1. The Essential Role of Water

No environmental factor influences a plant’s structure and function as profoundly as the availability of water. As the botanist E. Warming noted in 1909, “no other influence impresses its mark to such a degree upon the internal and external structures of the plant as does the amount of water present in the air and soil.”

Why Water is So Critical

Water is far more than just a drink for plants; it is the stage on which the chemistry of life unfolds. Its primary roles include:

• A Universal Solvent: Water dissolves the essential minerals and nutrients in the soil and within the plant’s cells, making them available for use.
• A Cellular Medium: It serves as the dispersion medium for the complex colloidal systems (like protoplasm) that make up living cells.
• A Partner to Proteins: Water forms hydration shells around proteins. Think of these shells as custom-fit scaffolding that holds a protein in the exact shape it needs to do its job. If the scaffolding is removed, the protein collapses and stops working.
• A Key Ingredient for Photosynthesis: Water is a raw material in the process that converts light energy into chemical energy.
• A Component in Hydrolysis: It actively participates in hydrolysis, the process of using water to break down complex substances into simpler building blocks.
• A Natural Coolant: Through transpiration (the evaporation of water from leaves), plants can cool themselves and avoid heat damage on hot days.

When Water is Scarce: The Effects of Water Stress

Even a slight reduction in available water can slow or halt plant growth. This condition, known as water stress, has several immediate and damaging consequences.

• Reduced Cell Growth: Water stress affects cell enlargement more than it affects cell division. While cells may continue to divide, they cannot expand in size without adequate turgor pressure, directly stunting the plant’s growth.
• Stomatal Closure: As turgor pressure drops, the stomates (small pores on the leaf surface) close. This closure conserves water but has two negative side effects: it dramatically reduces the intake of CO2, slowing photosynthesis, and it stops transpirational cooling. This means the plant loses its primary method of staying cool, directly connecting water stress to potential heat damage.
• Slower Photosynthesis: A reduction in the water content of leaves generally decreases the rate of photosynthesis, both by limiting CO2 supply and by directly affecting the biochemical processes involved.
• Disrupted Cell Processes: At the molecular level, water stress can prevent the net increase of RNA and slow the increase of DNA. This indicates that the fundamental cellular machinery for growth and multiplication is being impaired.

When Water is in Excess: The Problem of Flooding

Too much water can be as harmful as too little. Flooding lowers the oxygen supply to the roots, forcing plants to rely on anaerobic processes (energy production that doesn’t require oxygen). Plants have evolved different strategies to cope with these conditions:

• Flood-tolerant species tend to accumulate malic acid, an organic acid that can be stored in large quantities without causing cellular damage.
• Flood-intolerant species, in contrast, produce ethanol, which quickly becomes toxic and damages the plant.

Just as water provides the fundamental medium for life, temperature sets the pace for all the chemical reactions that life depends on.