Midday Closure of Stomata in the Oil PalmElaeis guineensis. Jacq.

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Abstract
Under natural climatic conditions in West Africa, midday closure of stomata in 3–6-year-old oil palms occurs only in the latter half of the dry season, when soil moisture is low and air temperatures are high. Using an infiltration method, attempts were made to determine the climatic factors which affect stomatal closure. The extent of midday closure is related closely to air temperature and shade temperature under the palm, agreeing closely with the findings of Meidner and Heath (1959) on onion, but the aperture-temperature curves are different for watered and unwatered palms in the dry season, the difference being greater at higher temperatures. Watering reduces closure by changing the aperture-temperature relationship. Under natural conditions this effect is usually obscured because of lower temperatures in the dry season following rain. During Harmattan weather, with very low relative humidities, the aperture-temperature relationship broke down. The results are discussed in relation to current views on midday closure of stomata.