Improved growth of hydroponically-grown rough lemon (Citrus jambhiri Lush) seedlings treated with kelp and vermi extracts

Abstract

A substantial number of South African citrus trees are rooted on rough lemon rootstocks. In order to ensure the successful establishment of a high percentage of young citrus trees, it is necessary to ensure that farmers are provided with young vigorous nursery trees that have a large and healthy root system. Vermicast extracts (VE) and liquid extracts from the giant seaweed, Ecklonia maxima (EM), have been used on various crops over the past decades to promote plant growth and development. It is widely reported that plant growth, seedling vigour, water-use efficiency and nutrient-uptake of these crops improves with EM and VE treatments, thereby reducing excessive fertiliser applications. With the improved uptake of nutrients such as nitrogen (N), which is the main component of chloroplasts and proteins involved in the Calvin cycle, it is likely that the application of EM and VE to plants could lead to an improvement in the photosynthetic capacity of plants. The benefits from applications of EM and VE have been attributed to the presence of plant growth regulators/hormones (PGRs) such as auxins and cytokinins. Since seaweed extracts are manufactured using different methods, some of which may include the use of elevated temperatures, acid and/or alkaline hydrolysis of plant material, it is not surprising that different seaweed extracts may contain various levels of PGRs which can lead to dissimilarities in their growth promoting effects. A hydroponic experiment was conducted to test if locally produced EM from two different suppliers and VE would increase root length, root volume, root dry mass, stem diameter, plant height, chlorophyll content, nutrient uptake and photosynthetic capacity of rough lemon seedlings. The other objective of this study was to determine if there was any difference in growth promoting effects of EM obtained from two different supplies due to the difference in the levels of plant growth regulators they contain. Since the action of EM and VE is dose dependant, it was also necessary to determine the optimum concentration of EM and VE for the growth of rough lemon seedlings. This experiment was conducted in a glasshouse at the University of Pretoria experimental farm. Liquefied Ecklonia maxima from different manufactures; Ecklonia maxima - Kelpak® and Ecklonia maxima – Afrikelp® (EM1 & EM2 respectively); and vermicast extracts (VE) were applied separately to rough lemon seedlings as a drench (at 0.5%, 1% and 2% dilutions), at 14 day intervals. Deionized water and full strength Hoagland solution were used as controls. Root volume, root length, stem diameter, plant height, chlorophyll content and photosynthesis were measured weekly for 12 weeks. At the end of the trial, the average root dry mass and shoot dry mass for each treatment was determined. Upon termination of the trial, oven dried leaf samples were analysed for nitrogen (N), phosphorus (P), potassium (K) and calcium (Ca). N analysis was done at an accredited commercial lab whilst P, K and Ca analyses were done at the University of Pretoria using ICP-OES.