The translocation and root exudation of leaf-applied sublethal concentrations of 14C-labeled clopyralid (3,6-dichloropicolinic acid) and 14C-labeled 2,4-D [(2,4-dichlorophenoxy)acetic acid] were compared in Canada thistle [Cirsium arvense (L.) Scop. var. horridum Wimm. and Grab. ♯ CIRAR] at the rosette stage over a period of 9 days. The rate of absorption and export of 14C out of the treated leaf was similar for both herbicides. However, the distribution of the herbicides throughout the plant was very different. After 9 days, 15 vs. 3% of the applied 14C from 14C-clopyralid vs. 14C-2,4-D, respectively, was isolated from the foliage of the treated leaf. In the roots, twice as much 14C was recovered from the 14C-clopyralid treatments as from the 14C-2,4-D treatment at all sampling times, with 33 vs. 15% being recovered, respectively, after 9 days. When the plants were grown hydroponically, 20% of the 14C-clopyralid vs. 48% of the 14C-2,4-D was recovered in the nutrient solution during a 9-day period. No metabolites of either herbicide were recovered from the foliage, root system, or nutrient solution. Large differences in translocation of these herbicides may account for the unequal toxicity to young Canada thistle plants.

The butyl ester of fluazifop {[(±)-2-[4-[[5-trifluoromethyl)-2-pyridinyl] oxy] phenoxy)propanoic acid} at 0.26 μM in nutrient solution inhibited root growth of hydroponically grown goosegrass (Eleusine indica Gaertn. ♯ ELEIN), large crabgrass [Digitaria sanguinalis (L.) Scop. ♯ DIGSA], and giant foxtail (Setaria faberi Herrm. ♯ SETFA). Treating the soil and plant foliage at 0.035 or 0.07 kg ai/ha did not result in greater phytotoxicity than exposing only the foliage of each grass to the herbicide. Foliar-applied fluazifop was retained on the foliage in similar amounts by each of the species. Translocation of 14C to all plant parts was detected 6 h after foliar application of the butyl ester of 14C-fluazifop to the grasses in the pretillering or tillering stage. The majority (90%) of 14C absorbed by each of the species remained in the treated leaf. In hydroponic studies, each species exuded 14C into nutrient solution following foliar application of the 14C-labeled herbicide. The exuded material was predominantly fluazifop with small amounts of compounds more polar than the butyl ester of fluazifop. Uptake and translocation studies suggest that the greater sensitivity of goosegrass to fluazifop may be related to higher concentrations of the herbicide present in plant tissue.

Absorption, translocation, and metabolism of AC 263,222 by leafy spurge were studied over 8 d. Based on the amount of herbicide applied and recovered from the leaf surfaces, 40% of applied AC 263,222 was absorbed by leafy spurge 2 d after treatment (DAT), with no further absorption observed by 8 DAT. Eight DAT, 19% of applied [14C]-AC 263,222 had translocated to below-ground plant parts while 4% was exuded from the roots into the sand media. AC 263,222 was not metabolized 2 DAT in the crown, root, and root buds, but 42% was metabolized in the treated leaves. Only 17% of recovered [14C] was AC 263,222 in treated leaves 8 DAT (83% metabolized), while AC 263,222 accounted for 70% of recovered [14C] in the root and root buds. HPLC analysis indicated that the balance of [14C] was associated with a single, polar metabolite. Total recovery of [14C] was 88% at 8 DAT.

Mesotrione is an effective herbicide for volunteer potato control but is not selective in several crops, including onion and carrot. Studies were conducted in 2006 to evaluate the effectiveness of wiper-applied mesotrione for control of volunteer potato in a potato crop. Surprisingly, nontreated potatoes growing adjacent to mesotrione-treated plants exhibited bleaching symptoms resembling mesotrione injury. Additional field trials confirmed injury to nontreated plants growing within 26 cm of potatoes treated with mesotrione applied with a wiper at 0.25, 0.5, and 1% (v/v) solutions. Greenhouse bioassays confirmed that mesotrione applied to potato leaves moved down through the plant and was exuded into perlite potting medium in sufficient quantities to injure potato plants that were exposed to the leachate from the perlite pots. In tracer studies, 52% of 14C-labeled mesotrione applied to potato leaves was absorbed into the potato plant by 15 d and 15% of the absorbed 14C-mesotrione was exuded into the soil and soil leachate. Mesotrione applied to potato by the wick application method has potential to injure neighboring susceptible plants from root uptake of exuded mesotrione.

Experiments were conducted in a growth chamber to study alachlor and metolachlor metabolism in soil and corn and to determine if alachlor and metolachlor and their metabolites are exuded from corn roots to the growth medium. Alachlor was more readily absorbed by corn than was metolachlor. The absorption of alachlor and metolachlor was 72 and 55%, respectively, 10 d after seedling emergence (DAE). Alachlor and metolachlor were rapidly metabolized in corn, although metabolism rates were higher with metolachlor than with alachlor. Ten similar alachlor metabolites were detected in roots and shoots. In addition, two metabolites were detected only in the shoots, and one metabolite was detected only in the roots. Metolachlor metabolism in corn produced fewer metabolites than did alachlor metabolism. At 5 DAE, 10 and 9 metabolites were detected in shoots and roots, respectively. The metabolism of alachlor and metolachlor in soil showed patterns similar to the metabolism in corn but produced fewer metabolites. One unique alachlor metabolite appeared in soil but not in corn. Roots of corn seedlings treated with 14C-alachlor or 14C-metolachlor released significant amounts of radioactivity to the surrounding growth medium 5 d after treatment. Plants treated with alachlor released more radioactivity than did plants treated with metolachlor.

We review four hypotheses for the control of carbon acquisition by roots, and conclude that the functional equilibrium hypothesis can offer a good description of C acquisition by roots relative to shoots, but is deficient mechanistically. The hypothesis that import into roots is solely dependent on export from the shoot, itself determined by features of the shoot alone (the ‘push’ hypothesis), is supported by some but not all the evidence. Similarly, the idea that root demand, a function of the root alone, determines import into it (the ‘pull’ hypothesis), is consonant with some of the evidence. The fourth, general, hypothesis (the ‘shared control’ hypothesis) – that acquisition of C by roots is controlled by a range of variables distributed between root and shoot – accords with both experiment and theory. Top-down metabolic control analysis quantifies the control of C flux attributable to root relative to source leaf. We demonstrate that two levels of mechanistic control, short-term regulation of phloem transport and control of gene expression by compounds such as sugars, underlie distributed control. Implications for the impact of climate change variables are briefly discussed.