Uptake by roots from soil

The uptake of herbicides by plant roots and their movement in the xylem or phloem is becoming increasingly well understood on the basis of their physicochemical properties. Uptake can take place from the soil either via the air or water phase. Knowing that diffusion constants are about 10,000 times greater in air than in water, and the ratio of concentration in air and water, it is possible to predict which phase is most important in root uptake. Hence, herbicides such as trifluralin, EPTC, triallate and bifenox are likely to move as vapour in moist soils, while movement will be by diffusion in the aqueous phase by monuron and simazine. Lipophilic herbicides with a high vapour pressure will be strongly absorbed by roots from a moist soil. Volatility should also be considered and soil incorporation is often necessary (e.g. with trifluralin) to prevent rapid loss by volatilisation.

Uptake via the soil aqueous route has been studied in detail by Briggs and colleagues at Rothamsted Experimental Station, UK, using barley plants grown in nutrient solution and radiolabelled test compounds (e.g. Briggs et al., 1982, 1987, 1994). Distribution of a non-ionised compound between the roots and the bathing solution was defined by the root concentration factor (RCF), where

Concentration in roots

Concentration in nutrient solution

It was found that the RCF was directly related to the log of the octanol/water coefficient (log Pow), thus uptake increased with increasing lipophilicity.

The uptake of acidic compounds by roots is very different from that of the non-ionised herbicides above, but is dependent on the pH of the soil solution. Thus, the uptake of 2,4-D into barley roots over a 24-hour period from nutrient solution was 36 times greater at pH 4.0 than at pH 7.0 (Briggs et al., 1987). This may be explained as an ion-trap effect, whereby weak acids are accumulated in compartments of higher pH by virtue of the greater permeation rates across membranes of the undissociated form compared to the anion (Figure 3.7).

Once inside the root hair, the herbicide has to be transported to the vascular system for long-distance transport to the target site. Two routes are possible via the apoplastic or symplastic pathways. The former entails movement down a concentration gradient along the cell walls, while the latter entails the cytoplasmic continuity of the root cortical cells via plasmodesmata. Water and solutes cannot enter the xylem by an entirely apoplastic route, but must move through the symplast of the endodermis. This is because the tangential walls of the endodermal cells are thickened by the deposition of suberin to form the waterimpermeable Casparian strip (Figure 3.8). Efficiency of transport from the root cortex to the xylem is low, at less than 10%, which further illustrates the difficulty that weak acids have in crossing biological membranes when they are largely ionised at a physiological pH.

Apoplast pH 5.0 (cell wall)

Symplast pH 8.0

(cell cytoplasm)

Symplast pH 8.0

(cell cytoplasm)

Figure 3.7 Accumulation of a weak acid (pKa4.0) within root cells by the ion-trap effect.
Figure 3.8 Cross section of a dicotyledonous root, showing the position of the endodermis and the Casparian strip (from Bromilow and Chamberlain, 1991, with kind permission of Springer Science and Business Media).

3.6 Herbicide translocation from roots to shoots

Movement in plants can be predicted from pKa and log Pow values and is therefore largely controlled by the physicochemical properties of the herbicide (Figure 3.4). As a general rule, lipophilic compounds (i.e. log Pow >4) are non-systemic, while compounds of inter-

Table 3.6 Metabolism of 2,4-D butyl ester and systemicity of its metabolites (from Bromilow et al., 1986 with permission).

Compounda

log Kw

pKa

Mobility

2,4-D butyl ester

4.4

Non-ionised

Immobile

2,4-D acid

2.9

3.0

Xylem/phloem

2,4-D glucose

0.6

Non-ionised

Xylem

Ring-OH 2,4-D

2.2

3.0

Xylem/phloem

Ring O-glucosyl 2,4-D

0.0

3.0

Xylem/phloem

Diglucosyl conjugate of -COOH and ring-OH

-2.3

Non-ionised

Immobile

a Structures of the metabolites of 2,4-D are on page 153.

a Structures of the metabolites of 2,4-D are on page 153.

mediate lipophilicity (log Pow -0.5 to +3.5) move in the xylem, and weak acids are also mobile in the phloem to the physiological sinks, or growth points, in the plant. Most compounds enter the phloem quite freely - in contrast to sucrose, which requires an active transport system - and leave it equally readily. Movement is therefore according to sink strength. Movement in the xylem is at least 50 to 100 times faster than in the phloem and is therefore dependent on the transpiration stream at any particular time. These rates can reach huge values, especially in trees where, for example, it has been calculated that an oak tree can transpire a ton of water a day, or a maple tree growing in the open was measured to transpire 48 gallons of water per hour! Even in young seedlings (2-3 leaf stage) of black-grass rates of 0.25 gm-2 leaf h-1 have been recorded from plants growing in greenhouse conditions (Sharples et al., 1997). Thus, water can move through the xylem at velocities of greater than 10 mh-1, while movement through the phloem may be 50 to 100 times less.

The above patterns of systemicity have been confirmed for many herbicides by autora-diography using radiolabelled molecules, and further details for individual herbicides can be found in Bromilow and Chamberlain (1991).

Metabolism of a herbicide within the plant will also influence its movement, since metabolism generally reduces lipophilicity. Whereas aryl hydroxylation may be expected to increase the mobility of the parent molecule, its subsequent conjugation to glucose, for example, may create a less mobile glucoside that becomes compartmentalised or inactivated within the cell vacuole. An example of the consequences of metabolism on mobility is illustrated in Table 3.6 .

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