Varietal Differences in Tomato Intumescence under Changing Water Conditions

Damage caused by intumescence has recently become a serious issue in tomato-producing areas (Wu et al., 2017; Misu et al., 2018). Intumescence is a physiological disorder and non-pathogenic disease that occurs in many plant species (Eguchi et al., 2016). These species include members of the Solanaceae family, such as the eggplant (Solanum melongena) (Eisa and Dobrenz, 1971), potato (Solanum tuberosum L.) (Petitte and Ormrod, 1986), and tomato (Solanum lycopersicum L.) (Lang et al., 1983). In tomato, intumescence causes abnormal outgrowths of the leaf epidermal and palisade parenchyma cell walls, and of the petiole or stem surfaces during early seedling growth or during cultivation after transplanting under greenhouse conditions. Blisters occur on the leaf abaxial surfaces in mild cases of intumescence (Fig. 1A, B), deformities of compound leaves can develop as the condition worsens (Fig. 1C, D), browning and necrosis appear in more severe cases, and leaf abscission occurs in extreme cases, resulting in a significant decrease in growth. It has been reported that there are differences between varieties in the occurrence of tomato intumescence (Ozawa et al., 2018). However, it is not clear why there are differences between varieties. Although intumescence reportedly results from cell hypertrophy and rupture (Balge et al., 1969; Eisa and Dobrenz, 1971; Lang and Tibbitts, 1983; Lang et al., 1983; Wetzstein and Frett, 1984; Pinkard et al., 2006; Craver et al., 2014; Suzuki et al., 2020), the underlying causes are not yet fully understood. Previous studies have indicated that a high relative humidity, high root medium water content, or a combination thereof are the causes of intumescence (Metwally et al., 1970; Eisa and Dobrenz, 1971; Misu et al., 2018). These reports have suggested that excess turgor pressure may be the primary cause of intumescence. Since intumescence involves the swelling and rupture of cell walls, it is likely that sudden variations in the plant water potential will influence the onset of intumescence. Plant water potential has been shown to be closely related to the water environment (Kramer and Boyer, 1995). For example, the water potential of tomato plants has been shown to be affected by the relative humidity and soil moisture content when grown in controlled climate chambers (Araki, 1993), and by attributes of the water environment, including weather and soil water suction pressure (pF value), during cultivation under field conditions (Fusao, 2003). Furthermore, the water potential of tomato plants is affected by water absorption and transpiration rates, as influenced by atmospheric and soil water potentials (Zhang et al., 2017). Lang and Tibbitts (1983), however, reported no differences in intumescence incidence at relative humidity levels of 30%, 80%, and 92%. Considering these findings, we proposed that intumescence does not occur merely owing to the persistence of high levels of humidity and soil moisture content, but rather because of a sudden fluctuation of these ambient variables from low to high levels. Tomato varieties exhibit different responses in water potential to changes in the water environment (Torrecillas