and iron. It has been demonstrated in some studies conducting in rats fed with soy products that the bio‐availability of zinc was low. Interactions of diets rich in oilseed crops are still under investigation as many factors could be of influence, such as food processing conditions or digestibility of the foods (Wang 2016).
Tannins not only could provide a bitter taste but also can react with trypsin and amylase or their substrates by the hydroxide radicals and reduce the utilization rate of protein or carbohydrate. They could also interfere with the absorption of some minerals such as iron ions and form insoluble complexes on the intestinal mucosal surface damaging the intestinal wall. Similarly, the combination of tannin with metal ions including calcium, iron, and zinc could form complexes with vitamin B12 and negatively affect their absorption (Wang 2016).
2.3 Factors Affecting Oil Yield
Oil yield depends on a number of interrelated factors. The potential yield is defined as a yield of a cultivar when grown in environments to which it is adapted; with non‐limiting nutrients and water; and with pests, diseases, weeds, lodging, and other stresses being effectively controlled (Evans and Fischer 1999). There is a gap between the potential yield and the actual yield of a crop. Once the type, variety, and location are determined, climatic conditions and agronomic practices have an impact on the amount of oil that can be obtained. The potential yield is primarily determined by the type of crop, as each has its characteristic fruit or seed density, and those have an average amount of percentage oil in them. Most oilseeds typically accumulate 20–50% of the dry weight of their seed tissue as storage oil, usually as triacylglycerol. However, there are some oilseed species, such as candlenut or sesame, which contain as much as 60–76% oil in their seeds (Walker 2008). Among the cultivated oil sources around the world, Palm fruit has the highest yield, followed by soybeans, olives, rapeseed, and sunflower, whereas poppy seed and sesame have the lowest yields (FAOSTAT 2018).
According to the definition, the potential yield is calculated in the absence of stresses, but there exist some abiotic factors – drought, extreme temperatures, and salinity – or biotic factors – diseases – that severely affect the growth and yield of the crops. Climatic periods of drought may affect crops around the world, and according to the studies in climatic changes, such extreme periods might affect crop yield in the near future (Müller et al. 2010). Drought conditions change physiological traits like stomatal conductance, turgor potential, relative water content, osmotic adjustment, crop temperature stability, and leaf temperature (Pasban 2009). As inflorescences, leaves, and roots respond differently to drought conditions, stress tolerance may depend on the type of crop (Azzeme et al. 2016). Olive trees are well adapted to drought conditions, and sunflower is usually characterized as a low‐medium sensitivity crop to drought, whereas rapeseed's yield decreases markedly with it (Balbino 2017). Nevertheless, in some crops like palm trees, resistance can be induced by the addition of adequate doses of boron and silicon.
It is well known that extreme temperatures may affect oil content, and also oil composition. In sunflower, high temperature during the development of the seed leads to stress on the biosynthesis of fatty acids, and not only is the yield decreased, but also the percentage of linoleic acid is decreased (Harris et al. 1978). For olive crops, the beginning of the oil accumulation period is the most sensitive to temperature increases. (García‐Inza et al. 2014) found that olive oil content decreased 1.1% for each 1 °C increase. Moreover, even a tropical crop such as coconut its resistance to extremely high temperatures depends on the humidity of the zone, being less productive in a dry zone than it is in a wet one (Pathmeswaran et al. 2018).
Salt stress alters the morphological, physiological, and biochemical performance of crops. It occurs as result of limited water uptake, which is caused by a reduction in hydrolysis and translocation of the nutrient reserves, thus reducing seedling vigor (Parveen et al. 2016). For instance, soybean grain yield decreased by 20% when salinity was 4.0 dS/m and by 56% at 6.7 dS/m. There are some methods, which can prevent salt stress, such as the addition of glutathione, which takes part in the detoxification system of the plant. When applied at 50 mM at the reproductive stage of soybean, it improved plant growth, yield, number of pods, and number of seeds (Akram et al. 2017).
Crops can be classified according to their lifecycle: annual or biannual, such as sunflower, groundnut, or rapeseed, or perennial, such as olive, coconut, or palm (Sharma et al. 2012). There are structural differences in several ways. In annual crops, new seeds for each growing season might be advantageous, while the potential yield for perennial ones is fixed for each planting cycle (Woittiez et al. 2017). The perennial crops have a yield cycle which depends on the age of the tree, and varies among the species. Four yield phases have been described: (i) the immature or building phase, before harvestable production, (ii) the young mature phase, when yield increases linearly along the years, (iii) the mature phase, when yield remains stable, and (iv) the phase of yield decline (Díez et al. 2016). In palm oil tree, the mature phase is reached at the eighth year after plantation and stands for eight years more (Goh et al. 2003). In coconut oil tree, full production is reached between the 15th and 20th years after plantation, and lasts until the plant's 80th year (Broschat and Crane 2017). On the other hand, as calculated by Vega et al. (2001), seed number per plant in annual crops can be related with their growth rate. Seed number varied from 0 to 890 in soybean, which was directly related with the growth rate of the plant. In sunflower it varied from 0 to 4096 in sunflower, and from 0 to 1348 in maize, which both have a curvilinear relationship with the growth rate.
Variety is an important factor that affects the average oil yield of a crop, which will be later affected by stresses. The oil yield has been one of the main objectives in genetics and breeding studies in oil crops all over the years (Knowles 1983; Murphy 2014). In soybeans, oil content between varieties may range from 0.9 to 11.9% (Kim et al. 2013). Regarding palm, the variety determines the thickness of the endocarp, whose development depends on the major effect shell gene. The thickness of the endocarp is highly correlated with the oil content, which may vary by 30% between varieties (Barcelos et al. 2015). Apart from the oil content of the seed, the amount of formed seeds per area unit also contributes to the yield of a crop. For instance, it is clear for sunflower that variety affects this trait (Khan et al. 2007), but some authors report that this is not a significant factor affecting sesame yield (Mesera and Mitiku 2015). However, breeders not only look for maximum yields, but also the stability of the cultivars across a range of production environments, including the cases where several stresses occur. Sometimes, maximum and stable yields do not occur in the same genotypes, soybean cultivars being an example (Gurmu et al. 2009).
Radiation also plays a role in potential yields, as it is the driving force for photosynthesis, which will allow converting carbon dioxide into organic compounds. Regions with eight sunshine hours/day would have 60% higher potential yield than regions with only three hours/day (Van Kraalingen et al. 1989). In palm‐oil, for instance, productivity is constrained if radiation is less than 5.5 hours a day. Above this, one additional hour a day yields an additional 15–20 kg bunch dry matter/palm/year compared with productivity under cloudy conditions (Paramananthan 2003). Light incidence has also a direct relationship with the cell number and cell expansion of olives, with the ones at the top of the canopy being the ones that yield the most oil (Reale et al. 2019). So, in general, under no‐stressed environmental conditions, the amount of dry matter produced by a crop is linearly related to the amount of solar radiation, specifically photo synthetically active radiation, intercepted by the crop (Campillo et al. 2012).
The ability of a plant to intercept the incident radiation depends on the leaf area available. To maximize it, most production strategies relay on achieving an optimal distribution of the plants in a way that canopies can capture sufficient solar energy (Hemming et al. 2007). In rapeseed crops, plant density has the greatest effect on yield. Seedling rate establishes the competition for nutrients and water within the canopy, and evenly distribution makes yield more stable because there are fewer losses in seed growth (Diepenbrock 2000). In olive orchards, density between 200 and 550 tree/ha is translated into a higher fraction of intercepted radiation per area, which leads to higher productivity (Gucci et al. 2012). Optimal density also depends
