Salt tolerance and avoidance mechanisms at germination of annual pasture legumes: importance for adaptation to saline environments

Abstract

Mechanisms for tolerance and avoidance of salinity at germination were examined in five selfregenerating annual pasture legumes of Mediterranean origin (Medicago polymorpha, Melilotus siculus, Trifolium subterraneum, T. michelianum and T. tomentosum). The maximum NaCl concentrations, for which no reduction in germination percentage occurred, were 300 mM for M. siculus, 240 mM for M. polymorpha and 120 mM for T. michelianum, T. subterraneum and T. tomentosum. Following 21 days exposure to 300 mM NaCl, imbibed seeds of T. subterraneum showed reduced germinability upon transfer to non-saline solution, whereas those of the four other species recovered full germinability. Following exposure to 600 mM NaCl, however, only T. michelianum and Melilotus siculus had some degree of recovery, with 38% and 31% germinability, respectively. Na + in imbibed seed tissues of all species increased markedly with increasing NaCl concentration, while K + decreased in all but Melilotus siculus. Seed coat impermeability ('hard seeds') acted as protection against the toxic effects of salinity. Melilotus siculus, Medicago polymorpha and T. tomentosum showed a delay in hard-seed breakdown ('seed softening') over the summer-autumn period, compared to T. subterraneum and T. michelianum, which acts to defer germination until soil surface salinity levels are likely to be lower. These three species also had higher levels of residual hard seeds. The results from this study support field observations that Melilotus siculus and Medicago polymorpha are the best adapted annual pasture legumes for highly saline soils in southern Australia, T. tomentosum and T. michelianum have some adaptation to moderately and mildly saline soils, respectively, whereas T. subterraneum has no adaptive traits for even mildly Plant Soil (2009) 315:241-255 saline environments. A model for annual legume germination on saline land in southern Australia is also presented.

Accumulation of ions during seed development under controlled saline conditions of two Suaeda salsa populations is related to their adaptation to saline environments

“…In a study of the seeds of the halophyte S. vermiculatus, for example, the Na + concentration was highest in the bracts and lowest in the embryos (Eddleman and Romo 1987). Impermeable seed coats also protect seeds from the toxic effects of salt by preventing Na + uptake (Song et al 2007;Nichols et al 2009). In the present study, more Cl − and Na + were compartmentalized in the pericarps of S. salsa plants in the intertidal zone than in the saline inland population when plants were cultured in 400 mM NaCl.…”

Microsite and litter cover effects on soil conditions and seedling recruitment in a saline agricultural system

“…Annual species which germinate following autumn and winter rains, when soil salinity levels are low due to leaching (Nichols et al 2009), are less likely to benefit from dense litter cover, reflected by their even distribution across microsites. Long-lived perennial species, such as Atriplex undulata and Atriplex nummularia., germinate following summer/autumn rains when temperatures are greater than 25°C (De Villiers et al 1994).…”

Salt tolerance in red clover (Trifolium pratense L.) seedlings

“…Each treatment was replicated three times. For each treatment, twenty scarified seeds were placed in 90 mm Petri dishes with filter paper and wet with 10 ml of the following concentration of NaCl: 0, 60, 120, 180, 240 mM NaCl (Nichols et al, 2009). Petri dishes were sealed with parafilm to prevent evaporation and the seeds were incubated in the dark at 20°C.…”