Water relations of lettuce seed thermoinhibition. I. Priming and endosperm effects on base water potential

Abstract

Lettuce (Lactuca sativa L.) seeds are inhibited from germinating above an upper temperature limit that is dependent upon cultivar, growing conditions and seed treatments. Thermoinhibition is accompanied by an increasing sensitivity of germination to reduced water potentials (ψ). We have employed a water relations analysis (hydrotime model) of seed germination rates to investigate the basis of thermoinhibition. Germination rates can be characterized by the distribution of base water potentials among seeds in the population (ψb(g)) and a hydrotime constant indicating the total accumulated hydrotime (MPa · h) above the base required for radicle emergence. The hydrotime model adequately described germination time courses across a range of ψ at both high and low temperatures. Increasing temperature caused the ψb(g) distributions to become more positive, accounting for the greater sensitivity to ψ and inhibition of germination. Increases in embryo osmotic potential and in the turgor yield thresholds of both the radicle and the endosperm/pericarp envelope contributed to this change. Seed priming (prehydration and drying) treatments speeded germination by reducing the hydrotime requirement. Priming also resulted in smaller increases in ψb(g) at high temperature, alleviating thermoinhibition by lowering the embryo yield threshold sufficiently to compensate for the increased endosperm resistance. The beneficial effects of priming in lettuce appear to occur primarily in the embryo, rather than in the surrounding envelope tissues.

Priming and temperature limits for germination of dispersal units of Urochloa brizantha (Stapf) Webster cv. basilisk

“…Based on the relationship between Tc (g) and base water potential (Ψb (g) ) (Alvarado and Bradford, 2002), the results presented here suggest that priming treatments did not affect the Ψb (g) distribution in the seed population of Urochloa brizantha, however further experiments must be conducted in a wide range of water potentials in order to determine a possible variation of the base water potential (Ψb (g) ) and, thus, evaluate the sensitivity of U. brizantha seeds to water deficit (Bradford and Somasco, 1994). The results presented here show that the response of U. brizantha seeds to temperature can be affected by priming, depending on the temperature and water potential conditions at which the seeds are pre-imbibed.…”

Key features of the seed germination response to high temperatures

“…This may have a mechanistic basis in that the value of Ψ b ( G ) is posited to relate to physical constraints to embryo growth within the seed (Bradford, ; Finch‐Savage & Leubner‐Metzger, ). Both increased physical constraints to embryo growth and an upwards shift in Ψ b ( G ) are associated with thermoinhibition at supra‐optimal temperatures (Bradford & Somasco, ).…”

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“…Consequently, this technique may be applicable to priming experiments in seed research. The priming mechanisms that have been investigated affect the endosperm (Cantliffe et al 1984, Guedes et al 1981 and/or the pericarp (Bradford andSomasco 1994, Drew andBrocklehurst 1987). However, the techniques used to study the effects of seed priming are destructive.…”