Water and Seed Survival

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Abstract
Between about −350 and −14 MPa the rate of loss of viability in orthodox seeds is a positive function of water potential. The relative effect of water potential has been analysed in an oily seed (lettuce) and a non-oily seed (barley) and found to be more or less identical. The lower limit for the relation in various species coincides with a seed moisture content (wet basis) between about 2 and 6%. Below this level there is little or no improvement in longevity with reduction in moisture content. The upper limit coincides with moisture contents of between 15 and 28%, depending on whether the seeds are oily or non-oily. A water potential of about −14 MPa is the threshold for respiration which increases more-or-less linearly with increase in water potential above this level. Above this threshold, and providing oxygen is available to sustain respiration, seed longevity increases with increase in water potential except that, unless the seeds are dormant, germination may be initiated at a water potential of about −1·5 to −0·5 MPa. In the absence of oxygen there may be a slight further decline in longevity with increase in water potential above −14 MPa before longevity reaches a minimum value Since they cannot be dried very much without immediate loss of viability, recalcitrant seeds survive longest in the presence of oxygen at maximum water potential commensurate with preventing germination. The threshold water potential for immediate loss of viability has not been determined for most species but it is probable that it is close to the water potential typical of the permanent wilting point in these plants, say −2 MPa

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