**5. Effect of environmental factors during seed filling**

Stress in plants was described by Hans Selve in 1936 as "unfavorable conditions and environmental constraints in plants". This general definition can be applied to all organisms, but the definition of stress in plants differs from that in animals and humans. Plants are sedentary and live in fixed locations. Therefore, they cannot escape abiotic stress conditions when exposed to them and are constantly exposed to these conditions without protection. Animals, on the other hand, are mobile and can avoid and escape these conditions when needed. Since plants are sedentary, they need mechanisms to protect themselves from stressful conditions so that they can continue their vital activities [170].

Global warming and drought in the world have become important inhibitors of agricultural production in recent years. The process of seed filling, which is affected by environmental factors, is becoming increasingly important for agricultural production because the potential heat and drought affect the, and rate and duration of seed filling. To overcome these adverse conditions, plant breeders are developing new varieties that are resistant to biotic and abiotic stress conditions while ensuring efficient water and nutrient use and good yields. After pollination and fertilization, seed development begins with cell division for embryo and endosperm development in the ovule and continues with cell expansion and differentiation to form seeds. Seed formation continues with the accumulation of storage reserves such as carbohydrates, proteins, and lipids. After accumulation is complete, desiccation occurs, during which the seeds lose moisture.

Flowering plants reproduce by the production, dispersal and germination of seeds. The cellular stage includes all processes involved in the formation and development of the various parts of a seed. At this stage, the storage reserves for the embryo are synthesized and seed filling takes place. Many factors influence seed production and seed content. The position of seeds on the inflorescence can affect the duration and rate of seed filling. Seeds farthest from the transport source, such as seeds on a cob, may remain small because they do not receive sufficient nutrients for optimal seed growth. In the early stages of seed development, a constant and adequate supply of nutrients is required for seed production. Seeds that do not receive an adequate supply of nutrients during the generation stage may fail to develop or develop poorly and have a smaller seed mass. Plants can be affected by abiotic stress at any stage of development, but the generative stage is the most critical period when plants respond to stress conditions. Stress conditions during the generative stage adversely affect pollen formation, pollination and fertilization rates, and reduce fruiting and seed set, resulting in yield losses. The generative stage is highly susceptible to drought, cold, and heat, and these stress factors reduce fertilization, seed development, and the filling process [171]. Heat stress has significant negative effects on meiosis during pollen development and could greatly reduce pollen fertility, pollen quantity and quality, pollen germination, and pollen tube development on the stigma [172, 173]. Heat stress can also significantly affect seed development during the seed filling stage due to reduced assimilate supply and reduce seed yield in many crops including cereals and legumes [174, 175]. The seed filling period is also closely related to the development of plant senescence [129]. Heat and drought stress during the seed filling period cause early senescence and also shorten the seed filling time [176, 177].

#### **5.1 Effects of drought stress during seed filling**

Drought stress limits vegetative growth by reducing leaf water content and stomatal conductance [178] in various crops such as cereals [179, 180] and legumes [181]. Decreased stomatal conductance increases leaf temperature, and both events

#### *Seed Filling DOI: http://dx.doi.org/10.5772/intechopen.106843*

lead to wilt symptoms [182, 183]. Drought stress damages cell membranes [184, 185], decreases chlorophyll content, photosynthesis, and reserve synthesis [178, 186–188]. Drought stress also impairs plant nutrient uptake [189, 190] and significantly reduces nitrogen fixation in legumes such as soybean [191] and pea [192]. The overall negative effects of drought stress reduce the production of assimilates and reduce the transport of reserves to the developing seeds of plants [193–195].

The generative phase of plants is more sensitive to drought stress than the vegetative phase. Drought stress reduces the number of flowers, fruits, and seed set and therefore could reduce seed yield [196, 197]. Decreased water content in tissues leads to a reduction in the activity of the acid invertase enzyme, which in turn prevents sucrose uptake into developing seeds [198]. Low sucrose and high ABA levels lead to poor seed development in cereals under drought stress [199]. Wheat plants subjected to drought stress during the seed filling period showed a significant decrease in cell wall and soluble invertase activities, and glucose, fructose, and sucrose contents of the drought-sensitive genotype were significantly lower [200]. Drought in the early stages of seed development leads to a reduction in seed size due to reduced number of endosperm cells. Seed yield was significantly reduced in plants subjected to drought stress during the seed filling stage [186, 201, 202]. Drought stress in the early stages of seed filling reduced germination percentage in soybean by up to 9% compared to the control [203]. Similarly, in chickpea, medium-sized seeds produced under drought stress had lower germination percentage and viability than control seeds [204].

Drought stress during embryogenesis and seed filling reduces the number of endosperm cells formed and thus the size and weight of the seeds [205]. At this stage, the duration and amount of seed storage reserves, such as starch accumulation in the endosperm decrease, and so does seed weight [206]. Drought stress during seed filling reduces the number and size of starch granules in endosperm cells [206]. Drought stress affects the composition of seed reserves. The starch content of wheat seeds subjected to drought stress during seed filling is significantly reduced [207]. Drought stress negatively affects phthosynthesis, and low phthosynthesis product content inhibits starch biosynthesis [208] and related activities such as reduced endosperm cell number and starch granule size [206] and lower starch amylase content [209].

Lipid content and fatty acid composition change due to lower content of soluble sugars such as glucose, fructose, and sucrose and reduced transport of sugars from the phloem to endosperm cells under drought conditions [210]. In peanuts, the content of linoleic and behenic acids decreased, while the content of stearic and oleic acids increased under drought stress [211]. In maize, drought stress resulted in a significant decrease in oil content, while the content of linolenic and oleic acids in the seeds increased. In addition to the oil content, the total tocopherol, flavonoids, and oil-soluble phenolics contents also decreased [212, 213]. In soybean, drought stress decreased the oil content and oleic acid content of seeds [214]. While drought stress reduced the starch and oil content of seeds, the protein content of soybean seeds grown under drought stress increased. Seed nitrogen supply depends on remobilization of nitrogen from vegetative tissues, while starch and oil biosynthesis depends on sugars from photosynthesis, which decreases under drought stress. For this reason, seed viability may also be affected by drought during late maturation and seed desiccation.

### **5.2 Effects of heat stress during seed filling**

Heat stress affects all stages of plant development from germination to senescence. Different plants have different sensitivities to heat stress during seed filling [215, 216]. Seed filling rate and potential seed mass are generally used as two selection criteria for heat stress tolerance [217]. High temperature stress could accelerate seed filling rate by shortening the duration of seed filling and could lead to yield reductions [216, 218]. Heat stress during seed filling significantly reduces seed weight, seed number and seed yield in legumes [219–221], cereals [222], and other crops [223]. In chickpea [224] and lentil [183] increased seed filling rate resulted in smaller seed size. Similarly, a reduction in seed filling time resulted in smaller seed sizes in soybean, pea, and white lupin [225]. Temperature also affects seed filling rate and duration. An increase in ambient temperature from 15.5°C to 26.6°C decreased seed filling duration in cowpea from 21 days to 14 days [226]. A 1.7°C increase in temperature shortened the duration of seed filling and accelerated maturation, but decreased seed yield in chickpea [227].

Starch accounts for more than 65% of the dry weight of cereal seeds [228]. Therefore, the main reason for yield reduction is mainly the reduction in starch accumulation. Heat stress during the seed filling period reduces seed size and mass in wheat [229] and rice [230], and also impedes starch biosynthesis and accumulation by altering the expression of genes in starch biosynthetic pathways [231]. As a result of altered gene expression, the amount of non-structural carbohydrates decrease, altering the balance between soluble sugars and starch [232]. Heat stress decreases the content of sugars such as fructose and hexose phosphate in wheat [233]. In some cases, up-regulation of starch hydrolyzing enzymes such as α-amilaz under heat stress is thought to be responsible for the increased sugar content during seed filling [234, 235]. Thus, heat stress negatively affects starch accumulation by altering gene expressions in metabolic pathways. These changes may vary depending on the duration of heat stress, the growing season, and the plant species.

Oil content and quality are severely affected by heat stress in oliferous crops [236]. The effects of heat stress may vary depending on location, altitude, precipitation, and differences between day and night temperatures during the seed filling period. Because oleic acid and linoleic acid are produced by the same pathway through desaturation, there is a strong and negative correlation between them, and temperature and precipitation during the flowering and seed filling periods have significant effects on the fatty acid composition of plants [237]. Growth experiments conducted at different temperatures (10, 16, 21, 26.5°C) with canola, flax, sunflower, safflower and castor bean during the seed filling period have shown that the fatty acid composition changes and the amount of unsaturated fatty acids is reduced, with the exception of safflower and castor bean [238, 239]. High temperatures during seed filling reduce linoleic acid content and increase oleic acid content in seeds, while palmitic and stearic acid content change insignificantly [240–242]. The fatty acid composition of rapeseed also changes depending on location and year. While low temperatures and precipitation decreased oleic acid content, high temperatures and low precipitation did not cause significant changes in linoleic and linolenic acid content [243]. The activities of oleayl-PC desaturase and linoleayl-PC desaturase, which catalyze the synthesis of linoleic and linolenic acids from oleic acid, are decreased by high temperatures [244]. Consequently, high temperatures have negative effects on linoleic and linolenic acids synthesis, whereas high temperatures have positive effects on oleic acid synthesis [240, 241, 245]. Linolenic acid is the major fatty acid in flaxseed, and increasing temperatures (15, 20, 25, 30°C) during seed filling reduced the oleic and linolenic acids content in flaxseed [246]. Increasing the growth temperature from 29°C to 35°C during seed filling in sunflower and soybean resulted in a 2.6% reduction in oil content in the seeds of these plants [214, 247].

#### *Seed Filling DOI: http://dx.doi.org/10.5772/intechopen.106843*

Heat stress reduces the duration of seed filling, the amount of protein accumulation and protein quality, but has no effect on the rate of accumulation [248]. The composition and quality of storage proteins change due to changes in nitrogen mobilization during seed filling in wheat under heat stress [228, 249]. A decrease in high molecular weight glutenins and increased gliadin accumulation decreased dough quality in wheat under heat stress [248]. Similarly, high temperatures caused denaturation and aggregation of several storage proteins (globulins, legumin, and vicilin) in pea [250] and loss of enzyme activities in protein synthesis in lupin [251].
