Low boron supply depresses seed viability in lupin (Lupinus angustifolius)

Boron deficiency is known to impair reproductive development of plants (Dell and Huang, 1997), but the harmful effects of low B are manifest at different reproductive stages. Differences in the stage of reproductive development that is most impaired by B deficiency can be attributed to the time when B supply in the soil becomes limiting, the phloem mobility of B in the plant, and the demand for B in the reproductive organs both in absolute terms and relative to other plant parts. One of the least studied consequences of low soil B supply is that of seed quality. Low B in peanut seed results in lesions on the internal face of the kernel known as hollow heart (Rerkasem et al., 1988). Similarly, low B soybean seed had hollow heart symptoms on the cotyledons and sometimes an external dimple on the testa (Rerkasem et al., 1993). In legumes, low Bin seed has two effects on the viability and vigour of the seed, depending on its B concentration. At very low B concentrations, seed viability was impaired in soybean and black gram (Bell et al., 1989; Rerkasem et al., 1997). Critical B concentrations were 7-10 and 6 mg/kg, respectively. Saarela (1985) also reported that oilseed rape seed from low B plants had decreased seed germination, however, no seed B concentrations were reported. It is suggested that at below the critical B concentration for germination, B supply to the embryo during seed development was too low for its normal formation, or that tissue B concentrations in the embryo were limiting during the early germination events after imbibition of seed (Rerkasem et al., 1997). At seed B concentrations higher than those critical for viability, seedling vigour after germination was depressed and this was manifest as seedling abnormality and decreased seedling emergence in soybean, green gram and black gram (Rerkasem et al., 1990, 1997). The effects of low seed B on vigour were only manifest when the seed was planted into low B soil. Evidently sufficient B can be absorbed from the soil to meet the needs of the embryo axis if the external B concentration is sufficiently high. In the legumes there was no evidence of either impaired germination or seedling vigour when seed B concentration exceeded 20 mg/kg. The aim of the present study was to determine the effect of soil B levels and fertiliser B supply on seed B concentrations in canola and lupin, and then to determine the consequence of these seed B concentrations for seed germination and seedling vigour.