Enumerate the salient features of water relations in crop plants.

I. Water Absorption by Roots

Water absorption primarily occurs through the roots, specifically via root hairs, which significantly increase the surface area for absorption. This process is governed by the water potential gradient between the soil and the root cells.

• Osmosis: Water moves from the soil (higher water potential) to the root cells (lower water potential) via osmosis.
• Apoplast and Symplast Pathways: Water travels through the root cortex via two main pathways:
  • Apoplast Pathway: Movement through the cell walls and intercellular spaces. This pathway is blocked by the Casparian strip in the endodermis, forcing water to enter the symplast.
  • Symplast Pathway: Movement through the cytoplasm of cells via plasmodesmata.
• Root Pressure: In some plants, particularly smaller ones, root pressure can contribute to water movement, especially when transpiration rates are low.

II. Water Transport in Plants

Once absorbed, water is transported upwards through the xylem vessels, driven primarily by the transpiration pull.

• Transpiration Pull: The evaporation of water from leaves creates a negative pressure (tension) in the xylem, pulling water upwards from the roots. This is the primary driving force for water transport.
• Cohesion-Tension Theory: Water molecules are cohesive (attracted to each other) due to hydrogen bonding, forming a continuous column in the xylem. This cohesion, combined with the tension created by transpiration, allows for efficient water transport.
• Adhesion: Water molecules also adhere to the xylem walls, further aiding in upward movement.
• Xylem Structure: The structure of xylem vessels – narrow diameter and lignified walls – contributes to efficient water transport and prevents collapse under tension.

III. Water Loss: Transpiration

Transpiration is the loss of water vapor from plant surfaces, primarily through stomata on leaves. It is an essential process for cooling the plant and facilitating nutrient transport, but also represents a significant water loss.

• Stomatal Regulation: Stomata are regulated by guard cells, which control the opening and closing of the pores. Factors influencing stomatal opening include:
  • Light: Generally, stomata open in the presence of light.
  • CO₂ Concentration: Low CO₂ concentration inside the leaf promotes stomatal opening.
  • Water Availability: Water stress causes stomata to close, reducing transpiration.
  • Abscisic Acid (ABA): This plant hormone promotes stomatal closure during water stress.
• Cuticular Transpiration: A small amount of water is lost through the cuticle, the waxy layer on the leaf surface.
• Lenticular Transpiration: Water loss also occurs through lenticels, pores on the stems of woody plants.

IV. Adaptations in Crop Plants

Crop plants exhibit various adaptations to regulate water relations and cope with water stress.

• Deep Root Systems: Plants like alfalfa have deep root systems to access water from lower soil layers.
• Reduced Leaf Area: Plants in arid environments often have smaller leaves or modified leaves (e.g., spines) to reduce transpiration.
• Thick Cuticle: A thick cuticle reduces cuticular transpiration.
• Stomatal Density and Distribution: Some plants have fewer stomata or stomata located in protected pits.
• C4 and CAM Photosynthesis: These photosynthetic pathways are more water-efficient than C3 photosynthesis, as they allow plants to fix CO₂ with reduced stomatal opening. (e.g. Maize, Sugarcane, Pineapple)