Water Management in Horticultural Crops (1+1)

1. Plant Criteria

• Plants show up certain characteristic changes in their constitution, appearance and growth behavior with changes in available soil water and atmospheric conditions.
  
• Different plant criteria considered to schedule irrigation are presented below:-
1.1.Plant appearance

• With water stress, some characteristic changes usually occur in the general appearances of plants.
  
• There may be changes in the normal colour of plant or distortions of plants such as wilting or drooping of plants and curling or rolling of leaves.
  
• Some crops like leafy vegetables are very sensitive to soil-water changes and develop scarcity symptoms easily, while others do not. Changes in colour appear first in the lower leaves.
  
• Water stress is also shown by temporary wilting of plants, as with sugar beet during the hottest part of the day.
  
• Fruit plants do not easily show up water stress by changes in appearance until serious retardation in growth takes place. However an experienced orchardist can detect early signs of stress by the appearance of the foliage especially during the period of peak transpiration demand.
  
• Young leaves are the most sensitive part in this regard.
  
• This technique is however quite simple and rapid, but suffers from many deficiencies. Changes in colour may be misleading since nutritional disorder, insect damage, disease attack and varietal character cause variable changes in foliage colour.
1.2.Plant water potential and water content

• Some crops show strong correlation between the water content of leaf or leaf sheath and the available soil water.
  
• The relative leaf water content (RLWC) and leaf water potential change with variations in soil water availability or owing to lag between water absorption by plants and evaporative demand of the atmosphere.
  
• Adverse physiological and growth phenomena specific to plant species have been reported with fall in the RLWC and water potential below certain critical limits.
  
• As mentioned in the 3rd schedule a drop of 8-10% moisture( -5 to -6 bars of leaf water potential)causes a mild stress and crop is to be irrigated before the critical RLWC is reached.
  
• The RLWC and Leaf Water Potential (LWP) values for the individual crops and their stages are to be standardized for scheduling irrigation. However, sophisticated equipment, intricate measuring devices, high cost and lack of proper standardization of instruments deter the use of this technique on a large scale.
1.3. Plant growth

• Cell elongation is considered as the growth process that suffers first with water stress in plant.
  
• Subsequently, retardation in growth of height or internodal length occurs.
  
• Timing of irrigation can be set as and when the normal growth rate is observed to decline.
  
• This is, however, possible in places where a continuous measurement of plant growth is maintained.
  
• This technique offers many difficulties owing to unavailability and high costs of equipment and so on.
  
• The serious objection to this approach of scheduling irrigation is that the plants suffer before they show any retardation in growth processes.
1.4. Critical crop stages of water need

• Irrigation scheduling may be decided based on stages of growth more conveniently in crops in which the physiological stages are distinct to locate the critical periods of water need. Scheduling of irrigation based on these critical stages is most convenient for ordinary Indian farmers who may need, at the most, some guidance or education initially. However, it may be a little difficult in crops where stages are not so well defined.
1.5. Indicator plant

• There are some plants sensitive to soil-water variations.
  
• They may be used for detecting the water stress in crops that do not show symptoms of water stress easily or exhibit the same when they have already suffered seriously.
  
• Sunflower plants are often used as indicator plants in onion crop.
  
• An indicator plant for irrigation should be such that it shows the water stress before the crop has suffered from it.
  
• When an indictor plant is grown in a crop field, care should be taken not to shade the plant by crop plants.
1.6. Stomatal aperture and Leaf diffusion resistance

• Opening of stomata in plants is regulated by soil water availability.
  
• Stomata remain fully open when the supply of water is adequate, whereas they start closing with scarcity of water in soils to restrict the transpiration.
  
• Water deficit in plants is directly related to availability of soil water and that may be used for scheduling irrigation in crops.
• A close relationship exists between leaf diffusion resistance (LDR) and plant water stress. LDR is a sensitive index of internal water balance in the mild to moderate stress range and holds a promise for scheduling irrigation.
1.7. Plant temperature

• Solar radiation received on earth heats up leaf tissues besides causing evapotranspiration and heating up the ambient air.
• With water deficit in plant the temperature of leaf tissues rises.
  
• Many investigations have shown that leaf or canopy temperature is a sensitive index of plant water status.