**2. Agrometeorological conditioners of soybean productivity**

### **2.1. Solar radiation**

In general, soybean production potential and risk is controlled by prevailing climatic condi‐ tions and genotypic performance. The basic source for crop production is solar radiation which acts as an energy source for photosynthesis. Light quality also acts to influence plant height and phenological development [3].

The light spectrum duration and quality besides the radiation intensity are determinants of morphological and phenotypic responses striking in soybean, such as plant height, induction of flowering and ontogeny [4]. The final yield of dry matter from the plant depends on the solar radiation absorbed by the leaves and the efficiency with which these convert radiant energy into chemical energy through photosynthesis.

Soybean is a qualitative short-day plant and must receive a certain day length or less so that developmental timing is optimal for the location [5]. This means that adaptability of each cultivar varies with latitude [6]. Movement of soybean to the central and northern regions of the country from the original southern region has faced the challenge of adaptation to shorter day lengths occurring in these regions (from Tropic of Capricorn to Equator Line). To deal with this challenge Brazilian agricultural scientists have sought to introduce the 'long juvenile period'. This characteristic gives soybean a longer developmental period under short days so that it can accrue enough dry matter for optimal yield [7].

Each variety has its critical photoperiod above which flowering is delayed. Flowering occurs anyway, but more rapidly as the days become shorter. It slows progressively, as the photo‐ period exceeds the critical period for each genotype.

### **2.2. Temperature**

In addition to photoperiod, temperature also influences growth and developmental tim‐ ing in soybean. This occurs due to the effect of temperature on the rate of metabolic re‐ actions, the diffusion rate of gases through aqueous media and the solubility of nutrients in the plant. Soybean grows best at temperatures between 20o C and 30o C. Greatest number of pods per plant is obtained under mild temperature conditions hav‐ ing a day/night temperature combination of 26/14o C [8]. Temperatures above 40o C dur‐ ing the vegetative stage (emergence to first flower) reduce growth and hastens flowering. High temperatures during the reproductive phase can cause reductions in seed number and seed weight, thus reducing grain quality and yield. If the high tem‐ perature is associated with a drought, the losses on grain production are even higher [7]. On the other hand, cold regions where the temperature is equal or below to 10C is not properly to soybean cultivation, because in these places, the vegetative growth and the development become small or null.

### **2.3. Water availability**

**2. Agrometeorological conditioners of soybean productivity**

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

In general, soybean production potential and risk is controlled by prevailing climatic condi‐ tions and genotypic performance. The basic source for crop production is solar radiation which acts as an energy source for photosynthesis. Light quality also acts to influence plant height

The light spectrum duration and quality besides the radiation intensity are determinants of morphological and phenotypic responses striking in soybean, such as plant height, induction of flowering and ontogeny [4]. The final yield of dry matter from the plant depends on the solar radiation absorbed by the leaves and the efficiency with which these convert radiant

Soybean is a qualitative short-day plant and must receive a certain day length or less so that developmental timing is optimal for the location [5]. This means that adaptability of each cultivar varies with latitude [6]. Movement of soybean to the central and northern regions of the country from the original southern region has faced the challenge of adaptation to shorter day lengths occurring in these regions (from Tropic of Capricorn to Equator Line). To deal with this challenge Brazilian agricultural scientists have sought to introduce the 'long juvenile period'. This characteristic gives soybean a longer developmental period under short days so

Each variety has its critical photoperiod above which flowering is delayed. Flowering occurs anyway, but more rapidly as the days become shorter. It slows progressively, as the photo‐

In addition to photoperiod, temperature also influences growth and developmental tim‐ ing in soybean. This occurs due to the effect of temperature on the rate of metabolic re‐ actions, the diffusion rate of gases through aqueous media and the solubility of

Greatest number of pods per plant is obtained under mild temperature conditions hav‐

ing the vegetative stage (emergence to first flower) reduce growth and hastens flowering. High temperatures during the reproductive phase can cause reductions in seed number and seed weight, thus reducing grain quality and yield. If the high tem‐ perature is associated with a drought, the losses on grain production are even higher [7]. On the other hand, cold regions where the temperature is equal or below to 10C is not properly to soybean cultivation, because in these places, the vegetative growth and

C and 30o

C [8]. Temperatures above 40o

C.

C dur‐

nutrients in the plant. Soybean grows best at temperatures between 20o

**2.1. Solar radiation**

Relationships

368

**2.2. Temperature**

and phenological development [3].

energy into chemical energy through photosynthesis.

that it can accrue enough dry matter for optimal yield [7].

period exceeds the critical period for each genotype.

ing a day/night temperature combination of 26/14o

the development become small or null.

Ninety % of the total soybean fresh weight (biomass) is water and water acts in all physiological and biochemical processes in the plant, working as a solvent, carrying gases, minerals and other plant solutes and acting as a thermal regulator to cool and maintain plant temperature [9].Water availability is important in three periods of soybean development: germination, emergence and flowering-grain filling. During the first period, both: excess or lack of water is detrimental to crop establishment and obtaining a good uniformity of the plant population and the surplus water more limiting than the deficit. Soybean seeds need to absorb at least 50% of its weight in water to ensure a good germination. At this stage, the water content in soil must not exceed 85% of the total maximum available, or must not be less than 50% [10]. The roots can reach over 1.5 m deep, however heavy and compacted soils hinder root pene‐ tration, further reducing the effective depth of the root system of soybean plants.

Another serious risk in Brazilian soybean production is drought stress. The problem is more pronounced in tropical and subtropical, semiarid and arid climates where up to 8mm/day may be lost by evapotranspiration.

The necessity of water needs by soybean gradually increase with plant development, peaking at 7 to 8mm/day during flowering through grain filling [11]. The negative effect of water deficiency on yield depends on the phenological stage in which soybean is affected by drought stress. For example, if drought occurs during the germination and emergence stages, plant stand is reduced. In the flowering period it can cause flower abortion and prevents anthesis while in the grain filling stage drought affects seed weight [12]. The lack of water induces these effects through reducing the efficiency of the photosynthesis [13].

An abundance of water is also harmful to yield by causing water logging of the soil. A very wet soil results in low aeration which reduces root growth, can cause nutritional deficiency and promotes the attack of root diseases.
