The effect of nitrogen sources and initial pH of the media with or without buffer on in vitro rooting of jackfruit (Artocarpus heterophyllus Lam.)

Abstract

The influence of nitrogen on growth and morphogenesis of tissue cultures has been well established. Availability of nitrogen and the form in which it is presented are of great importance. Nitrate has generally been considered to be the most essential form of nitrogen for the culture of tissues. However in many cases culturing tissues on nitrate alone as a source of nitrogen has not been very successful [9, 15, 16]. It has been found that a reduced nitrogen source such as ammonium is essential for the optimal use of nitrate ion by tissue cultures. A number of studies have shown that growth of tissue cultures is possible on a medium containing ammonium as the sole nitrogen source provided that a Kreb’s cycle dicarboxylic acid is also present in the medium [7]. Tobacco cells were found capable of growing on ammonium alone, provided citrate, malate and pyruvate were added to the culture medium [3, 6]. Later Dougall and Verma [4] demonstrated that suspension cultures of wild-carrot could be grown with ammonium as a sole nitrogen source even in the absence of any exogenous Kreb’s cycle acid. However a combination of nitrate and ammonium is still considered to be an ideal source of nitrogen for tissue cultures [10, 15]. George and Sherrington [8] suggest that as most tissue culture media have not been buffered, the concentration of ammonium and nitrate ions have probably been adopted more to obtain practical pH control than because of the requirement of plant tissues for one form of nitrogen or another. Uptake of ammonium and nitrate forms of nitrogen has been reported to be correlated with the pH of the medium. Uptake of nitrate requires an acid pH, but the medium drifts towards alkalinity [1]. In contrast, ammonium uptake shifts the medium pH towards acidity [4, 5, 7, 18] and consequently further uptake of ammonium is inhibited. Therefore it is evident that in an unbuffered medium efficient nitrogen uptake depends on the presence of both ions. Most of the studies on the effects of different nitrogen forms and related pH factors have been concentrated on cell culture and embryogenesis studies using unbuffered media. No definitive studies regarding these factors have been reported as far as in vitro rooting is concerned. Hence this study was undertaken to investigate the effect of sources of nitrogen and associated pH changes on in vitro rooting.