Chapter 4: Hydroponic production
4.4 Discussion
4.4.2 Crop productivity
145 cabbages (3057.63± 620.51 mg/kg; Figure 4.7). The sodium content of plant tissue has been found to increase as the sodium content of irrigation waters increases (Glenn et al. 1999, Silveira et al. 2009, Diaz et al. 2013). Sodium puts osmotic stress on plants and can reduce the availability of certain nutrient to plants such as Ca, K and Mg (Grattan & Grieve 1999).
High concentrations of sodium in saline waters increases the ratio of Na to nutrient cations such as Ca, K and Mg, resulting in increased Na uptake and decreased nutrient cation uptake (Maas & Grieve 1987, Alam et al. 1989, Ehret et al. 1990). Plants have to spend more energy on obtaining water and nutrients thus reducing the amount of energy spent on growth therefore reduced yield (Alam et al. 1989, Ehret et al. 1990, Grattan & Grieve 1999). The interaction between salinity and nutrient deficiencies is extremely complex and is influenced by crop type, soil type and the environment in which the crops are grown (Grattan & Grieve 1999).
146 the water (Musvoto et al. 2000, Van Rensburg et al. 2003). This results in a decrease in acidity of BE and an increase in pH (Musvoto et al. 2000, Van Rensburg et al. 2003). The high alkalinity of BE is a major issue because it is difficult to keep BE at a constant pH, without continuously adding acid, in order to optimise the availability of nutrients to plants when it is used as a hydroponic nutrient source. The continual addition of acid would increase the conductivity of BE and would put more osmotic stress on the plants in hydroponic systems.
One of the objective of this experiment was to determine how much sodium could be removed from BE by various crops. The sodium concentration increased in all hydroponic systems as the time that the plants were exposed to the solution increased.This increased in sodium concentration with time was probably due to a process called evapotranspiration, where salt concentration increases due to the removal of water (Lymbery et al. 2006, Muyen et al. 2011, Rozema 2014). The irrigation solutions from the hydroponic systems planted with saltbush had the lowest concentration of sodium, whereas the systems planted with millet and cabbage had similar sodium concentration to the hydroponic systems with No Crops. Halophytes such as saltbush have been shown to assimilate sodium from soil water complexes (Miyamoto et al. 1996, Brown et al. 1999, Glenn et al. 1999). However, this rate of sodium assimilation was slower than the concentrating effect caused by
evapotranspiration, which accounts for the increase in sodium in all the treatments (Rozema 2014).
Plant productivity
The level of sodium found in BE has been identified as a major restriction to the widespread use of BE as a water and nutrient source in irrigated crop production. Salinity poses two major threats to plants: osmotic stress due to the relatively high solute concentrations
147 irrigation waters; and ion specific stresses resulting from the k+/Na+ ratios and Na+
concentrations that are harmful to plants (Blumwald et al. 2000). The sodium leaf content was lower in millet, saltbush and cabbage plants grown in pH adjusted BE systems when compared to pH unadjusted BE systems. The influx of Na+ into root cells is passive, while the efflux of Na+ out of the root cells in active (Blumwald et al. 2000, Blumwald 2000, Apse &
Blumwald 2007). An increase in the extracellular Na+ concentration will cause an increased electrochemical gradient that will favour the movement of Na+ into the root cell (Blumwald et al. 2000). Sodium enters the cell through K+ carriers in the membrane because Na+ and K+ ions have similar hydraulic radii (Blumwald et al. 2000). The efflux of Na+ is an active process because they have to be transported against their electrochemical gradient (Cramer et al.
1985, Blumwald et al. 2000). Sodium extrusion is mediated by the plasma membranes H+- ATPase which pump out H+ generating an electrochemical gradient against the natural H+ gradient (Blumwald et al. 2000). This allows the Na+/H+ antiporters to couple the movement of H+ into the cell with the extrusion of Na+ (Hassidim et al. 1990, Blumwald et al. 2000). The increase in extracellular H+ has been found to aid in the efflux of sodium through the plasma membrane and to enhance the salt tolerance of cells (Nass et al. 1997, Blumwald et al.
2000). Therefore the pH adjustment of BE may have enhanced the efflux of Na+ in the root cells, thus decreasing the level of Na+ in the plant tissue. Future research should investigate the influence of irrigation water pH on the Na+ influx and efflux of plants and the salt tolerance of plants under acidic and alkaline conditions.
The growth of plants in hydroponic systems should be accompanied by a decrease in the nutrient concentration of the irrigation solutions. Phosphorous and total N concentrations decreased over time in all hydroponic systems. At the end of each cycle cabbage, saltbush and millet pH adjusted treatments had a lower total N and P concentrations than the No
148 Crop, cabbage and millet pH unadjusted hydroponic systems. Nitrogen and P are
macronutrients needed by plants to support growth (Lucas & Davis 1961, Epstein & Bloom 2005). There were also algae and other microorganisms that contributed to the nutrient decrease in all hydroponic systems. Millet grown on pH unadjusted BE hardly grew and did not remove more nutrients than the hydroponic systems with No Crops. This emphasises the importance of pH when using BE as a hydroponic irrigation source.
The pH of a hydroponic solution can influence the growth and health of plants. The CCI families of cabbage, saltbush and millet plants were all higher when grown in the pH adjusted hydroponic systems. Millet plants grown in pH unadjusted BE hydroponic systems hardly grew. Chlorophyll concentration index is a direct measurement of the photosynthetic potential of a leaf and can be indirectly related to the nitrogen nutrient status of a leaf (Filella et al. 1995, Moran et al. 2000). Chlorophyll concentration index is also closely related to plant stress and age (Peñuelas & Filella 1998, Merzlyak et al. 1999). The nitrogen content of cabbage, millet and saltbush plants was higher in pH adjusted BE systems which can be linked to the higher CCI of their leaves as most of leaf nitrogen is incorporated into
chlorophyll (Filella et al. 1995, Moran et al. 2000). The pH adjustment increased the
photosynthetic potential of millet and saltbush plants which intern means they were able to grown faster.