In the context of plant water relations, it is indeed the water that actually enters plant roots that counts for plant growth and physiological processes. While rainfall, irrigation, or any form of water application may provide water to the soil, it is the water that is absorbed by the roots that is crucial for plant water uptake and utilization.

Several factors influence how much water enters plant roots, including:

1. Soil moisture: The amount of water available in the soil is a primary determinant of root water uptake. Plants can only absorb water from the soil if it is present in a form that can be taken up by the roots, such as matric water or solution water.

2. Root system characteristics: The extent and distribution of the root system play a significant role in water uptake. Plants with extensive and deep root systems can access water from deeper soil layers, making them more resistant to drought conditions. Root density, root morphology, and root hairs all contribute to the efficiency of water absorption.

3. Root-soil contact: Good root-soil contact is essential for efficient water uptake. Factors that improve root-soil contact, such as proper soil compaction and adequate soil structure, enhance water movement towards the roots.

4. Transpiration: Transpiration, the process of water loss from plant leaves through stomata, generates a negative water potential gradient that helps pull water upward through the stem and into the roots. Higher transpiration rates increase water uptake by the roots.

5. Water potential gradient: The water potential gradient between the soil and the roots influences water movement. Water moves from areas of higher water potential to lower water potential. When the water potential in the soil is higher than that in the roots, water is absorbed by the roots.

6. Root anatomy and physiology: The structure and function of root cells, such as the presence of aquaporins (water channels), also play a role in water uptake. Aquaporins facilitate the movement of water across root cell membranes, enhancing water absorption.

7. Environmental conditions: Factors such as temperature, humidity, and wind can influence transpiration rates and, consequently, water uptake by the roots. High temperatures and low humidity increase transpiration, leading to greater water uptake.

Therefore, while the availability of water in the soil is essential, it is the water that is effectively absorbed and transported within the plant that ultimately contributes to plant growth and health. Understanding the factors that influence root water uptake can help optimize irrigation practices, improve water use efficiency, and ensure adequate water supply for plant growth and productivity.