  Citation: Farinon, B.; Picarella, M.E.; Mazzucato, A. Dynamics of Fertility-Related Traits in Tomato Landraces under Mild and Severe Heat Stress. Plants 2022, 11, 881. https://doi.org/10.3390/ plants11070881 Academic Editors: Amalia Barone and Frederic Delmas Received: 15 March 2022 Accepted: 22 March 2022 Published: 25 March 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). plants Article Dynamics of Fertility-Related Traits in Tomato Landraces under Mild and Severe Heat Stress Barbara Farinon , Maurizio E. Picarella and Andrea Mazzucato * Department of Agriculture and Forest Sciences, University of Tuscia, 01100 Viterbo, Italy; b.farinon@unitus.it (B.F.); picarella@unitus.it (M.E.P.) * Correspondence: mazz@unitus.it; Tel.: +39-0761-357370 Abstract: Studies on the reproductive dynamics under heat stress are crucial to breed more tolerant cultivars. In tomato, cultivars, breeding lines, and wild species have been evaluated for their response to heat stress. Here, we addressed the study to a panel of selected landraces representing traditional genotypes that usually show high adaptation to local environments. In two experiments, spaced by 12 years, we set-up an identical experimental design with plants transplanted at two different dates to expose the second field to thermic stress with natural fluctuations. Such a strategy resulted in both a mild and severe stress in the two years. The landraces showed wide variation for both vegetative and reproductive traits; all traits were affected by heat, mostly with a significant Genotype*Environment interaction. A high broad-sense heritability was estimated for plant height, stigma position, pollen viability, and fruit weight. Low heritability estimates were found for the number of flowers, fruit set, and yield. Despite the interaction, traits recorded under control and heat conditions were positively correlated. Multivariate analysis located the genotypes in a topography that was stable under all conditions, except under the harshest temperatures. The study revealed that landraces present a wide variability for the response of reproductive traits to thermic challenges and that such a variation could be useful to dissect the traits with higher heritability and identify quantitative trait loci for breeding more resilient varieties. Keywords: fruit set; heat stress; Solanum lycopersicum L.; stigma position; tomato 1. Introduction Due to their sessile nature, plants are highly vulnerable to harsh environmental conditions such as increasing temperatures. It is thought that the global scenario of planet warming may have significant consequences on the life of plants with serious implications in agricultural crops [1]. Tomato (Solanum lycopersicum L.) is one of the world’s most important vegetable crops. Due to the high value of its fruit in terms of versatility, making it suitable for fresh consumption and for numerous types of processed products, tomato is an important dietary source of nutritional and health-related compounds [2]. Although tomato plants can grow under a wide range of temperatures, optimal reproduction and fruit set are limited when the day’s maximum exceeds 32 ◦ C and the night’s minimum falls below 21 ◦ C[3]. High temperature impairs plant growth at both the vegetative and the reproductive level [4,5]. At the vegetative level, photosynthesis is very sensitive to high-temperature stress, and heat is one of the most important causes of dry matter reduction in many crops [6,7]. Estimates range up to a 17% decrease in crop yield for each degree Celsius increase in average growing season temperature [8]. In many vegetables, such as tomato, reproductive development is more sensitive to and therefore more vulnerable to heat than the vegetative growth [9,10]. Heat effects indicate that in tomato, the male function is generally more exposed to damage than the female one [3,11,12]. The most sensitive stage is at early stamen development, when the Plants 2022, 11, 881. https://doi.org/10.3390/plants11070881 https://www.mdpi.com/journal/plants