Question

value of RQ in succulents is (a) unity (b) infinite 1d less than unity (d) zero.

Short answer

The value of RQ in succulents is less than unity (c).

Step-by-step solution

Understanding RQ

The Respiratory Quotient (RQ) or Respiratory Ratio is a dimensionless number used in calculations of basal metabolic rate (BMR) when estimated from carbon dioxide production. It is used in plant physiology as well to indicate the type of respiratory substrate being used. The RQ is defined as the volume of CO2 evolved divided by the volume of O2 consumed while the substrate is being metabolized: RQ = (CO2 evolved)/(O2 consumed).

Identifying the Metabolic Process in Succulents

Succulents typically use the CAM (Crassulacean Acid Metabolism) pathway for respiration. In this pathway, CO2 is taken in during the night and is fixed into organic acids. During the day, these acids break down to release CO2 for photosynthesis in closed stomata, which minimizes water loss. As a result, the O2 is not consumed at the same time as CO2 is evolved.

Determining the RQ Value for Succulents

Given that succulents separate the timing of CO2 intake and O2 use, the respiratory quotient (RQ) is not a conventional measure since the CO2 evolved is not directly tied to O2 consumption in this same timeframe. However, the metabolic process still tends toward production of sugars (like in photosynthesis) and thus consumes more CO2 than O2 over time, leading to an RQ less than unity.

Key concepts

Basal Metabolic Rate (BMR)

Basal Metabolic Rate, or BMR, is a measure of the amount of energy expended while at rest in a neutrally temperate environment. In the context of human physiology, it is the rate at which the body uses energy for basic life functions like maintaining body temperature, repairing cells, and pumping blood. However, the concept of BMR extends beyond human biology and has applications in studying plant metabolism as well.

When considering plants, BMR can refer to the energy usage of plants for maintaining cellular activities and growth. It's crucial to note that while plants do not have circulatory or respiratory systems comparable to animals, they still undergo metabolic processes that demand energy. Photosynthesis and cellular respiration are the primary means by which plants produce and use energy, respectively, and both are influenced by environmental conditions, just as BMR is in animals.

Plant physiologists use measures like the Respiratory Quotient (RQ) to understand these processes better. An optimal BMR indicates a balance in a plant's energy production and consumption, essential for healthy growth and development.

Crassulacean Acid Metabolism (CAM)

Crassulacean Acid Metabolism, or CAM, is a carbon fixation pathway that evolved in some plants as an adaptation to arid conditions. The primary characteristic of CAM plants is their ability to fix carbon dioxide at night when the environmental conditions are less harsh, and the stomata are open to minimize water loss.

Stages of CAM

At night, CAM plants take in CO2 and convert it into various organic acids, storing them in vacuoles. Then, during the day, when the stomata are closed to conserve water, these acids are broken down, and the CO2 released is used for photosynthesis, even without external carbon intake.

This remarkable adaptation allows these plants, including succulents, to thrive in environments where water is scarce. Because CAM plants only open their stomata at night (when evaporation rates are lower), they can conserve water much better than their non-CAM counterparts. However, this unique mechanism also means that traditional measures of respiration, like the Respiratory Quotient (RQ), are less straightforward in CAM plants, as CO2 release does not occur simultaneously with O2 consumption.

Plant Physiology

Plant physiology encompasses the study of various functions and processes within plant organisms. It is a branch of botany that focuses on understanding how plants control and coordinate activities like photosynthesis, nutrient uptake, respiration, and growth responses to environmental stimuli.

In terms of respiratory efficiency, plants use unique pathways to manage energy production and resource use, with examples such as the CAM pathway mentioned previously. Not all plants have the same respiratory patterns; some, like those found in wet, shaded forests, rely on continuous gas exchange, while others, like succulents, have adapted to harsh, dry conditions by evolving specialized metabolic pathways.

Understanding plant physiology, especially metabolic rates and gas exchange, is essential for agriculture, horticulture, and conservation. Measures such as the Respiratory Quotient (RQ) can give insights into the metabolic activities occurring within the plant. By examining these processes, researchers and gardeners can better cultivate plants by creating optimal conditions that align with their physiological needs, whether that means adjusting lighting, water availability, or soil composition.