Seed dormancy is an evolutionary adaptation that prevents seeds from germinating during unsuitable ecological conditions that would be typically lead to the low probability of seedling survival .Dormant seed don’t germinate in specific period of time under a condition of environmental factors that are normally conducive to the germination of non ‒dormant seeds .
Seed dormancy is generally an undesirable characteristic in agricultural crops ,where rapid germination and growth are required .However ,some degree of dormancy is advantageous ,at least during seed development .This is particularly true for cereal crops because it prevents germination of grains while still on the ear of the parent plant (preharvest sprouting ),a phenomenon that results in major losses to the agricultural industry . Extensive domestication and breeding of crops species have ostensibly removed most dormancy mechanisms present in the seeds of their wild ancestors ,although under adverse environmental conditions ,dormancy may reappear .By contrast ,weed seeds frequently mature with inherent dormancy mechanisms that allow some seeds to persist in the soil for many years before completing germination.
Temperature and seed geminatio
Fluctuating temperature plays a critical role in determining the timing of seed germination in many plant species .However,the physiological and biochemical mechanisms underlying such a response have been paid little attention .Temperature affects germination in three primary ways :moisture,hormone,production and enzymatic activity.For seeds to germinate ,they need to imbibe water .For this to occur ,sufficient moisture must be present . A warmer climate may increase evaporation and decrease moisture , which would negatively affect germination .Temperature can affect the percentage and rate of germination through at least three separate physiological processes.
1.Seeds continously deteriorate and ,unles�� in the meanwhile they
Are germinated, they will ultimately die .The rate of deterioration depends mainly on moisture content and temperature .the q10 for the rate of loss of viability in orthodox seeds consistently increases from about 2 at ‐10 degrees C to about 10 at 70 degrees.
2.Most seeds are initially dormant .Relatively dry seeds continuously lose dormancy at a rate which is temperature dependent . Unlike enzyme reactions ,the Q10 remains constant over a wide range of temperature at least up to 55 degrees C ,and typically has a value in the region of 2.5 to 3.8.hydrated seeds respond quite differently :high temperature generally reinforce dormancy or may even induce it .Low temperature may induce dormancy in some circumstances ,but in many species they are stimulatory (stratification response),especially within the range 1 degree c to 15 degree C . Small , dormant ,hydrated seeds are usually also stimulated to germinate by alternating temperatures which typically interact strongly and positively with light (and often also with other factors including nitrate ions.
3.Once seed have lost dormancy their rate of germination shows a positive linear relation between the base temperature and the optimum temperature and a negative linear relation between the ceiling temperature .The optimum temperature for germination rate is typically higher than that required to achieve maxm percentage germination in partially dormant or partially deteriorated seed populations.
Much more needs to be learned about the key processes involved in germination and dormancy .Both germinating and dormant seeds must undergo many cellular and metabolic changes in common after imbibition ,and yet only the embryos of the former emerge from their surrounding structure .The real block to germinate in dormant seeds may occur at very last stage :radicle cell extension . New approaches that can be or are being tried in an attempt to identify germination and dormancy associated gene include T DNA mutagenesis ,differential display ,subtractive cDNA hybridization ,and the use of nondestructive reporter gene technology .Finally ,we need to determine how radicle extension occurs ,the ultimate manifestation of germination.