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 species is so successful in New Zealand native forest remnants. Over a two year period, the rate of shoot extension of T. fluminensis in a New Zealand mixed mahoe (Melicytus ramiflorus) coastal forest remnant showed a similar pattern to monthly mean values for mean daily air temperature 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 plant in the field was always > 250 mu mol per g dry weight. On high NO3- 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 irradiance). Changes in shoot to root dry weight ratio (S:R), specific leaf area (SLA) and leaf chlorophyll, carotenoid and protein content associated with decreased irradiance from 50 to 1% were similar to those associated with increased distance into the forest remnant and are discussed in relation to shade acclimation. Values for S:R (>30.1) and SLA (approximate to 900 cm(2) per g dry weight) were extremely high at low irradiance. These results support earlier conclusions that irradiance level is likely to be the primary factor limiting the extent of colonisation of forest remnants by T. fluminensis. Under glasshouse conditions, the growth response of T. fluminensis to different ammonium and NOS concentrations was similar to that previously reported for herbaceous species capable of rapid growth. Leaf nitrate reductase activity was within the range previously reported for fast growing species. Tradescantia fluminensis accumulated substantial amounts of NO3- in shoots with no depression in growth. This NO3- was utilised when nitrogen became limiting to growth. An 'invasion strategy' of T. fluminensis into N.Z. native forest remnants is proposed.