Plant nutritional status, specifically nitrogen (N), plays key roles in olive growth and productivity (Rodrigues et al., 2011a). Plants develop unique physiological and diverse developmental mechanisms that control N root uptake capacity, adapting to fluctuating N resource availability (Nacry et al., 2013). Because of the transient nature of N in the soil/plant system, Rodrigues et al. (2012) recommends that N be applied every year to increase the nutrient-use efficiency of olive trees. However, the requirement of optimal N concentration in leaves and roots is controversial. For example, in young olive trees ‘Meski’ and ‘Koroneiki’ N deficiency (no N applied) reduced net CO2 assimilation rate but did not significantly change the maximum quantum efficiency of PSII photochemistry (Boussadia et al., 2015). Although N-deficient olive seedling had higher levels of starch, mannitol, sucrose, and glucose the same cuttings had lower leaf N and chlorophyll a content, leaf dry weight, and photosynthetic capacity (Boussadia et al., 2010). Troyanos and Roukounaki (2011) found that olive cuttings subjected to high levels of N (6.25 g/tree N) had higher leaf dry weight and shorter root system compared with control-untreated cuttings.
The optimal N content is known as the minimum shoot N content required by a crop to reach maximum growth (Boussadia et al., 2015). Developmental root components including root hairs and lateral roots are particularly highly sensitive to the internal and external concentration of nutrients such as N and phosphorus (P) (López-Bucio et al., 2003). In olive, it has been shown that the leaf N concentration greater than 1.7% (dry weight) has negative effects on flower and oil quality (Molina-Soria and Fernández-Escobar, 2010). Olive cuttings fertilized with 0.75 g N per pot had higher shoot growth than those that received 2 g N (Fernández-Escobar et al., 2004). Interestingly, elongation of fine cuttings roots of cv. Koroneiki was enhanced under severe N deprivation (Boussadia et al., 2010).
In addition to N level, N form can significantly affect root and shoot growth and development (Lu et al., 2009; Takács and Técsi, 1992). Moreover, N form and nitrate (NO3−)/ammonium (NH4+) ratio play a key role in shoot and root development and the ultrastructure of chloroplasts (López-Bucio et al., 2003; Takács and Técsi, 1992). Nitrogen is available for plants in either organic (free amino acids) or inorganic (NO3− and NH4+) forms (Kiba et al., 2011), with NO3− being the main source of inorganic N (Andrews, 1986). The reduction rate of NO3− to ammonia in plant shoot to fulfill its essential functions as a plant nutrient depends on the level of nitrate supply, cultivar, and growth stage (Bouranis et al., 2004). Olive cuttings that received 16 mm urea-N had better growth compared with those that received either NO3−, or NH4+, or the combination of NH4+ + NO3−. However, at lower N levels using (1 or 8 mm), higher plant growth was noticed when they were fertilized with NO3−-N (Tsabarducas et al., 2017).
In agricultural soils, low N retrieval by plants increases the total N losses from the field, triggering undesirable effects on the environment (Fernández-Escobar et al., 2004). A study by Fernández-Escobar et al. (2012) found that, when N was applied annually to fertile olive orchards, N net mineralization was decreased and N net immobilization into soil organic matter increased. Consequently, N leaching increased because of excess of N applied, causing a disruption of the N balance in the soil and environmental damage. Slow-release N fertilizers provide a slow supply of N to plants for a longer period as compared with traditional N fertilizers. As a result, N-use efficiency increased and the total N losses by leaching decreased (Fernández-Escobar et al., 2004). Studies on the effect of N application on olive growth have been common recently (Troyanos and Roukounaki, 2011; Tsabarducas et al., 2017). It has been claimed that N fertilizer is not essential when olive orchard soils are fertile and leaf N content ranges from 1.22% to 1.35% N (% leaf dry matter) (Fernández-Escobar et al., 2012; Molina-Soria and Fernández-Escobar, 2010). However, few have included root components (length, diameter, surface area, and fork number) and leaf gas exchange analysis as an aid to determine the optimal N level for olive growth, specifically at early growth stages. The influence of N source on young olive plants is also not well known, especially, those from slow-release N. We hypothesize that low N levels can enhance root components and shoot growth. We also hypothesize that N form (NH4+ or NO3−) has differential effects on shoot and root growth. The objective of this research was to evaluate the influence of N source and level on shoot morphology and physiology, root component and plant N content of young olive cuttings cv. Arbequina.
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