Endotrophic Mycorrhiza – the cause for the phenomenon of the “Johannis-sprout” of apple trees?

Abstract

In the literature there are many explanations for the factors which influence the midsummer sprout, the so called “Johannis-sprout“. However there are no explanations for the fact that the shoot growth of apple trees stops 4 to 5 weeks after blossoming by developing terminal final buds. After a resting time they start a new but very unimportant shoot growth.

With respect to root colonizing by arbuscular mycorrhiza, damages in the rootlets are first of all not visible in the period of active root growth. This is also true if hypha, arbuscules and vesicles are occupying the whole root cortex. But with extending of the volume of vesicles there take place variations in their normal oval shape. These are mostly constrictions which occur if the root cortex is bursting by the enlargement of the vesicles and their occupying the holow spaces. These symptoms correspond mostly with a decreasing vitality of the rootlets. Consequently there exists the possibility that these mechanical damages of the rootlets may be the cause for disturbances in water and nutrient uptake and in the balance of plant hormones in the trees. Therefore the question arises, whether these processes may be responsible for terminating the first shoot growth period and accordingly for the succeeding second growth period, called Johannis-sprout.