Chilling hours and why these matter for fruit growers

With the post-monsoon season going dry in the temperate fruit-growing regions of Himachal, farmers are worried if the chilling-hour requirement of the deciduous fruit crop will be met or not. The average temperatures during November and December were quite high and the initiation of the chill-hour accumulation process did not start. However, the first fortnight of January did see some snowfall in higher hills, which plummeted temperatures below 7°C in the state, triggering the much-awaited chill-hour process.

Dormancy period

In the winter, days turn shorter and cooler due to the less availability of sunlight. At temperatures less than 12°C, deciduous plants stop growing, their leaves turn yellow and eventually fall and enter a phase of reduced metabolic activity called dormancy. The mechanism of chilling hours in plants is governed by a balance between growth inhibiting and growth promoting hormones. During the winter, cold temperatures trigger abscisic acid hormonal response in deciduous plants by boosting anti-oxidant activity, neutralising harmful effects of cold stress and accumulate protective substances like sugar, organic and amino acids, osmotic regulators that help maintain cellular health at low temperatures to withstand freezing. It inhibits ethylene production associated with stress and breakdown during chilling. Continuous chilling exposure causes abscisic acid to gradually decline and initiate growth during the spring.

What is chill hour?

A chill hour or chill unit (CU) is an hour wherein the air temperature around a dormant tree is within the chill range between December and March. The chill accumulation is the cumulative number of chill hours a tree needs during the winter to meet out its requirement. Chill units need not to be consecutive. Generally, temperatures between 0°C and 10°C help fulfil the chilling requirement of many plants, but the most efficient temperatures are between 0°C and 7.2°C. Freezing temperatures or temperatures above 12°C to 15°C for long periods are not effective and even negate accumulated chill units. Chilling hours are thus accumulated throughout the dormant season and summed up to determine the total count during the season.

Snowfall on the mountains, surroundings areas or in orchards help the air temperature go down close to 0°C or even below for a prolonged period. Besides, it increases the moisture content in the sub-soil following gradual melting of snow but only the consistent flow of cold air between 0°C and 7.2°C contributes to meeting out the chilling requirements of fruit plants.

Chilling requirements vary

These requirements vary widely from one variety to another. Among deciduous fruit trees, apple and cherry require higher chill units from 800 to 1,200 whereas, Golden Delicious needs 600 to 700 hours. Gala, Akane, Crisp Red, Pink Lady, Fuji cultivar require around 500 hours, and Granny Smith need 400 to 600 hours. Low chilling varieties, such as Anna or Golden Dorset, Tropical Sweet, Ein Shemar, Maayan and IPR-Julieta require 100 to 400 chill units, making them a good choice for low hilly regions. Pomegranate needs 100 to 200 hours, peach (200 to 800), plum (400 to 700), almond, pecan and nectarine (300 to 600), fig and apricot (300 to 900), persimmon and pistachio (700 to 800), pear (250 to 900), walnut (400 to 800), blueberry (200 to 600) while blackberry requires about 150 to 700 chill units. On the contrary, evergreen fruits such as citrus, mango, guava, papaya, olive, avocado etc., do not require chilling hours at all.

The accumulation of chill units is a non-linear process, moderate cool weather counts more than extreme cold or warm spells. Upon warming, dormant buds activate metabolic pathways, using cytokinin hormone for rapid growth prior to bud break in terminal buds. On achieving the required chill units, another growth promoting hormone gibberellin increases their levels to initiate bud break to develop viable flowers and fruits.

Effects of inadequate chilling

Insufficient chilling leads to delayed leaf emergence, sparse, unevenly or poor flower pollination, reduced pedicel length, fruit set and poor fruit yield. It influences flower initiation and fruit colouration, deterioration in fruit texture and taste. In recent years, some apple growers, in the absence of snowfall, use foggers to create icicles on fruit trees to mitigate the chill unit requirement, which is detrimental to plant growth and survival.

The chill units are air temperature-based, not ice-based, hence do not contribute to effective chill unit accumulation. Chilling is a physiological process, not a mechanical one and artificial ice can in no way mimic natural winter chilling. Remember that the chill units are accumulated by cool air temperatures over time and not by freezing ice on trees. Such a practice may cause harm by freezing injury, reduced bud viability and a severe injury to bark surface. Farmers should desist from using such a practice to avoid damage to the plants.

Generally, the farmers do not pay attention to the chill unit factor and cultivate new fruit varieties without ascertaining the chilling requirements, resulting in retarded growth, deficient foliation, small fruit size, flattened fruit shape and low production. For quality fruit harvest, farmers must select suitable varieties considering the total chill units received in the area. The higher chilling requirement varieties must be planted in colder areas to get proper quality fruit.

How to mitigate drought effect

To mitigate the adverse effects of drought, it is advised to delay fruit tree plantation, avoid tilling the basin area or pruning the trees, except removing diseased, dry twigs or branches until mid-March. Do not apply any fertilizer at this stage, except farm yard manure, until rain or snowfall. Mulch the tree basin with at least 5 cm to 10 cm dry grass or polythene mulching sheet/ mat. If possible, apply light irrigation until rain, in the tree basin area two to three times in a fortnight during the evening hours. Paint tree trunk with lime-copper sulphate-linseed/Horticultural Mineral oil paste up to the first main branch and the portion of bark directly exposed to the sunlight. These steps help minimise the drought impact for a considerable period.

(The write is a former professor of University of Horticulture and Forestry, Nauni.)