Most greenhouse crops consist of 85 to 95% of water, with the remaining part being dry matter. Some plant parts (e.g. cucumber fruit) are only 3% dry matter. Water is absolutely vital for plants and has many functions. The more we understand the roleof water, the better we can control plant growth. Water provides building blocks for plant tissue, fills plant cells, acts as a transport medium, provides firmness to leaves and cools the plant. Plant water status is the balance of water uptake and waterloss. In a greenhouse crop, these are controlled by the grower through irrigation and climate control. Water loss, or transpiration, depends on the weather (sun, humidity, wind), as well as on leave area and pores in the leaves (stomata). These stomataopen and close very dynamically to regulate the transpiration. Air humidity is, after radiation, the second most important factor that influences transpiration. Humidity is complicated because it has various "appearances' (relative humidity, vapour pressure, VPD, dew point) and is expressed in various units (%, g/m3, kPa). This article discusses the working of stomata and the effect of air humidity on transpiration. It explains the terminology around air humidity (see Table 7), and shows the unit conversion (Table 2]. In a future article we will discuss how to control air humidity in greenhouses.
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