**4.2 Ecophysiology of palm oil**

Palm oil is traditionally grown in areas with an annual rainfall of more than 2000 mm and yields are always higher in countries such as Malaysia and Indonesia, which have more uniform rainfall compared with countries such as Nigeria, the Republic of Benin, and Ivory Coast which are noted for dry seasons. Irrigation tests conducted in these countries have shown positive responses to irrigation in terms of growth and yield. The availability of water in soils of palm oil plantations plays an important role in its proper growth [1] and serves as a signal for sex representation [14]. In areas where water is scarce, a large number of male flowers are observed to be produced, which is combined with slower growth rather than poor productivity. Basic information regarding water stress response in palm oil is a hot topic that needs to be further explored for controlling water tolerance. Water scarcity is a major biological stressor that spreads across the world over more than 1.2 billion hectares, especially in rainfed areas [15–17]. A dehydrated environment has been reported to be a major determinant of plant growth and development before the loss of productivity, especially crop species [18–20]. However, the basic knowledge of morphological, physiological, and biochemical responses in palm oil when exposed to water stress or deficit is still meager.

In India, palm oil was promoted as an irrigated crop, as the rainfall is much less than 2000 mm in most of the areas it is cultivated. It is estimated that about 150 mm of water per month is required by palm oil to meet its evaporation requirements. Under Indian conditions, several months of the year receive more or less rain, while the other few months receive heavy rainfall. A month that receives less than 150 mm of rainfall without sufficient water reserves in the soil is called a deficit month. When there is not enough water for evapotranspiration of palm oil, like other plants, this controls outflow loss by closing the stomata. When stomata are closed, photosynthetic activity is affected, affecting both growth and yield [21].

When evapotranspiration exceeds rainfall, the soil water content decreases and may reach a point at which the palm cannot extract water from the soil quickly enough for transpiration to continue at the potential rate. The palm will then start to suffer from water stress and the plant water potential will decrease. Under such situations, seasonal water deficit becomes the most important climatic factor affecting palm oil growth and yield.

A variety of different irrigation methods have been used for irrigating palm oil. The simplest and cheapest method is to control the water table level by flooding or blocking drains, but this is only applicable in relatively flat and low-lying areas. Other methods involve significant capital investment in the form of pumps and piping. Corley [22] suggested that drip irrigation might be less effective than sprinkler or flood irrigation. Plants subjected to water deficit not only show a general reduction in size but also exhibit characteristic modifications in their structure particularly the leaves with reduced cell division and leaf area. In such cases, stomata get closed early and gaseous exchange between plant and atmosphere stopped, and photosynthesis decreases earlier than the soil moisture potential reaches to permanent wilting point. Reduction in photosynthesis accompanied by increased respiration reduces assimilation in the plants and reduces the crop yield. Depending on the stage of crop growth, moisture stress has variable effects on physiological and biochemical processes.

In India, palm oil has been regarded as a smallholder's crop under irrigated conditions that have a marked deviation from the traditional areas, *viz.,* Malaysia, Indonesia, etc. where it is grown as a complete rainfed crop in larger nucleus estates. It is being grown in traditional areas with a well-distributed annual rainfall of over 2000 mm with no marked dry spells to areas of regular seasons with an accumulated water deficit of 600 mm per annum. The water deficit can be compensated by the

#### *Management and Processing of Palm Oil (*Elaeis guineensis *Jacq): The Crop for Future DOI: http://dx.doi.org/10.5772/intechopen.108579*

provision of irrigation water. Also, palm oil requires a relative humidity of more than 45% for optimum transpiration. There will be a severe limitation of growth if the relative humidity is 30–35%. In addition to relative humidity, vapor pressure deficit also plays a vital role in influencing the growth rate, especially in the perennial crops.

Evapotranspiration is the sum of the evaporation and transpiration from plants. Transpiration is a continuous process caused by the evaporation of water from a palm leaf on one hand and its absorption by the roots into the soil on the other hand [23]. When evaporation exceeds the precipitation rate (rainfall or irrigation), a water deficit occurs. Groundwater deficit is the amount of water available from the soil in the active root zone of a crop. This is the actual amount of water needed to fill the root zone to bring the soil moisture level back to field capacity, which is the percentage of water remaining in the soil two or three days after the soil is saturated and the free drainage almost stops.

Water is an important component of plant tissues and a means of metabolism and metabolism in plants is essential for cell expansion by increasing its turgor pressure. In water deficiency, many physiological processes related to growth are affected and severe deficits can cause plant death. The effect of water deficit varies with the level and duration of water stress and the growth phase of palm oil. The leaf area can be reduced and this will reduce the amount of light that is intercepted. The reduced leaf water potential will close the stomata, so the plant will reduce its rate of transpiration, which is caused by an increase in leaf temperature, thus reducing biochemical processes. It will cause interference to separate the source and sink partitioning.

Even well-watered palms, as described by Corley [22], were found to close their stomata at noon when the sun was at its brightest and this could result in a 10% potential loss of yield. Despite high irrigation, low atmospheric pressures of relative humidity and high temperatures cause high vapor pressure (VPD) deficits, which can affect carbon accumulation. This was presented by Henson [24] who showed that the palm oil closed during the high VPD even though the soil moisture was not limited. Stomata begin to close when dehydrated, thereby affecting physiological processes [22]. Rees [25] showed that stomatal closure occurred during the second half of the day during the dry season under Nigerian conditions. Wormer and Ochs [26] reported that stomatal closure occurred during the dry season in Cote d'Ivorie. If the drought is prolonged, the palm can reach a critical threshold beyond which the water content of the tissue decreases rapidly. As a result, the leaflets get heated and dry out; hence, a large number of dried and broken leaves are observed as a result of severe stress. Finally, it entails the vegetative distribution lasting for several months or may cause the death of the palm.

#### **4.3 Irrigation management in palm oil**

Though palm oil is a typical humid tropical crop, it has been adapted to a wide range of climatic conditions ranging from tropical to semi-arid tropics. The climatic conditions prevailing in various palm oil-growing states of India are different from traditional palm oil-growing countries. In this context, irrigation management is one of the most critical aspects of palm oil cultivation. Irrigation is adopted to supplement the soil water reserve to meet the evapotranspiration demands of the crop, with an aim to increase plant growth and yield. A deficit or surplus of water would create stress on palm oil and adversely affect the yield of other crops. For palm oil irrigation without any deficit is considered optimum, which means that irrigation should be given at such rates and frequencies so that water is readily available for the plants with minimal losses.

