Soybean [
Glycine max
(L.) Merr.] growth rate and grain yield are modified by the interception and solar radiation use efficiency. Thus, it is desirable that the most of plant photosynthetic structures intercepting solar radiation
in order to have increment in carbon fixation and reflection on growth and yield. The goal of this study was to assess if soybean cultivars differ in grain yield in relation to
solar radiation interception.
Four soybean cultivars were evaluated at stages V6, V9, R2, R4, R6 and R8. To determine the photosynthetically active radiation interception
by the canopy
,
the plants were divided into two parts (upper and lower strata).
For grain yield components, the plants were divided into three parts (upper, middle and lower thirds). Of the photosynthetically active radiation intercepted by the vegetative canopy at the reproductive stages, the maximum observed intercept was 5.2% in the lower stratum of the plants. The number of infertile nodes increased in the lower third
of plants due to low interception of solar radiation in this plant region. Thus, the soybean cultivars more efficient in intercepting photosynthetically active radiation inside the vegetative canopy showed higher grain yields.
Glycine Max Leaf Area Index Efficiency of Solar Radiation Interception Extinction Coefficient Solar Radiation Use Efficiency1. Introduction
Plant growth depends on carbon assimilation, which is directly related to intercepted photosynthetically active radiation (IPAR). Part of photosynthetically active radiation (PAR) is absorbed and used in photosynthesis, and the rest is lost to the environment [1] . Plant growth rates can be obtained by estimating solar radiation use efficiency (RUE), which is defined by the ratio between biomass increase and radiation intercepted by the plant canopy [2] [3] .
Radiation interception efficiency (RIE), the fraction of light intercepted by the plants, can be high or low [4] , and depends on the leaf area index and morphological characteristics [5] . The shading decreases total soluble sugar concentration in soybean leaves, causing reduced numbers of pods; reduction of 42.5% in the number of pods is observed for plants shaded during the cycle, while the increase of light availability brought about increases of 59.5% in the number of pods [6] .
The determination of RIE must be evaluated not only for the plant-atmosphere edge, but also across plant canopy in a stratified manner. However, the IPAR of the whole plant has major importance for grain yield quantification. Most of the sugars synthesized by plants are transported short distances by the phloem [7] . In this way, the leaves supply the drains through direct vascular connections [8] , so the filling of pods in a node occurs mainly by photoassimilates produced by the leaves into which pods are inserted. Seed weights were reduced by removing 66% and 99% leaves, by the low production and translocation of photoassimilates during grain filling [9] . Therefore, greater shading and defoliation levels for crops at reproductive stages causes, reduced grain filling.
The soybean plant architecture varies with genotype, presenting a wide variety of forms, but a lack of knowledge on how much architectural features influence IPAR. The IPAR varies with leaf area index (LAI) and light extinction coefficient (k) [2] [10] [11] . The soybean showed k values of 0.52 and 0.93before and after flowering, respectively, due to increased LAI with the growth of the plants [12] . The higher the LAI, the greater the IPAR, but from a certain increase in the leaf area the shading of the lower leaves of the plant can be intensified [13] . Thereby, there might be an energy imbalance in plants since the shaded structures spend energy in respiration, without producing through photosynthesis. Leaf senescence in the canopy lower part is accelerated due to a decrease in the proportion of red and far red light, signaling that light conditions would not satisfy the primary metabolism demands [14] .
The knowledge about the solar radiation interception by strata of plant canopies can help to understand how this factor affects soybean growth and grain yield. Our hypothesis was that soybean plants with an architecture that allows more solar radiation interception inside of canopy would show better productive performances. Thus, the aim of this work was to evaluate if soybean cultivars differ in grain yield in relation to solar radiation interception.
2. Materials and Methods2.1. Study Subject and Experimental Design
Soybean cultivars with different agronomic traits were used. Seeds had germination percentages above 90% and vigour superior to 85%. For each cultivar, we followed the breeder company recommendations regarding the number of plants∙ha−1 and sowing time.
The treatments consisted of four cultivars BMX Ativa RR, BMX Potência RR, NA 5909 RG and 95R51. Cultivars BMX Potência RR, NA 5909 RG, and 95R51 have indeterminate growth habit, while BMX Ativa RR has the determinate habit. Maturation groups were different among cultivars, being 5.1, 5.6, 5.9, and 6.7 for cultivars 95R51, BMX Ativa RR, NA 5909 RG and BMX Potência RR, respectively.
The experiment was conducted in a randomized complete block design with five replicates, totaling 20 experimental units. Plots consisted of seven 10-m sowing rows spaced 0.45 m apart. Two outer rows were left as borders in addition to the outer 0.5 m at the end of each plot. Destructive measurements were carried out on plants within a 4-m row segment, leaving 5-m for final yield determination after harvesting.
2.2. Procedures
The study was conducted in the city of Passo Fundo, RS (Brazil) (28˚12'S and 52˚23'W, altitude 667-m). The climate is of the humid subtropical type [15] . The soil is classified as a humic dystrophic red latosol [16] .
Sowing was north-south oriented, being carried out in a no-tillage system, over wheat crop remains. At sowing, fertilization consisted of 6 kg∙ha−1 N, 60 kg∙ha−1 P2O5, and 60 kg∙ha−1 K2O. Seeds were inoculated with Bradyrhizobium japonicum and treated with insecticides and fungicides according to crop recommendations [17] .
The adopted seeding densities were 15.75 plants∙m−1 for BMX Ativa RR, 15.30 plants∙m−1 for NA 5909 RG, 17.10 plants∙m−1 for 95R51, and 12.60 plants∙m−1 for BMX Potência RR. Phytosanitary management was preventive so that there was no interference with plant growth, and mainly regarding the architectural traits of the cultivars [17] .
The evaluations took place at the following phenological stages: V6 (fifth fully expanded trifoliolate leaf), V9 (eighth fully expanded trifoliolate leaf), R2 (full bloom), R4 (fully developed pod), R6 (full grain) and R8 (physiological maturity) [18] .
2.3. Solar Radiation Interception Efficiency
The photosynthetically active radiation (PAR, μmol∙m−2∙s−1) was measured using a ceptometer (AccuPAR LP-80; DECAGON Devices) at V6, V9, R2, and R4 stages. The readings were made hourly from sunrise to sunset, always on the same plants, totaling 60 measurements per hour. The k and RIE were measured at 11 h, 12 h, and 13 h.
Plant dry mass (DM) was determined using 10 plants per plot, so totaling 50 plants per cultivar at all stages (V6, V9, R2, R4, and R6). The plants were collected following a sequence along the row length, being manually cut close to the soil and taken to an oven for drying at 60˚C until reaching a constant weight. After drying, each plant was weighed (g∙pl−1).
Leaf area (LA) was measured by a destructive method using the leaf area integrator (LI-3100C, LI-COR BIOCIENCE Inc.). Ten plants were collected following a sequence along the row length at V6, V9, R2, and R4 stages. Leaf area index (LAI) was estimated by the ratio between the total leaf area and the projected area of the soil (PA) (m2∙m−2) (Equation (1)):
LAI = LA / PA (1)
The interception of photosynthetically active radiation (IPAR) by canopy total was estimated from measurements of incident solar radiation (ISR) and transmitted fraction (TF) to the soil surface (Equation (2)):
IPAR = ISR − TF (2)
The IPAR in the upper stratum (US) was determined from incident solar radiation (ISR) (5 cm above the canopy) less the IPAR measured at half canopy height (HCH) (Equation (3)):
US = ISR − HCH (3)
The IPAR in the lower stratum (LS) was determined from HCH less TF (Equation (4)):
LS = HCH − TF (4)
The efficiency of solar radiation interception (RIE) was determined from the quotient between the intercepted photosynthetically active radiation (IPAR) and the total ISR on the vegetative canopy (Equation (5)):
RIE = IPAR / ISR (5)
The canopy extinction coefficient (k) was determined from the means of solar radiation interception efficiency and leaf area indexes (LAI), estimated by the theory proposed by [19] , based on the assumptions of the Beer’s Law [20] (Equation (6)):
Ln ( 1 − RIE ) = k * LAI (6)
Radiation use efficiency (RUE), expressed in g∙MJ−1, was estimated by regression analysis between plant-accumulated DM (g∙m−2) and IPAR sum (MJ∙m−2) during the crop cycle, according to [21] . In this study the IPAR used, was determined by the incident global radiation and transformed in PAR using the conversion factor 0.45 [22] . In addition, the interception efficiency of the plants of each cultivar was considered, because the canopy did not intercept all PAR. In this sense, the RUE is g∙MJ−1 of IPAR (Equation (7)):
RUE = DM / IPAR (7)
For RUE calculations, data had to be transformed from μmol∙s−1∙m−2 to MJ∙m−2∙day−1, using a single conversion value for each environment [23] , where t is time between PAR readings (3600 seconds) and 4.57 is the conversion value (Equation (8)):
PAR = ∑ day ( PAR ( μmol ⋅ s − 1 ⋅ m − 2 ) * t ( s ) ) / ( 4.57 ( MJ ⋅ m − 2 ⋅ day − 1 ) ) / 1000000 (8)
2.4. Fruiting
Effective fruiting was determined by counting reproductive structures (flowers and pods) at R2, R4, and R8 reproductive stages for the lower, the middle, and the upper thirds of the plant canopy. Plant height was measured and then divided into three parts (thirds), including the branches.
2.5. Grain Yield Components
At physiological maturity, 10 plants were collected following the sequence of the row in each experimental unit to evaluating the yield components by plant thirds. The number of fertile nodes (FN), infertile nodes (IN), grains number (GN), grain mass (GM), thousand-grain mass (TGM), and grain yield (GY) was determined. For GM estimation, moisture was corrected to 13% [17] . Finally, grain yield per hectare (kg∙ha−1) of each plant third was defined.
In addition to GY of each plant third, an area of 9 m2 was harvested using a WINTERSTEIGER Classic plot harvester. The samples were weighed and corrected for 13% moisture, and the GY (kg∙ha−1) and TGM (g) were definedfor whole plants.
2.6. Statistical Analysis
Data were subjected to analysis of variance (ANOVA), and means were compared by Tukey’s test at probability lower than 0.05 of error. Moreover, the Pearson correlation was calculated among variables lower than 0.05 error probability.
3. Results3.1. Solar Radiation Interception Efficiency and Plant Growth
Given the short stature of plants at V6 stage (maximum height of 18.3 cm), solar radiation availability was evaluated at above canopy and at ground level, to give the total amount intercepted by the canopy (Figure 1). During vegetative stage, there were no differences in IPAR measurements, however, during the reproductive stage, the IPAR of cultivars differed significantly.
As plants grew, the amount of IPAR at the lower stratum of canopy decreased (Figure 1). Regarding the lower stratum, all cultivars reached a maximum IPAR, nearly 18% total interception, at V9 stage. Conversely, at R2 stage, these values were 2.7%, 1.4%, 4.7%, and 1.1% for BMX Ativa RR, NA 5909 RG, 95R51, and BMX Potência RR, respectively; while in R4 stage, with the decrease of LAI, there were a slight increase in interception, being 4.4%, 1.0%, 5.1%, 1.5%, respectively.
The highest LAI values were observed at R2 stage, with variations among cultivars at 5.2, 6.9, 4.2, and 6.2 for BMX Ativa RR, NA 5909 RG, 95R51, and BMX Potência RR, respectively (Figure 2). There was a leaf fall at R4 stage that resulted in a LAI decrease of 1.1, 2.1, 0.4, and 1.5 for cultivars BMX Ativa RR, NA 5909 RG, 95R51, and BMX Potência RR, respectively. The highest k values were observed at V6 stage when plants showed high RIE, even under low LAI values. The same trend of k values continued until R2 stage in all cultivars, with the highest values at V6 stage, decreasing at V9 stage and increasing at R2 stage. Nonetheless, there was a decrease for cultivar 95R51 only at R4 stage, remaining virtually constant for BMX Ativa RR, while the other cultivars showed an increasing trend (Figure 2).
The lowest value of RIE (0.79) was observed for cultivar BMX Potência RR at V6 stage, whereas the highest value was 0.99 for all the cultivars at R2 stage (Figure 2). There was an alteration in RIE before and after flowering, averaging
from 0.86, 0.85, 0.89, and 0.81 to 0.98, 0.98, 0.96, and 0.98 for BMX Ativa RR, NA 5909 RG, 95R51, and BMX Potência RR, respectively.
The amount of PAR accumulated during the growing season was dependent on cycle duration and RIE. The total PAR for BMX Ativa RR, NA 5909 RG, 95R51, and BMX Potência RR were 825, 819, 775, and 878 MJ∙m−2, respectively, (Figure 3). BMX Potência RR has the longest cycle and achieved the highest accumulation value of PAR.
RUE showed1.6, 1.8, 1.8, and 1.5 g∙MJ−1 of IPAR for cultivars BMX Ativa RR, NA 5909 RG, 95R51, and BMX Potência RR, respectively (Figure 3). Although BMX Potência RR had intercepted more PAR than the other cultivars, it had the lowest efficiency in converting solar radiation to DM. To reach a shoot DM of 100 g∙m−2, the studied cultivars required 275 (BMX Ativa RR), 268 (NA 5909 RG), 319 (95R51), and 255 (BMX Potência RR) MJ∙m−2 IPAR. These results highlight the highest RUE of BMX Potência RR at the beginning of vegetative development. Conversely, to achieve a shoot DM production of 1000 g∙m−2, cultivars required 734 (BMX Ativa RR), 673 (NA 5909 RG), 679 (95R51), and
768 MJ∙m−2 of IPAR. Therefore, RUE of BMX Potência RR was reduced during the reproductive stage when compared to the other cultivars.
3.2. Dynamics of Pod Production
At R2 stage and full bloom stage, BMX Potência RR was superior in the number of reproductive structures when compared to the others (Figure 4). Nevertheless, this result had no effect on fruiting. Both BMX Ativa RR and 95R51 cultivars had the smallest number of reproductive structures at R2 stage, with the highest percentages of effective fruiting in the lower third of the canopy. Evaluating fruiting effectiveness, BMX Potência RR cultivar showed lower reproductive structures in the medium third, while BMX Ativa RR was the most productive in the upper third, without differing from NA 5909 RG. The maximum effectiveness of fruiting in the lower third was 22% for 95R51 cultivar, while for BMX Ativa RR this was achieved in the medium (81%) and in the upper (85%) thirds.
3.3. Grain Yield Components
Among the cultivars, 95R51 reached the highest productivity in the lower third due the least number of infertile nodes per plant (IN) (Table 1). The ratio between IN and fertile nodes per plant (FN) in the lower third was nearly 4 for BMX Ativa RR, 12 for NA 5909 RG, 1 for 95R51, and 2 for BMX Potência RR. Therefore, 95R51 had the highest grains number per plant (GN) and, consequently,
the highest GM per plant, with a yield about 500 kg∙ha−1 higher than that of BMX Potência RR in the lower third. NA 5909 RG, which showed high IN to FN ratio in the lower third, showed the lowest GM per plant, yielding about 1200 kg∙ha−1 less than 95R51.
Despite the differences in yield components in the middle third, the final yield showed no difference among cultivars for this plant region (Table 1). In the upper third, BMX Potência RR and NA 5909 RG showed superior performances in FN and GN, given their higher number of nodes and the lack of variation in IN among cultivars. Conversely, there was no difference for GM per plant among the cultivars, which was due to a lower thousand grain mass (TGM) for BMX Potência RR and NA 5909 RG, which may have influenced the final grain yield (GY). Evaluations of whole plants, through mechanized harvesting, showed that 95R51 obtained the highest GY, while BMX Potência RR was the least productive (Table 2).
Yield components per canopy third for soybean cultivars with different architectural characteristics
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