Heat tolerance evaluation in commercial African violet cultivars using physiological and pollen parameters

• First report on the evaluation of heat tolerance in commercial African violet cultivars.
• Thirty commercial cultivars of African violets were evaluated for heat tolerance using physiological and pollen parameters.
• The cultivars ‘Rob's Ooey Gooey’ and ‘Mac's Stop Sign’ exhibited greater vegetative and reproductive tolerance to high temperatures based on their cumulative temperature response indices.
• The identified cultivars are potential candidates to be incorporated into future breeding programs to produce novel heat tolerant genotypes.
• Screening process can be applied to a large number of African violet cultivars to identify additional heat tolerant individuals.

High temperatures negatively affect the vegetative and reproductive growth of African violets thus decreasing their aesthetic and commercial values. No scientific information is currently available on cultivar tolerance to high temperatures in this plant. This study was conducted with the objective of evaluating heat tolerance in commercial African violet cultivars using a combination of physiological and pollen parameters and ranking the cultivars based on their cumulative temperature response index (CTRI). Physiological parameters including cell membrane thermostability (CMT), canopy temperature depression (CTD) and chlorophyll stability index (CSI) were analyzed using fully expanded leaves. In vitro pollen germination percentage (PG) and pollen tube length (PTL) were estimated at between 10 and 40° C. Cardinal temperatures (Tmin, Topt, and Tmax) of PG and PTL, as well as maximum PG percentage (PGmax) and maximum PTL (PTLmax), were estimated using the modified bilinear model. Results showed the different cultivars varied significantly for their CMT (5.39–74.07%), CTD (−1.65 to 0.61° C) and CSI (70.16–81.06%). They also differed for PG (mean of 54.43%), PTL (mean of 3109.08 μm), and cardinal temperatures (Tmin = −2.5° C, Topt = 24.9° C, and Tmax = 49.9° C for PG; Tmin = 3.4° C, Topt = 26.0° C, and Tmax = 43.1° C for PTL). The CTRI of each cultivar was calculated as the sum of individual stress response index (ISRI) derived from CMT, CTD, CSI, and PGmax, PTLmax, Tmin, Topt, and Tmax for PG and PTL. Among the thirty cultivars screened, the cultivars ‘Rob's Ooey Gooey’ and ‘Mac's Stop Sign’ had high CTRI values for the physiological and pollen parameters, singly or in combination, so are potential candidates to be used for the development of novel heat tolerant genotypes.