Plant Physiology

Step 1: Understanding the Concept:
The question asks to identify a nitrogen-fixing bacterium that lives in a symbiotic relationship with sugarcane. This type of symbiosis, where the microbe lives inside the plant tissues without causing disease, is called an endophytic relationship.
Step 2: Detailed Explanation:
Let's analyze the options:

Frankia: This is a genus of nitrogen-fixing, filamentous bacteria (actinomycetes) that forms root nodules on actinorhizal plants, such as alder ({Alnus}) and casuarina. It is not associated with sugarcane.

Acetobacter: A specific species, {Gluconacetobacter diazotrophicus} (formerly in the {Acetobacter} genus), is a well-known endophytic bacterium that lives within the tissues of sugarcane. It can fix significant amounts of atmospheric nitrogen, contributing to the plant's nitrogen needs.

Anabaena: This is a genus of filamentous cyanobacteria. It is famous for its symbiotic relationship with the aquatic fern {Azolla}, where it fixes nitrogen within the fern's leaves. It is not associated with sugarcane.

Nostoc: This is another genus of cyanobacteria that can be free-living or form symbiotic relationships with fungi (to form lichens), bryophytes, and some vascular plants like cycads (in their coralloid roots). It is not the primary symbiont of sugarcane.

Step 3: Final Answer:
Based on established symbiotic relationships, {Acetobacter} (specifically {Gluconacetobacter}) is the correct nitrogen-fixing symbiont associated with sugarcane.

Step 1: Understanding the Concept:
The question asks for the specific biological term that describes the effect of light on seed germination.
Step 2: Detailed Explanation:
Let's analyze the given terms:

Phototropism: This is the directional growth of a plant in response to a light stimulus. For example, a plant stem growing towards a light source. It relates to growth, not germination.

Photoblasty: This term specifically refers to the response of seeds to the presence or absence of light for germination. Seeds that are stimulated to germinate by light are called positively photoblastic (e.g., lettuce), while those whose germination is inhibited by light are negatively photoblastic (e.g., onion). This perfectly matches the question's description.

Photonasty: This is a non-directional movement of plant parts (like leaves or petals) in response to changes in light intensity. For example, the opening and closing of some flowers during the day and night.

Nyctinasty: This is a nastic movement related to the diurnal cycle (day and night), often called "sleep movements." For example, the folding of leaves of a leguminous plant in the evening.

Step 3: Final Answer:
The correct term for the requirement of sunlight for seed germination is photoblasty.

Step 1: Understanding the Concept:
The question asks for a primary role of Gibberellic Acid (GA), which is one of the five major classes of plant hormones.
Step 2: Detailed Explanation:
Gibberellins are a group of plant hormones with a wide range of functions, but their most prominent effects are related to growth and development.

Stem Elongation: This is the most dramatic and well-known effect of GA. It stimulates internodal elongation in stems, causing plants (especially genetic dwarf varieties) to grow tall. This effect is achieved by stimulating both cell division in the apical meristem and the subsequent elongation of those cells.
Seed Germination: GAs break seed dormancy and promote germination by stimulating the production of hydrolytic enzymes (like -amylase) in the aleurone layer of cereal grains.
Flowering and Fruit Development: GAs can induce flowering in some plants (bolting in long-day plants) and are involved in fruit set and growth.
Let's evaluate the given options:

(A) It promotes cell division and elongation in stem tissues: This accurately describes the primary mechanism by which gibberellins cause stem elongation.
(B) It increases the levels of anti-oxidants: While hormonal changes can affect a plant's stress response, this is not a primary, defining role of GA.
(C) It decreases the levels of plant growth hormones: GA is itself a major plant growth hormone and often acts synergistically with others, like auxins. This statement is incorrect.
(D) It inhibits the synthesis of plant secondary metabolites: This is incorrect. The biosynthesis of gibberellins is itself a complex metabolic pathway, and its effects on other secondary metabolites are varied, not simply inhibitory.
Step 3: Final Answer:
The most accurate and primary role of Gibberellic acid listed is the promotion of cell division and elongation in stem tissues.

Step 1: Understanding the Concept:
The question asks to identify a facultative CAM plant. Crassulacean Acid Metabolism (CAM) is a photosynthetic adaptation to arid conditions. Obligate CAM plants use this pathway all the time. Facultative CAM plants, however, can switch between C₃ photosynthesis and CAM photosynthesis in response to environmental cues like drought or salinity.
Step 2: Detailed Explanation:
Let's analyze the photosynthetic pathways of the given plants:

(A) {Mesembryanthemum crystallinum (Common ice plant):} This is the classic textbook example of a facultative CAM plant. It performs C₃ photosynthesis when water is abundant but switches to the CAM pathway under drought or salt stress.

(B) {Opuntia (Prickly pear cactus):} This is an obligate CAM plant. It is a succulent that lives in arid environments and always uses the CAM pathway to conserve water.

(C) {Chrysanthemum:} This is a typical C₃ plant. It does not perform CAM photosynthesis.

(D) {Amaranthus edulis:} This is a C₄ plant, characterized by Kranz anatomy. It does not perform CAM photosynthesis.

Step 3: Final Answer:
{Mesembryanthemum crystallinum} is a known facultative CAM plant that uses C₃ metabolism in unstressed conditions and switches to CAM under stress.

Step 1: Understanding the Concept:
The question asks for another name for flooding stress in plants. Flooding stress refers to the adverse conditions experienced by a plant when its root zone is saturated with water for an extended period.
Step 2: Detailed Explanation:
When soil is flooded, the water displaces the air present in the soil pores. Plant roots require oxygen for cellular respiration to produce the ATP needed for nutrient uptake and other metabolic activities.

The lack of air in waterlogged soil leads to a condition of low oxygen (hypoxia) or no oxygen (anoxia) for the roots.
This directly translates to an "oxygen deficient stress" because the primary problem for the plant roots is the inability to respire aerobically.
Let's analyze the other options:

(A) Chilling stress: Caused by exposure to low, non-freezing temperatures. This is unrelated to flooding.

(B) Reactive oxygen species (ROS): ROS are harmful molecules produced as a byproduct of metabolism under various stress conditions, including oxygen deficiency. However, ROS are a {consequence} of the stress, not the stress itself.

(D) Drought stress: This is caused by a lack of water, which is the complete opposite of flooding stress.

Step 3: Final Answer:
The primary physiological challenge imposed by flooding is the lack of oxygen for the root system. Therefore, it is correctly known as oxygen deficient stress.

Step 1: Understanding the Concept:
This question requires matching essential mineral elements with their specific deficiency symptoms in plants.
Step 2: Detailed Explanation:


A. Calcium (Ca): Calcium is a structurally important component of the cell wall (as calcium pectate) and is relatively immobile within the plant. Therefore, deficiency symptoms appear first in young, growing tissues. A classic symptom is the necrosis (death) of young meristematic regions like root tips and young leaves. Thus, A matches with III.

B. Zinc (Zn): Zinc is required for the synthesis of auxin, a plant hormone that promotes cell elongation. Zinc deficiency leads to reduced auxin levels, causing stunted growth with short internodes, resulting in a rosette habit (leaves are clustered in a circular form). Thus, B matches with IV.

C. Manganese (Mn): Manganese is an activator for many enzymes and is essential for the water-splitting step in photosynthesis. Deficiency leads to interveinal chlorosis (yellowing between veins) and the development of necrotic spots. Thus, C matches with I.

D. Nickel (Ni): Nickel is a component of the enzyme urease, which is vital for metabolizing urea into ammonia. In the absence of nickel, urea can accumulate to toxic levels, leading to leaf tip necrosis. Thus, D matches with II.

Step 3: Final Answer:
The correct set of matches is A-III, B-IV, C-I, D-II, which corresponds to option (D).

Concept: Epistasis is the interaction between genes where one gene masks or modifies the expression of another. In dominant epistasis, a dominant allele of one gene masks the effect of another gene.
Step 1: Understanding dominant epistasis.}
In dominant epistasis, the presence of at least one dominant allele (A-) suppresses the expression of another gene (B/b), regardless of its alleles.
Step 2: Determining phenotypic ratio.}
A typical dihybrid cross (AaBb AaBb) under dominant epistasis produces a modified phenotypic ratio:
Step 3: Explanation of ratio.}

• 12 – Individuals with dominant epistatic allele (A-)
• 3 – Individuals with aaB-
• 1 – Individuals with aabb

Concept: In C4 plants, photosynthesis is spatially separated between mesophyll cells and bundle sheath cells. The initial fixation of carbon dioxide (primary carboxylation) occurs in mesophyll cells using a specialized enzyme.
Step 1: Understanding primary carboxylation.}
Primary carboxylation refers to the first step of CO fixation, where CO is incorporated into an organic compound.
Step 2: Identifying the enzyme in mesophyll cells.}
In C4 plants, phosphoenolpyruvate (PEP) combines with CO in mesophyll cells. This reaction is catalyzed by the enzyme PEP carboxylase, forming oxaloacetic acid (OAA).
Step 3: Eliminating other options.}

• RuBisCO – Functions in bundle sheath cells during the Calvin cycle.
• ATP synthase – Involved in ATP formation, not carboxylation.
• NADP reductase – Involved in reduction of NADP , not CO fixation.