What is germination rate exactly?

It is the latter – all 'rates' must be defined as something occurring per unit time. That's what 'rate' means lol. Germination percentage is just that...a percentage. I hope this helps :) 

Actually germination rate means total number of seeds germinated within a perticular time and u can calculate it in form of percentage. You follow the seed ISTA , 1993 protocal.

Yes, I agree, but you still have to express the % as a function of the time over which it occurred, which was my point lol :)

Germination rate= The number of seeds sprouted divided by the number of total seeds sprouted, then multiplied by 100. You will have to take the maximum germination taken in a minimum time , instead of final germination , and be expressed as germination /day as well, it is just an expression of germination... Andrew has stated very rightly..

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Generally the number of seeds which emerge divided by the tested sample gives a rate of germination which is usually expressed as a percentage. Use of 200 to 400 seeds with 3 or 4 replications give a reliable number will the ability to determine an error variance. 

Another concept which is supplementary is vigor which is the measurement of the strength of the germinated seedlings. This is best indicated by germinating using an adverse environmental stress. There is some correlation between germination and vigor. Vigor testing such as cold or warm emergence testing can differentiate stronger seed sources. 

Another approach is to use biochemical testing or pathological assay. All are very important to get a clear idea of what is happening in seed sources. 

Germination rate calculated by dividing total germinated seeds to total days from the first day of germination to the final germination day .This formula can considered as an expression for seeds vigor as well as germination media condition.

In several studies the definition of germination ranges from the appearance of the radicle through the testa to the establishment of a seedling. Tht latter was preferred by the ISTA( International seed testing association.) 1976 depending on the work you are doing I think germination can be defined for any time within radicle emergence and the establishment of the plant. Since germination is a developmental stage Germination rate can be defined as the part of the population that has reached that stage, divided by the time taken for the process. Thus G_50%/t, Would refer as the germination rate of 50 % of the population per unit time. Similarly G_80%/t  in reference to the germination of 80% of the population per unit time). In most cases the germination rate is defined as the reciptocal of time for 50% of the population to take place (Bierhuizen, 1973 Hegarty, 1973; Thompsom, 1978) An d usually varies linearly with Temperature.

Variable is the actual property measured by individual observation. It is a property with respect to which one individual in the sample differ in some ascertainable way. Derived variables are based on two or more independently measured variables. They are percentage, ratio, and rates. The evaluation of seed germination can follow a dimensionless proportionality expressed as percentage. It expresses the fraction of normal seedlings in the total of seeds under evaluation. In the control of seed quality you can express the final counting as percentage. Ratio is the relation between two variables expressed as a single value, for example 2:3 (read two to three) in the total 5 parts. Index is an average of two measurements: the germination index. In the evaluation of seed germination, rate is the limit (ΔG/Δt) when Δt goes to zero. G can be expressed by a function (F) and the events are recorded along the time.  (ΔG/Δt) = dF/dt. The physical dimension of rate is T^-1.    For example, in plant growth rate we have: (ln W₂ - ln W₁) / (t₂-t₁) where ln is dimensionless, and the relative growth rate (1/T) is T^-1.

germination by time

See the paper in the RG "Seed vigour: a biometric approach. 2003" for better understanding of the mathematical theory. Best regards

Germination is rate simply the number of seeds percentage germinated of the total seeds planted.

According to the oxford english ditonary one of definitions of rate is "The speed with which something moves or happens." Thus germination rate should include time in its definition, and should not  be confused with the capacity or potential of a population of seeds to germinate given the appropiate condition. So  firstly the developmental event must happen to a proportion of the population of seeds that germinate  p.eg. G_50%,  previously definedand secondly the time to reach that stage in population can be refered as germinatiion rate (1/T). This variable can be used to study the behaviour os the population of seeds  in variables environments.

Jorge good point, we need to define germination rate within a time frame..

As per the ISTA  procedure ,Germination percentage calculated by the number of normal seedlings were counted at the end of First and Final count  and expressed in percentage.(Ex: Maize germination test the first count is for 4 days and final count is for 7 days)but the germination percentage varies from speed of germination i.e daily count of germinated seeds was made up to final count.

Thank you all for your valuable inputs. It can be concluded from above discussion that germination rate is slightly different from germination percentage because germination rate is something which based on speed of germination per unit time. While germination percentage is indicator of seed viability and potential to emerge which is time independent.    

Mean germination time is the time spent by a seed to germinate. I t is calculated by sum of number of correspondent germinated seed * time from the start of the experiment to the last observation over sum of number of seeds germinated in the correspondent time.

Mean germination rate refers to the speed taken by a seed to germinate per unit time. It is calculated as the reciprocal of the mean germination time.

Germination rate is the average number of seeds that germinate over the a specific period of time (e.g., 7days or 14 days). It represents the speed of germination of a sample and is used as an index of seed vigor. The faster the seeds germinate, the higher the quality of the seeds. The speed of germination index is calculated following the formula described in the AOSA Seed Vigor Testing Handbook (2009).