Heat Stress-induced Abortion of Buds and Flowers in Pea: Is Sensitivity Linked to Organ Age or to Relations between Reproductive Organs?

Abstract A short period of heat stress can cause a significant increase in abortion frequency of floral buds and flowers in pea, but a large variability in sensitivity exists among reproductive organs (ROs) within a plant or between plants. We have analysed spatial distribution of abortion frequency in plants subjected or not subjected to heat stress in: (1) four controlled environment experiments in which apex temperature was increased to 31 °C for 6 h per day for 4 d (moderate stress); (2) one field experiment with a similar level of stress; and (3) one experiment with 2 d with 33/30 °C day/night (severe stress) in addition to treatment (1). Influence of neighbouring organs on sensitivity to heat stress was analysed by partial flower removal. Severe heat stress caused rapid interruption of RO development followed by abscission while, in moderate stress, at leat 4 d elapsed between cessation of stress and first symptoms of abortion. In both moderate stress and control treatments, abortion frequency followed a consistent pattern along the stem with no abortion on the lowest reproductive phytomeres, a temperature-dependent abortion frequency on intermediate phytomeres and 100% abortion on apical phytomeres. Regardless of temperature, this pattern was shifted by four positions if ROs were removed on the lowest four reproductive phytomeres, and no abortion was observed if young pods were sequentially removed. Moderate heat stress increased the effect of developing ROs on abortion frequency of younger ROs located above them. We suggest that the effect of mild stress should be viewed as the acceleration of a programme linked to the normal termination of phytomere production during the plant cycle, rather than as an abrupt event linked to stress.