The relationship between temperature and plant life is absolute. Temperature is a foundational environmental factor that dictates a plant’s ability to live, grow, and reproduce, standing alongside light and water as an environmental necessity. While plants do not generate their own heat, they require a specific thermal environment to drive the complex internal machinery necessary for development. Growth only occurs when the surrounding temperature falls within a functional range.
Heat as a Metabolic Catalyst
Plant growth is dependent on thousands of simultaneous chemical reactions within the cells. Heat provides the necessary kinetic energy to drive these metabolic processes at a rate that sustains life. Enzymes, which are complex proteins, act as biological catalysts to speed up these reactions, but their activity is acutely temperature-dependent. Below a minimum temperature, molecular movement slows significantly, causing enzymes to become sluggish and unable to process substrates.
This thermal dependence directly affects photosynthesis and respiration. Photosynthesis, the process of converting light energy into chemical energy, slows dramatically when temperatures are too low. Respiration, which converts stored chemical energy into usable power for the plant, is also governed by temperature. The rate of respiration typically increases with temperature. When temperatures are too low, both processes halt, and the plant enters a dormant state, unable to accumulate the biomass required for growth.
Defining the Optimal Temperature Window
For maximum productivity, every plant species operates within a specific thermal window defined by three points: a minimum, an optimum, and a maximum temperature for growth. Below the minimum, growth stops, and above the maximum, metabolic functions break down. Cool-season crops, such as wheat and barley, generally have an optimum range between \(15^{\circ}C\) and \(25^{\circ}C\), while warm-season crops like maize and rice thrive between \(25^{\circ}C\) and \(35^{\circ}C\).
Beyond instantaneous temperature, the total accumulated heat over a growing season drives plant development stages like flowering and fruiting. This accumulated heat is tracked using a metric called Growing Degree Days (GDD). GDD calculations subtract a base temperature—typically \(5^{\circ}C\) for cool-season plants and \(10^{\circ}C\) for warm-season plants—from the daily average temperature to quantify the heat energy available for growth.
The plant only develops when the temperature is above this base line, which represents the thermal minimum required for cell division and expansion. Tracking GDD allows farmers and horticulturists to predict when a crop will reach a specific developmental milestone, such as maturity. GDD accuracy is improved by capping the maximum temperature, often at around \(30^{\circ}C\), because growth usually does not accelerate beyond that point.
The Impact of Thermal Stress
When temperatures fall outside the optimal window, the plant experiences thermal stress, which manifests as two distinct forms of damage: cold stress and heat stress. Cold stress includes both chilling injury and freezing injury. Chilling injury occurs in sensitive tropical and subtropical plants, such as tomatoes and cucumbers, when exposed to low temperatures above freezing, typically between \(1^{\circ}C\) and \(15^{\circ}C\). This non-freezing cold disrupts cell membrane structure, causing leakage and leading to physiological disturbances in photosynthesis and respiration.
Freezing injury, which happens at or below \(0^{\circ}C\), is immediately destructive, as ice crystals form in the intercellular spaces. This ice draws water out of the cells, causing dehydration and, more severely, the rupture of the cell wall and membrane, leading to tissue necrosis. Conversely, heat stress occurs when temperatures exceed the optimum, causing a cascade of cellular failures.
Excessive heat damages the delicate three-dimensional structure of enzymes, a process known as protein denaturation, which inactivates the metabolic machinery. This damage is particularly noticeable in the photosynthetic apparatus, where the enzyme RuBisCO activase is highly sensitive to heat, inhibiting the plant’s ability to fix carbon dioxide. High temperatures also increase the rate of respiration disproportionately to photosynthesis, causing the plant to burn its stored energy reserves faster than it can create new sugars, leading to energy depletion and reduced yield. To cope, plants increase water loss through transpiration for cooling, but this often leads to stomatal closure, which further limits carbon dioxide uptake for photosynthesis.