Saline hydroponic systems
Coconuts were first cultivated in the islands of the Pacific. Coconut seeds crossed seas to spread along many coastal regions around the world. Being exposed to seawater, the plants have adapted to salinity.
Besides sodium chloride, seawater contains many nutrients. So why not use nutrient-rich seawater to grow coconut plants in a hydroponic system? But what are the salinity needs and limits of tolerance to salinity in coconuts? This understanding would help devise strategies to deal with the rising sea level affecting coconut cultivation in coastal areas.
A group of scientists from the ICAR-Central Plantation Crops Research Institute, Kasargod chose the Malayan green dwarf variety to testing salinity response . The plants are shorter and are resistant to the lethal yellowing disease.
The team grew six-month-old coconut seedlings in freshwater containing essential nutrients. After four months, some plants were grown in freshwater supplied with nutrients, and others with different combinations of freshwater and seawater. Some seedlings were grown with only seawater.
The researchers found that coconut plants grew well, without any damage in up to 10 per cent seawater. The plants could tolerate up to 25 per cent sea water with minimal damage, though leaf area and chlorophyll content reduced significantly. Leaves also tended to close stomata, signifying reduced transpiration. To overcome the injury and stress caused by excess sodium chloride, the plants accumulated osmoprotectants and antioxidants.
As the concentration of seawater was further increased to 50 per cent and above, the researchers found damage in the plants resulting in retarded growth.
“Coconuts have a good mechanism to compartmentalise the salts that are taken up. Sodium – which in high concentrations is toxic to the plants – is accumulated in the roots while potassium ions are transported to the leaves. Thus, the ionic balance is maintained”, says Balachandra Hebbar, ICAR-Central Plantation Crop Research Institute, Kasargod.
The trait of transporting potassium ions while retaining sodium ions is what makes sweet coconut water high in potassium content.
“Hydroponic cultivation of coconuts using sea water can reduce the cost of nutrients needed. But a salt concentration measured by an electrical conductivity of about 3 deciSiemens per metre seems to be optimum. And beyond 8 deciSiemens per metre, salinity is harmful for the tree’, says Selvamani V, ICAR-CPCRI, Kasargod.
In fact, farmers who add salt to the soil, when cultivating coconuts far inland, will also benefit from this research.
Scientia Horticulturae, 280: 109935 (2021);
K Sri Manjari
University College for Women, Koti, Osmania University