Pg 16-20 Early Detection of Litchi Graft Incompatibility

The South African litchi industry requires suitable, high quality litchi cultivars to extend the short harvesting period. In recent years, several cultivars, sourced from both Australia and Israel, were imported into South Africa. However, grafting onto ‘Mauritius’ rootstocks showed that some of the imported cultivars appeared to be incompatible with this rootstock. Compatibility is a pre-requisite for successful grafting which ensures a proper continuum between rootstock and scion, thereby facilitating water and nutrient uptake and ultimately tree performance and yield. Rootstock-scion compatibility is dependent on anatomical, biochemical, physiological and genetic variables.

The aim of this investigation is to determine which selected scion-rootstock combinations are incompatible and identify incompatibility factors (anatomical, biochemical and/or physiological) as soon as possible after grafting. Concomitant with these investigations, the genetic relatedness of the cultivars and rootstocks will be assessed and compared, together with the grafting observations. This will facilitate the development of a model for predicting compatible scion-rootstock combinations prior to the production of trees in nurseries and subsequent planting. The preliminary data are presented.

Summary:

• Research Objective: The study aims to identify incompatible scion-rootstock combinations in imported litchi cultivars (from Australia/Israel) early after grafting, using anatomical, biochemical, physiological, and genetic markers to predict compatibility before nursery production and field planting, addressing variable grafting success onto ‘Mauritius’ rootstocks.​​
• Background: Graft incompatibility disrupts vascular continuity (xylem/phloem), leading to poor water/nutrient uptake, tree breakage, reduced yield, and suboptimal performance; success requires proper callus formation and healing at the union, typically between taxonomically related materials.​​
• Materials and Methods: Tested 19+ cultivars (e.g., ‘Chakrapat’, ‘Souey Tung’, ‘Mauritius’) grafted to clonal ‘Wai Chee’ rootstocks (Sep 2019, Schagen Nursery) and seedling rootstocks (‘Brewster’, ‘Mauritius’, ‘McLean’s Red’, ‘Wai Chee’; Feb 2020, ARC-TSC); measured graft survival/shoot length (monthly to 6 months), Safranin O dye uptake (3/6 months post-graft), chlorophyll content, CO2 assimilation (IRGA at 400 µmol m^-2 s^-1 PAR, 28°C), with hierarchical clustering analysis.​​
• Key Results – Survival/Shoot Growth: Clonal ‘Wai Chee’: High success (57-100%) for Selection A (Israel, 100%), ‘Brewster’/’Fay Zee Siu’/’Late Seedless’ (71%); failures for ‘Mauritius’/Israeli Selections B/C (0%). Seedlings: 85-100% for ‘Fay Zee Siu’/’Souey Tung’ on ‘Brewster’, ‘Late Seedless’/’McLean’s Red’/’Brewster’ on ‘Mauritius’; low (10-41%) for ‘Sah Keng’/’Kim Cheng Mesa’ on ‘Brewster’, ‘Garnet’/’Kaimana’ on ‘McLean’s Red’; trends aligned with shoot length.​​
• Dye Uptake and Physiology: Dye visualized vascular staining (active xylem) across union; best performers (e.g., Selection A, ‘Souey Tung’/Brewster) showed strong uptake, worst (e.g., ‘Chakrapat’, ‘Yellow Red’/Wai Chee) poor; assimilation rates varied (high: 15-21 µmol CO₂ m^-2 s^-1 for ‘Souey Tung’/Brewster, ‘Garnet’/McLean’s Red; low: 1-4 for ‘Wai Chee’/McLean’s Red); chlorophyll varied but high inconsistency noted.​​
• Trends and Best/Worst Performers: Consistent patterns across metrics: Best (Selection A, ‘Fay Zee Siu’/Brewster, ‘Late Seedless’/Mauritius, ‘Souey Tung’/Brewster); Worst (‘Chakrapat’, ‘Sah Keng’/Brewster, ‘Yellow Red’/Wai Chee, Israeli Selections B/C); cultivar differences in light/temperature responses observed.​​
• Conclusions and Future Work: Preliminary data show compatibility trends but full dataset needed (grafting remaining combos 2021-2023 on ‘Mauritius’/’McLean’s Red’/’Brewster’/’Wai Chee’); suggests anatomical/biochemical/physiological/genetic factors interplay; potential for dwarfing rootstocks and shade net optimization.