Evaluation of drought and heat tolerance of apple cultivars in the south of Russia

Drought in the south of Russia recurs almost annually, and for the gardener, this is a direct risk of losing the harvest. Moisture deficit inhibits shoot growth, reduces fruit quality, and weakens trees. The most reliable way to stabilize a business is to initially select cultivars capable of enduring severe moisture deficit and extreme temperatures without a loss in productivity.

• Air temperature without leaf damage — up to +50 °C
• Maximum soil heating in the experiment — +60 °C
• Water loss of susceptible cultivars — 18–31 %
• Leaf death of Topaz cultivar at +60 °C — 100 %

Water-holding capacity and growth dynamics during drought

To evaluate the drought resistance of apple trees, a leaf-wilting express method is used. It shows how effectively tissues retain moisture under critical conditions. The less water a leaf loses during drying, the higher the overall resistance of the tree.

1. Determine the initial leaf water content before the start of the test.
2. Dry the cut leaves for 4 hours.
3. Calculate the percentage of moisture loss based on the difference in weight.

In the arid period of 2008, six cultivars were evaluated on trees planted in 2005 (M9 rootstock): Liberty, Florina, Enterprise, GoldRush, Topaz, and Imrus. Observations were conducted directly in the orchard to confirm laboratory tests. This allowed for an assessment of the plants' real reaction to moisture deficit.

The Liberty, Imrus, and Florina cultivars showed high water-holding capacity and minimal moisture loss. During the drought period, they maintained active and prolonged shoot growth. The Enterprise, GoldRush, and Topaz cultivars lost from 18% to 31% of water after 4 hours of drying. Their growth processes practically stopped, which indicates weak resistance to moisture deficit.

Heat resistance and leaf damage at critical temperatures

Summer heat in the south is often accompanied by extreme temperatures: the air can heat up to +50 °C, and the soil surface to +60 °C. Such overheating is dangerous due to dehydration, leaf scorch, chlorophyll degradation, and the death of plant proteins. Testing cultivars in August 2007 showed how trees react to heat stress.

At a temperature of +50 °C, the leaves of all tested cultivars remained undamaged. However, heating up to +55... +60 °C revealed significant differences in heat resistance. In the least adapted samples, leaf tissues turned brown and died off almost completely.