EFFECTS OF GROWTH PERIOD, PLANT-AGE AND CHANGES IN SOLUTION ALUMINUM CONCENTRATIONS ON ALUMINUM TOXICITY IN WHEAT

The effects of growth period (time between transplanting and harvestin g), plant age at which aluminium (Al) was added to solution, changes i n Al concentration, and solution culture techniques (monitoring and ad justing solution Al concentrations thrice weekly or weekly replacement of the solutions) were investigated using a low ionic strength (2.7 x 10(-3) M) solution culture technique. The wheat (Triticum aestivum L. ) cultivars Waalt (Al-tolerant) and Warigal (Al-sensitive), or the nea r isogenic lines bred from these cultivars (RR for the Al-tolerant lin e and SS for the Al-sensitive line) were grown. In all experiments and treatments, Al additions were required to maintain the nominal concen tration. The decline in solution Al concentrations was partially attri buted to formation of an Al-hydroxy-phosphate precipitate with an AI:P molar ratio of 2.8 to 4.0. Increasing the growth period from 14 to 28 days increased Al sensitivity in Warigal but not in Waalt. When plant s were exposed to Al for the same time, increasing the age of the plan ts that Al was added to solution decreased sensitivity to Al. Differen tial Al tolerance between the two lines was evident when solutions wer e monitored thrice weekly or replaced weekly. However, the Al concentr ation required to reduce relative yield by a given amount when the sol utions were replaced weekly was about twice that when the solutions we re monitored. With a constant growth period of 28 days, increasing sol ution Al concentrations for 3 or more days resulted in decreased yield s at harvest. The exact effect depended on the cultivar, plant part (t ops or roots), when solution Al concentrations were increased and the duration of the increase. For example, increasing Al concentrations fr om 5 mu M to 20 mu M for 10 days reduced yield in the RR line by appro ximately 50% in the tops and 30% in the roots beyond the effect of 5 M but had no effect in the SS line due to yields already being low at 5 mu M. Adding 10 mu M Al to solution for 6 days at the beginning of th e experiment reduced yield by 25% in the RR line and 50% in the SS lin e. In contrast, adding 10 mu M Al for 6 days in the middle of the grow th cycle had no effect on the RR line but reduced yield by approximate ly 25% in the SS line. These results show that growth period, the age of the plants at which Al is added and the technique used (monitored o r weekly replacement) all need to be considered when comparing results from different experiments. These results also show that the Al conce ntrations in solution need to be regularly monitored in long term expe riments.