Subtropical regions are the intermediate zone between the tropical and temperate regions where winters usually experience sub-zero temperatures with intense frosty situations. The evergreen plant species suffer huge frost damage in the subtropics, especially in the low-lying basin areas and the adjoining plains.

Fruit and nut crops have adapted to a wide range of soil, climate and moisture conditions irrespective of their place of origin. The limits of the distribution of these crops across the globe are largely governed by climatic factors such as temperature, wind direction, solar luminance level, length of day (latitude) and atmospheric water vapour content besides other factors. These climatic factors are considered critical in governing the distribution of plant species because not only they limit the growth and development of plant species but also determine the consistency of annual cropping and the quality output. 

Frost is a thermodynamic phenomenon that occurs in the atmosphere, especially under subtropical and temperate conditions during winter. To understand the horticultural implications of this meteorological event, it is necessary to understand the frost related terms and jargon, first. Some of the common terms used here in this book are described hereunder:

Earlier, it was a general perception that frost is something like snow that falls from the sky onto the Earth and causes cooling of the plant surface leading to freezing and lethal damage. But, with advancement in knowledge, it is now well understood that it is the cooling of a surface to a subzero temperature which leads to solidification of the water vapours present in the air onto it resulting in the formation of a white crystalline layer of ice molecules. The ice so formed further grows into the plant system causing damage to it through intracellular or intercellular freezing.

As discussed in previous chapters, the damage to the plants due to frost occurs as a result of frost-induced freezing and not merely because of the low temperature exposure. The cells of sensitive plant species get injured when the temperature drops below a critical level. When the damage caused is associated with sub-zero temperature, it is called freezing injury. And, when it is caused due to a temperature above 00 C, it is called chilling injury.

During winters, many of the evergreen species growing in the subtropics suffer chilling or freezing damage and the extent of damage however, depends greatly on their ability to perceive the low temperature stimulus and the capacity to modify the physiological or biochemical functioning at the molecular level;

All the plant biochemical processes are sensitive to change in temperature. Freeze or low-temperature stress in plants becomes lethal when the temperature is lowered beyond a critical threshold for a sufficient duration to cause irreversible damage to plant growth and development. This stress influences countless biochemical reactions which have certain temperature optima for their proper functioning.

As the vulnerability of different plant species to frost and low-temperature stress is quite variable, the symptoms are also variable, therefore. The development of visual symptoms depends largely upon the plant genotype, growing environment and tissue type being exposed to low-temperature stress.

Despite the tremendous advances in agriculture, the agrarian output is still weather and climate-dependent. Frost or low-temperature stress is a single weather-related phenomenon that causes losses greater than any other environmental or biological hazard. As the plants are immobile, they are bound to undergo this stress and suffer damage under natural condition. For avoiding and managing the adverse effects of frost-induced freezing in plants, a reliable early warning/prediction of intensity and duration of frost is necessary.

As discussed in the previous chapters, the global perspective of frost is a latitudinal phenomenon; it is more common in subtropical latitudes (23.50 to 350 N&S). The proneness of vegetation to frost usually increases as we move toward the poles. But, at the micro-level, there are several factors like proximity to a water body, type of soil, type of vegetation, prevalence of winds etc., which govern the temperature inversion and frost-proneness of a situation or site. 

The problem of frost damage to the crops is as old as agriculture is. Earlier, crop production was undertaken only as per the geographical distribution of the plant species and the cultivation of crops in non-conventional areas was not there. Later, with the expansion of agricultural activities, a number of introductions were made in the non-conventional areas. 

One of the important strategies to mitigate the adverse effects of frost is to develop tolerance in plant against frost stress. Tolerance is such a condition of frost vulnerable plant species, they develop certain ability to withstand the freezing stress by avoiding the states of inter or intracellular freezing. Thus, the process of cold hardening or acclimation together with ability to super cool at freezing temperatures is usually termed as frost tolerance development.

When avoidance or tolerance of frost is not possible and frost damage is inevitable, the active methods of frost protection are the way out for the protection of the fruit plantations. The methods which are employed just before or during a frost event for avoiding freeze damage are called Active Frost Protection Methods.

Augmentation strategies are generally used as a way to improve the effectiveness of passive or active frost protection methods. As the intensity of the frost is quite variable and sporadic in nature, simple application of the protection methods is not always helpful in giving adequate protection against freezing stress. 

If frost has hit an orchard during winters, it does not always imply that permanent loss has occurred. Plants do possess natural protective mechanisms due to which they survive the stress to a variable extent depending upon their nature and organ being exposed to the stress.

Global climate change has increased the weather variability across all geographical settings. The transitional regions between the tropics and temperate region are expected to buffer the changes comparatively for a extended duration of time.

Colour Plates

1. Koppen W. 1940. Köppen climate classification. https://www.britannica.com/science/Koppen-climate-classification 2. Anonymous 2010. http//www.business.gov.in/agriculture/ currentscenario