How do Plants Absorb Nutrients? | Botany

A physical or chemical proximity to the root is necessary for nutrient uptake.

Engagement between the root and nutrient ions can be made by:

(1) Contact exchange

(2) Exchange of soil ions with H in the mucigel

(3) Diffusion of ions in response to a chemical gradient

(4) Mass flow of ions to the root in response to a moisture gradient

(5) Extension of the root into the ion source

Root extension places newly formed absorptive tissues, par­ticularity the root hair zone, in an unexploited soil medium, enhancing ion uptake opportunity. Aboulroos and Nielsen (1979) found that fertilization with P increased yield and P uptake but also greatly increased root length, fineness, and density. The increase in P uptake may have resulted from more P concentration in the medium, or from increased root extension, or (more likely) both.

In any case the root must intercept the nutrient by one or more of the processes given previously. Their relative importance with respect to nu­trient uptake by varied with the particular nutrient, but mass flow (moving with water) was the primary process in the uptake of most nutrients.

However, chemical diffusion was primary for K in the Mollisol soil of this experiment. Mass flow of K would probably predominate in coarse-textured Spodosols, Entisols and Ultisols, in contrast to a Mollisol. Interestingly, the nutrient contribution from root extension is relatively low for all except Ca, which is immobile in the plant. Since methods of accurate quantification are difficult with fine roots and especially root hairs, the contribution made by root extension could conceivably have been under­estimated.

The processes only introduce the root to the nutrient, a necessary precondition for absorption. The process of absorption may be active, requiring respiratory energy and aerobiosis, or passive. In active ab­sorption, ions cross the cytoplasm membrane, the plasma lemma, by the ener­getics of high-energy phosphate bonds (e.g., ATP) produced in respiration (the ion pump).

Without ion uptake inhibition, Na or K concentration inside the cell may be many times that of the external concentration. Active movement between cells was through living connectors, the plasmodesmata; hence transport between cells can be active. The vacuole, a storage reservoir inside the cell for water and ions, functions to stabilize supply-demand balance.

The importance of a high respiration rate for active absorption is illus­trated in Table 5.4. After some initial absorption, low temperature inhibited uptake, as did anaerobic conditions.

The importance of radiation and high photosynthetic rate is also sug­gested. After shading tomato, barley, and wheat plants, root growth was the metabolic process first limited. Potassium up­take was reduced greatly, respiration moderately. Root extension appears to be affected more by anaerobiosis than is nutrient uptake.

Passive absorption is a physical process analogous to absorption of water by a sponge. Ions move with water without metabolic involvement.

The total capacity of passive absorption had two components:

(1) Outer space, defined as voids in the root tissue such as intercellular spaces but also including any nonliving tissues such as cell walls.

(2) Donnan free space, roughly defined as the CEC of cell parts exposed to ions in water in the outer space. The term apparent free space or free space is used to represent the sum of the two.

Minerals in high concentrations tended to move rapidly in the free space (apoplasm), finally pass the endodermis, and enter the tran­spiration stream of the xylem. The endodermis is a barrier to passive movement in the apoplasm because of the casparian bands, suberin deposits in the endodermis that ostensibly render it impermeable to free water movement.

Movement through the endodermis is apparently active in the symplasm, as illustrated in the model presented by Haynes. A severe rate limita­tion would seem probable, caused by the necessity of rerouting all transport through the endodermal plasmodesmata of the symplasm, but the nature of this process has not been fully clarified.

The CEC of the root, which influenced nutrient uptake, varied with species, variety, and age. Elements held in the mucigel of a young root can exchange with those electrostatically held by the soil particles, a process termed contact exchange. Once introduced to the root, the ion can move actively in the symplasm or passively in the apoplasm, the passive being faster and resulting in greater uptake.

Two theories are proposed for the movement of hydrophilic ions across lipoprotein membranes:

1. The carrier theory holds that molecules in the plasmalemma have bind­ing sites specific to certain ions, causing selectivity. A carrier-ion complex formed at the membrane interface carries the ion across the membrane and later discharges it into the cell. The process is driven by ATP and the enzyme kinase.

2. The ion pump theory holds that the energy released by the conversion of ATP to ADP by ATPase brings ions into a cell in response to the change in balance created as other ions leave the cell. The Na-K pump is a common example. Other ions enter the cell by chemical gradient. The ion absorption rate has been observed to be highly correlated to ATPase activity.