The effect of irrigation with lake water containing a variety of microcystins on accumulation of toxins, or toxin metabolites, and plant growth in ryegrass, clover, rape, and lettuce, was investigated in a glasshouse experiment. The plants were grown in sand culture and received either three or six applications of lake water, which was applied either directly to the sand surface or to the plant shoots. As determined by LC-MS, each plant received 170 mug of a mixture of 10 different microcystins per application. Microcystins in plant samples were extracted with 70% methanol and analyzed by Adda-specific ELISA. For the shoot application treatment, microcystins were not present at measurable levels in shoots of ryegrass or rape, but were present in lettuce [0.79 mg/kg dry weight (DW)] and clover (0.20 mg/kg DW). Total microcystin concentration in roots did not vary greatly depending on whether treatment water was applied directly to the sand, or reached the roots via run-off from the shoots. Microcystins in roots were highest in clover (1.45 mg/kg DW), intermediate in lettuce (0.68 mg/kg DW) and low in ryegrass (0.20 mg/kg DW), and rape (0.12 mg/kg DW). There was no evidence for root-to-shoot translocation of microcystins. Three applications of microcystins reduced shoot DW of ryegrass, rape and lettuce, and increased root DW of ryegrass and lettuce. Clover DW was not changed by treatment with microcystins. The results show that irrigation with water containing microcystins has the potential to move microcystins into farm animal and human food chains at concentrations that can exceed recommended tolerable limits.

Summary In phosphate‐deficient soils, VA mycorrhizas strongly stimulated nodulation and growth of Centrosema pubescent, Stylosanthes guyanensis, Trifolium repens and Lotus pedunculatus. The tropical legumes were much more dependent on mycorrhizas for growth than the temperate species and this difference seems to be related to the degree of root hair development. When Trifolium repens and Lolium perenne were grown together, mycorrhizas preferentially stimulated growth of the legume.

Agrostis capillaris, Arrhenatherum elatius, Bromus willdenowii, Cynosurus cristatus, Dactylis glomerata, Elytrigia repens, Lolium multiflorum, L. perenne, Phalaris aquatica, Phleum pratense, Schedonorus phoenix, and a ryegrass selected for high surface root mass, were grown in 1 m deep · 90 mm diameter tubes of sand. Tubes were irrigated with a low ionic-strength nutrient solution and 15 N-labelled nitrate was leached down the tubes prior to harvest. Shoot weights, root weights in 10-cm depth increments, and shoot and root nitrogen concentrations were determined. Plants of L. multiflorum were the heaviest and plants of A. capillaris were the lightest. Root system shape was analysed by comparing the decay constant from an exponential model fitted to the proportion of root mass in 10-cm depth increments, and, also, by analysis of the proportion of root mass in the top 10 cm. Cynosurus cristatus was strongly surfacerooted and the perennial ryegrass, selected for high surface root mass, had more root mass between 0 and 10 cm than did the unselected perennial ryegrass cultivar. There were only small differences in root shape between the other grasses. There was a strong and positive correlation between plant dry weight and the proportion of the pulse of labelled nitrate that was intercepted. The variation in root system shape shown in this experiment had no effect on nitrate interception. Nitrate interception per unit root weight was significantly higher in A. capillaris than in the other grasses. Developing winter-active grasses that have finely divided root systems should contribute to pastures with better nitrate retention characteristics.

Hydrogen cyanide (HCN) and iodine (I) concentrations in the herbage were determined for 51 white clover (Trifolium repens L.) cultivars that had been grown under uniform conditions in a glasshouse. HCN contents ranged from 120 to 1110ngHCN/g dry matter (DM). Cultivars that are agronomically successful in New Zealand, and cultivars of New Zealand origin, were mainly highly cyanogenic. There was evidence in 'Grasslands Kopu' and 'Aran' of a decline in cyanide content in plants raised from first generation seed, compared to plants from Breeders or Basic seed. This decline may result in part from contamination of seed crops by low HCN resident clovers. Iodine concentration in the white clovers ranged from 0.08 to 0.21 µg I/g DM with 77% of values being below 0.12 µg I/g DM. There was no correlation between I and HCN concentrations. It seems there is little potential to improve the I nutrition of stock by selecting for increased I content in white clover. The influence of cyanogenic clover on the metabolism of I, selenium (Se), and sulphur (S) in sheep is discussed. It is concluded that these factors, and safety margins against direct cyanide toxicity, warrant further study. A94090