María de los Ángeles Ruiz *; Alberto Daniel Golberg **; María Lía Molas **
*Estación Experimental “Guillermo Covas”, Instituto Nacional de Tecnología Agropecuaria. Argentina
**Facultad de Agronomía, Universidad Nacional de La Pampa. Argentina.
The initial phase of the life of a plant is a major event in theirlife cycle. The present chapter addresses this subject from an ecophysiological viewpoint. In this chapter we will first examine the germination process, and then we will discuss the dormancy of the seed, taking into account that this process has major consequences for the plant’s future individual fitness. The soil seed bank, including quiescent and dormant seeds, is the third topic. The differentenvironmental constraints that diminish or inhibit the ability of seed germination are discussed in the section dedicated to abiotic stress, and then we will examine some ecological topics such as the effect of fire on germination, recruitment of seeds, to finish the chapter examining some aspects related to seedling establishment.
The process that starts with the seedmaturation and ends with the seedling emergence is a highly complex event that can last for years if seeds have some mechanism of dormancy, or several months if an environmental factor is limiting. In the last case, the seed is in a quiescence state waiting for that factor –light, water, temperature, oxygen concentration- to meet the requirement. Hence, from the ecophysiological viewpoint, processespreceding the emergence become important. The purpose of this chapter has been to examine the various complex processes that occur when a seed comes in contact with surrounding water, thereby triggering germination until the seedling establishment.
In seed plants -spermatophytes, from Greek sperma: seed, fiton: plant- the germination of the seed is a process of enormousrelevance. In fact, this process starts the life cycle and places the future plant in the best environmental conditions to ensure ecological success of the future plant. Many seeds of non-cultivated species, and some cultivated as forage, are equipped with sophisticated mechanisms to monitor the environment in which they must survive and reproduce, considering that plants, unlike most animals aredestined to live in the place where they began their existence, that is, where germination occurred.
Germination can be defined as the set of events triggered by water absorption of the quiescent seed which concludes with the elongation of the embryonic axis [1, 2]. Thus, the visible sign that germination has been completed is the radicle going through the structures surrounding the embryo, whichis called visible germination. The seed contains an embryo, which represents a miniature plant. The embryo is structurally and physiologically equipped to regenerate one individual, so the seed must be well provided with reserves to sustain the growing embryo until the seedlings become independent of seed reserves and developed into an autotrophic organism .
The first physiological processrelated to germination is the absorption of water and, consequently, seed imbibitions. Once this event is complete, the seed that was in a state of quiescence quickly resets its metabolic activity, and the respiratory activity becomes important. After this restoration, respiratory rate decreases momentarily until the root penetration through the surrounding structures occurs and, finally, a newpeak of respiration takes place . These events are associated with the beginning of pentose cycle, glycolysis and the Krebs cycle, as well as the mobilization of seed reserves toward the embryo .
The emergence of the radicle through the structures surrounding the embryo is the event that determines the end of germination and marks the beginning of seedling growth . The elongation of...