Heat-stress effects on reproduction and seed set in Linum usitatissimum L. (flax)

Heat stress can detrimentally affect the reproductive capacity of many plants. The effect of a 7 or 14 d heat stress on flowering, seed set, pollen viability and germinability of flax (Linum usistatissimum L.) was assessed under growth chamber conditions. An incremental (2 degreesC/h), cyclical (daytime high 40 degreesC and night-time low 18 degreesC) heat stress was applied 12 d after the initiation of flowering. Although flower formation in flax was not affected by heat stress, boll formation and seed set were reduced with onset of the heat stress. On removal of heat stress the stressed plants showed a compensatory response, flowering and producing bolls at a greater rate than the control plants. Heat stress significantly prolonged flowering by 17 d. Boll weight and seed weight were reduced with heat stress and the number of malformed, sterile seed increased three-fold after 14 d of heat stress. Pollen viability and appearance were negatively affected after 6 and 10 d of heat stress, respectively. Pollen germinability decreased by the sixth day of heat stress, with no pollen germinating by the tenth day. Effects of heat stress on pollen viability and germinability alone, which did not occur until after the sixth day of the stress, could not account for the decreased boll formation due to heat stress in flax. These observations suggest that a combined effect of heat stress on both pollen and ovules contributes to decreased boll formation and seed set in flax.