Fallen Stars: Effects of High Temperature on Pomegranate Fruit Set

Fig. 1     Pomegranate (Punica granatum L. 'Wonderful') flowers on the ground.

A star is a promise of illumination - like a flower is an early promise of a fruit to come. This season, my pomegranate tree (Punica granatum L. 'Wonderful') growing in a large pot is producing numerous bright colored flowers.  However, this week the flowers are dropping like falling stars (Fig. 1) at a very high rate.   The only factor that I can attribute this to is the sudden rise in temperature - from the 70s to high 90s (F).

Fig. 2     Pomegranate at an early stage.

Temperature and humidity both play an important role in fruitfulness of orchard trees.  As a general rule, lower temperatures - without the frost - is most conducive to better flower initiation, successful pollination, and fruit set.  On the other hand, high temperatures (90+ degrees F) and low humidity can result in the abscission of flower buds and fruitlets (Fig. 1) - leading to unfruitfulness.  In our area, winter and early spring temperatures may be favorable but summer temperatures can be detrimental - hastening the deterioration of the embryo sac and the premature senescence of the ovules. 

Based on last year's observations,  Pomegranate 'Wonderful' blooms continuously from spring to fall (Fig.3).  Some of the earlier flowers managed to set already (Fig. 2).  I am assuming that there are more fruits yet to set when the temperatures relent once more.  But we'll see...after all, this is only the second blooming year for this tree.

 

Fig. 3    Pomegranate (Punica granatum 'Wonderful') in bloom.

 

High temperatures inhibit fruit set in a number of plants.  The reason for this was demonstrated in a study on flower development and fruit set in apricot (Prunus armeniaca).  Results showed that warmer temperatures hastened that external development of flowers without significant advances in pistil development.  For fertilization to take place, the pollen and the pistil need to be synchronized.  Under warm temperatures the flower can unfurl its petals that attract pollinators but the ovules are immature.  I hate to use this analogy but it is like a girl acting like a grown-up prior to puberty.  In fruit trees, this situation does not end up in a successful fruit set.

Abscission of flowers and fruitlets is a natural mechanism for the plant to retain enough fruits that is equivalent to its capability to support under the existing environmental conditions.