Introduction
Salinity is still one of the most serious environmental problems in agriculture worldwide (Ghassemi et al. 1995). At the crop scale, salinity reduces growth and productivity causing a serious decrease in yield. Many studies have analysed the effects of salinity on different functions at the leaf scale. The major impacts of increasing salinity are a reduction in leaf photosynthesis (Maggio et al. 2007), as well as in leaf transpiration and plant water status (Romero-Aranda et al. 2001; Maggio et al. 2004). In addition to these variations in leaf and plant function, an important aspect of salinity in plants is its strong effect on plant structure (Maggio et al. 2004; Romero-Aranda et al. 2006).
Increasing salinity has been reported to affect different morphological variables. For example, a reduction in stem elongation and thickening (Bartolini et al. 1991; Franco et al. 1993), total leaf area, leaf size and maximum leaf length (Maggio et al. 2004; Romero-Aranda et al. 2006), leaf growth rates (Munns 1993; Yilmaz et al. 2004, Chenu et al. 2008x), absolute and relative plant growth rates (Carneiro et al. 2004), and to increase leaf thickness (Sanchez-Blanco et al. 1991). The influence of salinity on organogenesis varies with the species. For instance, salinity decreases the number of leaves per plant in spring wheat (Grieve et al. 1994) but does not affect the number of leaves in lettuce (Jeronimo et al. 2005). In tomato, the number of inflorescences per plant, the number of flowers per inflorescence (Grunberg et al. 1995; Van Ieperen 1996) and fruit set, particularly on upper inflorescences (Adams and Ho 1992), are reduced by extreme salinity.
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All of the previous studies focused either on a local tissue or organ, or the whole plant or the whole crop. To our knowledge, no attempt has been made to study the effect of salinity starting from the architectural organs to the scale of the whole plant. The analysis of the effects of salinity on plant architecture can be performed starting from its architectural components up to the whole plant, including the developmental processes involved. A detailed analysis of these developmental effects is a prerequisite to predict the possible consequences of modifications in plant structure at the plant and canopy scale. Modelling plant architecture enables the integration of processes such as leaf initiation and elongation and their interactions, which could affect the development of the plant (Baker et al. 1985; Ritchie and Otter 1985). Plant architectural modelling has been used for tree (Kurth 1994; De Reffye and Houllier 1997) and herbaceous species (e.g., peas, Gould et al. 1992; cotton, Room et al. 1996; maize, Fournier and Andrieu 1998). However, only a few architectural models of tomato have been published (Dong et al. 2008).
Architectural modelling should enable investigation of the consequences of salinity on light interception and plant photosynthesis, including multiple interactions at different levels (tissue, organ, plant). A 3-D mock-up should be a useful way of obtaining an integrated vision of the architecture of the whole plant (Dauzat and Eroy 1997; Kurth 1994; Prusinkiewicz et al. 1997). Such a 3-D mock-up is required to link plant architecture to plant functioning by simulating light interception (Dauzat and Eroy 1997; Sinoquet and Rivet 1997; Sinoquet et al. 1998) and photosynthesis (Genard et al. 2000).
The objective of this work was to study how the developmental processes of the tomato plant are affected by salinity. Considering that the rachis internodes and the leaflet blades make up the composite leaf in tomato and assuming that the plant is a set of stacked modules (stem internodes, rachis internodes, and leaflet blades), the effects of salinity were described based on both the initiation of these modules (i.e., organogenesis processes) and their elongation (i.e., growth processes). We built a 3-D representation of the architecture of the tomato plant based on both quantitative and morphological observations. The model included two levels of information: (i) a typology of modules based on morphological and developmental characteristics; and (ii) a set of basic processes (initiation and growth). The model enabled us to link static and dynamic observations of the plant, and to scale the global consequences of the basic processes up to the whole plant. The simulated 3-D mocks-up were used to estimate light interception up the plant scale.
Materials and methods
Plant material and growth conditions The seeds of the two tomato varieties, 'Marmara' and 'Maxifort,' were germinated on 20 November 2005. Twenty days after germination, 'Marmara' was grafted onto 'Maxifort' rootstock. The hybrid [F.sub.1] 'Marmara' was planted on 10 January 2006 at the 3-4 leaf stage at a density of 2.40 plants.m-2 on 100 mm x 15 mm x 25 mm rock-wool slabs. The experiment was performed at Centre Technique Interprofessionnel des Fruits et Legumes (Ballandran, France, 43.75[degrees]N, 4.45[degrees]E), in four 250 [m.sup.2] greenhouse compartments. The nutrient solution contained (N-N[O.sub.3]) 15.0 mequiv. x [L.sup.-1], (N-N[H.sub.4]) 1.2 mequiv. x [L.sup.-1], (S-S[O.sub.4]) 5.5 mequiv. x [L.sup.-1], (Cl) 6.4 mequiv. x [L.sup.-1], (P) 2.3 mequiv. x [L.sup.-1], (K) 10.4 mequiv. x [L.sup.-1], (Ca) 13.5 mequiv. x [L.sup.-1], (Mg) 5.9 mequiv. x [L.sup.-1], (Na) 0.7 mequiv. x [L.sup.-1], with 3.5 mS x [cm.sup.-1] electrical conductivity (EC). Four treatments were applied (control; 4 mS x [cm.sup.-1], moderate; 7 mS x [cm.sup.-1], high; 10 mS x [cm.sup.-1]; and very high salt level: 13 mS x [cm.sup.-1]). These treatments are referred to below as T4, T7, T10, and T13, respectively. To obtain electrical conductivity of 4 mS x [cm.sup.-1] for the control, and 7 mS x [cm.sup.-1], 10 mS x [cm.sup.-1], and 13 mS x [cm.sup.-1] for the other treatments, sodium chloride was added to the same solution for all the treatments. Each treatment was applied in an individual compartment. Salinity treatment began just after initiation of the third inflorescence. Irrigation was controlled to obtain a 40% drainage rate. The drainage water was not recycled. To limit the development of apical necrosis, the K/Ca ratio of the treatments was decreased gradually from 0.5 (in January) to 0.1 (in May and June). The plants were trained to one stem, and older basal leaves were regularly thinned out before complete senescence, as is commonly done in commercial production. Flowers were pollinated three times a week …
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