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Seed Water Relations

Water uptake during seed germination investigated by NMR
Seed water uptake and ambient water potential
Hydrotime: Population based threshold model - germination dependence on water availability
Hydrotime modeling of tomato seed germination: Radicle growth and micropylar cap weakening at different water potentials


Water uptake during seed germination investigated by NMR





Seed water uptake and ambient water potential (water potential)

  • Water has by definition a water potential (water potential) of 0 MPa (zero MegaPascal). The pressure unit (1 MPa = 10 bar = 1MNewton/m2) is used to characterize water potential. Leaf tissue has a water potential around - 1 MPa. The water potenial of plant tissue or of soil is determined by the sum of their pressure potential (hydrostatic pressure, turgor pressure, positive), osmotic potential (osmotic pressure, negative), and matric potential (important for dry states, negative).

  • Dry seeds usually have water potentials (water potential) between -350 and -50 MPa. This huge difference between the dry seed tissue water potential and the ambient water potential (in the case of pure water) results in rapid water influx during imbibition (phase I). Water always flows from the higher to the lower water potential and the net flux will stop if the difference in water potential becomes zero (phase II). This leads to initial radicle extension due to reversible ("elastic") growth driven by osmotic water uptake.

  • Further embryo growth at and after the completion of seed germination requires cell wall loosening to allow phase III water uptake. The growth potential of the embryo and the constraining force of the endosperm and testa layers determine the completion of germination. Ambient water potential and temperature are of utmost importance for germination timing.

  • Imbibition at reduced ambient water potential (water potential) lowers seed water content, extends the lenght of phase II ("activation phase"), and delays/blocks entry into phase III. Radicle emergence and growth require a critical (minimum) seed water content. Below this germination is blocked (see figure below).

water potential





Hydrotime (hydrotime): Population based threshold model - germination dependence on water availability

  • Population based threshold models may provide a common framework to explain ecophysiological observations of the control of seed germination by the environment (Finch-Savage and Leubner-Metzger, 2006; Bradford, Weed Science 50: 248-260, 2002; Gummerson, Journal of Experimental Botany 37: 729-741). Such descriptive population based threshold models are established and are widely used to describe the seed germination responses with relatively simple mathematical equations.

  • For the hydrotime model see below. The thermal time model describes the dependence of germination on the temperature, and both models can be combined: hydrothermal time model (see webpage "Seed Modeling"). Similar models and equations describe the dependence of seed responses to other external or internal factors, e.g. ABA content, dormancy release, endosperm weakening (see webpage "Seed Modeling").

  • The hydrotime model describes the dependence of germination on the water availability: At constant temperature the germination time can be calculated from the amount of water potential waterpotential (in MPa) exceeding a base or threshold water potential waterpotential base (in MPa). Hydrotime hydrotime (in MPa-days) is often a constant value for all seeds of a population, but the base water potential waterpotential base varies among individual seeds in a normally distributed manner. Hydrotime hydrotime is the accumulation of MPa in excess of a base or threshold water potential, multiplied by the elapsed time to germination. The value of hydrotime is the total hydrotime (in MPa x h) required for each seed to complete germination.
          Hydrotime hydrotime (in MPa-days) = (waterpotential - waterpotential base) x time (in days) = constant value for all seeds of a population

          Base water potential waterpotential base is often normally distributed with a standard deviation sigma waterpotential base among seeds in the population



Hydrotime


Hydrotime


  • The figures above are derived from germination time courses at different ambient water potentials (seed imbibition in the presence of osmotica). From these plots tg, the times to the germination percentages g (10%, 50%, 90%, etc.) can be determined. The rate of germination GRg = 1/tg decreases linearly with water potential (waterpotential) to a base (waterpotential base). Hydrotime hydrotime is often the same for all seeds in the population (constant slope in the left figure above), but waterpotential base (g) varies between seeds in a normal distribution with a characteristic mean waterpotential base(50%) and a characteristic standard deviation sigma (sigma waterpotential base; calculated from the different waterpotential base(g) values obtained; right figure above).

  • Normalized hydrotime: Different germination curves at different waterpotentials waterpotential with the actual time as x-axis can be transformed and plotted as one combined germination curve with normalized hydrotime as x-axis. To do this one must first determine the base water potential waterpotential base(g) of the seed population as described above. Based on this the normalized hydrotime (in days or hours) can be culculated by multiplication of the actual time by (1 - ( waterpotential/waterpotential base(g)). An example for this is given in the next section "Hydrotime modeling of tomato seed germination" based on results published for tomato seeds.
          Time to germination g in water tg(0) = (1 - (waterpotential/waterpotential base(g))) x tg(waterpotential) with tg(waterpotential) = time to germination g at any waterpotential
  • Probit hydrotime analysis: An example for this is given in the next section "Hydrotime modeling of tomato seed germination" based on results published for tomato seeds. The variation of waterpotential base(g) closely approximates a normal distribution. The probit analysis technique is achieved according to the model:
          probit(g) = (waterpotential - (hydrotime/tg) - waterpotential base(50)) / sigma waterpotential base







Hydrotime modeling of tomato seed germination:
Radicle growth and micropylar cap weakening at different water potentials

  • An example for hydrotime modeling is given in this section. Hydrotime analysis of intact and cut (surgical removal of the micropylar cap) tomato seeds (line P) was performed. The original germination data at different water potentials were taken from Liptay and Schopfer, Plant Physiology 73: 935-938 (1983).

  • The left figure shows the germination time courses of intact and cut tomato seeds at different ambient water potentials waterpotential (in MPa) . From these plots tg, the times to the germination percentages g (10%, 50%, 80%, etc., in h) was determined. These values are used to calculate the rates of germination GRg = 1/tg (in 1/h).

  • The right figure shows the germination rate GRg dependence on the ambient water potentials waterpotential for three germination percentages g. Note that GRg decreases linearly with water potential (waterpotential) to a base (waterpotential base). The base water potentials for each g waterpotential base(g) can be determined from the intersections of the lines with the x-axis. These base water potentials waterpotential base(g) are normally distributed with a characteristic mean base water potential waterpotential base(50%) and a characteristic standard deviation sigma waterpotential base (calculated from the different waterpotential base(g) values obtained). Note that the presented mean base water potential calculated from the intersections of the regression lines corresponds in value to the base water potential waterpotential base(50%).

  • In the right figure the hydrotimes hydrotime can be calculated as the reverse of the slopes of the curves. Note that the presented mean hydrotime constants and standard deviations were calculated from the slopes of the regression lines for 10%, 20%, 30%, 40% 50%, 60%, 70%, and 80% g. Ideally, the slopes and therefore the hydrotimes hydrotime are constant for all germination percentages g.


Tomato hydrotime analysis


Hydrotime analysis of intact and cut (surgical removal of the micropylar
cap) tomato seeds (line P). Data were taken from Liptay and Schopfer,
Plant Physiology
73: 935-938 (1983) and analysed by G. Leubner

See the section above ("Hydrotime") for abbreviations and units.

Hydrotime analysis tomato





Standard deviation of mean base water potential


Normal distribution


 





  • Intact and cut tomato seeds differ in their germination responses to changing ambient water potentials waterpotential. Cut seeds have a ca. 10% higher mean base water potential waterpotential base(50%) and the standard deviation sigma waterpotential base is ca. doubled (see figure on the left). However, cut seeds have a three-fold smaller hydrotime constant hydrotime (see figure above).

  • Thus, the removal of the micropylar cap of tomato seeds confers only a small shift in the waterpotential base values at the time of radicle protrusion. The micropylar cap is therefore not a main constraint for water uptake. It is known that endosperm weakening is sensitive to changing ambient water potentials waterpotential. This is in agreement with the finding that cap removal lowered the hydrotime constant hydrotime.






Normalized hydrotime


  • Normalized hydrotime: Once the waterpotential base(g) distribution is known for a seed batch, germination time courses at different water potentials can be normalized on a common time course equivalent to that occurring in pure water (tg(0)). This is achieved by multiplication of the actual time tg(waterpotential) by (1 - ( waterpotential/waterpotential base(g)). See the section above ("Hydrotime") for formulas and units.

  • That intact and cut tomato seeds differ in their germination responses to changing ambient water potentials waterpotential is also evident from their different time courses based on normalized hydrotime (in h; see figure on the right).
 

Normalized hydrotime






Probit analysis of water relations


Probit analysis

 



  • Probit analysis of tomato germination at different water potentials comparing intact and cut seeds (see figure on the left): The probit analysis technique to estimate hydrotime, waterpotential base(50%), and sigma waterpotential base is described by Bradford (1995, in: Kigel and Galili (eds.) "Seed development and germination", pp. 351-396, Marcel Dekker, Inc., New York).

  • The probit-transformed percentages from germination time courses at different ambient water potentials are plotted as a function of waterpotential - (hydrotime/tg), which is equal to waterpotential base(g). Different values of hydrotime are used in repeted probit regressions until the optimal fit to all data is obtained.

  • The midpoint of the regression line (probit = 0, 50% germination) gives the value of  waterpotential base(50%). The inverse of the slope is sigma waterpotential base. To model germination in response to different ambient water potentials, these values are then used in the following equation to model the germination responses:
          probit(g) = (waterpotential - (hydrotime/tg) - waterpotential base(50)) / sigma waterpotential base
  • The estimated values for hydrotime, waterpotential base(50%), and sigma waterpotential base derived from the probit analysis with all data (see figure on the left) are similar to the values obtained with the above techniques.

  • Probit analysis can be used to model other external and internal factors that determine germination responses. See the webpage "Seed Modeling" for examples.
 
 
 


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