Quality Vs. Quantity of Sunlight

saoodhashimOctober 20, 2013

Many of the fruiting vegetables require full sun (atleast 6 hours of direct sunlight as they define it). However, I was just wondering if the intensity of the sunlight also has any effect on reducing this requirement? In a desert climate of Saudi Arabia - even in October - the sunlight is so intense that the leaves burn if not shaded. I have time and again read that vegetables plants prefer / appreciate shading in desert climates but then this raises the question as to how the sunlight quantity requirements (of atleast 6 hours) gets fulfilled to produce a comparable output with that of more moderate USDA zones (6-7-8)?

Whether this 6 hours requirement is for places where the intensity is moderate or is is applicable to vegetables grown in any climate zone? Can plants growing in desert climate work as good with 4 (or lesser) hours of direct sunlight compared to 6-8 hours of sunlight in more moderate climates? Can the intensity of the sun compensate for the reduced number of hours of direct sunlight? Is there any sort of scientific explanation for this phenomenon?



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Great question. Hopefully someone can answer. I have always wondered this. I also want to ask if you think Phoenix or Saudi Arabia us hotter? I think there cannot be any place hotter than here. I wonder why people would want to move here, except for the gardening.

    Bookmark   October 21, 2013 at 12:45AM
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I think Al Khobar, the city that I live is hotter than Phoenix. Average temperatures over here are 5-10F higher than in Phoenix. Riyadh, the capital city of Saudi is more extreme - with both hotter summer and colder winters than Al Khobar. So on that front I win (or lose - depends on which side you want to be) :-)

I posted this question on a few other forums of GW and while all the replies that I got were just opinions, one link someone forwarded me was a bit closer to being scientific. It does say and I will quote

"Subject to physiological limits, an increase in the intensity of light will result to an increase in the rate of photosynthesis and will likewise reduce the number of hours that the plant must receive every day (Manaker 1981)"

Below is the link.

I am still however, waiting for a more clearer answer, if possible.

Here is a link that might be useful: What is Light Intensity, Effects on Plant Growth

    Bookmark   October 21, 2013 at 1:28AM
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that question of intensity affecting required daylight hrs of sunlight is interesting. I seem to remember learning about this effect during one of my college classes, but I don't remember the details.

Seems that in our area, we worry more about how to reduce the intensity of the sunlight (shade cloths, lath houses, etc.) rather than worry if we are getting enough sun exposure.

I've noticed in more humid areas (like coastal Calif), the atmosphere humidity tends to "soften" the intensity of the sun, making it much more pleasant than the laser beam sunlight we get here in the desert.

    Bookmark   October 21, 2013 at 9:45AM
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From personal experience, and from my classes at the Desert Botanic Gardens ... even when a plant is rated for "full sun" it often needs shade in a Phoenix summer. I plant my chilis and eggplants for morning sun and afternoon shade. They get 4-6 hours of direct sun in the mornings.

You are 26ð17â²N and Phoenix is 33ð27â²N (so we're not wildly different in sun angle) Where we differ is in the humidity, with Al Khobar being seacoast and really humid. We are more likely to have frosts and killing low temperatures, your climate just cooks the plants. :)

There is probably an atmospheric physics formula that can calculate the difference between light intensity at different latitudes, but I have no idea where to look.

A website you might like is desert-tropicals.com because they report on what really grows in Phoenix.

If you are gardening, sunset.com might have helpful planting schedules (you are in Zone 13 for their purposes), and the Maricopa County Master Gardeners planting schedules might be useful.


    Bookmark   October 21, 2013 at 6:24PM
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GeeS 9b

Plants need light to engage photosynthesis. Extremely intense sunlight, in and of itself, does not harm these plants. It is the heat associated with intense sunlight that harms them. With that in mind, I would not curtail the duration of sunlight, but strive to limit the intensity.

    Bookmark   October 21, 2013 at 7:08PM
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What are you growing there in Saudi Arabia? What's the water situation there. We have little rain here but plenty of water gets wasted with people growing grass.

    Bookmark   October 23, 2013 at 11:35PM
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Thanks you all for your input.

At the moment, I am growing cucumbers, Armenian cucumbers, tomatoes and cherry tomatoes. The cherry tomatoes seem delicate and are finding it difficult in the extreme heat. I think they need extra protection. Also same is the case with hot peppers.

I am growing on a small scale in containers as this is my first experience with any kind of gardening. Water therefore is not a major issue. We have plenty of tap water :)

    Bookmark   October 24, 2013 at 2:14AM
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Tomatoes shut down fruit ripening when it gets too hot ... you might have betterluck with them in your "winter". There are a few exceptions - selections made to grow along the west coast of Mexico can handle hotter weather than

Armenian cucumbers seem to love the hot weather, English style cucumbers prefer cooler weather.

You might also have luck with cultivars from Israel, Egypt and India. I have seeds a Indian expat brought back with him and they thrived most of the summer.

    Bookmark   October 24, 2013 at 1:59PM
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