Plant growth (i. e. biomass production) is influenced by many (agronomic environmental and others, such as genetic) factors. Some of the most important factors that influence biomass production are: i) soil humidity, ii) soil and air temperature, iii) air humidity, iv) photoperiod, v) light intensity, vi) soil fertility, i. e. soil nutrient availability, and vii) genotype, and are fully analyzed below.

1.1. Soil humidity

Soil humidity is a very crucial factor influencing root growth, thus nutrient uptake and total biomass. Many plant species are more sensitive in soil humidity shortage during a particular (crucial) period of their growth. In olive trees, if soil humidity shortage happens early spring, shoot elongation, as well as the formation of flowers and fruits, are negatively influenced. If the shortage happens during summer, shoot thickening, rather than shoot elongation, is influenced. Finally, soil humidity shortage reduces olive tree canopy (in order to reduce the transpiration by leaf surface) and favors root system growth (in order to have the ability to exploit greater soil volume and to search for more soil humidity), so that the ratio canopy/root is significantly reduced [30]. On the other hand, under excess soil humidity conditions (waterlogging), when soil oxygen is limited, the root system may suffer from hypoxia, thus, nutrient uptake is negatively influenced. Under extreme anaerobic soil conditions, the presence of pathogen microorganisms, such as Phytophthora sp. may lead to root necrosis. According to Therios (2009) [31], for olive trees the mechanism of tolerance to waterlogging is based on the production of adventitious roots near to the soil surface.