Evaluation of the Effect of Day and Night Temperature Fluctuations in different Seasons of the Year on the Fruit Formation of Sweet Pepper (Capsicum annuum L.) Lines

Capsicum is a plant sensitive to temperature fluctuations at day and night, and temperature changes strongly affect the quality of the fruit. Identification of tolerant genotypes to temperature fluctuations that naturally produce parthenocarp and marketable fruit is important for use in breeding programs and the production of hybrids with appropriate fruit weight and size and high marketability.

In order to evaluate the reaction of the lines related to three populations of sweet pepper (A: red fruit, B: orange fruit and C: yellow fruit) obtained from five generations of self-polination (by generation management by single-seed bulk method), a greenhouse factorial experiment was conducted based on a completely randomized design with three different temperature conditions including optimal day and night temperature (day temperature 25± 2 and night temperature 20 ± 2 °C), low night temperature (day temperature 25± 2 and night temperature 11± 2 °C) and high day temperature (day temperature 40± 2 and night temperature 20 ± 2 °C). For this purpose, 100 lines from each population were planted in three separate greenhouses with the mentioned temperatures. Percentage of seedless fruit lines per population or Parthenocarp fruits (including seedless fruits that had at least 50% by weight of seeded and natural fruits and other seedless fruits that were deformed and small in size (knot) were removed), height Plant, day to ripening and number of fruit lobes per 100 lines of each population were measured in three different temperature conditions. Due to the fact that the lines within each population were different from the other population lines, so the data analysis was performed as a complex sequential-factorial design. Also, due to the importance of fruit characteristics in seedless fruit lines and seeded fruits, analysis of variance of these lines in a completely randomized design (15 treatments in 3 replications) using SAS v software 9.2 was performed and the comparison of the mean of the evaluated traits was performed using Duncan's multiple range test at 5% probability level.
 

The results showed that under optimal temperature conditions, all lines had good growth and no parthenocarpic plants were observed in the evaluated populations, but day and night temperature fluctuations outside the optimal temperature range caused significant changes in plant growth, fruit development. And seeds were formed. The effect of high day temperature on the evaluated characteristics was less than low night temperature. With a sharp drop in night temperature, population A produced the highest percentage of seedless fruit plants. The percentage of parthenocarp lines of populations B and C were significantly lower than population A at low night and daytime temperatures. Population C was less affected by adverse day and night temperatures than the other two populations. Fruit size, fruit weight and fruit shape index, which are the most important determinants of fruit marketing, were strongly affected by day and night temperature fluctuations. In all three populations evaluated, fruit length was significantly negatively affected by low night temperature more than high day temperature, which resulted in distortion of fruit shape index. Fruit shape index, which is the result of the ratio of length to diameter of fruit, in marketable fruits is 1-1.02. As the fruit length increases and the fruit diameter remains constant or decreases, the shape index increases from 1.02, and as the fruit diameter increases with decreasing fruit length, which is usually achieved under cold stress conditions, this number decreases below one. Based on the results, the three populations evaluated had different fruit lengths under optimal temperature conditions, which, with the proportion of fruit diameter to length, the fruit shape index was normal and produced marketable fruits. By decreasing the night temperature below the optimum growth temperature, fruit length decreased sharply in the three evaluated populations, and this decrease was greater in seedless fruits. According to Table 2, the highest percentage of fruit length reduction at low night temperature was observed in population A and in seedless fruits. In this temperature condition, fruit length decreased by 43% in seedless fruits and 17.5% in seeded fruits. The lowest decrease in fruit length at low night temperature was related to population C. Fruit length in seeded and non-seeded fruits of this population decreased by 12 and 24%, respectively. However, the percentage of fruit reduction in the total populations evaluated was 13.90 and 33.69% on average in seeded and seedless fruits, respectively. Although the length of the fruit was less affected by the high temperature during the day than the low temperature at night, but the trend of fruit length changes in these temperature conditions was similar to the low temperature at night. The average decrease in fruit length in the total population in seeded and seedless fruits was 10.41 and 31.52%, respectively, with population C having the least and population A having the most effect from unfavorable daytime temperature. Fruit weight was also affected by the unfavorable temperature of day and night, but the negative effect of low night temperature on fruit weight was more than the unfavorable temperature of the day. According to the results, the percentage of fruit weight loss in seeded and seedless fruits at low temperature at night was 21.19 and 50.06%, respectively, and at high temperature at day, 15.98 and 50.12%. As the results show, seedless fruits had the same effect of unfavorable temperature day and night and showed the highest percentage of weight loss. Also, fruit weight in population C showed the least effect of adverse temperature day and night and no significant difference was observed between populations B and A. Expression is associated with undesirable traits that can be due to the coherence of traits or pleiotropic effects of parthenocarpic genes or physiological or molecular changes. Although in population C the number of lines with Parthenocarp fruit was 1%, but Parthenocarp fruits consisting of size and shape index are more suitable than the other two populations. The C population also showed a low percentage of Knot fruits as well as slight differences in fruit weight and shape at low temperature at night and high temperature at day. Based on the results, the three populations evaluated have different potentials in terms of reacting to adverse low temperatures at night and high temperatures during the day, and this potential can be used in future research and breeding programs to produce hybrids that tolerate temperature fluctuations.