For most plants, more light means more food - at least to a point - Too little light, a plant starves. Too much light, many plants can be sunburned.

During photosynthesis -
 Light (energy) + carbon dioxide (source of carbon) 6
        Sugars (stored energy) + oxygen (by-product)

build carbohydrate reserve

Respiration
 carbos plus oxygen 6 energy for plant to do work (grow, pump water, defend against insects and diseases, etc.) + carbon dioxide

Note that plants store carbohydrates in roots, stems, even leaves and other parts of the plant.

Depletes carbohydrate reserves

Respiration is how the energy is released so the plant can take up water, build new cells and grow, flower, and basically run all other growth processes

 Houseplants adapted to very low light.
 This does not always mean that the plant can grow and flower with low light - but it does generally mean that the plant can survive long periods with very low light. Plants in your collection that will do well with little light:
Sansevieria spp., Philodendron scandens, Scindapsus, Algaonema,
 see page 8

Houseplants that prefer high light - at least for growth and flowering
 Cacti and succulents, especially - several other flowering plants
see page 8

Without high light, these plants will decline, and they probably will not flower.

Houseplants that tolerate intermediate light - usually bright but not direct sunlight - this is true for most house plants

African violet, bromeliads, begonias, peperomias, narcissus, and many more - see page 8 - be able to list 6 different plants from this category

 

Daylength - most plants seem to measure the length of the day - but actually they measure the length of the night - they can tell the difference between winter and summer. In fact, they can tell the difference between 12 and 12 hours and 15 minutes - perhaps even less in some cases.
Note that in Michigan, the length of the day varies from about 9½ (December 21) to 16 hours (June 21). The two equinoxes are on September 21 and March 21. On the equinox, the sun is perpendicular to the earth and the time from sunrise to sunset is 12 hours everywhere on earth - note how the lines cross for the examples given above.

Some plants only bloom in the summer while others only bloom in the winter.

Examples of winter blooming (short day) plants: Begonia rex, Sansevieria parva, Poinsettia.

Examples of summer blooming (long day plants) - many cacti, many outdoor perennials.

Examples of day neutral plants - those plants that bloom all year long - Oxalis regnellii, Azaleas.

Daylength - affects plant growth and development - but also affects the amount of light that a plant received - i.e. 9 hours is far less light for photosynthesis that 16 hours. IN winter plants grow slower since less photosynthesis

Daily light integral (DLI) - the total amount of light that is received by a plant each day - this is far greater in the summer than in the winter - since longer days and bright light.

Light quality - light comes in many colors - the composition of light from the sun changes little during the year. Light that first goes through plant leaves has specific wavelengths of light removed - increasing the relative amount of far-red light. One primary response of plants to far red light is to stretch. So - plants that receive too much far red light stretch - example - trees in forest grow tall and this whereas trees in full sun are short and branched.

Artificial light
Incandescent light bulbs - have more far-red light - this causes plants to stretch - also produce heat and can be dangerous

Fluorescent lights are probably the best and cheapest light source for house plants. Many house plants can thrive under fluorescent lighting - e.g. African Violets, Streptocarpus and many orchids will flower under fluorescent lights. They produce enough light to be useful without production of excessive heat.

You can combine fluorescent and incandescent to get very good light quality for raising plants - but usually this is not necessary. Best to rig up a system so the lights can be raised and lowered depending on your growing needs. Great for flowering house plants and for vegetable seedlings in the spring.

Light quality affects growth habit -

Plant responses to light

  1. Without adequate light - plants go hungry.
  2. High light inhibits plant growth - plants tend to grow taller when there is less light.
  3. Plants will usually grow towards light (phototropism). This is based in part on the fact that the dark side of the stem grows more than the lit side of the stem.
  4. Some low light plants can be injured by exposure to direct sunlight - for instance African Violets. Yellowing or bleaching of leaves caused by high light. It is easy to damage leaves in the spring if you put house plants outdoors and into the direct sunshine! Many houseplants simply cannot tolerate direct sunshine.

Tricks of the trade
  1. Clean windows let in more light
  2. Arrange plants so that larger plants do not block light to smaller plants
  3. Keep an eye on sun angle - this changes drastically in the winter
  4. Plants bend towards light - rotate regularly to expose all sides of the plants to the light
  5. Buy or rent a house with lots of sunlight and space for plants!