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

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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.