**Physiological Quality of Conventional and RR Soybean Seeds Associated with Lignin Content**

Cristiane Fortes Gris1 and Edila Vilela de Resende Von Pinho2 *1Federal Institute of Southern Mines 2Federal University of Lavras Brazil* 

#### **1. Introduction**

288 Soybean Physiology and Biochemistry

Torigoe Y., Shinji H. & Kurihara H. (1982). Studies on developmental morphology and yield

*Science*, Vol.51, pp. 89-96, ISSN 0011-1848.

determining process of soybeans. II. Developmental regularity of flower clusters and flowering habit from a view point of gross morphology, *Japanese Journal of Crop* 

> The sale of genetically modified soybean seed resistant to the Roundup Ready (RR) herbicide has revolutionized the worldwide soybean market in recent years. According to data from the International Service for the Acquisition of Agri-Biotech Applications-ISAAA (2009), in 2009, for the first time, more than three-quarters (77%) of the 90 million hectares of soybeans grown globally were biotech; followed by cotton, with almost half (49%) of the 33 million hectares being biotech; by maize, with over a quarter (26%) of the 158 million hectares grown globally being biotech; and finally by canola, with 21% of the 31 million hectares being biotech. These numbers indicate not only increases in hectares, but also a strong and growing adherence of farmers around the world to this technology.

> Considering the area planted to RR soybeans in the 2009/10 growing season throughout the world, from these 69.3 million hectares, a demand of approximately 4.2 million tons of RR soybean seeds may be estimated, which makes the international soybean seed market ever more expressive and competitive. In Brazil alone, up to November 2010, nearly 35% of the total soybean cultivars registered in the Ministry of Agriculture were RR genetically modified, this number having increased more than 443% in the last four growing seasons, a result of the increase in the number of breeding programs for obtaining RR cultivars.

> It is known that the physiological quality of soybean seeds is controlled in large part by the genotype or cultivar, features of the plant, and more specifically those of the pod and the seed itself, determining a differential response of each cultivar and its levels of tolerance to seed deterioration, to adverse field conditions and even to mechanized harvesting. Among seed characteristics, the seed coat is one of the principal conditioning factors for germination vigor and longevity of seeds, with its characteristics being associated with susceptibility to mechanical damage, longevity and potential for seed deterioration, which may be influenced by the lignin content and the degree of seed coat permeability. Understanding of the structure and properties of the seed coat has contributed to explaining and altering seed behavior under certain environmental conditions.

> In the case of soybeans, differences in the lignin content among seed coat have been observed by various authors (Tavares et al., 1987; Carbonell et al., 1992; Alvarez, 1994; Carbonell & Krzyzanowski, 1995; Panobianco, 1997; Menezes, 2008). In addition, a great deal of speculation has been generated in relation to the lignin content in the plant between RR genetically modified soybean cultivars and conventional cultivars (Coghlan, 1999; Gertz

Junior et al., 1999; Kuiper et al., 2001; Edmisten et al., 2006; Nodari & Destro, 2006), indicating overproduction of this substance of up to 20% more in RR cultivars. Such variation may occur not only in the vegetative parts of plants, but also in reproductive parts, such as pods and seeds.

The term lignin is used to designate a group of substances with similar chemical units indicated as polymers derived from "p-coumaryl", "conyferyl" e "sinapyl" alcohols (Lewis & Yamamoto, 1990). Impermeable to water, lignin is also very resistant to pressure and not very elastic and it is the most abundant plant polymer after cellulose, being found in greater quantity in the cell wall, around 60% to 90% (Egg-Mendonça, 2001), and its deposition occurs during the formation of the cell wall.

According to the authors, overproduction of lignin observed in the RR soybean plant in the US, and more recently in Brazil, is leading to deep stem fissures, with a significant number of plants in the field presenting bent or broken stems, and this effect possibly arises in the presence of water deficit and high temperatures.

Although the exact cause of the lignin behavior in this mechanism is still unknown, the hypothesis of overproduction of lignin in RR soybean plants is based on the fact of the precursors of the lignin molecule being formed in the same metabolic pathway, the pathway of shikimic acid, inhibited by the glyphosate herbicide. The inhibition of EPSPS enzymes by glyphosate present in this pathway leads to a deficiency in the production of amino acids and consequent death of the plants. That way, the sequence CP4 EPSPS, introduced in the genome of commercial soybean cultivars responsible for production of the protein CP4 enolpyruvylshikimate-3-phosphate synthase (EPSPS), an enzyme that participates in the biosynthesis of aromatic amino acids in plants and microorganisms, may be presenting the pleiotropic effect, thus modifying the lignin content in the plant.

Nevertheless, research in this area is still quite limited and the few results published do not compare conventional cultivars with their respective RR genetically modified versions, but refer to comparison between diverse genotypes and therefore do not isolate the effect of the inserted transgene. In this context, it is relevant to discuss the results of more recent research dealing with this issue in this chapter, principally looking at comparisons between conventional materials and their RR versions, which are essentially derivatives.

For that reason, in this chapter we will discuss results of research dealing with the physiological quality and the lignin content in RR and conventional soybean seeds submitted to different harvest times and spraying with glyphosate herbicide, produced in two different time periods and submitted to direct imbibition in water.
