Imbibition Period as the Critical Temperature Sensitive Stage in Germination of Lima Bean Seeds

Abstract

Sumiiimarv. Lima bean seeds (Phaseolus lutnatuts L.) and excised embryonic axes can be injured during imbibition at temperatures below 250. The early imbibitional stage is critical; imbibition at 250 followed by low temperature exposure does not cause injury. Sensitivity to chilling injury is conditioned by the pre-harvest seed history. Low vigor (bleached) seeds are most sensitive to injury, 'the effects of which can be intensified by restricted oxygen supply dulring early axis growth. The seed coat, by preventing water uptake, can permit the seed to avoid injury. This protective mechanism is most effective at low temperature and high moisture stress. Immediately following low temperature imbibition, injured axes lose organic materials, probably nucleotides. This organic leachate is a potential influence on soil microorganisms and, together with the temperature sensitivity, vigor, and seed coat effect undoubtedlv is important in controlling the potential variability in germination shown by a seed population. Temperature is probably the best known and most important environmental variable in biological systems. In addition to its gross quantitative aspects in controlling survival and rate of development, it also has many more subtle qualitative effects, as for example vernalizatioln and the control of physiological dwarfing (15). In spite of these many critical aspects of temperature response, surprisingly little is known of its detailed physiological and biochemical effects (11). Also imperfectly understood are the complex factors, collectively called vigor, which are expressed in rapidity of seed germination and the rate and environmental adaptability of early seedling growth. A possible experimental tool to approach both of these problems occurs in lima bean seeds. This species is known to be sensitive to germination temperature and some varieties can be commercially sorted into green and bleached (white) seeds. In agricultural practice , the latter are known to be of low vigor(18)but the physiological nature of this vigor is unknown. The recently reported relationship (8) between seed coat color and population vigor appears to be a separate phenomenon (Kannenberg, personal communication). The work reported in this present paper shows that the conventional temiiperature requirenmenit for germination of lima bean seedls is actually very comil-plex. Temperature cannot be considered as acting continuously to control rates of germination and of seedling development. Instead, temperature has a 1 Present address: Vegetable Seed Investigations, United States Department of Agriculture, National Seed Storage Laboratory, Fort Collins, Colorado 80521. more critical primary role restricted to the time of earlv imbibition but visibly expressed only later during seedling development. The effect of low temperature sensitivity can be recognized as chilling injury. occurring at very moderate temperattures and conditioned by the embryonic developmental history of the seed, e.g., conditions that lead to bleaching. This temperature sensitivity, particularly of the bleached seeds in a population, is a potential influence to the variability of that population. Variability is further complicated by seed coats acting at low temperature as an avoidance mechanism to restrict water uptake and hence injury from temperature. These specific factors in lima bean germination furnish a system which may be useful in investigating detailed mecha-niisms of temperature control of germination, seedling vigor, and factors that control population variability during germination. Materials and Methods The seeds used were Phaseolus lunatus L. of the cultivars and years of production listed in table I. These seeds were supplied by Ben Fish and Sons, Inc., Santa Barbara, California anid Asgrow Seed Coin-pany, New Haven, Connecticut. They were commiier-cially cleaned and sorted according to cotyledon color. The green and bleached portions within seed lots were separated and compared to minimize differences that might be due to genotype and production environment. In the laboratory, the seeds were further sorted by eye on an opal plastic plate over a fluorescent lamp, sized, and stored in sealed cans at-120 until used.