Starch accumulation is associated with adventitious root formation in hypocotyl cuttings of Pinus radiata

This study of the internal physiology of adventitious root formation in Pinus radiata was performed without the potential complications from microbial contamination and nutritional stress. Hypocotyl cuttings of radiata pine were cultured in half strength MS nutrient medium supplemented with IBA (indole-3-butyric acid), IBA + kinetin, kinetin, or without phytohormones (control). Averages of 8.35 and 0.08 roots per cutting were formed in IBA and in growth regulator-free treatments, respectively. No roots were formed in IBA + kinetin, kinetin, or sucrose-free treatments at day 30 after excision of hypocotyls. Changes in fresh weight, sugar, and starch content were measured at established developmental stages associated with adventitious root formation. Sucrose-supplemented medium was required for higher levels of sugar, starch, and root formation in rooting region of IBA-treated hypocotyls. Starch accumulation, in particular, seems to have potential as a biochemical marker before root primordium emergence in light of the following observations in the IBA treatment. Starch began to build up preferentially in cells involved in or in close proximity to potential sites of new root primordium formation (that is, the cells on the inside of the cortex and the pith) before any visible organized root primordia and then began to disappear during root primordium formation. The substantial starch accumulation associated with IBA treatment was not observed in the IBA + kinetin, kinetin alone, growth regula tor-free, sucrose-free treatments (nonrooting treatments) or in the nonrooting region of hypocotyls treated with IBA.