(a) What do you understand by water transport in xylem ? Explain the Cohesion-Tension theory. (20 marks) (b) How is the detailed…

Water Transport and Pedigree Selection in Plant Breeding

Water transport in xylem is the upward movement of water from roots to leaves, driven by transpiration. This process is best explained by the Cohesion-Tension theory, proposed by Dixon and Joly. The mechanism relies on three physical properties: cohesion, adhesion, and tension. Cohesion refers to the hydrogen bonding between water molecules, allowing them to form a continuous column. Adhesion is the attraction between water molecules and the hydrophilic walls of the xylem vessels, which helps maintain the column's integrity against gravitational pull. Transpiration at the leaf surface creates a negative pressure, or tension, within the xylem. This tension pulls the cohesive water column upwards. Evidence for this theory includes pressure bomb experiments, which demonstrate that xylem sap is under tension, and the observation that water columns break (cavitate) when tension exceeds the cohesive strength, a risk mitigated by the small diameter of xylem conduits and strong cohesion rather than adhesion alone. This efficient hydraulic system ensures that plants maintain turgor and metabolic functions, which is critical for the development of high-quality seeds in breeding programs.

Pedigree Selection Method

Pedigree selection is a method of plant breeding where individual plants are selected based on their performance and their progeny are tested over multiple generations. The core of this method is the maintenance of detailed records linking each selected plant to its descendants. This record-keeping ensures that the genetic history of every line is traceable, allowing breeders to identify superior genotypes with precision.

The procedure involves several stages. Initially, a cross is made between two parents to produce the F1 generation. The F1 plants are self-pollinated to produce the F2 generation, where segregation begins. From the F2, individual plants are selected based on desired traits. These selected plants are then harvested separately, and their progenies are grown in the next generation (F3). This process of selecting individual plants and growing their progenies in separate rows continues until the F6 or F7 generation. At each stage, records are maintained using a specific notation system, often involving plant-to-row and head-to-row identifiers, to track which progeny came from which parent.

A simplified schematic of this process is as follows:

P: Parent A x Parent B ↓ F1: Uniform hybrid (Selfed) ↓ F2: Segregation occurs; Select individual plants (P1, P2, P3...) ↓ F3: Progeny rows of P1, P2, P3...; Select best rows ↓ F4: Progeny of selected F3 plants; Continue selection ↓ ... ↓ F6/F7: Uniform lines; Yield trials

This systematic recording allows for the identification of stable, superior lines. The integration of water stress tolerance traits, discussed in part (a), is often a key selection criterion in these progeny tests, ensuring that the final varieties are not only high-yielding but also resilient.

Indian Seeds Act, 1966 and Industry Activities

The Indian Seeds Act, 1966, was enacted to regulate the quality of certain seeds and to ensure that seeds sold in the market meet specified standards. The Act establishes the Central Seed Committee and the Central Seed Laboratory, which are responsible for setting standards and testing seeds, respectively. It mandates the certification of seeds by authorized agencies to ensure that they meet minimum requirements for germination, purity, and viability for notified varieties. Certification is a voluntary process for growers, but it is essential for the sale of certified seeds. The Act also provides for the registration of seed varieties and the enforcement of truth-in-labeling, ensuring that the information on seed packets is accurate.

For the success of the seed industry in India, several essential activities are required. First, there must be a robust system for the production of breeder seed, which is the highest genetic purity seed used to produce foundation seed. Second, the multiplication of foundation seed into certified seed must be efficient and widely accessible. Third, adequate infrastructure for seed testing is necessary to verify quality parameters. Finally, strong linkages between farmers and extension services are crucial to ensure that farmers have access to quality seeds and the knowledge to use them effectively. The Seed Policy of 2002 further supported this by encouraging private sector participation in hybrid seed development, thereby enhancing the availability and variety of seeds in the market.

In conclusion, the physiological efficiency of water transport underpins the viability of breeding programs, while the pedigree selection method provides the genetic framework for developing superior varieties. The regulatory framework of the Seeds Act, 1966, ensures that these varieties are of high quality and reliably labeled, supporting the overall growth of the Indian seed industry.