**Abstract**

Plant seeds, being sessile, are simultaneously exposed to favorable or adverse conditions from sowing to harvest. The physiological quality of the seed is affected by the type of biotic and abiotic stress to which the mother plant is exposed, especially in the stages of embryogenesis, development and seed filling. Therefore, the behavior of their progeny will be reflected when the seeds are capable of maintaining acceptable viability standards with a high-germination potential to generate a normal seedling and establish themselves without difficulties under field conditions. Most of the species cultivated under abiotic stress conditions reduce their physiological quality; however, some species are salt dependent, and prolonged absence of NaCl in the soil inhibits seed development, results in lower seed quality and thus limits progenyseedling growth as is the case of Suaeda salsa, and typical annual extreme halophytic herb with succulent leaves develops well and produces high-quality seeds when grown under high salinity conditions. Consequently, the response of the plant to adverse factors depends on the genotype and its stage of development at the time of stress, the duration and severity of the type of stress and the environmental factors that cause it. Depending on the severity and duration of the stress, plants could activate mechanisms to adapt or tolerate abiotic stress conditions at the molecular, morphological, physiological and cellular levels.

**Keywords:** abiotic factors, climate change, heat stress, seed germination, soil salinity, water stress

### **1. Introduction**

High-quality seed is the main input to obtain high crop yields, by producing healthy plants, resistant to diseases and to adverse conditions [1, 2]. The physical, physiological, genetic, and sanitary quality of the seeds depend on the genetic material used in sowing, management of the mother plant, climatic conditions, such as temperature, humidity, and solar intensity; likewise, the edaphic characteristics or the fertility of the soils, presence or absence of stress during the stage of germination, emergence, growth, reproduction (flowering and pollinization/fertilization, seed filling period), and harvest and post-harvest management [3–8].

Different environmental stress during seed formation that affects the final seed quality can be due to water, mineral deficiencies, salinity, and extreme temperatures among the most important; water deficiencies during grain filling, flowering, or pod formation can reduce germination potential and seed vigor [9–11]. The seed's quality is also affected when the water deficit is complemented by high temperatures, causing the production of a high proportion of small seeds with low germination and vigor [6, 12, 13]. Internal plant temperature increase causes stomatal closure and causes damage to the seed embryo due to the decrease in the thermoregulatory effect of water, thus influencing seed germination. Photosynthesis is reduced due to high temperatures and reduces the flow of the substances produced toward the seed; in this condition they are used to sustain respiration, creating an imbalance in the stage of spike formation or seed filling, and affecting the quality and size of the seeds.

On the other hand, high salinity levels generally cause a reduction in seed germination or may retard the germination process or affect plant growth by interfering with seed germination, as well as enzyme activity and unbalance mitosis mainly in glycophytic plants [14, 15]. Some species of halophyte-type plants can tolerate high levels of salinity, but the levels of germination and vigor of the seeds can be affected in the absence of salts.

In response to abiotic stress conditions, plants can activate various stress-sensitive systems, such as upregulation of stress-related proteins that function as molecular chaperones (heat shock, abundant late embryogenesis, and protein dehydrins), production of enzymatic antioxidants (catalase, superoxide dismutase, and ascorbate peroxidase), and accumulation of compatible solutes known as osmolytes (proline and glycine betaine) in plant cells subjected to osmotic stress [16]. In the case of extreme temperatures, plants show a broad structural and physiological plasticity that allows them to adapt to different temperatures [12, 17, 18].

### **2. Incidence of abiotic factors in seed quality**

When plants grow under adverse environmental conditions, the oxidative/reductive state in their cells is altered and allows the processing of a type of protein and its transport to the nucleus to regulate the expression of genes related to the homeostasis of nitric oxide (NO) and the response to abscisic acid (ABA), a key plant hormone in the response to numerous abiotic stresses (solar radiation, temperatures, humidity, and salinity); it is a survival mechanism against different abiotic stresses during seed germination or subsequent plant growth. High radiation is rarely a problem, as long as water and nutrients are available. To obtain high yields, leaves grow and cover the soil surface as soon as possible after planting; if this process is delayed, solar radiation is lost as heat embodied in bare soil, evaporating soil moisture [19]. With high temperatures, crops need a greater amount of inputs (nutrients, water, and solar radiation) to be able to maintain its level of metabolism. During the filling of the seed or grains and as the temperature increases, the development accelerates more than the growth; even under optimal management conditions, yield can be reduced by up to 4% for every 1°C increase in mean temperature due to shortening the grain-filling period [20].

The damage caused by high temperatures is commonly associated with water stress; therefore, the availability of water becomes a critical factor for the survival and initial growth of plants and in the formation and development of seeds. Consequently, to the extent that plants can transpire freely, they will also be able to cope with high temperatures; with enough water available they can withstand air temperatures of 40°C; however, if water is a limiting factor, leaves may die at that temperature.

Regarding salinity, salts can destroy the soil structure causing the expansion of clays and the dispersion of fine particles that clog the soil pores through which water and oxygen circulate [19]. The increase in salts causes very high values of osmotic pressure in the soil water, generally causes a reduction in seed germination or can slow down the germination process, interfering with the growth of most crops and other plants specialized [11, 15].

The environmental effects of seed production are complex, in that environment, the mother plant has a significant influence on seed traits including seed size, dormancy, and germination. In many species, factors such as the age of the mother plant and the position of the seed in the fruit, inflorescence, or canopy can affect seed properties, often accompanied by dimorphism of the seeds themselves or of the fruits themselves arise [6].

#### **2.1 Water stress**

#### *2.1.1 Effect of water stress on seed germination*

Crop production is highly influenced by the water level in the soil, one of the best conditions being field capacity. There are three degrees of water stress based on the relative water content (WRC), a measure that normalizes the real water content in the tissue with respect to the water potential that the tissue could have: (i) mild stress: decrease in the water potential of some bars (tenths of MPa) or WRC by 8–10% compared to well-watered plants under slight evaporative demand; (ii) moderate stress: decrease in water potential more pronounced, although less than −1.2 to −1.5 MPa or a decrease in WRC between 10 and 20%, and (iii) severe stress: when the decrease in water potential is greater than 15 bars (−1.5 MPa) or decrease in WRC greater than 20% [21].

Water deficit during grain filling, flowering, or seed coat formation reduces germination and seed vigor [10]. When water deficiency is complemented by high temperatures, the plant produces small seeds of poor physiological quality [12, 13].

Seed germination in *Citrullus lanatus* is extremely sensitive to water stress. An osmotic ψ of −430 kPa completely inhibits germination; compared to other cucurbits, the seeds of *C. lanatus* are the most sensitive [22]. However, the sensitivity of *C. lanatus* seeds to water stress is reduced when the seed coat and inner membrane are removed. Exposure to water stress causes secondary dormancy (lettuce seeds) when insufficient water potentials are present; when seeds are exposed to the environmental signal, secondary dormancy is overcome, which means that secondary dormancy is a reversible process for the seed [22].

#### *2.1.2 Effect of water stress on the agronomic characteristics of the mother plant*

Water stress causes a decrease in stomatal conductance that causes a reduction in transpiration and photosynthesis as a consequence of the decrease in total carbon (C) fixation. Water deficit also causes loss of starch reserves and nonstructural sugars that support the growth and development of the mother plant, due to the decrease in the activity of the photosynthetic enzyme galactinol synthase [23, 24]. Sucrose import is blocked and remobilization of starch reserves to the ovary walls occurs when water


**Table 1.**

*Water stress and its effect on the physiological quality of seeds.*

stress occurs 5 days before anthesis in the case of maize [25]. These water stress conditions directly affect the photosynthesis processes in the carbohydrate translocation from the leaves (source) to the seeds (sink) (**Table 1**) [27, 28].

In the seed filling stage of *Glycine max*, there is a reduction in yield due to water stress, in addition, in pre-flowering, the abortion of flowers occurs due to damage to the pistil (ovule) and not to the pollen, which reduces the number of pods and therefore seed yield; however, they are not affected in the physiological quality in terms of germination, vigor, and viability of the seeds [12, 26, 29, 30].

In the maize case (**Table 1**), the action of CWIN (cell wall invertase) and glucose concentrations lead to ovary abortion and yield loss in the face of permanent water deficit [25]. When water stress occurs during flower induction and inflorescence development, panicle development and anther dehiscence are delayed, resulting in a substantial reduction in rice and sorghum seed establishment [27].

#### **2.2 Temperature stress**

### *2.2.1 Effect of high-temperature stress on plant physiology*

Most plant species are sensitive to temperature stress and suffer when these are low or very high with respect to the thresholds defined for each one. There are few production environments with ideal temperatures (5–25°C). In response to these environmental constraints, plants display a broad structural and physiological plasticity that allows them to adapt to different temperatures caused by geography, diurnal, and seasonal rhythms [17, 18]. The answers vary if it is a temporary or permanent stress, due to high night temperatures, daytime temperatures, and the daily average, or if there is an interaction between day and night temperatures. In general, four types of heat stress

*Water Stress, Heat, and Salinity in the Physiological Quality of the Seeds DOI: http://dx.doi.org/10.5772/intechopen.107006*

are recognized in plants: that caused by sustained high temperatures; frequent episodes of high temperatures ("heat shock"); damage due to cooling (from 0 to 10°C) or "chilling injury" in numerous fruits, foliage, and tropical flowers; and freeze damage at temperatures below 0°C, which causes ice formation in plant tissues [17].

High temperatures generate anatomical, morphological, and functional changes in plants, some similar to those produced by water stress. The opening of the stomata is a response of plants to stress due to high temperatures to cool their leaves through transpiration, while in response to drought they close their stomata to reduce water loss and under conditions of combined stress due to high temperatures and lack of water the stomata remain closed [31]. Heat waves are generally accompanied by periods of drought, accelerating soil water loss, and increasing the H20 vapor pressure deficit in the air, decreasing stomatal conductance [32]. Meristems, fruits, seeds, and young leaves are closely associated with mature leaves and with the whole plant in general, through complex networks of source/sink relationships. When moderate stress occurs due to high temperatures, photosynthesis and the activity of sources and sinks are reduced simultaneously because the assimilates destined for the reproductive or conservation organs are destined to solve maintenance respiration and osmotic adjustment [33]. The photosynthetic apparatus is very susceptible to high-temperature stress; photosystem I, II, and carbon assimilation (Calvin Cycle) are the most affected processes. Consequently, the cumulative effects of these changes usually result in poor growth and reduced plant productivity [34, 35].

#### *2.2.2 High temperatures in the physiological seed quality*

The physiological seed quality can be altered due to a shorter time in the grain filling period caused by high temperatures. This stress type affects the transport and accumulation of reserve substances in the seed due to a greater fluidity of the lipids in the cell membranes of the seeds and a greater loss of electrolytes so that the seed produced under these conditions has less vigor by the low amount of reserve substances accumulated by the seeds [36, 37].

High temperatures affect the development of gametes and the double fertilization process and therefore the number of seeds formed. It also affects pollen production and viability, stigma receptivity, pollen germination, and pollen tube elongation. Therefore, exposing plants to high temperatures in early reproductive stage results in an irreversible loss of yield while in late agronomic stage losses can be reduced with some agronomic practices [38–40]. Likewise, high temperatures cause a greater production of superoxides (ROS) that could reduce the metabolic activity of the seed necessary for the germination process. Hydrogen peroxide plays important role in the germination process, however, high levels of H2O2 can be toxic to seeds [41, 42].

Reduction of the thermostability of the plasmatic membrane delays the activity of the Ca + 2 signaling, kinases, and heat shock factors are responsible for the decrease in germination and seed vigor due to thermal stress [43]. On the other hand, phytohormones related to the germination process, such as ABA and GA, have a negative interaction, especially in response to stress conditions [44]; exposure to high temperatures increases ABA concentration and decreases GA biosynthesis in seeds [45].

Likewise, substances from the group of polyamines, including putrescine, spermidine, and spermine, are recognized for their relationship with seed development and tolerance to high-temperature stress [46]. Glutathione has also been identified in tolerance to heat stress, in the regulation of redox signaling and defense processes in *Zea mays*, *Coleus blumei*, *Fagus sylvatica, Triticum aestivum,* and Vigna radiata [47–49].


#### **Table 2.**

*Temperature stress effect on physiological quality of seeds.*

High temperatures have favorable and in some cases favorable effects on the physiological quality of the seeds (**Table 2**). For example, in late pea, rice, and Lupinus sp. and in plants of the Brassica genus, stress due to high temperatures in the field affects crop phenology, fruit set, viability, germination, and the number of abnormal seedlings [41, 50, 54, 56]. In lentils (*Lens culinaris*), exposure to high temperatures reduces the grain filling period, as well as seed growth, the number of seeds per plant, seed weight, carbohydrate, protein, and mineral content; abnormalities in flowering reduce retention capacity of legumes, lower oil content in seeds in canola, and higher incidence of pathogens in seeds [53, 57–59].

In *Medicago truncatula*, the increase in temperature from 15 to 25°C favors the increase in the properties of the seed coat, regulating imbibition and physical dormancy [52]. Likewise, the stress environment with high temperatures in *Medicago* sp. is positive for faster and more uniform germination compared to seeds produced under optimal conditions [54]. In soybeans, exposure to high temperatures of 36/24°C and 42/26°C from the beginning of flowering to harvest, presents a lower accumulation of lipoxygenase, the β-conglycin, sucrose binding protein, and Bowman-Birk protease inhibitor compared to soybeans of the control under 28/22°C [62].

#### **2.3 Salinity**

Osmotic and water stress in plants generated by soil salinity is one of the main causes of the deterioration of arable land. It is estimated that there are more than 800 million hectares globally including 20% of cultivated land, with salinity being the cause of yield losses estimated at 20% worldwide in arid and semi-arid regions of the world [63–67].

*Water Stress, Heat, and Salinity in the Physiological Quality of the Seeds DOI: http://dx.doi.org/10.5772/intechopen.107006*

The process of accumulation of K and Na salts in soils, with a predominance of Ca and Mg, can be of natural origin or the product of anthropogenic activities, mainly due to the indiscriminate use of chemical fertilizers, inadequate irrigation practices, and flooding of soils with seawater in coastal regions are the main causes of soil salinization [15, 68].

There are plants that can tolerate or be completely susceptible to salinity. Most cultivated plants are salt-sensitive glycophytes. In contrast, halophytic species such as quinoa (*Chenopodium quinoa* Willd.) and *Suaeda aralocapsica*, are capable of reducing stomatal density when grown under hypersalinity conditions [5, 11, 69]. Quinoa can tolerate high levels of salinity (6 d S m -1), without a significant decrease in seed yield and total biomass [11].

Salt stress affects seed germination rate, germination initiation, and seedling establishment due to osmotic stress, ion toxicity, and oxidative stress [11, 14]. The presence of salts in the soil can reduce seed germination by decreasing the amounts


#### **Table 3.**

*Salinity stress effect on physiological quality of seeds.*

of gibberellic acids (GA), increasing the levels of abscisic acid (ABA), and altering cell membrane permeability and water behavior within the seed [64]. However, the presence of salts may not always be detrimental to all crops (**Table 3**); some species require salts from the germination stage [5, 11]. In halophytic plant types such as *S. salsa*, the prolonged absence of NaCl inhibits development and affects seed quality; as a consequence, it limits the growth of progeny seedlings in terms of biomass and seed yield [76, 81]. In C. quinoa and *S. aralocapsica* with 0.3 M NaCl and 1.5 M NaCl, the percentage of seed germination is reduced between 10 and 20% [5, 11, 69]. In the case of rice seeds, they are relatively tolerant during germination and very sensitive at the seedling stage; after establishment, plant tolerance increases progressively until panicle differentiation, and decreases again until the flowering stage [82].
