Nutrient solution temperatures may reach injuriously high levels during summers, or damaging low levels in winters, which strongly influence growth and survival of the whole plant. This parameter not only depends on solar radiation and aerial temperature but also on the characteristics of the system. In general, soilless systems are exposed to larger daily variations in root temperatures than soil systems (Kafkafi, 2001) but possibilities for accurate control of root temperatures are more easily carried out in soilless cultures than in soils systems (Olympios, 1999), through cooling or heating systems. However, sometimes an excessive energy input is spent to protect the crop from incorrectly established temperature ranges. In order to optimize the use of energy in greenhouse production, it is necessary to know the range of nutrient solution temperatures, specific for each crop cultivar (Kafkafi, 2001), which permits plant growth and promotes high yields. In general terms, root zone temperatures below 18°C and above 28°C may seriously impair uptake and root growth, hence, temperatures outside this range should be avoided (Bar-Yosef, 2008). In some cases, though, a higher product quality may be obtained on exposing roots to infra- or supraoptimum temperatures during a short period of time. For example, a treatment of one week of low temperature stress in spinach plants increased the leaf concentrations of quality compounds like sugars, ascorbic acid and Fe2+, at the same time reduced the leaf concentrations of compounds considered harmful for human health like NO3~ and oxalic acid (Hidaka et al., 2008).
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