No abstract

Arsenic levels were determined in waters, sediments, aquatic macrophytes and other organisms taken from the Waikato River system, North Island, New Zealand. All water samples contained elevated levels of arsenic relative to 0.0017 1-Lg g-1 , the background level for arsenic in aquatic systems. Most water samples taken between Lakes Aratiatia and Whakamaru had arsenic levels above the World Health Organization (WHO) limit for arsenic in drinking water (0.01 1-Lg ml -1). Arsenic levels in the water of the river decreased downstream from the Wairakei and Ohaaki geothermal power stations, indicating that these are major sources of arsenic to the river. The arsenic concentration of the sediments was typically around 30 1-Lg g-1 , though there was a large variation. Sediments appear to be a sink for arsenic. Some aquatic macrophytes ( Ceratophyllum demersum and Egeria densa) had arsenic concentrations up to 1 ,000 1-Lg g-1 , (0.1 %) in dried tissue. These plants may be used as bioindicators of arsenic pollution in aquatic systems. Water cress taken near Broadlands (Ohaaki) contained around 300 1-Lg g-1, (dry weight) arsenic, and should not be consumed. Shellfish taken from the mouth of the Waikato river had significantly higher arsenic concentrations than the same species taken further down the coast at Raglan.

Cellular responses of the native New Zealand grass species Festuca novae-zelandiae (Hack.) Cockayne to a water deficit and re-hydration treatment were investigated. Leaf extension ceased after 28 d of withholding water, while mitotic activity within the intercalary meristem still continued. By 35 d, no evidence of mitotic activity could be detected. At the apical meristem, cell division commenced 24 h post re-watering after 49 d of dry-down and was accompanied by synthesis of storage oligosaccharides of the fructan type. Changes in water-soluble carbohydrates over the course of the water deficit were examined in consecutive leaf segments comprising the leaf base (meristem region), elongation zone, the enclosed and the exposed lamina, as well as basal sheath segments from the two next oldest leaves. In fully hydrated leaf tissue, the fructan pool was mainly composed of the low molecular weight fructans of the inulin and neokestose series with higher concentrations towards the leaf base. Fructan concentrations decreased over the course of the water deficit with the leaf base retaining significantly higher concentrations than any other tissue, until tissue water content fell below 45%. Sucrose content increased in each tissue during the course of the dry-down, and was highest at the leaf base, where a concentration of 200 μmol g DW–1 was measured after 49�d of dry-down.In 1.5 h after re-watering, levels of 1-kestotriose increased relative to the level of sucrose at the leaf base, indicating re-synthesis of fructans from accumulated sucrose. By 24 h post re-watering, the ratio of sucrose to 1-kestotriose declined from six in non-watered plants to two and higher molecular weight fructans became detectable. The negative correlation between fructan and sucrose content, which indicates an inter-conversion depending on tissue water content, suggests that, in this species, fructans serve as a carbohydrate pool, while sucrose stabilises the meristem during extreme water deficit.

The dispersal unit of Dacrycarpus dacrydioides (A. Rich.) de Laubenfels (Podocarpaceae) and the significance of the fleshy receptacle. Dncyarpus darrydioides (formerly Podwarpus darcydioides) is an arborescent gymnosperm endemic to New Zcaland. 'I'hc Iiigh water content (43";,) and sensitivity of viability towards desiccation, suggest the seeds arc or the 'recalcitrant' type. The 'fruits' comprising a seed borne on a fleshy receptacle arc shed in large numbers. The development of the seed precedes the full development ofsucculence in the receptacle and at maturity the seed has a high relative water content (RWC) relative to the reccptaele. Within the maturing reproductive unit, the receptacle bufkrs the seed against the efkcts ofwater stress. After shedding, seeds are rapidly desircated in moving air, and viability is impaired below approximately 80",, RWC and abolished at 34",, seed RWC. The presence of tlie receptacle during drying confers resistance to des tion-associated damage. Five of desiccation sensitivity are distinguished i n recognitio f tlie protective role of the reccptaclc. I t is suggcstcd that the advantages associated with prolonging serd viability may havc contributed to tht. evolutionary development of succulence in the reproductive unit. 'l'his might be considered as a selection pressure in a manner similar to the proposal that such fleshy structures are associated with serd dispersal hy birds.