A plant's survival is determined by its ability to tolerate stress that arises from physical, chemical and biological events. For example, nutrient limitation affects cellular functions, and consequently, plant development. This could be due to nutrient depletion or inaccessibility as nutrients such as phosphorus and iron could be locked up in complex compounds in the soil. In addition, plants have to withstand harsh environmental conditions such as heat, winds, torrent storms and drought. Disease-causing pathogens and pesticides are another threat that reduce a plant’s fitness.