Relative humidity (RH)

Relative humidity is a term used to describe the amount of water vapor that exists in a gaseous mixture of air and water vapor, compared to the upper limit of what it could be at the same temperature and bulk pressure.

also means  the ratio of actual water vapour content to the saturated water vapour content at a given temperature and pressure expressed in percentage (%).


Effect of Relative Humidity on Crop Production
Leaf Growth





1.Leaf growth not only depends on synthetic activities resulting from biochemical process but also upon the physical process of cell enlargement.


2.Cell enlargement occurs as a result of turgor pressure developed within the cells.


3.Turgor pressure is high under RH due to less transpiration. Thus leaf enlargement is high in humid areas


Photosynthesis




1.Photosynthesis is indirectly affected by RH. When RH is low, transpiration increases causing water deficits in the plant.


2.Water deficits cause partial or full closure of stomata and increase mesophyll resistance blocking entry of carbon dioxide.


Pollination


1.Moderately low air humidity is favourable for seed set in many crops, provided soil moisture supply is adequate.


For example, seed set in wheat was high at 60 per cent RH compared to 80 per cent when water availability in the soil was not limiting.




1.At high RH pollen may not be dispersed from the anthers


Pests


1.The incidence of insect pests and diseases is high under high humidity conditions.


2.High RH favours easy germination of fungal spores on plant leaves.


For example


The blight diseases of potato and tea spread more rapidly under humid conditions. Several insects such as aphids and jassids thrive better under moist conditions.




Grain Yield




Very high or very low RH is not conducive for high grain yield. Under high humidity, RH is negatively correlated with grain yield of maize. The yield reduction was 144 kg/ha with an increase in one per cent of mean monthly RH. Similarly, wheat grain yield is reduced in high RH. It can be attributed to adverse effect of RH on pollination and high incidence of pests. On the contrary, increase in RH during panicle initiation to maturity increased grain yield of sorghum under low humidity conditions due to favourable influence of RH on water relations of plants and photosynthesis. With similar amount of solar radiation, crops that are grown with irrigation gives less yield compared to those grown with equal amount of 'water as rainfall. This is because the dry atmosphere, which is little affected by irrigation, independently suppresses the growth of crops.




Very High Relative humidity:


•Reduces evapotranspiration


•Increases heat load of plants


•Stomatal closure


•Reduced CO2 uptake


•Reduced transpiration influences translocation of food materials and nutrients.


•Moderately high RH of 60-70% is beneficial.


•Low RH increases the evapotranspiration