Absorption of Mineral Salt in Plants (With Diagram)

The following points highlight the six factors affecting absorption of mineral salt in plants. The factors are: 1. Temperature 2. Hydrogen Ion Concentration 3. Light 4. Oxygen Tension 5. Ion Interaction and 6. Growth.

Factor # 1. Temperature:

Generally, an increase in temperature up to a certain limit brings about an increase in rate of salt absorption. However, this effect of temperature on salt absorption is confined only to a relatively narrow range. Beyond a certain limit, further increase in temperature will have a detrimental effect on salt absorption so much so that salt absorption may be completely in­hibited.

It is because at higher temperatures, the enzymes involved either directly in salt ab­sorption or indirectly in the synthesis of some necessary components of the process of salt absorption may get denatured. Both passive and active salt uptake are affected by change in temperature.

Factor # 2. Hydrogen Ion Concentration (pH):

The availability of mineral ions in soil solution is markedly affected by pH of the soil solution. The ionization of electrolytes or the valence number of different ion species are influ­enced by changes in pH. For instance, phosphorous is readily absorbed by plants in monova­lent phosphate H₂PO₄^– when pH of the soil solution is acidic.

As the pH of the soil solution increases or become more alkaline, production of divalent phosphate HPO₄^2- and then trivalent phosphate PO₄^3- is favoured. Phosphorus is only sparingly available to plants in divalent state while in trivalent state it is not absorbed at all. The reversible ionization of H₂PO₄^– is shown below.

Likewise, absorption of boron in the form of H₂BO₃ ion is believed to be favoured by a lower pH. On the contrary, absorption of cations is favoured by an increase in pH. Salt absorption is inhibited at pH values outside the physiological range due to damage to plant tissues and carriers.

Factor # 3. Light:

The effects of light on salt absorption are indirect through opened stomata and on photo­synthesis. Opened stomata facilitate mass flow of water in transpiration stream which in turn may increase salt absorption. The energy provided by photosynthesis and oxygen released during the process are all conducive to active uptake of ions.

Factor # 4. Oxygen Tension:

In absence of oxygen, active absorption of salts is inhibited. In-fact, this observation greatly supported earlier theories on mechanism of active uptake of ions. Hopkins (1956) had shown strong influence of oxygen on active uptake of phosphate by excised barley roots in phosphate solutions (fig. 7.14).

Factor # 5. Ion Interaction:

The absorption of one ion may be affected by the presence of another ion in the nutrient solution. In an experiment made on the uptake of KBr in excised barley roots, Viets (1944) found that absorption of potassium was inhibited by the presence of calcium, magnesium and other polyvalent cations in the external nutrient medium. Viets observed a dual effect by cal­cium on the uptake of both potassium and bromine.

In the absence of calcium, the uptake of both potassium and bromine was less. But, as the concentration of calcium was increased upto a certain level, the uptake of potassium and bromine was increased. Thereafter, further increase in calcium concentration resulted in sharp decline in uptake of both potassium and bromine. This effect of calcium on salt absorption was also noticed by other scientists. Olsen (1942), also observed adverse effect of calcium on the absorption of magnesium.

It is believed that interaction among several ions such as potassium (K), rubidium (Rb) and cesium (Cs) or barium (Ba), calcium (Ca) and strontium (Sr) occurs as competition for binding sites on the carriers and is associated with availability and specificity of binding sites on carriers.

If there are enough binding sites on carries, the interaction of ions will not be appar­ent and the ions will be absorbed without hindrance. Similarly, if binding site of an ion is highly specific for that ion, the absorption of that ion will seldom be inhibited by the presence of other ions. The phenomenon wherein one ion species present in excess in the nutrient medium may depress the uptake of other ion species has been called as ion antagonism.

Factor # 6. Growth:

Salt absorption is also affected by different phases of development and growth of the plant and specific need of mineral elements at those particular periods of time.