Mechanism of phloem transport | Botany Optional for UPSC PDF Download

Introduction

Mineral nutrients are essential for the growth and development of plants. They play a vital role in various physiological processes, including the synthesis of chlorophyll, regulation of pH, and enhancement of protein synthesis. In this article, we will explore the different types of mineral nutrient transportation within plants, with a special focus on the intriguing mechanism known as phloem transport.

Mineral Nutrient Transportation

Mineral absorption in plants is a natural process that mirrors the uptake of water. Plants acquire these essential minerals through both their roots and leaves. However, the transportation of mineral nutrients occurs through distinct methods, namely Active Transport and Passive Transport.

Types of Transportation

Plant cells carry out the process of mineral nutrient uptake or transport through two distinct methods: Active Transport and Passive Transport.

1. Active Transport: Active transport involves the utilization of metabolic energy by plant cells for the transfer of nutrients. This method is essential for moving ions both into and out of plant cells. There are various ways through which minerals can be transported to plants.

• Root Interception:
  • Soil aggregates consist of soil particles of different sizes, which contain essential nutrients.
  • Many plants cannot access the extensive surface area of these soil aggregates directly.
  • Instead of penetrating the aggregates, plant roots tend to grow around them.
  • Consequently, only a limited number of minerals can directly reach root hairs.
  • Root interception is not the most efficient method of nutrient transport.
• Mass Flow:
  • Transpiration is the process through which plants lose water vapor from their leaves.
  • To compensate for this loss, plant roots absorb and take up water from the soil.
  • Soil water contains three negatively charged ions: sulphate, nitrate, and borate.
  • However, roots are unable to assimilate all the water in the soil, resulting in limited contact between these ions and the root surface.
  • As a result, the quantity of these three nutrients reaching the root surface via mass flow can vary.

• Diffusion:
  • Plant physiologists have observed that the root surface of plants has a lower nutrient concentration compared to the rest of the plant.
  • Some positively charged nutrients like Ca++, K+, and Mg++ are also present in soil aggregates.
  • Soil aggregates have a higher concentration of these nutrients on their surface.
  • The diffusion process allows these ions to move towards the roots.
  • Plants rely on the absorption of mineral ions against a concentration gradient to sustain growth.

2. Passive Transport: In contrast, passive transport is a method of nutrient absorption that occurs without the expenditure of metabolic energy.

Transport of Phloem

•  Apart from the two methods mentioned earlier, there is another mechanism for moving nutrients and food materials from leaves to different parts of plants. Leaves are responsible for synthesizing food, which is then transported to other plant cells using a specialized tissue called Phloem. 
• This transportation primarily involves the transfer of sucrose. The movement of nutrients from the source to the sink is facilitated through the Phloem tissues. It's worth noting that phloem transport can occur in both directions, making it bidirectional.
• For instance, during the early spring, nutrients flow upward from the roots to the developing buds. Minerals absorbed by the roots follow a network of tube-like capillaries to reach the leaves.

Importance of Mineral Nutrients

Plants need both macro and micronutrients for various important reasons:

• Maintain Osmotic Pressure Balance: Nutrients play a crucial role in regulating osmotic pressure within plant cells, ensuring the right balance of water and solutes.
• Assist in Chlorophyll Synthesis: Nutrients are essential for the synthesis of chlorophyll, the green pigment crucial for photosynthesis.
• Increase Nutritional Value: Nutrients contribute to the enhancement of the nutritional value of fruits and seeds, making them more beneficial for consumption.
• Promote Growth and Development: Nutrients are vital for overall plant growth and development, from roots to shoots.
• Regulate Root Sap pH: They help in maintaining the pH balance of root saps, ensuring optimal conditions for nutrient absorption.
• Enhance Protein Synthesis: Nutrients are involved in protein synthesis within plants.
• Promote Root Development and Fruit Ripening: Nutrients also play a role in encouraging root growth and the ripening of fruits.