There are many aspects attributed to yield increase, such as higher temperature, solar radiation, adapt plant density, CO2 concentration, temperature, and leaf area index. Among them, solar radiation regards as one of the important factors which largely influence the growth of crops. The radiation crops receive to determine the yield result. That explains that more and more growers concern about using grow lights to improve the harvest yield of grown crops.
The Netherlands is one of the frontier countries apply supplement light in the greenhouse. In the last 30 years, the Dutch growers have usually taken a 1% additional light results in 1% additional growth and production. According to their experience, by lettuce, cucumber, rose, freesia, poinsettia, and Ficus, it is important to have high light levels during their growing process. All growers are convinced that the relative effect of light is larger in winter than in summer.
To test the 1% additional light results in 1% additional growth and production principlethe scientists choose soil-grown vegetables, fruit vegetables, cut flowers, and bulb flowers to test quantification of the growth response to the light quantity of greenhouse Grown Crops.
Soil grown Vegetables (Lettuce, Radish)
For soil grow vegetables by increasing 1% radiation leads to 0.6% to 1.4% increase in the yield. The formation of radish tubers is strongly dependent on light. The effects of light have more influence on tuber weight than on sprout weight, and consequently, the shoot/tuber ratio increases at low light. In the final stage of tuber development has a stronger effect on the final tuber weight than light during the initial phase of the crop. However, when a lack of light, a 1% decrease in radiation leads to a reduction in both the fresh and dry weight of lettuce heads by about 0.8%.
Fruit Vegetables (Cucumber, Tomato, Sweet Pepper)
Green LEDs with high PPF (300 µmol/m-2/S-1) are most effective to enhance the growth of lettuce. Supplementation of green light enhanced lettuce growth under red and blue LED illumination.
When reducing light during the growth of cucumber, it will change in dry matter partitioning and water content, hence, the light reduction has a stronger effect on cucumber yield than duration during a short period. The reduction in cucumber yield due to 1% less radiation varied in most cases between 0.6 and 1.2%. Pepper is slightly more sensitive to light than are tomato and cucumber, since when reduce 1% light, reducing the yield of tomato between 0.6 and 1.1% and 0.8 and 1.3% sweet pepper. For fruit vegetables in general, a 1% light reduction leads to an average yield reduction in the range of 0.8 to 1%.
Cut Flowers (Rose, Chrysanthemum)
LEDs can play a key role in floriculture by providing a suitable light spectrum (quality and duration). Light controls the circadian rhythm of plants which means the clocking of plants to day (light) and night (dark) cycles, and this circadian rhythm influences photomorphogenesis.
Red and far-red light have been shown to affect photomorphogenesis, thus, the ratio of red and far-red light plays an important role in the regulation of flowering. Flowering in plants is mainly regulated by phytochromes (a group of plant pigments), which occur in two forms: Pr (responds to red light) and Pfr (responds to far-red light). These two pigments (Pr and Pfr) convert back and forth. Pr is converted into Pfr under red light illumination and Pfr into Pr with far-red light.
The active form which triggers flowering is Pfr. Pr is produced naturally in the plant. The ratio of Pr to Pfr is in equilibrium when the plant receives light (day) because Pr is converted into Pfr by red light and Pfr is converted back to Pr by far-red light. Back conversion of Pfr is however also possible in a dark reaction, so it is the night (dark) period which mainly affects the ratio of Pr to Pfr and controls the flowering time in plants
In terms of cut flowers such as rose, whether flowers can get enough light affects quality effects. Similar to soil grow vegetables a reduction of 1% light reduced yield between 0.4 and 1.2%. A decrease in radiation results in fewer shoots, lower shoot weights, and less quality in rose. Moreover, when flower buds receive not enough light, it can cause blind shoots. In winter, lack of light have a great influence on flowers. In chrysanthemum, the effects of a 1% reduction in radiation led to yield reductions varying between 0.3 and 1.0%, but in most cases, a yield reduction of 0.6% fitted reasonably with the observations.
Bulb Flowers (Freesia, Lily)
For several long-day plants, the addition of far-red light (700-800 nm) to red light (600-700 nm) extend the day length promotes flowering and growth. The higher energy efficiency and longer lifetime are the most important advantages of LEDs in floriculture.
When reducing 1% light, it decreases around 0.5% fresh weight of shoots and till 1.2%. These data were based on summer experiments. In winter the effects of light are likely to be stronger.
In conclusion, a 1% light increment results in 0.5 to 1% increase in the harvestable product can apply to most of the crops. As a rule of the thumb, the following values may be used: 0.8-1% for soil-grown vegetables, 0.7-1% for fruit vegetables, 0.6-1% for cut flowers, 0.25-1.25 for bulb flowers, 0.5-1% for flowering pot plants and 0.65% for non-flowering pot plants. These are average values.