Kirti , an interesting question. Roots, stems and leaves are functionally interdependent and these three systems maintain a dynamic balance in biomass which reflects relative abundance of above-ground resources (light and CO2) compared with root-zone resources (water and nutrients) . Whole-plant growth rate and summary measures such as root:shoot ratio are thus an outcome of developmental stage and of environmental influences.The root:shoot ratio is one measure to help you assess the overall health of the r plants. the identified  control group of plants will provide  a "normal" root:shoot ratio for each of  plant types, any changes from this normal level (either up or down) would be an indication of a change in the overall health of those  planst. It is important to combine the data from the root:shoot ratio with data from observations to get an accurate understanding of what is happening with identified  plants.

Change in root:shoot ratio during a plant’s life cycle is part of an intrinsic ontogeny, but growth rates of roots and shoots continually adjust to resource availability with photoassimilate (hence biomass). In herbaceous plants, root:shoot ratios typically decrease with age (size) due to sustained investment of carbon in above-ground structures (root crops would be a notable exception). Developmental morphology is inherent, but expression of a given genotype will vary in response to growing conditions (hence phenotypic plasticity).

Consistent with shoot response to above-ground conditions, root biomass is influenced by below-ground conditions where low availability of either water or nutrients commonly leads to greater root:shoot ratio. For example, white clover (Trifolium repens) growing on a phosphorus-rich medium increased root:shoot ratio from 0.39 to 0.47 in response to moisture stress; and from 0.31 to 0.52 when moisture stress was imposed in combination with lower phosphorus . A positive interaction between low phosphorus and low water on root:shoot ratio was also evident in perennial ryegrass (Lolium perenne) grown on high nitrogen. In that case, root:shoot ratio increased from 0.82 to 3.44 in response to moisture stress when plants were grown on low phosphorus in combination with high nitrogen. Few points to consider....

Root shoot ratio is generally genetically fixed for the particular cultivars. However some factors (Biotic and abiotic) may change this ration and can help the plant in either way. If root ratio is high plant will absorb more nutrients from the soil and this will help in increasing above ground biomass. 

In general, it seems drought and salinity resistant or tolerant varieties  have large volume and length of roots and high value for root shoot ratio.

Please have a look at this useful ResearchGate link for insight.

https://www.researchgate.net/publication/251449023_The_Role_of_Mineral_Nutrition_on_Root_Growth_of_Crop_Plants

Article The Role of Mineral Nutrition on Root Growth of Crop Plants

Thanks alot sir for the paper. 

Kirti , an interesting question. Roots, stems and leaves are functionally interdependent and these three systems maintain a dynamic balance in biomass which reflects relative abundance of above-ground resources (light and CO2) compared with root-zone resources (water and nutrients) . Whole-plant growth rate and summary measures such as root:shoot ratio are thus an outcome of developmental stage and of environmental influences.The root:shoot ratio is one measure to help you assess the overall health of the r plants. the identified  control group of plants will provide  a "normal" root:shoot ratio for each of  plant types, any changes from this normal level (either up or down) would be an indication of a change in the overall health of those  planst. It is important to combine the data from the root:shoot ratio with data from observations to get an accurate understanding of what is happening with identified  plants.

Change in root:shoot ratio during a plant’s life cycle is part of an intrinsic ontogeny, but growth rates of roots and shoots continually adjust to resource availability with photoassimilate (hence biomass). In herbaceous plants, root:shoot ratios typically decrease with age (size) due to sustained investment of carbon in above-ground structures (root crops would be a notable exception). Developmental morphology is inherent, but expression of a given genotype will vary in response to growing conditions (hence phenotypic plasticity).

Consistent with shoot response to above-ground conditions, root biomass is influenced by below-ground conditions where low availability of either water or nutrients commonly leads to greater root:shoot ratio. For example, white clover (Trifolium repens) growing on a phosphorus-rich medium increased root:shoot ratio from 0.39 to 0.47 in response to moisture stress; and from 0.31 to 0.52 when moisture stress was imposed in combination with lower phosphorus . A positive interaction between low phosphorus and low water on root:shoot ratio was also evident in perennial ryegrass (Lolium perenne) grown on high nitrogen. In that case, root:shoot ratio increased from 0.82 to 3.44 in response to moisture stress when plants were grown on low phosphorus in combination with high nitrogen. Few points to consider....

Do you consider higher root-to shoot ratio is good for aquisition of certain immobilie nutrients like phosphorous..?

What is the significance of root-shoot ratio in crops?

It can be an indicator of the healthy of your plants growth. It can also be an indicator of whether your plants have suffered environmental stresses. The root-shoot ration will change when plants encounter stresses as mentioned by other people above. Some researchers use it as a method to measure plant growth. See link: https://www.sciencebuddies.org/science-fair-projects/project_ideas/PlantBio_measuring_growth.shtml

It's important because the root system plays a fundamental role in taking nutrients and water, and as the absorption at the root level is more intense, the higher the biomass, hence the agricultural production.

Greater root/shoot ratio means greater root density and root interception for nutrient uptake. Luliana good comments. 

Root-shoot ratio is the amount of plant tissues that have supportive functions to the amount of those that have growth functions. Plants with a higher proportion of roots can compete more effectively for soil nutrients, while those with a higher proportion of shoots can collect more light energy. Large proportions of shoot production are characteristic of vegetation in early successional phases, while high proportions of root production are characteristic of climax vegetational phases.

For crops, the gain in harvestable crop  is typically due to changes in biomass partitioning. If too much is taken from the roots then an imbalance can occur. Of course, fertilization and irrigation can make any imbalance tolerable (these treatments also change the root:shoot ratio).  As above the root:shoot ration changes with ontogeny so allometric principles need to be used in analyses particularly if treatments (including variety) alter overall growth rate.   

Respected Kurt Henry Sir Thanks for your answer, Kindly explain the mechanism behind the difference in root shoot ratio in crops irrigated with different concentrations of waste water.

More below ground resources increases allocation to aboveground resources

to know biomass of plant

Yes , greater  root biomass will ensure consequently greater above-ground biomass....

Root:shoot ratio value determine according to plant conditions. root:shoot ratio for example will high under water stress due the plant response to water stress by elongation root and at same time high root:shoot ration reflect lower P availability and other nutrients.

there different explanation for higher root:soot ration.

I also should say that the R:S ratio changes as the plant grows. So a bigger plant will have a different root shhot ratio than a smaller plant. Thus differences in R:S ratios might be deceiving. You need to use allometry to analyze the data.

i have studies on heavy metal stress and the response of cereal crops.

Anyone can explain that when we calculate the root to shoot ratio we should take which parameter , i means to calculate the root to shoot ratio regarding heavy metals or regarding biomass root to shoot ratio? which one is best to put in my data.

thanks

Shakir, r to s ratio changes with plant size. Plot ln r:s x ln biomass to give you the correct results.

Kurt Henry Johnsen thank you sir, but i have found some contrast between the statment of different articles ,some says with incresasing heavy metals the biomass R/S decreases while some says that with increasing conc of HM biomass root to shoot ratio increases. which is the right statement ?

thanks .

I have also same question like Shakir Ullah I am also working on heavy metals, I have three factors, heavy metal stress, pH and different cultivars. the results which I got for root shoot ratio are quit complex. In one of the heavy metal stress root shoot dry weight ratio and root shoot length ratio is increasing but at the same time in another heavy metal stress it is decreasing. while if we talk about pH ranges combined with heavy metal, same is the case like heavy metal stress. Root shoot ratio is different among heavy metal stress under pH ranges.

Can somebody explain the root shoot ratio according to my study