greenhouses I shade net I hydroponics I aquaponics A poor canopy as result of poor nutrition, disease or climatic conditions will have a reduced capacity to produce energy required for fruit development.
Keeping the balance of leaf canopy to root mass crucial in achieving yield Taking cucumber plants as an example to explain the advantages of keeping the balance of leaf canopy and root mass, the information in this editorial becomes apparent.
C
ucumber roots have a high oxygen requirement and sudden temporary wilt, ‘epinasty’, of greenhouse cucumber plants is often noted. A high solution temperature also reduces the amount of dissolved oxygen in the solution. Oxygen diffuses through air 10 000 times the rate at which it diffuses through water, thus waterlogged soils and media are undesirable. Oxygen content of a fully aerated solution at 10˚C is 13 ppm and at 30˚C it is 7ppm. This may not seem significant but at higher temperatures the oxygen requirement increases as respiration increases. Alcohol, hydrogen sulphide and ethylene are produced in the roots in waterlogged soils and media and hormone production is also disturbed. Media such as sawdust often compact over time, which leads to water logging and results in low oxygen levels in the root zone and impaired potassium, phosphate, manganese and iron uptake. The ability of water to move into the roots is reduced almost threefold in the absence of oxygen in the root zone, mainly under waterlogged conditions.
8 Undercover farming I
July/August2020
I Volume 17 No 4
The combined effects of these changes are seen as stunted or wilted tops and root death; roots will often be at the surface of the media where oxygen is available. In such circumstances, the balance of leaf canopy to roots is compromised. In addition, Pythium is an opportunistic fungus and often takes hold of plants stressed by a combination of high temperatures and low oxygen levels in the root zone. Bearing the above in mind, basics such as media amelioration should be considered mandatory to further improve media properties and boost oxygen levels. Oxygen is part of the respiration process whereby carbohydrates are broken down to provide energy for the plant, this being the opposite process of photosynthesis. Products of respiration are carbon dioxide as well as a wide range of carbon compounds exuded from the roots.
Carbohydrate + O² ↔ CO² + H²O + energy Growers often avoid removing the lower fruit on cucumber plants in a bid to get quick yields, this load on the plant at such early developmental stages
draws energy from the plant that would normally be used for root development, thus reducing the root mass. Similarly, loss of leaves in the lower extremities of the plant are often evident, due to low light or infection, yet growers allow fruit to develop on these nodes where leaf abscission is noted. These fruit have no “neighbouring” leaves to supply the energy for the development of these fruit, and thus draw energy from the plant that would usually be used by the root system, thus causing a decline in the efficacy of the root system. Conversely, a poor canopy as result of poor nutrition, disease or climatic conditions will have a reduced capacity to produce energy required for fruit development and in turn having reduced energy for root development and root exudates, further exacerbating the problem and diminishing the yield potential. In such cases growers should be able to “read” the plant and remove fruit to allow the plant to carry a suitable fruit load without compromising the root system or plant health. By Mike Haupt
