Two lines of 'gigas' wheat, the uniculm 492 and oligoculm 380, were compared with a normal cultivar of comparable duration to anthesis, Yaqui 50 or its dwarf isogenic line 50E, under a range of temperatures, daylengths and irradiances in the Canberra phytotron in order to define the conditions under which gigas characteristics are expressed.

In the gigas lines the characteristics of large ears with many grains were expressed most strongly at low temperatures and were associated with fewer, heavier shoots than in Yaqui 50E. At higher temperatures both gigas lines had more tillers and ears, not fewer as in Yaqui 50E and most cultivars, and yielded more grain per plant than Yaqui.

Both gigas lines were sensitive to daylength, the oligoculm showing the greater reduction in time to anthesis, the uniculm the greater increase in stunting and reduction in tillering with longer days. However, long days were not detrimental to the full expression of the gigas characteristics in the oligoculm.

Grain number and weight per shoot increased with increasing irradiance in the order: uniculm > oligoculm> Yaqui 50. However in terms of yield per plant or per unit area this order was reversed, reflecting the different capacities for tiller and ear production of the three genotypes.

Gigas ears were larger throughout their development. The flowering gradient along each spikelet was similar in gigas and Yaqui ears, but the initial lag in growth of the upper floret grains was shorter in the oligoculm, resulting in more synchronous grain growth, which was also more prolonged in the gigas ears.

Partial removal of grains from gigas ears led to a fall in photosynthetic rate of the flag leaf and to its earlier senescence, whereas removal of the lower leaves resulted in faster photosynthesis and delayed senescence. Thus even when supporting gigas ears, flag leaf photosynthetic rate was not maximal, nor was the mobilisation of reserves. Maximum harvest index was no higher in the gigas lines than in Yaqui 50E or in modern wheat varieties. The gigas characteristics appear to derive from the capacity to utilise the savings from restricted tillering for both greater leaf and ear growth per shoot.