Soybean Phenology
Dr. Saratha Kumudini, Plant Science Department, Rutgers University
Dr. Thijs Tollenaar, Department of Plant Agriculture, University of Guelph
Vintage: January 2000
An understanding of the developmental processes of a soybean plant is important in evaluating its yield potential. The use of indeterminate versus determinate as well as plants of various maturity groups allows growers to maximize the yield potential within their growing region.
Determinate versus Indeterminate Soybeans
In a determinate genotype, the onset of reproductive development
results in the production of flowers on the main axis and conversion of
the apical meristem into a reproductive primordia resulting in the termination
of the main axis in a floral bud. Determinate types are generally grown
in southern latitudes where the duration of the growings season is not
a limiting factor.
Indeterminate genotypes, however, continue to maintain vegetative growth on the main axis while floral buds develop on the axillary buds. Indeterminates are generally grown in Northern latitudes where they are better suited to eploit the short growing season. The indeterminate growth habit allows for the early establishment of reproductive primordia thus increasing the duration of the seed filling period while not compromising vegetative growth for maximum radiation interception. In Ontario, reproductive development is generally initiated prior to full canopy closure.
Soybean Developmental Stages
The developmental stages in soybeans are characterized
by the standards established by Fehr and Caviness (1977). The life cycle
of the soybean is split into vegetative (V Stage) and reproductive (R Stage)
stages. The stages begin with VE, defined as seedling emergence, the appearance
of the seedling above the soil surface. The next stage is the VC stage,
which marks the opening of the cotyledonary leaves. The VC stage (Fig.
2) is when the cotyledonary leaf is considered open, i.e., when the
node above it has a leaf which is unrolled. In an unrolled leaf, the edges
of the leaf blade must not be touching. The edges of the leaf blade in
Fig.
1 are touching and, therefore, this leaf is not considered unrolled.
Following the VC stage all other V stages are numbered according to the
number of nodes on the main axis (Vn) with a fully developed leaf. A fully
developed leaf is defined as one that has a leaf above it (at the next
node) with an unrolled leaf. The V1 stage is defined as when the node above
the cotyledonary leaf, the first true leaf is fully opened (has a node
above it with a fully unrolled leaf). The first true leaf is an oppositely
arranged, unifoliate leaf. By this system, the V5 stage is when the fifth
node above the cotyledonary leaf is fully opened (has a node above it with
a fully unrolled leaf (Fig. 3).
The R stages are split into two flower stages (R1 and R2), two pod stages (R3 and R4), two seed stages (R5 and R6), and two maturity stages (R7 and R8). The R1 stage is defined as the stage at which one open flower appears at any node on the main stem (Fig. 4). The R2 stage refers to an open flower at one of the two uppermost nodes on the main stem with a fully developed leaf (Fig. 5). The R3 is the stage at which a pod of 5 mm (3/16 inch) is apparent on one of the four uppermost nodes on the main stem with a fully developed leaf (Fig. 6). The R4 stage occurs when the pod reaches 2 cm (3/4 inch) at one of the four uppermost nodes of the main stem with a fully developed leaf (Fig. 7). The R5 stage occurs when the seed within the pod is 3 mm (1/8 inch) long at one of the four uppermost nodes on the main stem with a fully developed leaf (Fig. 8). At the R6 stage the seed fills the pod cavity at one of the four uppermost nodes on the main stem with a fully developed leaf (Fig. 9). The R7 stage is considered to be the point at which one normal pod on the main stem reaches its mature pod colour (Fig. 10). The normal colour of a mature pod can range from tan to brown depending on the genotype. The R8 stage is when 95% of the pods have reached their mature pod colour.
Temperature and Photoperiod
The duration of the soybean life cycle (from seed to seed)
and duration of the individual phases of development within the life cycle
are dependent on photoperiod and temperature.
Photoperiod
Plants are defined as either short-day or long-day plants
based on the response of flower initation to duration of the photoperiod
(actually, it is duration of the number of hours during the nightthat determines
the photoperiod reponse). Soybeans are short-day plants and, therefore,
they flower earlier under short daylengths (i.e., during the month of June
daylength in Southern Ontario is about 16 hours, which is a long photoperiod,
whereas it is about 12 hours at locations near the equator; in contrast,
photoperiod is much shorter during the months of August and September).
There is evidence to suggest that developmental stages other than flowering
are also photoperiod sensitive in soybean. Indeed, the seed filling period
and, therefore, the time to harvest maturity is also photoperiod sensitive.
Soybean cultivars are classified according to their response to photoperiod.
There are 10 maturity classes: in Ontario, the earliest maturing cultivars
that are adapted to Northern Ontario are designated 000, and later maturing
cultivars that are adated to more southern locations are designated either
00, 0, I, or II; maturity class increases when moving further south,
to VIII in he Southern US.
Temperature
Duration of the life cycle and the individual phases
of development within the life cycle are also influenced by air temperature.
Rate of development in plants is generally slower under cool temperatures
and faster under warm temperatures, resulting in either longer or shorter
durations of growing phases, respectively. There are a number of methods
that quantify the effect of air temperature on rate of development or,
alternatively, the effect of accumulated temperature sum on duration of
development. Crop heat units (CHU) is the temperature-sum method that is
used in Ontario (CHU [137 KB]).
Soybean cultivars released in Ontario are categorized as to the number
of CHU required to mature. Therefore, the producer can select the cultivar
that is best suited for the farm based on the average crop heat units received
in that area (CHU
map). This ensures that the crop will mature when grown on that farm
while minimizing the risk of loss to frost.
Dry matter accumulation, Harvest Index and Yield
Yield is the product of total dry matter accumulated and
allocation of these assimilates into the economically important components.
The production of dry matter is dependent on a plant's ability to capture
radiation energy from the sun through the process of photosynthesis. Therefore,
the plant may accumulate more dry matter by either or both (i) increasing
the duration of the plant's life cycle and (ii) by increasing the efficiency
by which the plant converts solar radiation into dry matter. Increasing
photosynthetic efficiency is generally the plant breeder's domain. Therefore,
the producer may attempt to optimize dry matter accumulation by maximizing
the duration of the cultivar's life cycle while ensuring that the crop
is not unduly exposed to the risk of frost. The grower does this by selecting
a cultivar that is designed to mature within his/her growing region. Physiological
processes governing yield may act through determination of total dry matter
(DM) accumulated or the partitioning of accumulated assimilates to the
grain (i.e., harvest index). Harvest index (HI) has been reported to be
associated with soybean yield in only a limited number of studies (Gay
et al., 1980; Morrison et al., 1999; Shiraiwa and Hashikawa, 1995). Gay
et al. (1980) and Shiraiwa and Hashikawa (1995) reported that both DM accumulation
and HI were associated with seed yield. Many factors influence the ability
of a plant to accumulate DM such as rate of photosynthesis, duration of
leaf area and stress tolerance. Contradictory findings have been reported
about the association between dry matter accumulation and yield (Cregan
and Yaklich, 1986; Weber et al., 1966; Shibles and Weber, 1965). Those
studies which showed no correlation between yield and total DM accumulated
were based on studies in which the treatments imposed affected DM accumulation
prior to the seed filling period (defined as the period from R4/5 to R7).
Egli (1993) reported that neither the duration of the vegetative period
nor DM accumulation prior to seed formation was correlated with seed yield.
After the onset of the seed filling period (SFP), however, numerous reports
have shown a positive association between DM accumulated and yield (Board
et al.,1996; Hayati et al., 1995; Hardman and Brun,1971). Hardman and Brun
(1971) reported that CO2 enrichment treatments during the post-flowering
period was the most effective in increasing seed yields.
Therefore, it is apparent that the SFP is a critical period of the soybean life cycle for yield. The SFP is distinguished by the shift in sink demand from being primarily roots and new leaves to a heavy demand for assimilates by the seed sink. Factors that increase the duration of the SFP have been attributed to yield increase. These factors can be due to genetics (improved genotypes with longer SFP) or environmental (cooler temperatures during the SFP which prolong the SFP).
References
Board, J.E., W. Zhang, and B.G. Harville. 1996. Yield rankings for soybean cultivars grown in narrow and wide rows with late planting dates. Agron. J. 88:240-245.
Cregan, P.B., and R.W. Yaklich. 1986. Dry matter and nitrogen accumulation and partitioning in selected soybean genotypes of different derivation. Theor. Appl. Genet. 72:782-786.
Egli, D.B. 1993. Cultivar maturity and potential yield of soybean. Field Crops Res. 32:147-158.
Gay, S., D.B. Egli, and D.A. Reicosky. 1980. Physiological aspects of yield improvement in soybeans. Agron. J. 72:387-391
Hardman, L.L., and W.A. Brun. 1971. Effect of atmospheric carbon dioxide enrichment at different developmental stages on growth and yield components of soybeans. Crop Sci. 11:886-888.
Hayati, R., D.B. Egli, and S.J. Crafts-Brandner. 1995. Carbon and nitrogen supply during seed filling and leaf senescence in soybean. Crop Sci. 35:1063-1069.
How a soybean plant develops. Sp. Rpt. #53. Iowa state university of science and technology. Cooperative extension service. Ames, IA.
Morrison, M., H.D. Voldeng, and E.R. Cober. 1999. Physiological changes from fifty-eight years of genetic improvement of short-season cultivars in Canda. Crop Sci. 39:(in Press)
Shibles, R.M., and C.R. Weber. 1965. Leaf area, solar radiation interception and dry matter production of soybeans. Crop Sci. 5:575-577.
Shiraiwa, T., and U. Hashikawa. 1995. Accumulation and partitioning of nitrogen during seed filling in old and modern soybean cultivars in relation to seed production. Jpn. J. Crop Sci. 64:754-759.
Weber, C.R., R.M. Shibles, and D.E. Byth. 1966. Effect of plant population and row spacing on soybean development and production. Agron. J. 58:99-102.
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