Photosynthetic responses of heat-tolerant and heat-sensitive cultivars of impatiens hawkeri and viola x wittrockiana to high temperature exposures

Flowering of many plant species is reduced or inhibited by exposure to high temperature. This reduction in flowering may be due to reduced photosynthesis, resulting in a reduced carbohydrate supply for floral initiation and/or development. Photosynthetic light response curves for heat-tolerant and heat-sensitive genotypes of Impatiens hawkeri Bull. and Viola x wittrockiana Gams. were developed. Plants were moved from a 20°C growth chamber to another chamber maintained at 35°C for varying lengths of time, and at different times during the day or night. High temperature exposures were: 2 h during the middle of the day (1200-1400 h) or night (2400-0200 h), 12 h during the day (0700-1900 h) or night (1900-0700 h), 24 h (beginning at 0700 h), or 72 h (beginning at 0700 h). After high temperature exposure, plants were returned to the 20°C chamber. High temperature exposures as brief as 2 h reduced net photosynthetic (P_n) rates of some cultivars the day after the exposure ended. A 72 h exposure to 35°C reduced P_n rate of all four cultivars the day after the exposure ended. Impatiens hawkeri photosynthetic rate was reduced to a greater extent than Viola x wittrockiana, and did not recover to the same photosynthetic rate as the untreated plants, as occurred for Viola.