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Agrometeorology :: Relative Humidity and PlantGrowth

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Relative Humidity

Relative humidity: It is the ratio of actual water vapour content to the saturated water vapourcontent at a given temperature and pressure expressed in percentage (%).

Diurnal variation in relative humidity:

Mean maximum relative humidity occurs in the early morning.1.Mean minimum, relative humidity occurs in the early afternoon.2.Low RH in the afternoon is due to expansion of air and thus increases the total watervapour capacity

3.

Distribution of RH

Maximum RH is in the equatorial region due to high evaporation.1.Decreases towards poles upto 30° N and S due to subsiding air mass.2.RH increases in poles due to low temperature.3.

Effect of Relative Humidity on Crop Production

Relative humidity (RH) directly influences the water relations of plant and indirectly affects leafgrowth, photosynthesis, pollination, occurrence of diseases and finally economic yield.

The dryness of the atmosphere as represented by saturation deficit (100-RH) reduces dry matterproduction through stomatal control and leaf water potential.

Leaf Growth

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

1.

Cell enlargement occurs as a result of turgor pressure developed within the cells.2.Turgor pressure is high under RH due to less transpiration. Thus leaf enlargement is high inhumid areas

3.

Photosynthesis

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

1.

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

2.

Pollination

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

1.

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

At high RH pollen may not be dispersed from the anthers1.

Pests

The incidence of insect pests and diseases is high under high humidity conditions.1.High RH favours easy germination of fungal spores on plant leaves.2.

For exampleThe blight diseases of potato and tea spread more rapidly under humid conditions. Severalinsects 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 isnegatively correlated with grain yield of maize. The yield reduction was 144 kg/ha with anincrease in one per cent of mean monthly RH. Similarly, wheat grain yield is reduced in high RH. Itcan be attributed to adverse effect of RH on pollination and high incidence of pests. On thecontrary, increase in RH during panicle initiation to maturity increased grain yield of sorghumunder low humidity conditions due to favourable influence of RH on water relations of plants andphotosynthesis. With similar amount of solar radiation, crops that are grown with irrigation givesless yield compared to those grown with equal amount of 'water as rainfall. This is because thedry atmosphere, which is little affected by irrigation, independently suppresses the growth ofcrops.

Very High Relative humidity:

Reduces evapotranspirationIncreases heat load of plantsStomatal closureReduced CO2 uptakeReduced transpiration influences translocation of food materials and nutrients.Moderately high RH of 60-70% is beneficial.

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Agrometeorology :: Relative Humidity and Plant Growth http://agritech.tnau.ac.in/agriculture/agri_agrometeorology_relativehumi...

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Low RH increases the evapotranspiration

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Agrometeorology :: Relative Humidity and Plant Growth http://agritech.tnau.ac.in/agriculture/agri_agrometeorology_relativehumi...

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