The distribution of current assimilates between competing zones of potential growth in the peach tree (Prunus persica (L.) Batsch) was studied using limb girdling, which altered the balance between reproductive growth and vegetative growth in a similar manner to the aging process. Fruit matured earlier, and leaf senescence and abscission were advanced in girdled limbs. which supported normal fruit loads but had only half the leaf area. Lateral growth and secondary thickening were reduced by 50% but vegetative growth approached normal rates at times when fruit growth was minimal, indicating that girdling reduced the ability of vegetative growth to compete with reproductive growth for assimilates. Starch and soluble sugars did not accumulate above the girdles. The data are consistent with the hypothesis that girdling alters the balance between endogenous growth regulators which favour either vegetative or reproductive development. We suggest that the initial effects on the girdled limb are attributable to accumulation of growth regulators produced above the girdle. The reduced flow of growth regulators to the roots eventually results in lowered levels of root-produced hormones which subsequently causes effects throughout the tree.
The transport of ions across the plant root can be inhibited by cytokinins, abscisic acid and cycloheximide. It appears that secretion of ions into the xylem vessels is a different kind of process from active transport into the cortical cells, as it is not affected to the same extent by these compounds. In the present paper it is shown that p-fluorophenylalanine (FPA) also inhibited transport across the root without reducing uptake to the cells, and hence it is concluded that transport involves a specific protein. From the time needed to produce inhibition with FPA it appears that this protein has a short effective life. Measurement of distribution of 36Cl between stele and cortex showed that these inhibitors did not stop movement of ions into the stele. It seems then that the site of action of the proteins is within the stele, probably between xylem parenchyma and xylem vessels.
In studies of the characteristics of the uptake of boric acid by plant tissues (Wildes and Neales, unpublished data) we used disks cut from carrot roots. Bacon, MacDonald, and Knight (1965) have emphasized the necessity of using storage tissue free of bacteria for such physiological studies. In the course of our experiments we therefore investigated the extent of bacterial contamination in carrot disks immediately after cutting, and also examined the effects of chloramphenicol and calcium chloride in the washing solution on the extent of the development of bacterial contamination in the disks. We also made measurements of various properties that gave evidence of their normal biological activity. The loss of this activity, in our experience, was characterized by a final loss of turgor, browning of both tissue and washing solutions, and a failure to respond in respiration rate to both the addition of salts and 2,4-dinitrophenol.The inclusion of 50 /Lg/ml of chloramphenicol has been shown (Edelman and Hall 1965;Leaver and Edelman 1965;Atkinson et al. 1966) to prevent the multiplication of bacteria in disks of storage tissue. Also Marinos (1962) and Jones and Lunt (1967) have shown that the maintenance of cell membrane structure requires a supply of calcium. In its absence there is a loss of cell contents (Skou 1963;Van Steveninck 1965a;Garrard and Humphreys 1967; van Goor 1968) and also an impairment in the ability of barley roots to absorb potassium (Epstein 1961). Therefore we investigated: (1) the extent of bacterial contamination with washing time; (2) the effellt of the inclusion of chloramphenicol in the washing solution and the effect of a spectrum of antibiotics on the growth of the contaminating bacteria; and (3) the effect of calcium chloride in the washing solution on the deterioration of carrot disks. MethodsCarrot disks were cut from the phloem tissue of intact roots, obtained periodically from the local market. The roots were thickly peeled and cut into slices 1 mm thick. Disks 10 mm in diameter were then cut from these slices with a sterilized cork-borer, and placed into 4 litres of washing solution maintained at room temperature (c. 18°C). This washing solution was changed several times in the first 3-4 hr, and then daily until the disks were used.
Disks prepared from the storage tissues of carrot (Daucus carota) were able to absorb boron from boric acid solutions (0·1 mM). At 25°C, net influx ceased 3-4 hr after the tissue was placed in boric acid solution. At equilibrium the tissues were able to maintain an internal concentration of diffusible boron in excess of that in the external solution. Boron uptake was inhibited by anoxia, 2,4.dinitrophenol, and at 1°C; and, in the presence of these inhibitors, the equilibrium concentration of boron in the tissues only approached that in the external solution. The capacity of the tissues to accumulate boron developed 2-3 days after the beginning of a washing treatment. Boric acid (0·1 mM) in this washing solution inhibited the development of this capacity.It is deduced that there are two components of boron uptake from boric acid solutions in these tissues: a passive diffusion and an active transport process. A model is proposed for the uptake of borate, incorporating the active transport of the B(OH)~ ion and the passive diffusion of B(OH)a.
Earlier reports of inhibition of Cl- transport across the root to the xylem by p-fluorophenylalanine (p-FPA) were used to support the need for continual production of a protein used as a carrier. It is now shown that o-FPA and m-FPA are also effective as inhibitors of transport, ruling out the possibility that p-FPA was acting by inhibition of phenylalanine ammonia-lyase activity and so changing endogenous concentration of phenolic compounds.
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