One of the changes seen in some plants grown under boron deficiency is the outgrowth of lateral buds resulting in plants with a bushy or rosette appearance. In pea, this was associated with a sharp decrease in IAA-export from the apex (Li et al., 2001). It is generally accepted that the outgrowth of lateral buds is inhibited by polar auxin transport in the stem and that disruption of this transport by decapitation or auxin transport inhibitors results in the outgrowth of lateral buds (Tamas, 1987).
Cotton ovules which otherwise develop fibres when cultured, commence extensive callus formation when placed on a medium deficient in boron. On the other hand, the growth rate of callus cultures of Helianthus annuus and Daucus carota (Krosing, 1978), and cell cultures of sugar cane (Bowen, 1979), was much reduced when boron was not present in the growth medium. Boron influences the development of the suspensor of somatic embryos in Larix deciduas (Behrendt and Zoglauer, 1996). Boron had no influence on the induction of embryogenesis in Daucus carota but altered the development of embryos: root development was promoted at low concentrations and shoot development at high. This coincided with a high and low auxin-cytokinin ratio, respectively (Mashayekhi and Neumann, 2006).
Adventitious root formation. Boron is thought to promote the destruction of auxin. Although auxin is required for the formation of adventitious root initials, boron is necessary in light grown-plants for the growth of these primordia (Middleton et al., 1978); possibly boron enhances the destruction of auxin in these circumstances, which in high concentrations is inhibitory to root growth (Jarvis, 1986). An interaction between boron and auxins in the rooting of cuttings has been noticed in several species (Hemberg, 1951; Weiser, 1959; Weiser and Blaney, 1960; Bowen et al., 1975; Josten and Kutschera, 1999) and a supply of exogenous borate has been shown to be essential (Ali and Jarvis, 1988). However, excessive boron concentrations lead to a reduction in the number of roots formed (Jarvis, 1986). Boron deficiency had no observed effect on the rooting of Eucalyptus microcuttings (Schwambach et al., 2005).
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