The Biological Basis for Aeroponics
The Biological Basis for Aeroponics
The following constitutes a general explanation of the basis of aeroponics, alongside a comparison of its benefits versus hydroponics. We are always happy to discuss this topic in further depth if you'd like so please get in touch.
The fundamental justification for yield improvements in aeroponic systems, versus hydroponic alternatives, is the superior access that roots have to oxygen. At any one time, the optimal amount of irrigation can be derived from balancing the plant’s requirement for sufficient nutrients (of a given type), water, and gaseous exchange to meet biochemical requirements within the plant root cells.
Hydroponic systems immerse plant roots in a nutrient rich solution of water, which is either maintained as a narrow film or flooded and drained to expose the plant roots intermittently, to air and then water. In such systems, the roots can use only the oxygen dissolved in nutrient solution, the concentration of which is very low due to its low solubility, roughly 8mg/l at room temperature. This restriction is exacerbated by higher temperatures, which increase the respiratory demand for oxygen by the roots (approximately doubling for each 10C rise in temperature, up to about 30C), and crop demand, which is higher when the photosynthetic activity increases. A decrease below 3 or 4 mg/l of dissolved oxygen, dramatically inhibits root growth and causes a visual change in root appearance, to a brown colour, which can be considered the first symptom of oxygen deficiency.
As previously communicated, in an aeroponic system, the delivery of nutrients and water is a function of the duration, manipulation, and intensity of irrigation. To a point, increasing supply of these resources increases growth rate. However, the effect of gaseous exchange upon growth rate acts in opposition, with increased irrigation waterlogging the root bed and generating hypoxic conditions, where roots cannot access sufficient oxygen for aerobic respiration. The effect of this stressor has been examined in natural waterlogged conditions and shown to stimulate anaerobic respiration in plant roots, resulting in the creation of fermentation products - ethanol, alanine, and lactate. Aside from accumulation of these potentially toxic by-products, the growth of the plant is limited fundamentally by this reliance on anaerobic respiration, which is up to 15 times less efficient than the aerobic equivalent (Smith et al, 2010).
Experimental validation of the impact of this process upon yield has been observed by Yin et al (2009) where two cultivars of Chrysanthemum plants were waterlogged and growth compared to that of plants in naturally aerated soil. Accumulation of toxic by-products, reduced energy conversion from anaerobic processes, and increased exposure to Reactive Oxidative Species (ROS) generated in waterlogged tissues, contributed to increased rates of senescence in the waterlogged plants. In summary:
“Hypoxia is the primary stress factor under waterlogging (Shiono et al., 2008). When tissues are hypoxic, the aerobic energy generating system sharply decline and the functional relationship between roots and shoots is disturbed (reviewed in Vartapetian and Jackson, 1997). In most cases, oxygen deprivation affects directly the roots and indirectly the shoots.”
The roots of plants grown within hydroponics exist in a constant state of oxygen deprivation, which has been shown to have significant impact on plant health, growth rate, and therefore yield.
Conventional High Pressure Aeroponic (HPA) systems address this issue by growing roots in a gaseous environment supplied by nutrient dense mist, an improvement, but have little control over mist behaviour and subsequent deposition onto root. Similarly, this lack of control does not allow the farmer to adjust nutrient deposition throughout the plant's growth, or respond to inconsistencies in growth, which fundamentally will impact on yield - and therefore a farmers bottom line.
HPA systems operate by forcing nutrient rich water through numerous valves embedded in pressurised tubing, with the mist generated then depositing upon the plant roots. This is simple on a small scale, but that pressurised system becomes seriously difficult to scale into a productive vertical farm. Most vertical farmers have sensibly steered clear of this complex solution when operating at scale.
This presents a significant opportunity for innovation.
Ritter, E. Angulo, B. Riga, P. Herran, C. Relloso, J. San Jose, M. (2001). Comparison of hydroponic and aeroponic cultivation systems for the production of potato minitubers. Potato Research.
Hosseinzadeh, S. and Verheust, Y. and Bonarrigo, G. and Van Hulle, S. (2017). Closed hydroponic systems: operational parameters, root exudates occurrence and related water treatment. Rev Environ Sci Biotechnology.
Smith, A. and Coupland, G. and Dolan, L. and Harberd, N. and Jones, J. and Martin, C. and Sablowski, R. and Amey, A. (2010). Plant Biology, Garland Science.
Dongmei Yin. and Sumei Chen. and Fadi Chen. and Zhiyong Guan. and Weimin Fang. (2009). Morphological and physiological responses of two chrysanthemum cultivars differing in their tolerance to waterlogging. Environmental and Experimental Botany.
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