Seed is a developmental stage that is highly protective against external stresses in the plant life cycle. In this study, we analyzed toxicity of essential (Cu 2+ and Zn 2+ ) and non-essential heavy metals (Hg 2+ , Pb 2+ and Cd 2+ ) on seed germination and seedling growth in the model species Arabidopsis. Our results show that seedling growth is more sensitive to heavy metals (Hg 2+ , Pb 2+ , Cu 2+ and Zn 2+ ) in comparison to seed germination, while Cd 2+ is the exception that inhibited both of these processes at similar concentrations. To examine if toxicity of heavy metals is altered developmentally during germination, we incubated seeds with Hg 2+ or Cd 2+ only for a restricted period during germination. Hg 2+ displayed relatively strong toxicity at period II (12-24 h after imbibition), while Cd 2+ was more effective to inhibit germination at period I (0-12 h after imbibition) rather than at period II. The observed differences are likely to be due in part to selective uptake of different ions by the intact seed, because isolated embryos (without seed coat and endosperm) are more sensitive to both Hg 2+ and Cd 2+ at period I. We assessed interactive toxicity between heavy metals and non-toxic cations, and found that Ca 2+ was able to partially restore the inhibition of seedling growth by Pb 2+ and Zn 2+ .

Polymorphic seeds of Atriplex triangularis were germinated at various temperatures (5–15 C, 5–25 C, 10–20 C, 20–30 C) and salinity regimes (0 to 1.5% NaCl) in order to determine their germinability and early seedling growth under these conditions. Larger seeds generally had a higher germination percentage in saline medium. The rate and percentage of germination decreased with increased salinity stress. A thermoperiod of 25 C day and 5 C night, 12 hr/12 hr, temperature enhanced germination of seeds. Early seedling growth is promoted in larger seeds at lower salinity, and at high‐day and low‐night temperatures. Polymorphic seeds have different physiological requirements which provide alternative situations for seed germination in natural habitats.

Recovery of seed germination from NaCl salinity of desert shrubs (Haloxylon recurvum and Suaeda fruticosa, and the herbs Zygophyllum simplex and Triglochin maritima was studied under various thermoperiods. The percentage of ungerminated seeds that recovered when they were transferred to distilled water varied significantly with variation in species and thermoperiods. Zygophyllum simplex had little recovery from all NaCl concentrations in all thermoperiods. Haloxylon recurvum, S. fruticosa, and T. maritima showed substantial recovery. Percentage recovery was highest in S. fruticosa, followed by T. maritima, and H. recurvum. Thermoperiodic effects varied with the species investigated. There was little thermoperodic effect on the percentage recovery of S. fruticosa, except in the higher salinity treatment at higher thermoperiods. Variation in thermoperiod appears to play an important role in recovery of germination of halophytes from salt stress when seeds are transferred to distilled water.

Limonium stocksii (Boiss.) Kuntze (Plumbaginaceae) is a perennial, woody shrub distributed at Hawks Bay, Karachi, Pakistan. Experiments were carried out to investigate seed germination responses of L. stocksii at different salinities (0, 100, 200, 300, 400, and 500 mmol/L NaCl) and under different temperature regimes (10:20, 15:25, 20:30, and 25:35 °C), both in a 12 h dark : 12 h light photoperiod and in complete darkness. The highest percentage of germination (about 100%) was obtained at 0, 100, and 200 mmol/L NaCl at 20:30 °C, and a further increase in salinity resulted in a gradual decrease in germination. Less than 5% of seeds germinated at 500 mmol/L NaCl. Germination under salinity treatment at 15:25 °C was slightly more inhibitory than the optimal temperature regime, whereas under both 10:20 and 25:35 °C temperature regimes, seed germination was substantially reduced and few seeds germinated at concentrations higher than 200 mmol/L NaCl. Germination rate was fastest at 20:30 °C and slowest at 10:20 °C. Relatively low seed germination was obtained in the dark in comparison to seeds germinated in a 12-h photoperiod under saline conditions. Recovery experiments showed that exposure of seeds to various salinity and temperature regimes had little effect on viability of seeds.Key words: germination, light, Limonium stocksii, NaCl, recovery, temperature.

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