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A laboratory experiment was carried out to determine the effect of different constant temperatures on germination and early seedling establishment and to study the variation among parameters of thermal time model parameters for two contrasting chickpea cultivars . Seeds were subjected to six constant temperatures from 10 o C to 35 o C . A complete randomized design was used with four replication. Analysis of variance showed significant differences among treatments for all characters studied. The final germination percentage significantly increased with increasing temperature up to 25 ° C, and thereafter there was a sharp decrease in final germination at 30 ° and 35 ° C. Desi type cultivar (small seeded) “Jabel Marra” significantly exhibited higher final germination percentage and lower germination rate compared with the kabui type cultivar “Shendi” at all temperatures. The median (θ T(50) ) of the thermal time was significantly differ between the two chickpea cultivars. The large seeded cultivars (shendi) recorded significantly higher median thermal time than the small seeded cultivars (Jabel Marra). The results also revealed a significant differences between the two cultivars in all thermal time model parameters. The small seeded cultivar (Jabel Marra) scored lower total dry matter and temperature tolerance index (TTI) compared to the large seeded cultivar (Shendi) at all temperatures studied.

Chickpea (Cicer arietinum L.) is the third most extensively planted grain legume in the world after dry bean and field pea [

The establishment of plants is often limited by temperature when moisture conditions are favourable [_{b}), maximum temperature (T_{c}) that germination rate at above of that will be zero and optimum temperature (T_{o}) at which the germination rate is the highest [

Seed size is a genotypic character which varies between and within species. It has been well known that larger or heavier seeds usually germinate faster than smaller or lighter seeds of the same species [

The objectives of this study were to test the assumption of the thermal model using two contrasting chickpea cultivars and to determine the effect of thermal requirement on germination and early growth of this crop.

Two chickpea cultivars were obtained from the Agricultural Research Corporation (ARC) in Sudan, namely; Shendi (kabuli-type) and Jabel Marra (desi-type). Shendi cultivar; large seeded cultivar is described as a late mature and small seeded variety and the other cultivar (Jabel Marra); small seeded cultivar is described as an early mature variety. Germination tests of fresh mature seeds were carried out by putting seeds in Petri dishes with two layers of moistened filter papers (Whatman, No.1). The Petri dishes were randomly put in six incubators (Sanyo Versatile Environmental Chamber MLR-350 H), six temperatures from 10˚C to 35˚C, at 5˚C interval, were randomly assigned to each incubator. The relative humidity inside the incubator was 32% and the photoperiod is 12 hr with 30 µmol photons. m^{−2}・s^{−1}, 400 - 700 nm. A randomized complete design (RCBD) was used with four replicates of 25 seeds each, at 7-day interval between replicates in each incubator. Clear plastic bags were used to seal Petri dishes to reduce water evaporation. Seeds were considered germinated when 2 mm of radicle was visible [

Final (cumulative) germination percentages were transformed using log_{10} Probit [_{b}) was determined by plotting germination percentage (on a probit scale) against log-thermal time (log10(T − T_{b})t_{g}), where T_{b} is unknown and estimated by changing the value of T_{b} until the minimum residual variation is obtained [

where T_{b} ≡ base temperature,

These parameters were estimated for each seed size class of each cultivar for the construction of the thermal time model. General linear model (GLM) in SPSS was used to test differences in base temperature (T_{b}) between subpopulations and the effects of genotype and seed classes on thermal model parameters (T_{b}, ^{2}). R^{2} was calculated as: 1-(SS residual/SS total), where, SS residual represents residual sum of squares and SS total is the total sum of squares, while SS residual = observed value − predicted value and SS total = observed value − mean. Germination rate index (GRI) which is independent on thermal time model, is calculated as:

where G_{tot} is the total number of germinated seeds in a dish at the end of the germination test; P is the total number of seeds incubated; g_{i} is the number of germinated seeds accumulated between time (d) _{i}. The effect of cultivars on GRI was also analyzed using general linear model (GLM) in SPSS (version 16).

Temperature tolerance index (TTI) was calculated as total plant (shoot + root) dry weight at different temperatures compared to the total plant dry weight obtained at optimum temperature as indicate below:

where TDW = total dry weight; T_{i} = control treatment (at optimum temperature); T_{x} = x treatment (at corresponding temperature).

Statistical Analysis:

Hundred Seeds of each cultivar were put in incubators at six different temperatures and arranged in a complete randomized design (CRD) with four replicates. If there was a significant difference at p < 0.05, the means separation was done for different treatments using least significant (LSD) method. To validate the thermal time model, correlation test was done and accordingly, the coefficient of determination (R^{2}) and level of significance (P) were calculated. All statistical tests were done using a statistical package (SPSS version 16). Figures were constructed by using the computer software (Excel).

The final germination percentage significantly increased with increasing temperature up to 25˚C, and thereafter there was a sharp decrease in final germination at 30˚C and 35˚C (

Germination rate (d^{−1}) of subpopulations, GR(g), calculated from the predicted germination time course correlated strongly (R^{2} = 0.92, P < 0.01) with temperature at the suboptimal range 10˚ to 25˚C (_{b} for cultivar Shendi (1.2˚C) was about 1.8˚C lower than that of Jabel Marra (3.0˚C). Also, there were significant differences in

Germination rate index was significantly different between the two cultivars due to the difference in their seed sizes at various temperatures (

The mean total dry weight was significantly different between the two cultivars at different temperatures (

It has long been known that seeds can germinate over a wide range of temperatures, but maximum germination and rate of germination typically reduce at the extremes of the range [

Cultivar | Parameters of Thermal Time Model | ||||
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Base Temperature in Water ( | Median Thermal Time ( | Standard Deviation of Thermal Time ( | *R^{2} | **p-value | |

Shendi (large seedsd type) | 1.2a | 117.5 a | 0.83b | 0.87 | < 0.01 |

Jabel Marra (small seeded type) | 3.0b | 102.2 b | 1.70a | 0.97 | < 0.01 |

***LSD_{(0.05)} | 0.56 | 8.5 | 0.7 |

T_{b}_{(0)}: base temperature, θ_{T}_{(50)}: median thermal time, σ_{θT}: Standard deviation of thermal time. *R^{2}: Coefficient of determination; **p-value: level of significance.

TDW (g) | TTI (%) | ||||||||||
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10˚C | 15˚C | 20˚C | 25˚C | 30˚^{ }C | 35˚C | 10˚C | 15^{ }˚C | 20˚C | 25˚C | 30˚C | 35˚C |

0.75 | 1.17 | 2.31 | 2.48 | 2.05 | 1.80 | 30.24 | 87.5 | 93.14 | 100 | 82.66 | 72.58 |

0.41 | 1.00 | 1.70 | 1.90 | 1.68 | 1.31 | 21.58 | 84.21 | 89.47 | 100 | 88.42 | 68.95 |

and modified the thermal time model (which first proposed by [_{b} was often considered as a constant may be for the ease of modeling [

The authors are grateful for funding received from Prof. Mustafa Salih, University of King Saud.

Atif Hassan Naim,Faisal El Gasim Ahmed, (2015) Variation in Thermal time model Parameters Between Two Contrasting Chickpea (Cicer arietinum) cultivars. Agricultural Sciences,06,1421-1427. doi: 10.4236/as.2015.612138