Effects of gelling agents on growth, mineral composition and naphthoquinone content of in vitro explants of hybrid walnut tree (Juglans regia × Juglans nigra)

Abstract

Gelling agents affect growth of walnut in vitro cultured shoots. Gelrite promoted shoot elongation and bud production, whereas agar inhibited growth, induced mature leaf formation and necroses. The 2 gelling agents differed significantly in mineral content. They altered the chemical composition of the medium as well as that of the explants. A pronounced accumulation of Na and several microelements was observed in leaves after 16 d of culture on agar, probably due to a disturbance in the K selectivity mechanism and membrane permeability. Moreover, on agar, the level of hydrojuglone glucoside, an indicator of juvenility in walnut, was reduced drastically in the callus. Mineral element accumulation and reduction of hydrojuglone glucoside were evident after growth inhibition, indicating that they are a result of, rather than a cause of this inhibition. Lack of growth, mature foliar morphology, Na and microelement accumulation and hydrojuglone glucoside decline support the hypothesis that agar accelerates the ageing of in vitro propagated walnut trees.