Abstracts
The objective of this study was to evaluate the effect of priming treatments on the upper and lower thermal limits for germination of Urochloa brizantha cv. basilisk, and testing the hypothesis that pré-imbibition affect thermal parameters of the germination. Pre-imbibed seeds both in distilled water (0 MPa) and PEG 6000 solution (–0.5 MPa) were put to germinate in different temperatures. It is suggested that U. brizantha seeds have low response to priming when they were placed to germinate in medium where water is not limiting. The response of U. brizantha seeds to priming is dependent on the temperature and water potential conditions at which the seeds are pre-imbibed, as well as on the germination temperature. The optimum temperature for germination of U. brizantha shift toward warmer temperatures in primed seeds. Priming effect was more pronounced at temperatures closer to the upper and lower limit for germination, but probably that response cannot be accounted for changes in the thermal time constant (θT(g)) and ceiling temperature (Tc(g)). Otherwise, a decrease in the base temperature (Tb) was observed in primed seeds, suggesting that the Tb distribution in U. brizantha seeds is influenced by priming.
palisade grass; osmoconditioning; osmotic potential; thermal time
O objetivo deste estudo foi avaliar o efeito do tratamento pré-germinativo priming sobre os limites térmicos inferior e superior para a germinação de Urochloa brizantha cv. basilisk, e testar a hipótese de que a pré-embebição afeta os parâmetros térmicos da germinação. Sementes pré-embebidas, tanto em água destilada (0 MPa) quanto em solução de PEG 6000 (–0,5 MPa) foram colocadas para germinar em diferentes temperaturas. Os resultados sugerem que sementes de U. brizantha apresentam baixa resposta ao priming quando colocadas para germinar em meio onde a água não é limitante. A resposta de sementes de U. brizantha para o priming é dependente das condições de temperatura e potenciais hídricos em que as sementes são pré-embebidas, bem como para a temperatura de germinação. A temperatura ótima para germinação de sementes de U. brizantha altera-se para temperaturas mais altas em sementes pré-embebidas. O efeito de priming foi mais pronunciado em temperaturas mais próximas do limite superior e inferior para a germinação, mas, provavelmente essa resposta não foi responsável por mudanças na constante tempo térmico (θT(g)) e temperatura teto (Tc (g)). Por outro lado, uma diminuição na temperatura base (Tb) foi observada em sementes pré-embebidas, sugerindo que a distribuição Tb em sementes de U. brizantha é influenciada pelo priming.
grama paliçada; condicionamento osmótico; potencial osmótico; tempo térmico
1 Introduction
In general, the seeds exhibit a minimum, an optimal and a maximum temperature for
germination (the cardinal temperatures). A thermal time (or degrees-day) approach
have been used to describe the distribution of the times to germination at different
temperature regimes according to the models θT(g) =
(T-Tb)t(g), for suboptimal temperatures, and θT =
(Tc(g)-T)t(g), for supraoptimal ones, where
θT(g) is the thermal time required for (g) percent of the seeds
germinate, T is the temperature, Tb is the minimum or base temperature,
t(g) is the time for (g) percent of the seeds germinate and
Tc(g) is the maximum or ceiling temperature corresponding to a
percentage fraction (g) (Garcia-Huidobro et al.,
1982Garcia-Huidobro, J., Monteith, JL. and Squire, GR., 1982. Time,
temperature and germination of pearl millet ( S. and H.). I. Constant
temperatures. Pennisetum thyphoidesJournal of Experimental
Botany, vol. 33, p. 288-296.
http://dx.doi.org/10.1093/jxb/33.2.288.
http://dx.doi.org/10.1093/jxb/33.2.288...
; Bradford, 1995Bradford, KJ., 1995. Water relations in seed germination. In KIGEL,
J. and GALILI, G. (Eds.). Seed development and germination. New York: Marcel
Dekker. p. 351-396.). Once the
model parameters Tb, θT(50) (median thermal time), σθT
(standard deviation in thermal time), Tc(50) (median Tc) and
σTc (standard deviation in Tc(g)) are known, the
germination time courses at different temperatures can be normalized on a common
thermal time scale which allows the germination rate at any temperature regime can
be predicted (Bradford, 1995Bradford, KJ., 1995. Water relations in seed germination. In KIGEL,
J. and GALILI, G. (Eds.). Seed development and germination. New York: Marcel
Dekker. p. 351-396.).
The seed germination is also strongly sensitive to the water potential (Ψ) of its
environment, and the germination responses to Ψ have been analyzed in a manner
similar to thermal time (Gummerson, 1986Gummerson, RJ., 1986. The effect of constant temperatures and
osmotic potential on the germination of sugar beet. Journal of Experimental
Botany, vol. 37, no. 6, p. 729-741.
http://dx.doi.org/10.1093/jxb/37.6.729.
http://dx.doi.org/10.1093/jxb/37.6.729...
;
Bradford, 1995Bradford, KJ., 1995. Water relations in seed germination. In KIGEL,
J. and GALILI, G. (Eds.). Seed development and germination. New York: Marcel
Dekker. p. 351-396.). Accordingly, the
germination time of a given percentage (tg) is inversely proportional to
the difference between Ψ and Ψb(g) (the base or threshold Ψ capable of
preventing a percentage (g) to germinate), and the variation in
germination rates among seeds in the population can be accounted by shifts in
Ψb(g) distributions (Bradford and
Still, 2004Bradford, KJ. and Still, DW., 2004. Applications of hydrotime
analysis in seed testing. Seed Technology, vol. 26, p. 75-85.; Finch-Savage, 2004FINCH-SAVAGE, WE., 2004. The use of population-based threshold
models to describe and predict the effects of seedbed environment on germination
and seedling emergence of crops. In BENECH-ARNOLD, RL. and SANCHEZ, RA. (Eds.).
Handbook of seed physiology. The Haworth Reference Press. p.
51-95.).
Alvarado and Bradford (2002)Alvarado, V. and Bradford, KJ., 2002. A hydrothermal time model
explains the cardinal temperatures for seed germination. Plant, Cell &
Environment, vol. 25, no. 8, p. 1061-1069.
http://dx.doi.org/10.1046/j.1365-3040.2002.00894.x.
http://dx.doi.org/10.1046/j.1365-3040.20...
proposed
that the distribution of Ψb(g) among the seeds accounts for the
distribution of Tc(g) since above T optimum the accumulation of thermal
time would stops and the temperature effects on the germination would be primarily
due to changes in Ψb(g).
No progress toward germination occurs when Ψ < Ψb(g), but this does not
encompass the phenomenon known as seed priming (Bradford, 2002Bradford, KJ., 2002. Applications of hydrothermal time to
quantifying and modeling seed germination and dormancy. Weed Science, vol. 50,
no. 2, p. 248-260.
http://dx.doi.org/10.1614/0043-1745(2002)050[0248:AOHTTQ]2.0.CO;2.
http://dx.doi.org/10.1614/0043-1745(2002...
), in which the seeds are placed in a solution containing
a solute that reduces the kinetic energy of water molecules. In general, the
polyethylene glycol has been widely used for not interfering in the germination
process. Prehydration or priming treatments, followed by drying improves the
germination rate when seeds are imbibed again, suggesting that metabolic events
which cause progress toward germination occur at Ψ < Ψb(g) and are
retained during subsequent drying and rehydration. The priming effects can be
related to changes in the germination responses to temperature. According to Hardegree and Van Vactor (2000)HARDEGREE, S. and VAN VACTOR, SS., 2000. Germination and emergence
of primed grass seeds under field and simulated-field temperature regimes.
Annals of Botany, vol. 85, no. 3, p. 379-390.
http://dx.doi.org/10.1006/anbo.1999.1076.
http://dx.doi.org/10.1006/anbo.1999.1076...
, priming
increased low-temperature germination rate for many crop species and significantly
lowered thermal time requirements and Tb for germination of grass seeds. Ellis and Butcher (1988)Ellis, RH. and Butcher, PD., 1988. The effects of priming and
‘natural’ differences in quality amongst onion seed lots on the response of the
rate of germination to temperature and the identification of the characteristics
under genotypic control. Journal of Experimental Botany, vol. 39, no. 7, p.
935-950. http://dx.doi.org/10.1093/jxb/39.7.935.
http://dx.doi.org/10.1093/jxb/39.7.935...
and Dahal et al. (1990)Dahal, P., Bradford, KJ. and Jones, RA., 1990. Effects of priming
and endosperm integrity on seed germination rates of tomato genotypes. I.
Germination at suboptimal temperature. Journal of Experimental Botany, vol. 41,
no. 11, p. 1431-1439. http://dx.doi.org/10.1093/jxb/41.11.1431.
http://dx.doi.org/10.1093/jxb/41.11.1431...
also observed that priming
decreased thermal-time requirements for germination although they did not show
whether priming reduced base temperature thresholds. Priming effects on germination
rate are also known to be reduced at supra optimal temperatures, thus minimizing the
relative benefit of priming at that range (Hardegree and Van Vactor, 2000HARDEGREE, S. and VAN VACTOR, SS., 2000. Germination and emergence
of primed grass seeds under field and simulated-field temperature regimes.
Annals of Botany, vol. 85, no. 3, p. 379-390.
http://dx.doi.org/10.1006/anbo.1999.1076.
http://dx.doi.org/10.1006/anbo.1999.1076...
). The temperature in which priming occurs
also affects the germination response. For example, Cucurbita pepo
submitted to priming at relatively high temperatures presented lower thermal
requirements for germination than seeds primed at lower temperatures (Zehtab-Salmasi, 2006Zehtab-Salmasi, S., 2006. Study of cardinal temperatures for Pumpkin
() seed germination. Cucurbita pepoJournal of Agronomy, vol. 5,
no. 1, p. 95-97. http://dx.doi.org/10.3923/ja.2006.95.97.
http://dx.doi.org/10.3923/ja.2006.95.97...
).
Urochloa spp are largely utilized as pasture grasses in Brazil. The
species U. brizantha was introduced in Brazil in the 1960s as
fodder for cattle, becoming invasive and changing the landscape and species
interactions and composition of native flora (Barbosa et al., 2008Barbosa, EG., Pivello, VR. and Meirelles, ST., 2008. Allelopathic
evidence in and its potential to invade the Brazilian Cerrados.
Brachiaria decumbensBrazilian Archives of Biology and
Technology, vol. 51, no. 4, p. 625-631.
http://dx.doi.org/10.1590/S1516-89132008000400021.
http://dx.doi.org/10.1590/S1516-89132008...
). The germination is indifferent to light
conditions (Lima and Cardoso, 1996LIMA, VLD. and CARDOSO, VJM., 1996. On the germination and dormancy
of dispersal units of Stapf. Brachiaria decumbensArquivos de
Biologia e Tecnologia, vol. 39, p. 595-606.) and non
dormant seeds of U. brizantha are capable of germinating in
temperature intervals ranging from 7 – 10 °C to 40 – 45 °C (Horibe and Cardoso, not
published). The germination rate of the U. brizantha cultivars
‘marandu’ and ‘xaraes’ is favored by imbibing the seeds previously in water
potentials of 0 and/or –0.5 MPa at 25 °C during 48 h (Lima, 2007Lima, AES., 2007. Condicionamento osmótico de sementes de
Brachiaria brizantha (Hochst. ex A. Rich.) Stapf. Campo
Grande: Universidade Federal de Mato Grosso do Sul. Dissertação de Mestrado em
Botânica.), and Bonome et
al. (2006)Bonome, LTS., Guimarães, RM., Oliveira, JA., Andrade, VC. and
Cabral, PS., 2006. Efeito do condicionamento osmótico em sementes de
Brachiaria brizanha cv. Marandu. Ciência Agrotécnica, vol.
30, no. 3, p. 422-428.
http://dx.doi.org/10.1590/S1413-70542006000300006.
http://dx.doi.org/10.1590/S1413-70542006...
reported that priming at Ψ < -1 MPa for 12 h improve both
the germinability and germination rate of the cv. marandu. The aiming of this study
was to describe the effect of priming treatments on the temperature dependence on
the germination of Urochloa brizantha cv. basilisk through the use
of the thermal time model. Our working hypothesis was that pre-imbibition can widen
the “thermal window” for germination by reducing Tb and increasing
Tc(g).
2 Material and methods
2.1 Plant material
Urochloa brizantha cv. basilisk dispersal units (referred to as seeds) were obtained in March, 2010 from ProSementes Company, from Araçatuba, SP, Brazil). The seeds were harvested in April, 2009 and stored at 24 °C and 60% of air relative humidity. After being purchased the seeds were stored during 15 months at 9±3 °C and maximum germinability was 70%.
2.2 Germination assays
Two assays were performed: in the assay I we tested the priming effect at
optimum, minimum and maximum temperature for germination as determined from
previous experiments. Seed germination (primary root protrusion) was recorded on
five repetitions of 50 seeds each in 18 combinations involving three priming
temperatures (8 °C, 32 °C and 39.5 °C) and two osmotic potentials (0.0 MPa and
–0.5 MPa), and three germination temperatures (8 °C, 32 °C and 39.5 °C). The Ψs
of –0.5 MPa was maintained using a polyethylene glycol (PEG 6000) solution
prepared according to Michel and Kaufmann
(1973)MICHEL, BE. and KAUFMANN, MR., 1973. The osmotic potential of
Polyethylene Glycol 6000. Plant Physiology, vol. 51, no. 5, p. 914-916.
PMid:16658439
PMid:16658439.http://dx.doi.org/10.1104/pp.51.5.914.
http://dx.doi.org/10.1104/pp.51.5.914...
. Seeds were fully immersed during 20 h in aerated PEG solution
or distilled water (Lima, 2007Lima, AES., 2007. Condicionamento osmótico de sementes de
Brachiaria brizantha (Hochst. ex A. Rich.) Stapf. Campo
Grande: Universidade Federal de Mato Grosso do Sul. Dissertação de Mestrado em
Botânica.) and
after priming the seeds were quickly rinsed with water and dried during 72 h at
25 °C under forced air flow. The moisture levels from seed samples were both
determined before and after the drying by oven drying at 105±3 °C (Brasil, 2009BrasilMinistério da Agricultura, Pecuária e Abastecimento2009Regras
para análise de sementesBrasília398p). For germination assays,
seeds were put in plastic boxes (gerbox) on two layer of filter paper kept
saturated with distilled water and maintained in germination chambers under
continuous white light (≅ 33 µmol.m–2.s–1 at seed level)
at the constant temperatures of 8 °C, 32 °C and 39.5 °C, close respectively to
minimum, optimum and maximum temperatures for germination. Non primed seeds
(control) were germinated at the same temperatures. The number of germinated
seeds (radicle protrusion) was recorded daily up to it ceased. The germinability
(G) or germination capacity was the maximum accumulated germination expressed as
percentage.
Another assay (assay II) was designed to find the thermal time parameters. Seeds
were primed in 0.0 and –0.5 MPa solutions at temperatures of 20 and 30 °C and
placed to germinate at different isothermal conditions. Priming and germination
conditions were as described above (assay I). Primed and non-primed seeds were
assayed at the temperatures of 13 °C, 20 °C, 30 °C, 33 °C, 35 °C and 38 °C, and
the resulting germination time courses were fitted by Weibull function (Dumur et al., 1990Dumur, D., Pilbeam, CJ. and Craigon, J., 1990. Use of the Weibull
function to calculate cardinal temperatures in faba bean. Journal of
Experimental Botany, vol. 41, no. 11, p. 1423-1430.
http://dx.doi.org/10.1093/jxb/41.11.1423.
http://dx.doi.org/10.1093/jxb/41.11.1423...
). The time (days) to
50% of the seeds to germinate (t50%) were estimated from each curve
and the germination rate (GR) was calculated as the reciprocal of
t50%, or t5% if the germinability was low. The GR
values were regressed against suboptimal temperatures, and the x-intercept from
each of these regressions was the estimated base temperature (Tb) (Steinmaus et al., 2000Steinmaus, SJ., Prather, TS. and Holt, JS., 2000. Estimation of base
temperatures for nine weed species. Journal of Experimental Botany, vol. 51, no.
343, p. 275-286. http://dx.doi.org/10.1093/jexbot/51.343.275.
PMid:10938833
http://dx.doi.org/10.1093/jexbot/51.343....
), whereas the
thermal times (θinfra) to germination were equal to the inverse of
the slopes of the lines (Bradford,
2002Bradford, KJ., 2002. Applications of hydrothermal time to
quantifying and modeling seed germination and dormancy. Weed Science, vol. 50,
no. 2, p. 248-260.
http://dx.doi.org/10.1614/0043-1745(2002)050[0248:AOHTTQ]2.0.CO;2.
http://dx.doi.org/10.1614/0043-1745(2002...
). Similarly, the values of Tc(50) and θsupra
were estimated from the regression line of GR against supra-optimum
temperatures. The expected germination rates (1/t50) at different
temperatures (Figure 1) were calculated
according to the equations: 1/t50 =
(T-Tb)/10(probit(50)-a).b, for infra optimum temperatures; and
1/t50 = [10(probit(50)-a).b – T]/θ, for supra optimum
temperatures, where a and b are, respectively,
the intercept and the slope of the regression line of the cumulative germination
percentages transformed to probit on log θ(g), for infra-optimum T,
or on log Tc(g), for supra-optimum T (Ellis et al., 1987Ellis, RH., Simon, G. and Covell, S., 1987. The influence of
temperature on seed germination rate in grain legumes. Journal of Experimental
Botany, vol. 38, no. 6, p. 1033-1043.
http://dx.doi.org/10.1093/jxb/38.6.1033.
http://dx.doi.org/10.1093/jxb/38.6.1033...
). Probit(50) is the median
probit corresponding to 50% germination.
Temperature dependence on the germination rate (1/t50%) Urochloa brizantha seeds primed in PEG 6000 –0.5 MPa (A, C) and distilled water (B, D) at the temperatures of 20 °C (A, B) and 30 °C (C, D). The germination rate of non-primed seeds is also presented (E). Germination under continuous white light. Lines were fitted according to the thermal time model (see Material and Methods for details).
A completely randomized experimental design was used in the assays (Vieira, 1999Vieira, S., 1999. Estatística experimental. 2. ed. São Paulo: Atlas.). The germinabilities and average germination rate were tested for homogeneity of the variances (Bartlett test) and submitted to Anova if the variances were homogeneous. The Tukey’s test (α = 0.05) was applied to the significant differences.
3 Results
3.1 Assay 1
When the seed water content was measured just after priming, lower water content was observed in seeds pre imbibed at 8 °C relative to that imbibed at 32 °C and 39.5 °C (Table 1). The water uptake during priming was improved by imbibiton in distilled water (DW) as compared to polyethylene glycol 6000 (PEG), except at 8 °C in which no difference occurred between DW and PEG treated seeds. After drying the differences in seed water content among the pre-imbibitions treatments disappeared and the seed moisture was approximately 13% (Table 1).
Water contents of Urochloa brizantha cv. Basilisk seeds primed at the temperatures of 8 °C (8), 20 °C (20), 30 °C (30), 32 °C (32) and 39.5 °C 39.5) in distilled water (DW) and –0.5 MPa polyethylene glycol solution (PEG). Data were taken either immediately after the seeds were removed from priming treatments (not dried) or after the primed seeds were dried 72h at 25 °C (dried).
The germinability of Urochloa brizantha at 8 °C did not exceed 10% and attained the highest values in seeds primed at 32 °C in PEG (32-PEG), which germination was significantly higher than in not primed seeds (Figure 2A). The germination rate at 8 °C was significantly promoted by priming at 32 °C both in distilled water (32-DW) and PEG 6000 solution (32-PEG) as compared to not primed seeds (control) and other priming treatments (Figure 2B). At the germination temperature of 32 °C the germinabilities did not differ statistically amongst the priming treatments and between primed and not primed seeds, with exception of the seeds primed in PEG at 39.5 °C (39.5-PEG), which germination was slightly lower although it did not differ from control (Figure 2C). Priming in distilled water at 8 °C (8-DW) and 32-DW improved significantly (P < 0.05) the germination rate in comparison to not primed seeds (Figure 2D). At 39.5 °C, the highest germination (around 30%) was obtained for seeds primed at 32 °C in polyethylene glycol solution, which germinability was higher than control (Figure 2E). The germination rate at that supra-optimal temperature was promoted by priming in 32-DW, as compared to control (Figure 2F).
Germinability (A, C, E)) and germination rate (B, D, F) of Urochloa brizantha seeds primed in distilled water (W) and polyethylene glycol –0.5MPa solution (P) at 8 °C (8), 32 °C (32) and 39.5 °C (39), and placed to germinate at 8 °C (A, B), 32 °C (C, D) and 39.5 °C (E, F) under continuous white light. C = not primed seeds. Small letters (Tukey test, α = 0.05) compare treatments within each germination temperature.
3.2 Assay 2
The water content measured immediately after priming was lower in seeds primed in PEG solution than in DW at the temperature of 30 °C, whereas no difference between the priming media occurred at 20 °C (Table 1). After drying the seed water content was similar (≅ 9.5%) among pre-imbibitions treatments (Table 1).
The germinability at the temperatures of 13 °C (infra-optimal), 30 °C (optimal) and 38 °C (supra-optimal) was not affected by priming treatments whereas the germination rate (1/t50%) both at 13 °C and 38 °C was promoted by priming in distilled water at 20 °C (Table 2). The germination rate (GR) of seeds put to germinate at 30 °C was not increased by priming the seeds both in –0.5 MPa PEG solution and distilled water (Table 2). Priming in PEG at 30 °C (30-PEG) inhibited the GR of seeds placed to germinate either at 13 °C, 30 °C or 38 °C (Table 2). Otherwise, when compared to not primed seeds, the pre-treatment 30-PEG caused a decrease in the base temperature (Tb) as well as an increase of thermal time requirement for 50% (θ50) of the seeds to germinate at the infra-optimum temperature range (Table 2). Pre-treatment with distilled water at 30 °C (30-DW) also resulted in a significant (α = 0.05) decrease in Tb, although it did not affect θ50 (Table 2). Both the ceiling temperature (Tc50) and θ50 for supra-optimal temperatures were not influenced by priming treatments (Table 2). The analysis of the temperature dependence on the germination rate of Urochloa brizantha based on the predicted curves according to the thermal time model suggests a displacement of the optimum temperature for germination (To) toward higher values in response priming (Figure 1). The model in general matched actual germination rate, as can be seem in Figure 1 which compares actual (symbols) and predicted (smooth line) values.
Thermal parameters from germination data (Assay II) of primed Urochloa brizantha seeds. The priming treatments were: distilled water, at 20°C (20-DW) and 30 °C (30-DW); and PEG 6000, at 20 °C (20-PEG) and 30 °C (30-PEG). Not primed seeds were used as control (C). Means ± sd. Small letters compare treatments (Tukey test, α = 0.05)
4 Discussion
The water content of Urochloa brizantha seeds soaked for 24h in PEG
and DW was affect both by temperature and priming medium. Relatively low priming
temperatures caused lower water content (WC), whereas imbibition in PEG (–0.5 MPa)
tended to reduces WC as compared to distilled water, although no effect of the
osmotic medium was observed at relatively low (8 °C and 20 °C) priming temperatures.
That response was expected since earlier reports show that temperature affect the
rates of water uptake primarily by changing the water viscosity, although the
increase in water uptake with temperature can be discontinuous from 5 °C to 35 °C as
reported for pine embryos in which the slope of the regression line of the water
uptake on temperature was significantly steeper above 20 °C (Murphy and Noland, 1982Murphy, JB. and Noland, TL., 1982. Temperature effects on oxidative
metabolism of dormant sugar pine seeds. Plant Physiology, vol. 70, no. 5, p.
1410-1412. http://dx.doi.org/10.1104/pp.70.5.1410.
PMid:16662689
http://dx.doi.org/10.1104/pp.70.5.1410...
). Once the previously imbibed
U. brizantha seeds were dried for 72h at 25 °C the seed water
content was found to be similar among priming treatments within each assay, ensuring
that WC was similar among the seeds before use in the germination tests. Otherwise,
the seeds used in the assay I and II differed in water content probably due to the
assays were performed in different times, and the storage environment conditions
must have influenced the results.
In the assay I the priming temperatures were the same used in the germination assays.
Under the conditions described in the present assay the germination response of
U. brizantha seeds to priming can be influenced by the
temperature and priming medium as well as by the germination temperature. When the
priming osmotic potentials of 0.0 MPa (DW) and –0.5 MPa (PEG) were compared to each
other the former tended to be more effective than the latter as priming agent
chiefly when the U. brizantha seeds were placed to germinate at 32
°C, an “optimum” temperature (Nakao, 2012NAKAO, EA., 2012. Temperatura e osmocondicionamento na germinação de
sementes de (STAPF) Webster CV. Basilisk. Urochloa brizanthaRio
Claro: Universidade Estadual Paulista “Júlio Mesquita Filho”. Dissertação de
Mestrado em Ciências Biológicas (Botânica). Available from:
<http://www.athena.biblioteca.unesp.br/exlibris/bd/brc/33004137005P6/2012/nakao_ea_me_rcla.pdf>.).
Otherwise, when the effect of the priming treatments were compared to not primed
control, and taking into account the effect both on the germinability and
germination rate, the priming effect on U. brizantha seeds appeared
to be more pronounceable both nearby the lower and upper thermal limits reported for
germination of the species (Nakao, 2012NAKAO, EA., 2012. Temperatura e osmocondicionamento na germinação de
sementes de (STAPF) Webster CV. Basilisk. Urochloa brizanthaRio
Claro: Universidade Estadual Paulista “Júlio Mesquita Filho”. Dissertação de
Mestrado em Ciências Biológicas (Botânica). Available from:
<http://www.athena.biblioteca.unesp.br/exlibris/bd/brc/33004137005P6/2012/nakao_ea_me_rcla.pdf>.),
what is partially in agreement with Hardegree and
Van Vactor (2000)HARDEGREE, S. and VAN VACTOR, SS., 2000. Germination and emergence
of primed grass seeds under field and simulated-field temperature regimes.
Annals of Botany, vol. 85, no. 3, p. 379-390.
http://dx.doi.org/10.1006/anbo.1999.1076.
http://dx.doi.org/10.1006/anbo.1999.1076...
which reported that the germination rate of some
grasses was enhanced by priming, and the effect was most notable in the cooler
temperature treatments. Furthermore, assays with other species, such as
Capsicum annuum L (Posse et
al., 2001Posse, SCP., Silva, RF., Vieira, HD. and Catunda, PHA., 2001.
Efeitos do condicionamento osmótico e da hidratação na germinação de sementes de
pimentão (Capsicum annuum L.) submetidas à baixa temperatura.
Revista Brasileira de Sementes, vol. 23, p. 123-127.) indicate that the use of primed seeds is only viable when they
are sowed at sub or supra-optimal temperatures. The results presented here also are
in accordance with a previous report with U. brizantha cv. Marandú
(Lima, 2007Lima, AES., 2007. Condicionamento osmótico de sementes de
Brachiaria brizantha (Hochst. ex A. Rich.) Stapf. Campo
Grande: Universidade Federal de Mato Grosso do Sul. Dissertação de Mestrado em
Botânica.) which shows a dependence on
the exposure time and concentration of the priming agent of the seed response to
priming. The relative low effect of the priming temperature of 8 °C on the
germination of U. brizantha as compared to 32 °C can be accounted
for the lower water in the former, suggesting that the advancement of early
physiological processes related to seed germination during priming can be limited by
the seed water levels. Otherwise, the lack of response of U.
brizantha seeds to a supra optimum priming temperature was not related
to the seed moisture since the percentage of water was similar between the priming
temperatures of 32 °C and 39.5 °C. More investigations are required in order to
access for the lower and upper temperature limits to priming of U.
brizantha, considering that some physiological and metabolic activities
related to progress toward germination show a threshold-type at low Ψ and
temperature (Bradford, 2002Bradford, KJ., 2002. Applications of hydrothermal time to
quantifying and modeling seed germination and dormancy. Weed Science, vol. 50,
no. 2, p. 248-260.
http://dx.doi.org/10.1614/0043-1745(2002)050[0248:AOHTTQ]2.0.CO;2.
http://dx.doi.org/10.1614/0043-1745(2002...
), although no
model has yet been proposed that uses parameters for ceiling temperatures.
In the assay II the seeds were primed in 0.0 and –0.5 MPa at the temperatures of 20
°C and 30 °C chosen based on Zehtab-Salmasi
(2006)Zehtab-Salmasi, S., 2006. Study of cardinal temperatures for Pumpkin
() seed germination. Cucurbita pepoJournal of Agronomy, vol. 5,
no. 1, p. 95-97. http://dx.doi.org/10.3923/ja.2006.95.97.
http://dx.doi.org/10.3923/ja.2006.95.97...
, to access for a possible effect of the priming temperature on the
T-dependence on the germination of U. brizantha as described
through the thermal time model. The temperature dependence on the germinability of
U. brizantha changed with priming since the optimum temperature
for germination shifts toward warmer temperatures in primed seeds. Reinforcing the
results obtained in the assay 1, priming effects on the germination rate of
U. brizantha seeds were observed only at the germination
temperatures of 13 °C and 38 °C, close to the lower and upper thermal limits for
germination, respectively (Nakao, 2012NAKAO, EA., 2012. Temperatura e osmocondicionamento na germinação de
sementes de (STAPF) Webster CV. Basilisk. Urochloa brizanthaRio
Claro: Universidade Estadual Paulista “Júlio Mesquita Filho”. Dissertação de
Mestrado em Ciências Biológicas (Botânica). Available from:
<http://www.athena.biblioteca.unesp.br/exlibris/bd/brc/33004137005P6/2012/nakao_ea_me_rcla.pdf>.).
Those results can neither be explained in terms of variation in the ceiling
temperature (Tc50) or the amount of thermal time (θT(g))
required for germination, since Tc50 values were similar among the
treatments and θT(g) values were not reduced by priming. Otherwise, a
decrease in the base temperature (Tb) was observed in primed seeds, supporting the
hypothesis that priming could improve the germination rate of U.
brizantha by reducing Tb. However, the results presented here did not
support our hypothesis that a priming effect on the germination of U.
brizantha can be accounted for a decrease in θT(g), as
reported by Hardegree and Van Vactor (2000)HARDEGREE, S. and VAN VACTOR, SS., 2000. Germination and emergence
of primed grass seeds under field and simulated-field temperature regimes.
Annals of Botany, vol. 85, no. 3, p. 379-390.
http://dx.doi.org/10.1006/anbo.1999.1076.
http://dx.doi.org/10.1006/anbo.1999.1076...
in some grasses, and an increase in Tc(g), considering that the
germination rate is directly proportional to Tc(g) and inversely
proportional to Tb for a given T and θ (Garcia-Huidobro et al., 1982Garcia-Huidobro, J., Monteith, JL. and Squire, GR., 1982. Time,
temperature and germination of pearl millet ( S. and H.). I. Constant
temperatures. Pennisetum thyphoidesJournal of Experimental
Botany, vol. 33, p. 288-296.
http://dx.doi.org/10.1093/jxb/33.2.288.
http://dx.doi.org/10.1093/jxb/33.2.288...
). In onion (Allium cepa L.) seeds the major
effect of priming was to reduce θ(g) at both sub- and supra-optimal
temperatures (Ellis and Butcher, 1988Ellis, RH. and Butcher, PD., 1988. The effects of priming and
‘natural’ differences in quality amongst onion seed lots on the response of the
rate of germination to temperature and the identification of the characteristics
under genotypic control. Journal of Experimental Botany, vol. 39, no. 7, p.
935-950. http://dx.doi.org/10.1093/jxb/39.7.935.
http://dx.doi.org/10.1093/jxb/39.7.935...
), but
this feature was not observed in the present study, suggesting such parameters may
have a strong genotypic basis which is relatively unaffected by environmental
factors prior the visible germination in U. brizantha. However,
further assays testing more priming conditions are required to ascertain that
hypothesis. Based on the relationship between Tc(g) and base water
potential (Ψb(g)) (Alvarado and
Bradford, 2002Alvarado, V. and Bradford, KJ., 2002. A hydrothermal time model
explains the cardinal temperatures for seed germination. Plant, Cell &
Environment, vol. 25, no. 8, p. 1061-1069.
http://dx.doi.org/10.1046/j.1365-3040.2002.00894.x.
http://dx.doi.org/10.1046/j.1365-3040.20...
), the results presented here suggest that priming
treatments did not affect the Ψb(g) distribution in the seed population
of Urochloa brizantha, however further experiments must be
conducted in a wide range of water potentials in order to determine a possible
variation of the base water potential (Ψb(g)) and, thus, evaluate the
sensitivity of U. brizantha seeds to water deficit (Bradford and Somasco, 1994Bradford, KJ. and Somasco, OA., 1994. Water relations of lettuce
seed thermoinhibition. I. Priming and endosperm effects on base water potential.
Seed Science Research, vol. 4, no. 01, p. 1-10.
http://dx.doi.org/10.1017/S0960258500001938.
http://dx.doi.org/10.1017/S0960258500001...
).
The results presented here show that the response of U. brizantha seeds to temperature can be affected by priming, depending on the temperature and water potential conditions at which the seeds are pre-imbibed. The general conclusions can be drawn from the two assays are that priming tend to improve the germination at temperatures closer to the upper and lower limit for germination and, with exceptions, it has no effect when the seeds are placed to germinate at temperatures close the optimum. The thermal time model described relatively well the isothermal germination rate seeds, although the variation in the germination responses to priming treatments was not consistently explained by the thermal-time and Tc(g) variation. However, the Tb distribution in U. brizantha seeds can be influenced by priming.
Acknowledgments
The authors thank COORDENAÇÃO DE APERFEIÇOAMENTO DE PESSOAL DE NÍVEL SUPERIOR – CAPES, for the master grant awarded to EAN.
-
(With 2 figures)
References
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» http://dx.doi.org/10.3923/ja.2006.95.97
Publication Dates
-
Publication in this collection
Jan-Mar 2015
History
-
Received
06 Aug 2013 -
Accepted
10 Oct 2013