Mediterranean Long Shelf-Life Landraces: An Untapped Genetic Resource for Tomato Improvement

Abstract

The Mediterranean long shelf-life (LSL) tomatoes are a group of landraces with a fruit remaining sound up to 6-12 months after harvest. Most have been selected under semi-arid Mediterranean summer conditions with poor irrigation or rain-fed and thus, are drought tolerant. Besides the convergence in the latter traits, local selection criteria have been very variable, leading to a wide variation in fruit morphology and quality traits. The different soil characteristics and agricultural management techniques across the Mediterranean denote also a wide range of plant adaptive traits to different conditions. Despite the notorious traits for fruit quality and environment adaptation, the LSL landraces have been poorly exploited in tomato breeding programs, which rely basically on wild tomato species. In this review, we describe most of the information currently available for Mediterranean LSL landraces in order to highlight the importance of this genetic resource. We focus on the origin and diversity, the main selective traits, and the determinants of the extended fruit shelf-life and the drought tolerance. Altogether, the Mediterranean LSL landraces are a very valuable heritage to be revalued, since constitutes an alternative source to improve fruit quality and shelf-life in tomato, and to breed for more resilient cultivars under the predicted climate change conditions.

Keywords: Mediterranean landraces; drought tolerance; extended fruit shelf-life; fruit quality traits; gas exchange; tomato; yield.

Figure 1

Morphological variation in fruit size and shape in the Balearic LSL landrace ‘de Ramellet.’ Left: ‘de Ramellet’ fruit (blue dots) as compared to diverse tomatoes with variable size and shape, based on the first two Principle Components (73% of total variation explained) resulting from morphological data collected from transverse sections with Tomato Analyzer (Brewer et al., 2006). Each accession is represented by an average of all scanned fruits. Representative fruits are shown along the plot. PC1 and PC2 mainly explain fruit size and fruit elongation, respectively. Modified from Conesa et al. (2010). Right: Variation found in a prospection across the Balearic Islands to create the UIB-collection. Modified from Ochogavía et al. (2011).

Figure 2

Different ways to store hung long shelf-life tomatoes across the Mediterranean. (A) Typical ‘de Ramellet’ strings with fruit pedicels needle-sewn to a main rope (Banyalbufar, Mallorca; courtesy: Aina Socies - Associació de Varietats Locals de Mallorca). (B) ‘de Ramellet’ hung by pedicels on wild olive tree branches (Artà, Mallorca; courtesy: Toni Muñoz). (C) Needle-sewing of ‘de Penjar’ strings (Alcalà de Xivert, Castelló; courtesy: Associació de Productors i Comercialitzadors de Tomata de Penjar d'Alcalà de Xivert). (D) Typical Sicilian ‘da Serbo’ trusses (Sicily). (E) Typical ‘piennoli’ of ‘Pomodorino del Piennolo del Vesuvio’ (Ercolano, Napoli; courtesy: Rosario Custro).

Figure 3

Water use efficiency as inferred from δ¹³C (‰) isotopic composition of leaves from tomato plants grown under full irrigation (WW) and water deficit (WD) treatments in Fullana-Pericàs et al. (2019). Tomato accessions (n = 171) were separated in five groups depending on the accession type, including non-long shelf-life cherry (Cherry; n = 29), fresh market (Fresh; n = 50), long shelf-life from eastern Iberian Peninsula and Balearic Islands (LSL-big; n = 63) and from Italy (LSL-cherry;n = 16), and processing accessions (Processing; n = 13). (A) Percent of δ¹³C in WD as compared to WW. ANOVA differences between WD and WW were significant in all groups (P < 0.001). (B) Values of δ¹³C in WW and (C) in WD. For each accession group, boxplots represent the average and the median (red and black lines inbox, respectively), the 75% interval (box), the 90% interval (error bars) and the outliers (isolated points). In each plot, letters on top indicate ANOVA-Tukey differences among groups (P < 0.05).

Figure 4

Cross sections of a ‘de Ramellet’ leaflet formed under (A) well-watered and (B) water stress (WS) conditions. (C) Detail of the mesophyll cells and airspaces under WS. Notice the chloroplasts (bright) tightly positioned against the cell membrane and cell wall. (D) Magnification of a few cells in C to highlight intercellular airspaces, that have been whitened to ease visualization. Black bar represents 200 µm in A and B, 40 µm in C and 10 µm in D. All images from the experiment described in Galmés et al. (2013).

Figure 5

Relationship between intrinsic water use efficiency (net photosynthesis and stomatal conductance ratio, A_N/g_s) and the ratio between the mesophyll cell surface exposed to airspaces (S_c) and the Rubisco concentration. Dots are different “de Ramellet” accessions grown under well-watered (black, soild regression line) and water-stress (grey, dashed regression line) conditions. The triangles represent a processing accession used as control. Plotted from data in Galmés et al. (2013).

Figure 6

Resistance of ‘de Ramallet’ fruits during post-harvest storage. (A) Fruit with slight wrinkling due to water loss six months after harvest. (B) Fruit with important scarification a month after a microbial attack, showing the zone of attack (left) and the rest of the fruit (right) remaining intact, without wrinkling. (C) Fruit with an important microbial attack emptying half part of the fruit (left) but not the other part (right) which remains intact. Fruits as in B and C can remain as in the picture up to 6 months. (D) Fruit completely dry after suffering a microbial attack during postharvest storage. Notice that the dark part was attacked and that the clear half of the fruit maintained integrity until complete water loss. The fruit remains as in the picture for decades with no further deterioration.