Chapter 5 GERMINATION AND SEEDLING DEVELOPMENT Judith M. Bradow' and Philip J. Bauer2 . , • USDA, ARS, 'Southern Regional Research Cent~r, New, Orleans, La. 70179 and LCoastal Plams, Soil, Water, and Plant Conservation Research Center, ,Florence, s.c. 29502 1. INTRODUCTION 2. PHYSIOLOGY OF GERMINATING COTTON SEF~DS The responses of cotton (Gossypium spp.) seeds to the gehnination environment depend upon (1) the pointin the Under promotive environmental condtions, the four germination-through-emergence. sequence at which condi- sequential phases of cotton seed germinatioil arid seedling tions cease to promote germination and seedling develop- emergence occur during a relatively brief period (i::a. four ment, (2) the magnitude and duration of the deviations from to six days) in the physiological progression from fertilized conditions promotive of germination, and (3) seedling de- ovule to the mature plant that. produces the next crop of velopment 'success' potentials detehnined by the genetic seeds and fiber. When a quiescent, but viable, seed is plant- and seed vigor of a particular cotton seed lot and genotype. ed (Baskin et aI., 1986; Delouche, \ 1986; Association of Suhoptimalenvironmental factors, both abiotic and biotic, Official Seed Analysts, 1988; McCarty and Baskin, 1997), modulate. delav or tenninate cotton seed germination and the return of the embryo and the sustaining seed stDrage tis- seedJingde'lel~;ment dmi.ng any of the four universal phas- sues to active metabolism is initiated by water imbibition, es of seed germination: (1) imbibition, (2) mobilization of the first step in seed germination (Ching, 1972; Bewley seed reserves (cotyledonai-y lipids and proteins in cotton), and Black, 1978; Pradet, 1982; Simon, 1984; Christiansen (3) radicle protrusion and elongation through resumption of and Rowland, 1986). However, in 'hard" seeds of some cell division, and (4) hypocotyls and cotyledon emergence cotton species and varieties, this chalazal pore is plugged above the soil with the shift from metabolic dependence on seed. storage compounds to photosynthetic autotrophy~ with water-insoluble parenchymatous. material (tran and In cotton and other oilseeds, cotyledonary lipid mobiliza- Cavanaugh, 1984). The presence and persistence of the tion depends upon subcellular organelle-cooperativity and plug can produce 'hardseed' or 'seed-coat' dormancy, a membrane-transport phenomena elucidated as the gluco- form of dormancy in which there is no or minimal water neogenic glyoxylate cycle of oilseed species. uptake (Christiansen and Moore, 1959; Benedict, 1984; Among the environmental factors that affect cotton Christiansen and Rowland, 1986, Delouche et aI., 1995). seed germination and seedling establishment are tempera- Seed coat impermeability can also be induced in cotton ture, water availability, soil conditions such as compac- when seed water content is reduced to ::;10% before plant- tion, rhizosphere gases, seed and seedling pathogens, and ing or germination testing (De10uche, 1986; Delouche et interactions among theses and other biotic and abiotic fac- af., 1995). tors that are present in the seed bed and post-emergence micro-environments. This chapter refers to earlier reviews 2.1 ./ Early Imbibition of cotton seed germination and seedling establishment and provides a guide to recent investigations of the two essen- In the presence of adequate water and oxygen, vi- tial physiological processes, seed germination and seedling able, non-dormant cotton seeds, depending 0.1 the ambi~ establishinent, that ultimate:y determine both the yield and ent temperature, require four to six hours for full hydra- the quality of a crop. lMcD. Stewart et al. (eds.), Physiology a/Cotton, . DOll 0.1007/978-90-481-3195-2_5, ~"Springer Science+Business Media B.Y. 2010 Chapter 5. Germination and Seedling Development 49 tion (Benedict, 1984; Christiansen and Rowland, 1986). (Christiansen, 1967). A similar period of increased sensitiv- Initially, seed rehydration is it consequence of the matric ity to chilling was observed at 28 to 32 hours when Pima potential ('IIm) of the cell walls and cell contents of the seed seeds were germinated at 35°C and 40 to 56 hours when (Bewley and Black, 1978). Thus, the earliest phase of imbi- germination was at 25°C (Buxton et ai., 1976). Respiration bitional water can occur in both in dead and viable seeds. and water uptake both increase rapidly after radicle protru- Exposure of imbibing Upland cotton seeds to tempera- sion and the resumption of cell division in embryo tissues. tures below 5°C during the initial phase of imbibition re- These processes occur in Upland cotton seeds after a re- sults in seedling death (Christiansen, 1967; Christiansen, hydration/germination period of approximately 48 hours at 1968). Depending on the duration of chilling exposure, im- 30 to 31°C, the temperature considered optimal for cotton bibitional temperatures below 10°C cause radicle abortions seed germination (McCarty and Baskin, 1997). One field or, in cotton seedlings that survive chilling injury, pecrosis study also identified a third, later period of chilling sensitiv- of the tap root tip and abnormal lateral root proliferation ity at ca. 140 to 170 h after planting (Steiner and Jacobsen, (Christiansen, 1963). Germination of Pima cotton seeds is 1992). Under the conditions of that study, the third period inhibited by exposure to temperatures of 5 to 10°C at the of sensitivity corresponded to the 'early crook' stage of de~ beginning of the imbibition period (Buxton et ai., 1976). velopment for the chilling-stressed seedlings when the hy- Pima seeds have been reported to be resistant to chilling pocotyls were near the soil surface and ready to emerge. damage after four hours of warm imbibition. Significant chilling injury is induced by exposure of cot- 2.3 Glyoxylate Cycle and Storage Lipid ton seeds to cold water during the initial hours of imbibition. Metabolism Chilling during earliest seed imbibition has also been asso- ciated with increased leakage of solutes from seeds (Simon, In the lipid~storage tissue of cotton cotyledons, mito- 1979,1984). Both chilling injury and solute leakage reduce chondrial respiration is intergrated with glyoxysomal glu- seedling vigor and increase seed and seedling susceptibility coneogensis (Trelease, 1984; Trelease and Doman, 1984). to pathogens. Both processes are manifestations of events Thus, lipid mobilization during cotton seed germination during the first few hours of imbibition, and the severity of involves four subcellular compartments, i.e., lipid bodies, both chilling injury and solute leakage may be reduced if glyosomes, mitochondria, and cytosol. As germination and the seeds are preconditi(med under waml"germination-pro- seedling development progress, lipases associated with the motive conditions (Christiansen, 1968; Simon, 1979). lipid bodies liberate fatty acids stored in the cotyledons as triaclyglycerides during seed development. The free fatty 2.2 Post-Imbibitional Periods of acids are transported across the membranes of the lipId Sensitivity to Chilling Temperatures body and glyoxysome into the glyoxysomal matrix. Within the single unit membrane of the glyoxysome are the en- In viable seeds only, the initial phase of imbibition is zymes necessary for the ~-oxidation of the fatty acids to followed by an apparent 'lag phase' characterized by reduc- acetyl-CoA and the specialized glyoxylate cycle by which tion in the rate of water uptake, rapid increases in metabolic the acetyl-CoA is metabolized with the result of net synthe- activity, e.g., protein and mRNA synthesis, and reactiva- sis of succinate within the glyosome (Goodwin and Mercer, tion of preexisting organelles and macromolecules (Ching, . 1983; Trelease and Doman, 1984). 1972; Bewley and Black, 1978; Prad~t, 1982; Simon, Enzymes needed for further metabolism of succinate 1984). Significant water uptake resumes when protrusion of synthesized during the glyoxylate cycle are not located in the radicle through the seed coast signals 'true' germination the glyoxysomes, and succinate must be transported across with the concomitant resumption of cell division in embry- the glyoxysomal and mitochondrial membranes for con .. onic axis coupled with rapid mobilization of seed storage, version the oxaloacetate in the tricarboxylic acid cycle of reserves (Ching, 1972; Simon, 1984). mitochondrial respiration (Goodwin and Mercer, 1983; During the germination process, seed respiration rates Benedict, 1984; Trelease, 1984; Trelease and Doman, follow a triphasic curve similar to the cubic rate of imbi- 1984). Additional information on the biochemistry of em- bition (Ching, 1972; Simon, 1984). The initial period of bryogenesis and the development of glyoxylatecycIe or- high respiration overlaps the rapid initial stage of imbibi- ganelles and enzymes during seed maturation are discussed tion and the second germination phase characterized by the in Chapter 25 of this book. reactivation of preexisting macromolecules and organelles. Nearly all lipid reserves in oil-rich seeds like cotton are The post-imbibitional phase in seed germination represents mobilized after radicle protrusion (Treleaseand Doman, a 'steady state' for both water uptake and respiration dur- 1984). Once lipid mobilization is initiated during the im- ing which preexisting metabolic systems synthesize the bibition period, lipid utilization is rapidly completed over a substrates needed for biogenesis of new proteins, mRNA, relatively brief period during