Factors Affecting Herbicides Absorption and Translocation
1) Plant Factors:
a) Branching Habit:
Most broad-leaf species have open or horizontal branching habit with expanded leaves and exposed growing regions or apical buds. This facilities retention of spray droplets and easy surface coverage. Whereas , grass species have minutely ridged surface on the leaves. They are vertically arranged and growing regions are enclosed by sheaths ( or bases of older leaves) which serve as protective cover. Therefore, non-polar herbicides are more effective against grasses as they tend to have greater surface coverage.
b) Plant Surface:
The surface of leaves and stems are commonly covered by waxy and fatty substances. Hence , aqueous sprays which are polar are repelled by most plant surface , their droplets are spherical and tend to run-off. This is especially of aqueous sprays on grasses. This problems can be solved by the addition of wetting agents (Surfractants) to the sprays resulting in lowered surface tension and better spread. Or non polar herbicides are effective in such weeds.
c) Plant Maturity or Age of the Plant:
Herbicides move faster in young plants with green and succulent leaves and stem than in the older plants with hard cuticle. Rapid root and shoot growth or active period of growth of the plant also favour rapid herbicide absorption and translocation.
d) Plant Species and Varieties:
Some plant species and some cultivars within species show difference in absorption and translocation of herbicides. E. g Cotton is moderately , tolerant species to prometryn while soybean is sensitive.
2) Environmental Factors:
a) Temperature and Humidity:
Plant species and varieties vary greatly in their requirement of optimum temperature and humidity. These factors affect the transpiration , respiration and evaporation of water, which is directly concerned with rate of absorption and translocation of herbicides. Therefore, very high as well as very low temperatures and humidity affect the absorption and translocated of herbicides.
b) Rainfall:
Rain occurring soon after application of foliage applied herbicides reduce the effectiveness of herbicides due to washing away of spray deposits on the foliage. Polar herbicides like dalapon, 2,4-D salts etc. which are highly soluble in water and penetrate leaf surfaces very slowly can be washed away by rain occurring within 4 to 6 hrs. of application.
The absorption of herbicides like paraquat, 2,4-D ester which penetrate very rapidly is not seriously affected in most species by rain occurring even within one hour of application. Generally herbicides formulated in oil or oil emulsions are less affected by rains than the aqueous solutions. Heavy rains after application of herbicides may cause leaching and runoff losses of herbicides.
c) Wind:
Wind has direct effect on evapotranspiration. Wind also causes rapid drying of spray solution on the foliage as well as uniform applications of herbicide resulting in reduce absorption.
d) Light:
Sunlight assists herbicide penetration by stimulation stomatal opening. Light also activates photosynthesis and increases movement of food material and herbicides from the leaves to roots and other parts of the plant.
3) Soil Factors:
a) Soil Water and Temperature:
Soil water stress (Inadequate moisture) and temperature affect the absorption of herbicides by roots and foliage and also subsequent movement in the plant body. The excess moisture or heavy irrigation may cause leaching and runoff losses of herbicides. Therefore, presence of adequate moisture in the root zone of the crop is essential for proper absorption and translocation of herbicides. Water stress may also lead to increased thickness of cuticle which results in reduced entry and translocation of foliage applied herbicides.
b) Soil PH:
Soil PH has profound effects on the uptake of herbicides by roots. Change in soil PH affect the cation exchange capacity of soils.
c) Soil Organic Matter and Clay Type:
Soils with high organic matter and montmarillonite clay has high cation exchange capacity which affects the absorption and translocation of herbicides therefore, these soil require more dose of herbicides.
4) Chemical Factors:
Chemical structure or molecular structure of herbicide and other character of the herbicide may affect the absorption and translocation of herbicides. Therefore, herbicides differ in their weed control efficiency under different environmental conditions in which plants are grown.
E. g: Ester form of 2,4-D is rapidly absorbed by the foliage than the sodium salt.
Use of suitable adjuvants also increase the efficiency of herbicide by increasing its absorption and translocation in the different plant parts.
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