Hello everyone,

I'm trying to find the percentage of sunlight needed for a proper
photosynthesis and for plants to grow properly.
For example on a cloudy day where not all of the sunlight gets to the
soil, plants apparently still grow fine (or not?) so what is the
minimum need for a plant?
If the sun was only producing 50% (or 60, 70%...?), would the plant
still have enough light for photosynthesis?
I'm not taking into account the heat generated by sunlight and also
obviously needed by the plant. I just want to know about the percentage
of light needed.

I guess it's different for every species but I'd like an approximate
answer if possible. If you answer with message, please take into
account the fact that I'm not at all a scientist, so try to make it
basic please.

Thanks in advance.

Chris

Plant-light relations are complex because light levels vary throughout
the day, from day to day and throughout the year in natural
environments. Different plant species also differ greatly in their
light requirements. Too much light will harm obligate shade plants,
such as African violet. There are also obligate sun plants, such as
sunflower, which grow very poorly or die with too much shade. Many
plants are very adaptable to different light levels. Tree seedlings may
survive and grow very slowly for many years in deep shade in a forest.
When surrounding trees die or are cut down, the seedlings then can grow
quickly in the higher light. Many of the ornamental plants marketed as
plants for shade, such as impatiens and caladiums, may grow faster in
full sun than shade, if they have sufficient water.

Assuming other factors, such as water, mineral nutrients, temperature,
carbon dioxide, etc., are not limiting plant growth, most sun plants
will grow faster with more light. For a single leaf, photosynthesis
will usually reach a maximum (Light Saturation Point or LSP) and start
to level off well before full sunlight (see Reference 1). In reference
1, LSP is about 50% of full summer sunlight for bean (Phaseolus
vulgaris), a sun plant. For some sun plants, the LSP for the whole
plant will be higher than the leaf LSP. The higher whole-plant LSP
allows leaves deep within the canopy to have sufficient light for
maximum photosynthesis. On the same plant, LSP is higher for sun leaves
than shade leaves. Sun plants will have a higher LSP than shade plants.


The light compensation point (LCP) is one way to quantify the minimum
plant need for light. The LCP is the light level where photosynthesis,
usually measured in a single leaf, just equals cellular respiration.
Cellular respiration is basically the reverse of photosynthesis because
sugar is broken down to release energy for the plant and carbon dioxide
is released. LCP varies depending on the plant species, the light
environment and other environmental factors. Sun leaves will usually
have a higher LCP than shade leaves. Shade plants usually have the
lowest LCPs.

Figure 3 of reference 2 indicates that the LCP for sun leaves of a
sweetgum tree in June was about 30 micromoles per square meter per
second of PPFD (photosynthetic photon flux density) and 15 for shade
leaves. PPFD is a measure of light level. Full summer sunlight at noon
on a cloudless day is about 2,000 micromoles per square meter per
second of PPFD. Thus, the LCP of the sun leaves was 1.5% of full summer
sunlight, and the LCP of the shade leaves was 0.75% of full summer
sunlight.

The daily (24-hour) LCP for the whole plant would be higher in order to
allow for extra photosynthesis to supply the nonphotosynthetic parts
such as roots and stems and to store some photosynthates for the daily
dark period. The leaf LCPs give an indication that the plant LCP can be
low, probably less than 5% of full summer sunlight.

LCP may also be given in foot candles. For comparison, full summer
sunlight is about 10,000 foot candles. Shade tolerant houseplants, such
as pothos and philodendron, may have LCPs of about 10 foot candles (see
page 56 of reference 3). That is 0.1 % of full summer sunlight.

Reference 4 reported that for eight sun plant species LSP was
2,000-2,500 foot candles and LCPs were 100-150 footcandles. For five
shade plant species, LSPs were between 400 and 1,000 foot candles and
LCP was about 50 foot candles.

Reference 5 has several informative graphs including ones showing how
LSP and LCP can differ for the same species grown in high and low light
and between sun and shade species.

David R. Hershey


References

1. Light Saturation Point graph
http://chemsrv0.pph.univie.ac.at/hort/aq.html

2. Herrick, J.D. and Thomas, R.B. 1999. Effects of CO2 enrichment on
the photosynthetic light response of sun and shade leaves of canopy
sweetgum trees (Liquidambar styraciflua) in a forest ecosystem. Tree
Physiology 19: 779-786.
http://face.env.duke.edu/PDF/tp19-99.pdf

3. EFFECT OF LIGHT INTENSITY ON PHOTOSYNTHESIS/RESPIRATION RELATIONS OF
SUN VERSUS SHADE PLANTS
http://generalhorticulture.tamu.edu/...ght/light.html

4. Bohning, R.H. and Burnside, C.A. 1956. The Effect of Light Intensity
on Rate of Apparent Photosynthesis in Leaves of Sun and Shade Plants.
American Journal of Botany 43: 557-561.
http://links.jstor.org/sici?sici=0002-9122(195610)43:8%3C557:TEOLIO%3E2.0.CO;2-7

5. Koning, Ross E. 1994. Photosynthetic Environment. Plant Physiology
Information Website.
http://plantphys.info/Plant_Physiology/photoeco.html (6-2-2006).



wrote:
Hello everyone,

I'm trying to find the percentage of sunlight needed for a proper
photosynthesis and for plants to grow properly.
For example on a cloudy day where not all of the sunlight gets to the
soil, plants apparently still grow fine (or not?) so what is the
minimum need for a plant?
If the sun was only producing 50% (or 60, 70%...?), would the plant
still have enough light for photosynthesis?
I'm not taking into account the heat generated by sunlight and also
obviously needed by the plant. I just want to know about the percentage
of light needed.

I guess it's different for every species but I'd like an approximate
answer if possible. If you answer with message, please take into
account the fact that I'm not at all a scientist, so try to make it
basic please.

Thanks in advance.

Chris