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Thread: root zone chilling(handeling the heat)

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    Default root zone chilling(handeling the heat)

    found this info online thought i would share.


    Root Zone Chilling: Handling the Heat

    Heat loading in indoor hydroponic gardens is a common problem. The combination of high intensity lighting, small enclosed spaces, limited air exchange and sweltering summers all take their toll on plants. Small volumes of recirculating nutrient and restricted growing containers also make a contribution to heat build-up in the root zone, often to the point where roots can cook without the grower even being aware of the problem. A hot environment is hard to handle, often rather expensive to keep cool, and if combined with high humidity, the tropical conditions have some nasty consequences for plant growth and development. However, hydroponic systems can offer some significant advantages when it comes to a little manipulation of a plant’s physiology to boost yields and keep crops happy. Root zone temperature strongly affects shoot growth. In fact root zone temperatures play more of a role in growth and development than that of the air surrounding the plant. This is because the root tissue sends numerous `non hydraulic’ messages to the shoot, which influence the way the shoot responds to the environment. So with many plant functions under the control of what goes on down in the roots, temperature in the root zone becomes an extremely important factor to be keeping an eye on.

    Ignorance is not bliss

    Many of us have probably seen the effects of high air temperatures of crops—wilting, reductions in growth, tip burn, premature bolting, loss of chlorophyll (leaves going pale and yellow), flower and fruit let drop, overall losses in yield and increases in root disease pathogens such as pythium that prey on stressed root tissue. At high temperatures, plant tissue has a rapidly increased rate of respiration that burns up sugars produced during photosynthesis, leaving much less for growth and development.


    Lettuce being grown in a deep flow system with chilled nutrient solution under tropical conditions, showing the impressive root structure of aeroponic production.
    While most growers keep a regular check on the air temperature surrounding their plants and aim to keep this within an ideal range for the species being grown, few realize the potential of root zone temperature control. Nutrient temperature build-up can be surprisingly fast and become excessive under hot lights and in systems with very limited root zones, so regular root zone temperature checks are a good idea. Research has shown that even a short duration, less than 30 minutes of root zone heat build-up, can have a very negative effect on many crops, which can not be compensated by having a low daily temperature average. Just a few minutes a day of root zone temperatures over 86°F have been shown to retard the growth of some heat sensitive crops such as lettuce and parsley.

    Root zone temperature physiology

    It seems that chilling in the root zone via the nutrient solution affects the plant’s physiology in a number of different ways, which allows the aerial portion of the plant to withstand higher than optimal temperatures. Firstly, we know a cooler nutrient solution holds more dissolved oxygen for root uptake, so that oxygen starvation is likely to be far less of a problem than it would be with a warm nutrient. Secondly cooling of the roots to well below ambient air temperatures seems to allow higher assimilation rates by reducing both photo inhibition and stomatal closure that typically occur once the plant becomes temperature stressed. What is even more interesting is that the positive effects of nutrient chilling seem to be largely the result of changes in the production of plant growth hormones abcissic acid (ABA) and cytokinins that control a range of plant responses. Cytokinins are mainly synthesized in roots and any environmental stress in this tissue such as high temperature, results in a shortage of this hormone being sent up to the shoots. Very low cytokinin concentrations in the roots of even warm season crops have been found after only five days at 90 to 100°F suggesting that the response of cyctokinin deficiency to high temperatures is very rapid. Cytokinins are important as they stimulate cell division, cell enlargement, chloroplast formation, synthesis of chlorophylls and proteins and in general, drive plant growth and development. The small leaf area, stunted growth and leaf yellowing often seen in heat stressed plants may be at least particularly attributable to a reduction in cytokinin synthesis in the roots. ABA is known as a stress hormone and has been found to increase in leaves when roots are exposed to high temperatures. ABA is inhibitory to growth and results in stomatal closure and the reduction in photosynthesis that results as the plant starts to shut down.

    Tricking plants into handling the heat

    Apart from keeping a close check on root zone temperatures to make sure they are not over heating and cooking the roots, there is a well proven technique that can be used to fool the physiology of many plants into handling higher then optimal air temperatures. Root zone chilling of the hydroponic nutrient solution is a technique being used commercially by many growers in warm or tropical climates, most often with cool season crops such as butter head lettuce, herbs and other vegetables. In Singapore NFT, aeroponic and deep flow culture systems are utilized with extensive nutrient chilling to grow butter head and Romaine lettuce, crops that otherwise do not grow or yield well at ambient air temperatures. Chilling the nutrient solution down to as low as 61 to 64°F, allows the cool season vegetables to crop well at ambient air temperatures that are often well above optimal for these crops (82 to 97°F). Without nutrient chilling, the root zone usually warms to the level of the air and this give numerous growth problems including slow growth, lack of heart formation, bolting, tip burn and low marketable yields. Other researchers have reported that nutrient chilling of lettuce also reduces the occurrence of the fungal root disease Pythium aphanidermatum. Chilling the nutrient tricks the physiology of the plant into growing in air temperatures that would otherwise not be economic. However, trials have shown that while root zone chilling via nutrient cooling can have these effects, the root zone cooling must be applied soon after early crop establishment and maintained for the life of the crop for maximum effect.

    “...we know a cooler nutrient solution holds more dissolved oxygen for root uptake, so that oxygen starvation is likely to be far less of a problem than it would be with a warm nutrient.”While this technique of root zone chilling is perhaps more economically viable with high value crops in tropical climates such as cool season lettuce, herbs and other vegetables, it has also proven to be beneficial for other species. Warm season plants such as capsicum and cucumber have also been shown in research trials to respond in a similar way to root zone chilling when ambient air temperatures are higher than optimal for their growth and development. Cucumber is a warm season plant whose optimal root zone temperature lies around 77°F; once root temperatures increase to 95°F, severe reductions in shoot growth can begin to occur. Cucumbers grown at high air temperatures (100°F) have been found to have a larger leaf area and higher yield when the root zone was cooled to below 77°F. Capsicum (sweet pepper), another warm season crop has been found to respond to nutrient chilling when grown under warm aerial conditions in tropical Singapore. It was found that capsicum plants grown at a root temperature of 68°F had more leaves, greater leaf area and dry weight than plants grown at ambient root zone temperatures of 77 to 104°F even though both were under the same ambient tropical conditions of high temperature and light intensities. Under normal growing conditions for capsicum (71 to 79°F), having a nutrient solution temperature of 68°F would result in growth reductions, however, under the high temperature tropical conditions, root zone chilling to 68°F had significant benefits to crop growth and development via altering the physiology of the plant to handle the temperature stress. Red basil plants, while normally considered to be a fairly warm season crop, have been found to have increased vegetative growth, more intensive development of purple anthocycanin pigment as well as increased development of aromatic compounds in the leaves when grown in a chilled nutrient at 77 to 79°F under tropical conditions.

    Cool season vs. warm season crops and nutrient chilling

    Obviously for the technique of root zone chilling via the nutrient solution to work effectively the correct temperature needs to be constantly maintained and this optimal temperature is likely to not be the same for all species. Cool season crops such as lettuce may benefit from nutrient chilling to a cooler temperature than warm season crops and there is evidence to suggest that root zone chilling is only beneficial when ambient air temperatures are consistently higher than optimal. Furthermore, as with any technique or treatment, overdoing it can have some fairly drastic results—chilling the nutrient in a crop that is not under high air temperatures could potentially result in growth reductions or chilling damage, particularly with warm season plants. Such over chilling results in root tissue damage and decay, reduction in nutrient ion absorption and water uptake.

    Best systems for nutrient chilling

    Various studies on the effect of nutrient or root zone chilling for hydroponic crops under high heat conditions have shown that deep flow water culture gives the best results for many crops such as lettuce, herbs and other vegetables. Aeroponics and nutrient film techniques are also commonly utilized for root zone chilling; however, these seem less effective than deep flow methods. It is possible that the larger volume of chilled nutrient solution in deep flow systems maintains cooler temperatures for longer, while the thin (two to three millimeters deep) flow of NFT and fine mist of aeroponics tend to heat up more during application. Another possibility is that the more of the root system that is submerged in the chilled nutrient, the greater the effect on the physiology of the plant. In general, however, solution culture methods are the only effective way of providing consistent chilled nutrient solution and in insulated systems can be reasonably cost effective.

    By Dr Lynette Morgan

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  3. #2

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    nice bit ov info there
    pollenmanandy
    peace out

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    Yeah nice one!

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    Something abit different mate! Cheers

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    think i should have posted it in the hydro section, my bad, if a mod see's this could you move it for me? ta EB

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    man i need to know just the opposite,i have my stuff in a hidden place outside,and my roots are to cold,in my homemade boxes,and i am currently unemployed without any cash flow,has any one any ideas-please help

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    Cracking read that mate

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