Ecological and physiological studies of Tradescantia Fluminensis Vell. : A thesis submitted in partial fulfilment of the requirements for the degree Master of Applied Science at Lincoln University

Abstract

Tradescantia fluminensis Vell. is a herbaceous perennial, native to tropical South America which has become established in many native forest remnants in the North Island and northern South Island of New Zealand. Tradescantia has been implicated in suppressing regeneration of these remnants. Laboratory and field studies of growth, nitrogen nutrition and acclimation of Tradescantia to different irradiance levels were conducted with the objective of determining reasons for the success of Tradescantia in New Zealand forest remnants. The field site was a native forest remnant located in Akaroa, Canterbury, New Zealand (National grid reference NZMS 260 N36, Akaroa 076113). Over a two year period in the field, Tradescantia displayed a very seasonal growth pattern, which correlated positively with temperature. Length increase of individual plants was estimated at around 0.65 m yr⁻¹. However, plant length and dry weight (d.wt) changed little over the two year period. Nitrate (NO⁻³) content was measured in all tissues during the first year of the study. Stem tissue always contained over 0.3 mmol NO⁻³ g⁻¹ d.wt. In the glasshouse, plant d.wt and shoot to root ratio increased with increased NO⁻³ 0.1 to 5 mol m⁻³. At low concentrations (0.1-1 mol m⁻³) ammonium (NH4⁺) gave similar growth to NO⁻³ but at 5 mol m⁻³, NH4⁺ toxicity symptoms developed. Growth rate at high NO⁻³ was almost an order of magnitude greater than that found in the field. Nitrate content of all tissues increased with increased applied NOj concentration. Nitrate reductase activity (NRA) of all tissues increased with increased applied NOj from 0.1 to 0.5 mol m⁻³ then changed little with further increases in applied NO⁻³ thereafter: activity was greater in leaves than in stem or root at all concentrations of applied NO⁻³. The shoot was the main site of NO⁻³ accumulation (>95% total plant NO⁻³) and NRA (around 95% total plant NRA). Tradescantia utilised accumulated NO⁻³ when external NO⁻³ supply was reduced or withdrawn. In the glasshouse, plant d.wt at low (0.5 mol m⁻³) and high (5 mol m⁻³) NO⁻³ supply increased with increased irradiance 1 to 26% (open ground photosynthetically active radiation= 100% relative irradiance) then changed little with increased irradiance thereafter. Values were greater at high than at low NO⁻³ at all but the lowest irradiance level. Regardless of treatment, 87% or more of plant d. wt was partitioned fairly evenly between leaves and stem. The major part of irradiance effects on leaves occurred over the irradiance range 1 to 26%. At both low and high NO⁻³ specific leaf area (SLA), chlorophyll per unit d.wt and carotenoids per unit d.wt decreased with increased irradiance over this range, while protein per unit d. wt increased with increased irradiance 1 to 3%, then decreased with increased irradiance 3 to 26%. At low NO⁻³, chlorophyll and carotenoids per unit area decreased with increased irradiance 3 to 26%, at high NO⁻³, chlorophyll per unit area did not change with irradiance while carotenoids per unit area increased. Regardless of NO⁻³ supply, protein per unit area increased with increased irradiance 1 to 26%. For all measurements, values were greater at high than at low NOj at all but the two lowest irradiance levels. At both low and high N0⁻³ the chlorophyll to carotenoids and chlorophyll to protein ratios decreased with increased irradiance 1 to 26%. The chlorophyll a to chlorophyll b ratio was not affected by either irradiance or N0⁻³. On high N0⁻³ supply outdoors, the chlorophyll a to chlorophyll b ratio increased with increased irradiance. Changes in SLA, chlorophyll (a+b), carotenoid and protein content associated with increased distance into a forest remnant were similar to those obtained with decreased irradiance in both experiments. Chlorophyll a to chlorophyll b ratio decreased with increased distance into the remnant. It is concluded that; a) growth is highly influenced by seasonal temperature, b) in an established sward, growth at the apex is balanced by death at the base, c) Tradescantia shows features of N nutrition characteristic of ruderal, nitrophilous, and tropical species, d) Tradescantia is capable of acclimation to and growth at a wide range of irradiance levels and e) low irradiance appears to be the major factor limiting further colonisation of forest remnants. These data are discussed in relation to a possible strategy employed by Tradescantia in invasion of native forest remnants.