A study of effects of low temperature stress on seed development and yield in wheat (Triticum aestivam L.) : a thesis presented in partial fulfilment of the requirements for the degree of Master of Agricultural Science in Seed Technology at Massey University

Temperature affects the growth, development, fertility and yield of cereals. The degree of sterility and subsequent yield reduction caused by extreme temperature stress depends upon the minimum level and duration of the stress temperature applied and the stage of plant development at the time of stress. An experiment was conducted in which three low temperature regimes (-4°, -2°C and +3°C) were applied at 5 different stages of plant growth (from 1 day before anthesis to 9 days after anthesis) for a period of 6 hours with pre- and post- conditioning periods of 6 and 4 hours respectively. The results showed that the minimum temperature reached determined the nature and severity of temperature injury in Karamu wheat. Complete floret sterility was evident when a -4°C temperature was imposed at the pre-anthesis or anthesis stages of plant development; florets in any position of the head being equally affected. A -4°C temperature stress applied 3 days after anthesis produced 50% and 5% seed formation in primary and secondary heads, respectively. This seed formation mainly occurred in the basal florets of the apical and central spikelets of the head, however the seeds formed did not develop after stress and subsequent viable seed yield was zero. At the later stages, 6 or 9 days after anthesis a -4°C temperature stress had no significant effect on seed numbers. However there was a substantial negative effect on seed development and viability so that subsequent viable seed yield was zero. Temperature stresses of +3°C and -2°C had no significant effects on seed formation, development and viable seed yield when stresses were applied at any of the stages of plant development tested. The percentage of seed formation was highest in the two basal florets of the central and apical portions of the head compared to that in the two basal florets of the bottom of the head and to the distal florets of all spikelets. The percentage sterility in terms of relative sterility (percentage 'D + R' type ovules) and sterility index (percentage 'D' type ovules) was also described. It was found that in 'Karamu' wheat 16% to 33% rudimentary florets were a common feature, such structures included tiny basal, sterile spikelets and the terminal florets of all spikelets. Morphological and anatomical differences in ovules harvested at different stages of development from different treatments were observed. Ovules were classified into 6 groups for assessment of seed development. (A = apparently not fertilised, B = swollen and conical shaped, C = developing, D = shrivelled and shrunken, E = shrunken with reduced conical shape, R = rudimentary). Possible pathways to seed formation and development can be estimated from the data. A probable pathway to normal seed development is A to B to C. However, in the case of unsuccessful seed formation and development, the pathway is likely to be A to D,A to B to D or A to B to C to D. Further detailed electron microscope work is needed to enable a complete description and understanding of the pathways of seed development in stressed and unstressed plants. Such knowledge is needed to provide a logical basis for the development of cultivars with increased cold tolerance, fertility and yield