Reproductive potential of the functionally female native Croatian grapevine 'Grk bijeli'

A native Croatian grapevine 'Grk bijeli', sharing a parent-offspring relationships with 'Tribidrag' (aka 'Zinfandel'), is grown exclusively on the Adriatic island of Korčula. It is one of the grape cultivars with female-only functional flowers, causing reduced fertilisation and leads to problems in grape production and wine quality. A typical cluster of 'Grk bijeli' at maturity contains fully developed seeded berries, in addition to a highly variable proportion of undersized seedless berries. The aim of this study was to identify the cause of the reduced reproduction potential of 'Grk bijeli' in order to establish a better growing environment for improved yield and grape quality. 'Grk bijeli' female gametophyte develops normaly and at maturity contains both egg and central cell, together with two synergid cells and three antipodal cells. On the other hand, 'Grk bijeli' pollen grains show developmental deviations. Specifically, most of the pollen grains undergo the first pollen mitosis and contain one vegetative cell and one generative cell, while 20 % of ungerminating grains also accomplish the second pollen mitosis, giving rise to two sperm cells and one vegetative cell. Morover, 'Grk bijeli' pollen has acolporate morphology, which prevents germination and contributes to reduced reproduction. Furthermore, fertilisation after pollination with other varieties results in various degrees of ovule abortion depending on the pollinator, revealing Croatian cultivars 'Plavac mali crni' and 'Pošip bijeli' as favourable varieties. Although this study describes a highly valuable cultivar of local importance, it also contributes to fundamental knowledge of grapevine reproductive biology and offers a strategy for improvement of wine production and oenological performance of semi-fertile varieties in general.