Impact of environmental stress on reproductive development in sweet pepper (Capsicum annuum L.)
This investigation was aimed at determining the impact of environmental stresses such as high temperature, low irradiance and drought on reproductive development in sweet pepper, particularly var. Blue Star. Special attention was given to abscission of the primary and secondary flowers. The role of assimilate accumulation and partitioning and the endogenous growth regulator ethylene in mediating stress effects on flower abscission were investigated. The hypothesis that flower abscission is promoted by these stress factors and that abscission is mediated by increased ethylene production and reduced assimilate partitioning to the flowers was tested. Imposition of a mean daily temperature (26 °C) from the third true leaf stage accelerated the development of the first primary flowers to anthesis when combined with high irradiance (4.9 MJ m-² d-¹). However, abscission was increased by 17 % as compared to lower temperature treatments at the same irradiance. The combination of high temperature and low irradiance (2.4 MJ m-² d-¹) induced complete abscission of the primary flowers. Although flower abscission was reduced at the lowest temperature examined (14 °C), development of the primary flowers to anthesis was slower than at higher temperatures. Both varieties, Blue Star and Bell Boy, were able to grow over a wide range of temperatures, as indicated by the large difference (c. 35 °C) between the base and maximum temperatures for growth indicated by a germination trial. In Blue Star, the base (Tb), optimum (To) and maximum (Tm) temperatures were 6.0, 27.5 and 41.5 °C respectively, whereas in Bell Boy, the corresponding values were 8.5, 23.0 and 44.0 °C. Severe water stress imposed progressively after the appearance of the first flower bud promoted the initial development of the primary, but not the secondary flowers to anthesis, but induced early and increased abscission of both primary and secondary flowers shortly afterwards. The high percentage abscission of the primary flowers was partially offset by the lower abscission of secondary flowers. Percentage abscission increased as the severity and duration of water stress increased. However, short exposures to stress did not reduce abscission, or advance anthesis. A more advanced stage of flower development (4.0 mm diameter) proved more susceptible to early abscission than younger flower buds (1.0 mm) when exposed to severe stress. Temporary osmotic adjustment occurred soon after the imposition of water stress, during which osmotic potential decreased sharply from -1.15 to -1.80 MPa, and noticeable reductions in turgor were observed in all treatments between 11 - 22 d after the imposition of stress. Although water stress reduced vegetative growth under low irradiance, complete flower abscission occurred after anthesis. The advancement of anthesis in stressed plants was associated with a decrease in dry matter accumulation in the leaves and stems. However, at the onset of flower abscission, assimilate accumulation and partitioning were not significantly affected by water stress, and flower abscission was not directly related to any reduction in assimilate production or its distribution within the shoot. Instead, prior to flower abscission in severely stressed plants, ethylene evolution in the flowers increased by 8-fold as compared to unstressed plants, and by 40-fold relative to severely stressed plants measured just before anthesis. The application of the ethylene releasing substance, 2 chloroethylphosphonic acid (CEPA), mimicked the effects of severe water stress, as reflected by a surge in ethylene evolution prior to abscission, followed by increased bud abscission. Sweet pepper flowers were also capable of forming abscission zones at the base of their pedicels in response to elevated ethylene production, whilst mature leaves were apparently incapable of this response. Foliar application of silver thiosulphate (STS) to water stressed plants and STS pre-treatment of plants subsequently sprayed with CEPA blocked the action of elevated ethylene resulting from severe stress or CEPA application in inducing flower abscission.