**3. Edaphoclimatic Factors**

Expansion of soybean into tropical areas of Brazil has been made possible by development of genotypes having the long juvenile or late flowering trait in short-day photoperiods. This was required to allow soybean to have a sufficiently long developmental period to achieve enough size for optimal yield [37]. Soybean cultivars are affected by photoperiod, humidity, temperature, planting date, altitude, latitude, level of soil fertility and other factors [35]. Ac‐ cording to [44], these are the main elements responsible for the variability and difficulties for growing the crop in Brazil. Environmental factors interact with soybean's developmental periods in affecting the growth and yield production of the crop.

Data for Value for Cultivation and Use (VCU) of soybean BRS 278RR for the state of São Paulo indicated that it belongs to a medium maturity group (maturity group 9.4), with the number of days to maturity ranging from 115 to 127 days [26]. According to these authors, the average yield of soybean tested in 24 environments of evaluation was 2,973 kg/ha, and 5.2% more productive than the standard transgenic BRS 271RR, and 5% less productive than

A Comprehensive Survey of International Soybean Research - Genetics, Physiology, Agronomy and Nitrogen

**Cultivar Productivity (Kg/ha) Relative productivity (%)**

**Table 9.** Average grain yield (kg / ha) and relative yield (%) of BRS 278RR, BRS and BRS 271RR turtle in the agricultural years 2004/05 to 2006/07 of the southern regions of the Maranhão, Piauí southwestern and northern Tocantins.

Because of these pioneering studies to develop cultivars adapted to different regions of northern Brazil, soybean production has expanded rapidly throughout the region. For all the soybean growing regions of Brazil, the greatest expansion has occurred in the North and Northeast regions of the country [8]. Across the region, between the 2008/2009 growing sea‐ son to the 2009/2010 season, the area of production increased by 8.9% from 2,105,600 to 2,292,600 hectares. A more recent survey by [7] highlights the states of Bahia, Piauí and Mar‐ anhão. In the state of Piaui area there was a 16.6% increase in soybean acreage between 2011 and 2012, with the entire state having a production area of 447,300 hectares for soybean. In the state of Bahia, the largest producer of oilseeds in the Northeast, the area planted to soy‐ bean increased by 6.6%, from 1,040,000 to 1,110,000 hectares across the same period. The same type of expansion occurred in Maranhao where the growing area increased 12.2%,

Development of adapted cultivars for Northeast Brazil has greatly aided this expansion into tropical regions. This has made a significant contribution to strengthening the regional agri‐ cultural economy. Soybeans provides oil for the food industry and livestock meal for meat production, enhances agricultural development in undeveloped areas, and helps to add a protein-rich source of human food to ameliorate the widespread protein deficiencies com‐

Expansion of soybean into tropical areas of Brazil has been made possible by development of genotypes having the long juvenile or late flowering trait in short-day photoperiods. This was required to allow soybean to have a sufficiently long developmental period to achieve

**2004/05 2005/06 2006/07 Média BRS 278RR** 2,960 3,059 2,890 **2,973 105.2 BRS Tracajá** 3,083 3,298 2,942 **3,115 110.2 BRS 271RR** 2,720 3,154 2,568 **2,825 100.0**

the conventional BRS tracajá (Table 9).

from 518,200 to 581,400 hectares.

mon to Northeast Brazil [3].

**3. Edaphoclimatic Factors**

Source: [26]

Relationships

384

Although soybean originated in the temperate world, it has a wide range of adaptability and can be grown well in tropical and subtropical locations. Average temperatures optimal for the best soybean development are between 20 and 35o C. At temperatures outside this range there can be physiological disorders with flowering and rhizobial inoculation, as well as poor growth. [35] claimed that at temperatures below 24o C flowering is retarded by two to three days for each decrease of 0.5°C. Such conditions can happen in the Northeast at alti‐ tudes greater than 500 m in the rainy winter period. Such cases occur in Areia-Paraíba, Ga‐ ranhuns-Pernambuco and Triumfo-Pernambuco, among other cities.

Temperature has a strong influence on the rates of all the metabolic and physiological proc‐ esses occur during development. This has a direct effect on growth rate and yield. Tempera‐ ture also has a significant effect on the duration ofthe different developmental periods that make up soybean's crop cycle. In particular, the periods from emergence to flowering and flowering to maturity are affected by it [16]. It is recommended that the planting of soybeans should not be done when the soil temperature is below 20 o C so that germination and plant emergence are not compromised. Once temperatures reach this level, the germination rate increases exponentially with increasing temperature [21]. Thus, the number of days from sowing to the state of emergency (LV) can vary from about 5-15 days, depending on temper‐ ature [35]. Also according to the authors, temperature is the main factor influencing plant development, where low temperatures delay and high temperaturesaccelerate seedling emergence and leaf development.

The time course of the vegetative activity of plants is adjusted to local conditions during the growing season. In the dry tropics and subtropics, the growing season is limited by the in‐ tensification of water stress when the dry season begins [21]. Under the conditions of north‐ eastern Brazil, the predominant climate is hot and dry. According to some authors [14, 35], for soybean temperatures above 40 o C have an adverse effect on the rate of growth, the rate of formation of the nodes and internode growth and floral initiation. This causes problems with flowering, pod formation and retention, and results in lower yield. These problems are accentuated with the occurrence of water deficits.

Differences in flowering date between yearsby a cultivar sown in the same season are due to temperature variations. Thus, high temperatures can cause soybean to flower too early, which may cause a decrease in plant height,and accelerate the maturity of the crop [14]. The nodulation and nitrogen fixation in soybean are greatly affected by soil temperature and the growth of Bradyrhizobiumjaponicum is limited by temperatures above 33 o C [35]. When fac‐ ing environmental stress, there is always a genotype x environmental interaction which must be considered during the breeding program for cultivar development [32].Solar radia‐ tion is a critical environmental factor because it directly affects the crop growth rate and the ability to obtain enough dry matter for optimal yield potential. Water availability is also very important, because it affects leaf expansion, photosynthetic rate, crop growth rate, and nitrogen fixation [30].

Not only is the level of light important, but length of day, or photoperiod, is also important. Photoperiod affects many developmental processes such as flowering, seed germination, growth of stems and leaves, formation of storage organs and assimilate partitioning [38]. It also affects leaf expansion, photosynthesis, senescence, dormancy of buds, and other proc‐ esses [16]. Photoperiodism is a term describing all these plant responses to day length or photoperiod. Photoperiod will be determined by the latitude of where the crop is grown and the planting date.For soybean, the effective photoperiod includes the time from sunrise to sunset and some of civil twilight. Three types of photoperiodic response are recognized: short-day response in which flowering is induced and/or accelerated at a certain critical day length or less; long-day response in which flowering is induced and/or accelerated at a cer‐ tain critical daylength or longer; and day neutral in which flowering time is unaffected by day length. Soybean is a short-day plant.

Since vegetative growth in soybean occurs between emergence and the start of seed filling, it is important that soybean be planted at a latitude and planting date where day lengths will be long enough to allow enough time to the start of seed filling so that dry matter accumula‐ tion is large enough for optimal yield potential. The major problem with the expansion of soybean into northern Brazil is in these low latitudes, day length is seldom longer than 12 hours per day. Ideally, it is best to plant in October or November to take advantage of the lengthening days that occur at this time and continue until the summer solstice in late De‐ cember. Since the critical short-day period to hasten flowering occurs when the photoperiod is shorter than 13.5 to 14.5 hours (the length varies with maturity group and cultivars within maturity groups), the typical soybean planted in this region will have too short a time to flowering and the start of seed filling to achieve enough size for optimal yield. For this rea‐ son, much research in Brazil has concentrated on incorporating the "long-juvenile" trait into soybean cultivars. The juvenile period is the initial period in the plant's growth when it's developmental rate does not accelerate in response to short days. Thus, cultivars having this trait will have a longer time to flowering and the start of seed filling compared to those that do not. Incorporation of this trait into Brazilian cultivars has greatly facilitated soybean ex‐ pansion into northern Brazil. [25, 15, 14] .

The physiology of the mode of action of the long juvenile period is still unknown. The trait is either caused by a delayed ability of the plant to respond to short days or a requirement for more short day cycles to induce and promote flowering relative to other soybeans [14].The problem of short-day induced premature flowering in northern Brazil was ap‐ proached by selecting genotypes with insensitivity to photoperiod and/or having a long ju‐ venile period [27]. Several breeding programs have contributed to the development of highyielding cultivars adapted to different agro-climatic conditions of Brazil. For expansion into central and northern Brazil, breeding programs have followed a strategy of developing gen‐ otypes adapted to low latitudes, through the incorporation of the long juvenile trait. Re‐ search conducted by [33] concluded that the germplasm bank of soybeans genetic variability has remained relatively constant over the last 30 years.

The availability of water is another important environmental stress for soybean culture in northern Brazil. The first critical period is seed germination and seedling development. Soy‐ bean seeds need to absorb enough water to achieve 50% moisture content to ensure good germination. Soil water content needs to be between 50-85% of total available water. Ac‐ cording to [25], an annual rainfall 700-1200 mm that is well distributed during the soybean cycle (500 to 700 mm) will meet the crops water needs. Soybean's water requirement increas‐ es with plant development, reaching a maximum during the flowering and grain filling peri‐ ods (7-8 mm / day), and decreasing thereafter [12].

The growing season in northern Brazil is largely determined by occurrence of the rainy and dry seasons of the region. The dry season occurs between May to September and any agriculture during this time requires irrigation [16]. Drought is usually the main factor re‐ sponsible for crop losses. The two most critical periods for drought stress in soybean are during seed emergence to seedling establishment and the grain filling period. During ger‐ mination, both excess and lack of water are harmful to crop establishment. Soils having a low water storage in general are unfit for soybean production for most cultivars and planting dates [13].

The soybean plant requires more water as growth and development proceed. Peak demand is during flowering and early pod formation and remains high until physiological maturity. The most critical period is during flowering and early pod formation. Drought stress at this time will cause abortion of flowers and pods, resulting in lower seed production and re‐ duced yield [27]. Rainfall data from the state of Bahia indicates that there is a 80% chance for receiving sufficient rainfall to avoid drought stress during the critical periods [36]. The most important variable to be considered when dealing with drought stress is water retention in the soil during the growing season [27]. In summary, obtaining optimal yield in a given en‐ vironment, depends upon maximizing the genetic potential of a given cultivar [31]. This in‐ volves having a long enough time for vegetative growth so that the prevailing photoperiod does not reduce yield, and the avoidance of drought stress [34]. Experimental trials in Brazil have shown there is a genetic yield potential of more than 5,000 kg/ha grain [2]. Genetic po‐ tential for most cultivated crops has increased greatly over the last 100 years. However, real‐ izing this potential will depend on coping with environmental stress [27]. Soybean is recognized as having wide genetic diversity and with proper genetic and breeding research it can be adapted to a wide range of environmental conditions. Efforts in Brazil over the last 40 years to deal with problems presented by photoperiodic adaptation and coping with oth‐ er environmental stresses have shown how this can happen through cooperative research ef‐ forts involving breeders, physiologists, geneticists, and agronomists.

## **4. Conclusion**

very important, because it affects leaf expansion, photosynthetic rate, crop growth rate, and

A Comprehensive Survey of International Soybean Research - Genetics, Physiology, Agronomy and Nitrogen

Not only is the level of light important, but length of day, or photoperiod, is also important. Photoperiod affects many developmental processes such as flowering, seed germination, growth of stems and leaves, formation of storage organs and assimilate partitioning [38]. It also affects leaf expansion, photosynthesis, senescence, dormancy of buds, and other proc‐ esses [16]. Photoperiodism is a term describing all these plant responses to day length or photoperiod. Photoperiod will be determined by the latitude of where the crop is grown and the planting date.For soybean, the effective photoperiod includes the time from sunrise to sunset and some of civil twilight. Three types of photoperiodic response are recognized: short-day response in which flowering is induced and/or accelerated at a certain critical day length or less; long-day response in which flowering is induced and/or accelerated at a cer‐ tain critical daylength or longer; and day neutral in which flowering time is unaffected by

Since vegetative growth in soybean occurs between emergence and the start of seed filling, it is important that soybean be planted at a latitude and planting date where day lengths will be long enough to allow enough time to the start of seed filling so that dry matter accumula‐ tion is large enough for optimal yield potential. The major problem with the expansion of soybean into northern Brazil is in these low latitudes, day length is seldom longer than 12 hours per day. Ideally, it is best to plant in October or November to take advantage of the lengthening days that occur at this time and continue until the summer solstice in late De‐ cember. Since the critical short-day period to hasten flowering occurs when the photoperiod is shorter than 13.5 to 14.5 hours (the length varies with maturity group and cultivars within maturity groups), the typical soybean planted in this region will have too short a time to flowering and the start of seed filling to achieve enough size for optimal yield. For this rea‐ son, much research in Brazil has concentrated on incorporating the "long-juvenile" trait into soybean cultivars. The juvenile period is the initial period in the plant's growth when it's developmental rate does not accelerate in response to short days. Thus, cultivars having this trait will have a longer time to flowering and the start of seed filling compared to those that do not. Incorporation of this trait into Brazilian cultivars has greatly facilitated soybean ex‐

The physiology of the mode of action of the long juvenile period is still unknown. The trait is either caused by a delayed ability of the plant to respond to short days or a requirement for more short day cycles to induce and promote flowering relative to other soybeans [14].The problem of short-day induced premature flowering in northern Brazil was ap‐ proached by selecting genotypes with insensitivity to photoperiod and/or having a long ju‐ venile period [27]. Several breeding programs have contributed to the development of highyielding cultivars adapted to different agro-climatic conditions of Brazil. For expansion into central and northern Brazil, breeding programs have followed a strategy of developing gen‐ otypes adapted to low latitudes, through the incorporation of the long juvenile trait. Re‐ search conducted by [33] concluded that the germplasm bank of soybeans genetic variability

nitrogen fixation [30].

Relationships

386

day length. Soybean is a short-day plant.

pansion into northern Brazil. [25, 15, 14] .

has remained relatively constant over the last 30 years.

Brazil has become the second largest producer of soybeans in the world. This has occurred largely because of our success at developing soybean cultivation in the low latitude regions (1°20'00'' and 19º00'00'' S) in central and northern Brazil. Much of the area opened to soy‐ bean production lies in the Cerrado region which is Brazil's agricultural frontier.

Northeast Brazil is also an area for agricultural development. It has a wide variety of envi‐ ronmental zones and is suitable for mechanized agriculture on small- or medium-sized farms. It has potential for production of many crops ranging from pastures for dairy cattle to bio-energy crops like sugar cane (ethanol) and biodiesel (soybean). Our progress in the agri‐ cultural development of this region is due to concentrated investment of financial and hu‐ man resources in genetic improvement of cultivars and their adaptation to different climates in this region. The largest effort has been the development of cultivars with the long-juve‐ nile trait which has overcome the problem of the short photoperiods common to this tropi‐ cal region. Thus, short-day induced premature flowering is no longer a problem, and soybean can be grown in northern Brazil that achieve a size suitable for optimal yield. The greatest expanse of soybean production in northern Brazil has occurred in the states of "Ba‐ hia, Maranhão and Piauí". It is in these states that the public and private research efforts have been focused. Similar efforts have now been started in other northern states such as "Ceará, Sergipe, Paraiba, Pernambuco, Alagoas and Rio Grande do Norte". University and state research efforts are now being conducted to develop adapted soybean cultivars for the small and medium-sized farms of this region. Much effort will also have to be invested into marketing and transport. We firmly believe that with continued research efforts, soybean production in northern Brazil will expand into the future and that Brazil will become the largest soybean producer in the world within the next 20 years.
