Fig 7.4. Boron deficiency effect (relative values) on specific water content (SWC), water potential (y), water saturation deficit (WSD), transpiration (E), diffusive resistance (DR) and proline content (Pro) in cabbage (Brassica olcracea L. var. Capitata cv Pride of India) (After Sharma et al. 1984).
on resupply of boron. Observed effects of boron deficiency on tissue hydration (RWC) are not consistent. In early works (Leaf, 1953; Backer et al. 1956), leaves of plants exposed to boron deficiency were reported to have low water content. Opposite results were reported subsequently. Sharma et al. (1984b) and Sharma and Ramachandra, (1990) reported increase in RWC in leaves of boron-deficient plants, but this was associated with decrease in leaf water potential.
Boron-deficient plants also show enhanced accumulation of proline (Sharma et al. 1984b), which is a typical feature of water-stressed plants. The decrease in water potential of boron-deficient plants, inspite of their high tissue hydration, has been interpreted to be caused by its increased partitioning into the bound form, as had been proposed earlier by Backer et al. (1956).
7.4.12. Reproductive Development, Seed Yield
When plants are exposed to moderate deficiency of boron, their reproductive development is inhibited to a greater extent than vegetative development or dry matter production (Gauch and Dugger, 1954; Sharma et al. 1981, Sherrell, 1983; Loomis and Durst, 1992). Relatively high concentration of boron in reproductive parts of flowers such as anthers, ovary and stigma (Gauch and Dugger, 1954; Syworotkin, 1958) and induction of floral abnormalities in boron-deficient plants (Adams et al. 1975; Xu et al. 1993) also point to an involvement of boron in plant reproductive development. Inadequate supply of boron manifests in the form of delayed and restricted flowering, premature bud abscission, pollen sterility, decreased seed set and poor development of seeds. In plants subjected to boron deficiency, both number and size of flowers are severely restricted (Adams et al. 1975; Zhang et al. 1994). Decrease in the number of flowers in boron-deficient plants may be caused by restricted emergence of flower bearing branches and premature bud abscission.
Boron involvement in pollen development and fertility has been central to boron nutrition for a long time. Way back in 1937 (Lohins, 1937) reported atrophy of anthers in boron-deficient plants of several cereal crops. Agarwala et al. (1981) showed that in boron-deficient maize plants, emergence of tassels and anthers is delayed, anthers lack sporogenous tissue and many stamens turn into floral appendages or staminodes. Several other studies substantiate a role of boron in microsporogenesis and male fertility. Boron-deficient plants of oilseed rape show an abnormally developed tapetum (Zhang et al. 1994) and arrest of microsporogenesis beyond the pollen mother cell stage (Xu et al. 1993).
In boron-deficient maize, pollen size and germination percentage is severely reduced (Agarwala et al. 1981; De Wet, 1989). Decrease in pollen germination is observed even before induction of visible symptoms of boron deficiency (Agarwala et al. 1981). Boron-deficient plants of wheat show poor development of anthers and inhibition of floret fertility (Huang et al. 2000). Inhibition of pollen germination due to boron deficiency has also been reported in perennials, e.g. avocado (Smith et al. 1997) and Picea meyeri (Wang et al. 2003). Low in vitro germination of pollen grains is reflected in poor fertilization (Garg et al. 1979; Agarwala et al. 1981; Huang et al. 2000). Boron deficiency induced cytoskeletal changes in tips of meristematic cells (Baluska et al. 2002; Yu et al. 2002) and reported interaction between cytoskeleton, membranes and cell walls of pollen grains (Li et al. 1997; Franklin-Tong, 1999) suggest possible involvement of boron in cytoskeletal changes preceding pollen germination and pollen tube growth.
Not only does boron play a role in microsporogenesis and pollen germination but it also affects pollen receptivity of the stigma, pollen tube growth through the stylar tract and development of the megagametophyte. The germination of pollen grains on the stigma of boron-deficient Campsis grandiflora is reported to be inhibited because of enhanced accumulation of phenolic compounds on the stigmatic surface (Dhakre et al. 1994). In boron-deficient oilseed rape, the stigmatic papilla shows morphological aberrations (Xu et al. 1993) and the rate of pollen tube elongation is retarded (Shen et al. 1994). Based on cross fertilization experiments, Vaughan (1977) had attributed the poor setting of grains in boron-deficient maize plants to non-receptiveness of silks. Robbertse et al. (1990) described a gradient of boron concentration along the style and suggested that this facilitated the growth of pollen tubes. High concentration of boron in stigma and style is also reported to cause inactivation of callose by forming a boron-callose complex at the interface of the pollen tubes and styles (Lewis, 1980b). Boron deficiency is reported to inhibit the development of ovules in cotton (Birnbaum et al. 1974) and oilseed rape (Xu et al. 1993). The boron-deficient plants of oil seed rape are reported to develop abnormal embryo sacs (Xu et al. 1993).
Role of boron in reproductive development of plants has a direct bearing on seed yield (Mozafar, 1993; Rerkasem et al. 1993; Cheng and Rerkasem, 1993; Rawson, 1996a). Low seed yield of wheat in certain areas of warm subtropical Asia is attributed to male sterility caused by reduced transport of boron to the flowering parts, where it is critically required for microsporogenesis and pollen fertility (Huang et al. 1996; Rerkasem, 1996; Rawson, 1996b, Subedi et al. 1998). Boron is also important for post-fertilization development and seed maturation. In sunflower, deprivation of boron supply, even as late as the time of anthesis, produces morphological aberrations in seeds and reduces the seed content of non-reducing sugars, starch and oil (Chatterjee and Nautiyal, 2000). Seeds of low boron sesamum plants show enhanced accumulation of phenolic compounds and decrease in oil content (Sinha et al. 1999). Seeds of low boron black gram plants showed poor germination and lack of vigour (Bell et al. 1989). Foliar application of boron during the reproductive phase of sunflower leads to enhancement in seed yield because of boron involvement in reproductive processes (Asad et al. 2003).
Severe limitation in reproductive development and seed yield of boron-deficient plants could result from several factors restricting the supply of boron to the reproductive organs. Transpiration, and through it the passive delivery of boron to the reproductive organs, may be restricted because of the foliar coverings of the reproductive parts and the inflorescence architecture (Hansen and Breen, 1985) or due to the environmental constraints such as temperature, light, humidity and availability of water during the critical stages of reproductive development (Rawson and Noppakoonwong, 1996; Dell and Huang. 1997). Other major factors that may cause shortfall in boron supply needed for reproductive development are poor translocation of boron from leaves and other mature tissues to the floral parts (Brown et al. 1999) and poor access of the pollen grains and the embryo sacs to the vascular supply (Rawson and Noppakoonwong, 1996; Dell and Huang, 1997; Brown et al. 2002). As the vascular supply to the anther terminates at the tapetum and that to the ovules at the hypostate, the male and the female gametophytes are deprived of access to boron supply directly through the vascular channels.
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