SUN SHADE ACCLIMATION AND NITROGEN NUTRITION OF TRADESCANTIA FLUMINENSIS, A PROBLEM WEED IN NEW-ZEALAND NATIVE FOREST REMNANTS

Growth, sun/shade acclimation and nitrogen nutrition were examined in Tradescantia fluminensis to gain greater understanding of why this spe cies is so successful in New Zealand native forest remnants. Over a tw o year period, the rate of shoot extension of T. fluminensis in a New Zealand mixed mahoe (Melicytus ramiflorus) coastal forest remnant show ed a similar pattern to monthly mean values for mean daily air tempera ture and day length. Growth at the shoot apex was balanced by death at the shoot base. During the first year, nitrate (NO3-)content of the p lant in the field was always > 250 mu mol g(-1) dry weight. On high NO 3- supply in pot experiments, in a glasshouse or outdoors, total plant dry weight increased with increased relative irradiance from 1 to 30- 50% (open ground photosynthetically active radiation = 100% relative i rradiance). Changes in shoot to root dry weight ratio (S:R), specific leaf area (SLA) and leaf chlorophyll, carotenoid and protein content a ssociated with decreased irradiance from 50 to 1% were similar to thos e associated with increased distance into the forest remnant and are d iscussed in relation to shade acclimation. Values for S:R (>30.1) and SLA (approximate to 900 cm(2) g(-1) dry weight) were extremely high at low irradiance. These results support earlier conclusions that irradi ance level is likely to be the primary factor limiting the extent of c olonisation of forest remnants by T. fluminensis. Under glasshouse con ditions, the growth response of T. fluminensis to different ammonium a nd NOS concentrations was similar to that previously reported for herb aceous species capable of rapid growth. Leaf nitrate reductase activit y was within the range previously reported for fast growing species. T radescantia fluminensis accumulated substantial amounts of NO3- in sho ots with no depression in growth. This NO3- was utilised when nitrogen became limiting to growth. An 'invasion strategy' of T. fluminensis i nto N.Z. native forest remnants is proposed.