Root tips

DCPA inhibits the growth of grass species by dismpting the mitotic sequence, probably at entry (190). DCPA influences spindle formation and function (181) and causes root-tip swelling (182) and britde shoot tissue (191). It has been reported that DCPA, like colchicine and vinblastine, attests mitosis at prometaphase and is associated with formation of polymorphic nuclei after mitotic arrest (192). Pronamide also inhibits root growth by dismpting the mitotic sequence in a manner similar to the effect of colchicine and the dinitroanilines (193,194). Cinmethylin and bensuhde prevent mitotic entry by unknown mechanisms (194). 

Protoanemonin, which has been isolated from Anemone pulsatilla and Ranunculus spp., was reported to inhibit root growth by slowing down metabolism and blocking mitosis 35). Erickson and Rosen 35) observed cytological effects in corn root tips at concentrations of 10M and lower. Cells undergoing division appeared to accumulate in the interphase or prophase stages. Metaphase, anaphase, and telophase stages were not observed. Cytoplasmic and vacuolar structures were disturbed and the presence of mitochondria could not be demonstrated in treated tissue. Thimann and Bonner 141) reported that protoanemonin was 10 to 30 times more inhibitory than coumarin in coleoptile and split pea stem tests, and that BAL prevented the inhibitory action. 

Inhibitors must possess chemical and physical properties that will ensure absorption by root tips or penetration by foliar surfaces, and translocation to the active site. Once there they must assume the precise spatial configuration required to complement the molecular architecture of the active center if they are to block the key reaction. A comprehension of comparative biochemistry and information on how plants differ in the architecture of the reactive sites should assist in developing truly selective herbicides. 

Despite large decreases in the osmotic potential profile of the root tip... 

Fig. 8. ABA accumulation in detached root tops (apical 20-30 mm) of Commelina as a function of root tip turgor. Root tips were excised from well-watered plants and dried in air at 23 °C in the dark until different percentages of fresh weight had been lost. This process took between 5 and 20 min. The samples were then maintained at the various water contents for 7 h prior to measurements of water relations and ABA content. Points are means s.e. of at least four measurements. Modified from Zhang Davies (1987).

Fig. 10. Leaf water potential and abaxial stomatal conductance (upper figure), and water potential and turgor of secondary and tertiary root tips (lower figure) of maize plants growing in 1 m deep soil columns, watered daily (A) or not watered after day 0(A). The roots were sampled from the upper 20 cm of the soil column. Plants were 20 days old at the beginning of the experimental period. Points are means s.e. Modified from Zhang Davies (1989).

Zhang, J. Davies, W.J. (1987). Increased synthesis of ABA in partially dehydrated root tips and ABA transport from roots to leaves. Journal of Experimental Botany, 38, 2015-23. 

Calcium was present throughout at 0.5 mol m. NaCI and KCl were present at 10 mol m and ABA at 25 mmol m" where indicated. For excision treatment root tips (1 cm) were incubated in 0.5 mol m CaCl2. All measurements were taken after 24 h of the appropriate treatment. 

Fig. 1. Decline in tensiometric extensibility of live wheat root tips (2-7 mm) following excision from the plant. Each point is the mean of 10 determinations performed on methanol-killed tissue.

Roberts, J.K.M., Callis, J., Wemmer, D., Walbot, V. Jardetzky, O. (1984). Mechanisms of cytoplasmic pH regulation in hypoxic maize root tips and its role in survival under hypoxia. Proceedings of the National Academy of Sciences, USA, 81, 3379-83. 

Both H-thymidine incorporation and radiolabeled leucine incorporation techniques have been recently used to determine bacterial activity and growth in the rhizosphere of barley seedling (28), Bacteria were initially released from the rhizosphere using homogenization and centrifugation before adding the labeled substrates. The cell incorporation rate was twice as high in the rhizosphere than in bulk soil. In addition, both the leucine and thymidine incorporation rates increased with the distances from the root tip (28). 

The permeation of soil at the root-soil interface by mucilage from the root cap may affect structure, and it may oppose the damaging effects of compression and shearing, but little is known. Another suggested role is that the mucilage assists root-cap cells or acts in concert with them to decrease the friction between the growing root tip and soil (51) or, conversely, that the mucilage acts as a lubricant. 

G. M. Whiteley, The deformation of soil by penetrometers and root tips of Pisiim sativum. Plant Soil 7/7 210 (1989). 

J. Xia and P. H. Saglio, Characterization of the hexose transport system in maize root tips. Plant Physiol 06 1 (1988). 

I. Aluminium-induced specilic. secretion of oxalic acid from root tips. Plant Phvsiol. 7/7 745 (1998). 

J. H. Xia and J. K. M. Roberts, Improved cytoplasmic pH regulation, increased lactate efflux, and reduced cytoplasmic lactate levels are biochemical traits expressed in root tips of whole maize seedlings acclimated to a low-oxygen environment. P/anr P/iy.vto/. 105 651 (1994). 

P. H. Saglio, P. Raymond, and A. Pradet, Metabolic activity and energy charge of excised maize root tips under anoxia control by soluble sugars. Plant Physiology 7 6 l()53 (1980). 

P. H. Saglio, M. C. Drew, and A. Pradet, Metabolic acclimation to anoxia induced by low (2-4kPa partial pressure) oxygen pre-treatment (hypoxia) in root tips of Zea mays, Plant Physiology S6 61 (1988). 

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