Tag: mineral absorption

Stomata are usually confined in more number on the lower sides of the leaves. The lower surface is called as abaxial surface and the upper surface is called as the adaxial surface. Hence, maximum percentage i.e., 80- 95% of transpiration occurs from the lower surface.

Passive absorption is absorption by the pull from transpiration without the active involvement of root cells, The shoots and leaves of the plant experience transpiration due to evaporation of water from their surface.

Water is required to generate the reducing power, which is used to reduce carbon dioxide during photosynthesis. The water molecules are split up and oxygen released as a by-product. The protons are used to synthesize reducing power. Now all the water that is absorbed is never used up. In fact, the water that is actually used is a small fraction of water that is absorbed. Hence, transpiration ratio is calculated to find out the effectiveness of stomata in maximizing photosynthesis and minimizing loss of water.

According to transpiration pull theory, due to transpiration, the water column inside the plant comes under tension. This is called 'transpiration pull'. On account of this tension, the water column is pulled up passively from below to top of the plant(almost like a rope). A tension of one atmosphere is sufficient to pull water to a height of about $20$ meters.

Loss of water from the leaves causes water stress to develop from the top of the trees which forces more and more water to rise from the roots to the leaves. The column of the water is maintained due to cohesion and adhesion forces and the water moves up due to transpiration pull.

An action spectrum is a graph of the rate of a physiological activity plotted against the wavelengths of light. it shows which wavelengths of light is most effectively used in a specific reaction. In photosynthesis, chlorophyll is much more efficient at the blue and red wavelengths of light to carry out photosynthesis. Since transpiration is indirectly related to photosynthesis, its action spectrum would be similar to photosynthesis.

Translocation of organic nutrients from the region of source or supply to the region of sink or utilisation is mostly in the form of sucrose. Phloem (sieve tubes or sieve cells) is the pathway for this translocation, as found put by stem girdling, sieve tube puncturing, radioautography and chemical analysis of sieve tube sap.

• The cohesion-tension theory is a theory of intermolecular attraction that explains the process of water flow upwards (against the force of gravity) through the xylem of plants. 
• Transpiration pull, utilizing capillary action and the inherent surface tension of water, is the primary mechanism of water movement in plants. 
• Hence, The cohesion transpiration pull theory operates only in Conditions favouring transpiration.
• So, the correct answer is 'Conditions favouring transpiration'.

In vascular plants, water and minerals move through xylem, whereas sugars move through phloem. Ringing experiments prove that sugars move through phloem. If the phloem tissue is removed in a ring or girdle, the sugars cannot be transported to roots from aerial parts and plant dies. 

Dixon explained ascent of sap in trees based on transpiration, cohesion, adhesion theory. Mac Dougal supported transpiration pull theory through experimental proof. Cohesive force and adhesive force works continuously in the cavity (lumen) of xylem. Both the forces are responsible for maintaining unbroken continuity of water column from the roots to the leaves.