Abiotic stresses due to environmental factors could adversely affect the growth and development of crops. Among the abiotic stresses, drought and heat stress are two critical threats to crop growth and sustainable agriculture worldwide. Considering global climate change, incidence of combined drought and heat stress is likely to increase. The aim of this study was to shed light on plant growth performance and leaf physiology of three tomatoes cultivars (‘Arvento’, ‘LA1994’ and ‘LA2093’) under control, drought, heat and combined stress. Shoot fresh and dry weight, leaf area and relative water content of all cultivars significantly decreased under drought and combined stress as compared to control. The net photosynthesis and starch content were significantly lower under drought and combined stress than control in the three cultivars. Stomata and pore length of the three cultivars significantly decreased under drought and combined stress as compared to control. The tomato ‘Arvento’ was more affected by heat stress than ‘LA1994’ and ‘LA2093’ due to significant decreases in shoot dry weight, chlorophyll a and carotenoid content, starch content and NPQ (non-photochemical quenching) only in ‘Arvento’ under heat treatment. By comparison, the two heat-tolerant tomatoes were more affected by drought stress compared to ‘Arvento’ as shown by small stomatal and pore area, decreased sucrose content, ΦPSII (quantum yield of photosystem II), ETR (electron transport rate) and qL (fraction of open PSII centers) in ‘LA1994’ and ‘LA2093’. The three cultivars showed similar response when subjected to the combination of drought and heat stress as shown by most physiological parameters, even though only ‘LA1994’ and ‘LA2093’ showed decreased Fv/Fm (maximum potential quantum efficiency of photosystem II), ΦPSII, ETR and qL under combined stress. The cultivars differing in heat sensitivity did not show difference in the combined stress sensitivity, indicating that selection for tomatoes with combined stress tolerance might not be correlated with the single stress tolerance. In this study, drought stress had a predominant effect on tomato over heat stress, which explained why simultaneous application of heat and drought revealed similar physiological responses to the drought stress. These results will uncover the difference and linkage between the physiological response of tomatoes to drought, heat and combined stress and be important for the selection and breeding of tolerant tomato cultivars under single and combine stress.

Co-occurrence of cold and drought stress can alter the response of plants at morphological, physiological and molecular levels, which finally affect crop production, more than individual stress. Understanding the responses of crop to combined stress is necessary to improve tolerance and maintain crop production especially in the field where combined stress frequently occurs. We aimed to clarify the underlying leaf physiological and molecular mechanisms of tomato by imposing combining cold and drought on one popular tomato cultivar ‘Jinlingmeiyu’ as an example. The physiological and genetic responses were identified in tomatoes after 42 h exposure to control, cold, drought and combined treatments. As compared with control, water loss rate at the three stresses including cold, drought and combined stress significantly decreased until 40 min after taking samples from the plants. The content of H2O2, zeatin riboside (ZR) and melatonin in all stress treatments were significantly higher than the control. Drought stress alone and combined stress induced the accumulation of abscisic acid (ABA) and auxin (IAA) as compared with control. The individual cold and combined stress significantly decreased the maximum quantum efficiency of PSII (Fv/Fm), quantum yield of PSII (Fq′/Fm′) and electron transport rate (ETR). In total, 7141, 1850 and 7841 genes were involved in the stress response to cold, drought and their combination. Functional analysis of the stress-inducible genes provided more insights concerning the complex regulatory mechanisms that were involved in combined stress. The expression level of 12 genes were validated by quantitative real-time PCR (qRT-PCR). We found that the expression of stress-specific genes changed with physiological variation, indicating the close crosstalk between physiological and genetic response especially under combined stress. This study provides new knowledge on the complex regulatory mechanism genes in tomato (‘Jinlingmeiyu’) leaf to abiotic stresses.

Stevia (Stevia rebaudianaBertoni) is well known for its very sweet steviol glycosides (SGs) consisting of a common tetracyclic diterpenoid steviol backbone and a variable glycone. Steviol glycosides are 150–300 times sweeter than sucrose and are used as natural zero-calorie sweeteners. However, the most promising compounds are biosynthesized in small amounts. Based on Illumina, PacBio, and Hi-C sequencing, we constructed a chromosome-level assembly of Stevia covering 1416 Mb with a contig N50 value of 616.85 kb and a scaffold N50 value of 106.55 Mb. More than four-fifths of the Stevia genome consisted of repetitive elements. We annotated 44,143 high-confidence protein-coding genes in the high-quality genome. Genome evolution analysis suggested that Stevia and sunflower diverged ~29.4 million years ago (Mya), shortly after the whole-genome duplication (WGD) event (WGD-2, ~32.1 Mya) that occurred in their common ancestor. Comparative genomic analysis revealed that the expanded genes in Stevia were mainly enriched for biosynthesis of specialized metabolites, especially biosynthesis of terpenoid backbones, and for further oxidation and glycosylation of these compounds. We further identified all candidate genes involved in SG biosynthesis. Collectively, our current findings on the Stevia reference genome will be very helpful for dissecting the evolutionary history of Stevia and for discovering novel genes contributing to SG biosynthesis and other important agronomic traits in future breeding programs.