Botany

Last updated on February 23, 2014

Seed characteristics

Shape = round to oval components seed coat large embryo virtually no endosperm

Seed coat has 3 layers

  • No air exchange due to impermeable cuticle
  • Epidermis = cuticle + palisade cells function = protection
  • Hypodermis = consists of "hourglass" cells function = support
  • Inner parenchyma = flattened cells little function

Embryo = 3 main parts

  • Two cotyledons modified seed leaves provide nutrients for germination and early growth
  • Hypocotyl = stem tissues below cotyledons includes radicle 6
  • Epicotyl = tissues above the cotyledons consists of plumule + two welldeveloped leaves that are called unifoliolates

Germination and vegetative growth

Seed imbibes water and expands the radicle passes through the micropyle and grows downward, developing a tap root system

Soybean: tap root system

  • Has some lateral roots (top 12")
  • May extend 56' into the soil

Roots form nodules and fix N

  • In general, soybeans are regarded as having a relatively small amount of root growth, but a relatively large amount of top growth

Shortly after radicle emergence, the hypocotyl begins to move towards the soil surface, carrying the cotyledons with it

The hypocotyl hook is the first structure to break the soil surface

Soybean emergence is epigeal cotyledons are carried aboveground by elongation of the hypocotyl

Peas hypogeal cotyledons do not emerge

Soybean Seedling

Cotyledons opposite on stem

Unifoliolates one leaflet/leaf (simple leaves) opposite on stem

Trifoliolates 3 leaflets/leaf = compound leaves alternate on stem

Pods eventually develop from axillary buds that are located in the leaf axils

Pubesence covers the stems, leaves, and pods of most soybean varieties, but there are some glabrous types

Reproductive growth

Soybean flowers are perfect and complete

  • Perfect = both stamens and pistil in the same flower
  • Complete = has all 4 sets of floral organs sepals green tissue under petals petals (corolla) purple or white 2 wings 2 keels 1 standard
  • Stamen = filament + anther 10 stamens 9 in a ring and 1 free pistil = stigma, style, and ovary

Soybean flowers develop from axillary buds that occur in leaf axils

  • 3 to 15 flowers form at an axil
  • Most flowers (1/2 to ¾) abort before forming pods.
  • Can pgr's reduce flower drop? Soybeans are highly self pollinating flowers open fairly early in the day and pollinate shortly after opening

Fertilization occurs 12 hrs after pollination

Growth habit determinate vs. Indeterminate

Determinate soybeans general characteristics (vs indeterminate)

  • Have a terminal cluster of 58 pods at the top of the plant in addition to pods on lower axils
  • Because vegetative growth ceases when the stem terminates in a cluster of flowers, determinates usually have little or no increase in height after flowering is initiated
  • Shorter, stockier plants with fewer stem nodes
  • Thicker stems
  • Branch more extensively
  • Less likely to lodge on highly fertile soils
  • Flower over a shorter period of time
  • Lower basal pod height
  • Prevalent in SE U.S.

Indeterminate soybeans

  • Stem does not terminate in a terminal cluster of pods; have 13 pods at terminal apex
  • Taller, more stem nodes
  • Thinner stems that lodge more easily
  • Less branching and narrower canopy
  • Flower over a longer period of time
  • Continue to increase in height after flowering is initiated
  • Higher basal pod height

In the corn belt, indeterminate varieties usually have more yield stability and drought tolerance because they have a longer flowering period

  • Indeterminate varieties are grown in the Northern U.S. because of the need for sufficient plant height
  • Determinate varieties are preferred in the Southern U.S. because indeterminate varieties grow too tall in these areas
  • Some soybean varieties are classified as semideterminate have characteristics of both types

Photoperiod response: Soybeans are photoperiod sensitive soybeans are classified as short day plants

  • Actually, they are long night plants, as they require at least 10 hrs of darkness before they will flower
  • As the nights lengthen past 10 hrs, growth switches from vegetative to reproductive

Reproductive growth eventually predominates

  • Soybeans initiate reproductive growth when the days begin to shorten after June 21, regardless of what maturity group they are in
  • Maturity groups = 10 maturity groups: 00 to VIII

Summary: Soybean growth and development

Soybeans are best adapted to

  • Warm summers plenty of sunshine
  • Sufficient rainfall (amount and distribution)
  • Will yield well when temperatures are commonly in the upper 80's if moisture is sufficient
  • Mean daily temperature of 7577of during the summer appears to be optimum
  • Like corn, soybeans do not grow when temperatures drop below 50oF

Soybeans have good drought tolerance

  • Need plenty of moisture for germination and emergence
  • Will do reasonably well on limited moisture thereafter

Soybeans have good tolerance to low fertility acid soils (preferred range is 6.0 to 6.8)

Soybeans have better Aluminum tolerance than cotton - variety differences in Al tolerance

Soybeans also have good tolerance to soils that are not well drained, however, phytophthora and iron chlorosis may be problems


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