Dictionary of botanic terminology - index of names

After hardiness ratings, soil and watering requirements one of the most important environmental factors to consider for plants growing is the amount of sun exposure they need or can tolerate.
Light is critical for plant growth and blossoming.

• "Full sun" generally means that plants should receive and tolerate at least six hours or more of direct sunlight  per day given that the most intense sunlight occurs mostly in the afternoon (with a little in the late morning), shadows during these hours have a big effect on how much light plants receive. Six hours of direct sun during the morning is very different from the same six hours in the afternoon. Morning sun, even if it is six hours worth of direct sun is still not strong enough to be considered anything more than partial sun (or part shade) The afternoon sun has a stronger intensity which makes it more powerful.
• "Bright shade" An environment that receives between 4-6 hours of direct sun, such as a very bright location with a high, diffuse canopy of trees.
• "Partial shade" (Also "light shade" or "half shade") An environment that receives between 2-4 hours
  of direct sunn (morning sun and  afternoon shade) or a filtered shade through a canopy of shade trees in summer. Species which require some protection from prolonged hot sun, but which will tolerate some sun . Often these species prefer morning sun and afternoon shade.
• "Full shade" is an environment with no direct sun exposure or receiving  little direct sun, usually less than 2 hours such as a ground among buildings or dense evergreen tree growth which often prevent anything but dappled light from reaching the ground.

Some plants are particular as to light required while others are more adaptable. Species that will tolerate only a few hours of morning sun in the summer, if that. These species will often tolerate considerable winter sun and generally do well planted so that they can take advantage of the annual shift in the sun’s angle.

Sun exposure requirements that are appropriated fort mature plants, may not  been suitable for young seedlings and plants that don’t receiving adequate care (water, fertilizer). Pay also attention to ventilation and heat,  plant grown  in a hot green house with a bad air circulation necessitate protection in the most intense time of the day (11 to 2 pm)

Sun exposure is a very  important planning consideration. Trees, fences, and buildings can all affect exposure. Matching plants to light levels will go a long way toward creating a successful and healthy plant collection.  Taking time to see the light of a garden is very worthwhile.

Placing a plant in light conditions that are outside their ideal range will likely cause stress and could in extreme cases cause death. Sun-loving plants growing in shade will be lanky and flower poorly; shade lovers in sun will probably turn a light green, wilt easily, and exhibit epidermis scorch. Make sure that plants to be placed in areas receiving full afternoon sun can tolerate lots of heat and light.

The intensity of the sun does vary according to geographic location and period of the year. An afternoon of full sun in Egypt is not the same as an afternoon of full sun in England. In this case, the distance that separates the two locations has a considerable effect on the intensity of light. Plants that might not tolerate full sun in Egypt could be very happy in England without any protection. And also plants that need to be grown in shade or partial shade in summer can be grown and take advantage in a full sun exposure during the winter. 

Many plants grow their best in partial sun (or part shade). In this situation, temperatures are moderated slightly and relief from the blazing sun is present for part of the day. Flowers can last longer and colours often are more intense in part shade locations. Filtered light during the most intense time of the day (11 to 2 pm) is much more beneficial than any other time of the day. Bright light (not direct sun) during mid-day often produces a better garden than full sun in the morning or late afternoon. It is the intensity of this mid-day light that has the most impact on the growth of plants.

See also:  Filtered sun