Stock scion interaction in Piper nigrum L. grafts

Abstract

Black Pepper (Piper nigrum L.) also called ‘King of Spices’ and ‘Black Gold’, is one of the most widely used spices in the world, occupying a position that is supreme and unique. India is one among the countries where black pepper is being widely cultivated. However our productivity is low compared to other pepper producing countries in the world. Among the various factors limiting the productivity of Indian pepper, a soil borne disease called foot rot caused by Phytophthora is of prime importance. Piper nigrum is highly susceptible to Phytophthora foot rot. It has been reported that some South American Piper spp. are immune to Phytophthora. Interspecific crossing between Piper nigrum and immune species has not been successful so far. Grafting on resistant rootstock is a method to escape soil borne inoculum.

The present study was conducted to assess the stock and scion interaction in grafts of Piper nigrum L. on different South American species of Piper immune to Phytophthora foot rot and to standardize the best time of grafting. The study was carried out as three experiments, viz., grafting studies, anatomical studies on Piper species and anatomical studies on graft union at different stages of development.

Orthotropic as well as plageotropic shoots of P. nigrum, variety Panniyur

1 was grafted on three South American Piper species viz., P. colubrinum, P. auduncum and P. arboreum. Grafting of Piper nigrum on the same species was also done to study the feasibility of such grafting. The trial was laid out in completely randomized design. When orthotropic shoot of P. nigrum was used as scion, highest grafting success was recorded by the grafts of P. nigrum on the same species (100%) followed by the grafts on P. colubrinum (96.66%), P. auduncum (66.66%) and P. arboreum (46.66%). Graft survival after one year of grafting was 90.00, 83.33 and 33.33 per cent for P. nigrum/P. nigrum, P. nigrum/ P. colubrinum and P. nigrum/ P. auduncum grafts respectively. Survival of P. nigrum grafts on P. arboreum was very limited. None of these grafts survived for more than three months after grafting. Irrespective of rootstocks, the best month

identified for grafting orthotropic shoot of P. nigrum was March. Although, self grafts showed highest per cent success in grafting, based on the growth of grafted plants (six month old), grafts on P. colubrinum was found to be the best with highest plant height (120.55 cm), number of nodes (18.91) and internodal length (6.32 cm).

The graft recovery was less when plageotropic shoot of P. nigrum was used as scion. The highest per cent success was recorded by the grafts of P. nigrum on the same species (93.33%) followed by the grafts on P. colubrinum (66.66%), P. auduncum (36.66%) and P. arboreum (26.66%). March was identified as the best month for grafting plageotropic shoot of P. nigrum on P. nigrum, P. auduncum and P. arboreum. However, grafts on P. colubrinum performed best when grafted during June. Graft survival after one year of grafting was 66.66, 30 and 16.66 per cent for P. nigrum/P. nigrum, P. nigrum/ P. colubrinum and P. nigrum/ P. auduncum grafts respectively. Survival of P. nigrum grafts on P. arboreum was very limited. As far as the growth of grafted plants was concerned, highest growth was shown by the grafts of P. nigrum on the same species.

In the anatomical studies of Piper spp. it was observed that, Piper species exhibited a peculiar arrangement of vascular bundles. Two rings of vascular bundles comprising of an outer, cortical and an inner, medullary bundles were observed in all the four Piper species examined. Mucilage canal was present only in P. nigrum while an additional medullary bundle was observed in P. colubrinum. Large pith region was the characteristic feature of P. auduncum and P. arboreum.

Examination of graft union at three, six and twelve months after grafting revealed that, graft union formation was completed by six months after grafting. One year old P. nigrum/ P. colubrinum grafts and P. nigrum/ P. auduncum grafts exhibited the formation of a dense necrotic area at the graft interface.

Based on the observations, it can be concluded that, due to the peculiar arrangement of vascular bundles in Piper species, matching of all the vascular bundles of stock and scion is difficult. However, complete union of parenchymatous tissues of stock and scion was formed within six months after grafting. But, a proper cambial connection between the grafting partners was not evident. Further studies need to be conducted to find out the reasons for the development of dense necrotic area at the graft interface.