Tomato Plants: Does age of seed affect the amount of fruit?



• Background and Objectives To better understand the regulation of fruit growth in response to environmental factors, the effects of temperature and plant fruit load on cell number, cell size and DNA endoreduplication were analysed.

• Techniques Plants were grown at 20/20 °C, 25/25 °C and 25/20 °C day/night temperatures, and inflorescences were pruned to two (‘2F’) or five (‘5F’) flowers.

• Major Findings and Conclusions Despite a reduced fruit development rate at 20/20 °C, temperature had no effect on ultimate fruit size due to cell number and size adjustment. The larger cell number at 20/20 °C (90 106 vs. 79 106 at 25/25 °C and 77 106 at 25/20 °C) was attributable to a longer duration of cell division, while the smaller cell size was related to a shorter period of expansion rather than a reduced expansion rate. By contrast, the lower fruit growth rate and size of 5F fruits compared with 2F fruits resulted from the slow down of cell expansion, whereas the number of cells was hardly affected in the proximal fruit. Nevertheless, the decreasing gradient in fruit size from proximal to distal fruits within the inflorescence was caused by a reduction in cell number with comparable cell size. Fruit size fluctuations within each treatment were always positively linked with cell number variations, but not with cell size variations. Negative relationships between cell size and cell quantity indicated that tomato pericarp cells may be seen as a population of competing sinks. By comparing 5F fruits to 2F fruits, mean ploidy was somewhat delayed and decreased. It reached a high of 25/25 °C and a low of 25/20 °C. The treatments did not influence ploidy and cell size in the same manner, although there were positive associations between mean ploidy and cell size within each treatment, however only in the 2F-25/20 treatment.



Much attention has been paid to the environmental influence on fruit growth in greenhouse tomato crops, and optimum temperature and light regime have been defined for fruit production (Pearce et al., 1993; Adams et al., 2001; Adams and Valdés, 2002). Since diverse mechanisms are involved in the regulation of growth throughout fruit ageing, the sensitivity to environmental fluctuations is predicted to vary during fruit development, as shown by temperature (De Koning, 1994; Adams et al., 2001). Moreover, compensation among the many components of growth may result in an underestimating of the fruit response. For example, a rise in temperature effectively increases the maximum tomato growth rate, but this is offset by a shorter growth time, thus fruit weight may not be considerably altered (Ho, 1996; Adams et al., 2001). To improve our knowledge and management of fruit growth in response to environmental changes, we must better explain the different mechanisms involved during fruit development.

Tomato is a fleshy fruit made up of many tissues: the epidermis, the pericarp (meat), and the placenta, as well as locular tissue containing seeds (pulp). The growth of pericarp tissue, which accounts for more than two-thirds of the total fruit weight, and biophysical constraint by epidermal extensibility cause an increase in fruit volume (Thompson, 2001). (Ho and Hewitt, 1986). Tomato growth is influenced by both division and expansion activity in pericarp tissue. Cell division in the pericarp is confined to a brief time of fruit development and is localized in the exterior tissue surrounding the vascular bundles and in the hypodermis, while cell division in the epidermis occurs throughout fruit development. After cell division ceases, cell expansion takes over as the primary method of increasing fruit size. Large endoreduplicated cells are seen in the tomato mesocarp (Bünger-Kibler and Bangerth, 1983).

Endoreduplication is an incomplete cell cycle that leads to the increase of nuclear DNA content (D’Amato, 1964; Galbraith et al., 1991), which in fruit pericarp reaches levels up to 256C (C is the haploid nuclei DNA content) in cherry tomatoes as well as in large-size fruit cultivars (Bergervoet et al., 1996; Joubès et al., 1999). Endoreduplication may be involved in fruit growth regulation since it has been proposed to determine the size limit of a cell (Traas et al., 1998). Endoreduplication and cell size were shown to be positively related in Arabidopsis epidermal cells (Melaragno et al., 1993). Endoreduplication in cotyledon cells and seed dry weight or mean cell volume were shown to have a linear relationship across pea seed genotypes (Lemontey et al., 2000). Endoreduplication and fruit size have minimal experimental evidence of a direct link in tomato (Bünger-Kibler and Bangerth, 1983; Bertin et al., 2003). Yet, since endoreduplication has received little attention, it cannot be ruled out that it is involved in the regulation of cell development in response to environmental changes.

Studies of fruit growth in response to environmental variations have been mainly focused on fruit expansion processes (Ehret and Ho, 1986; Pearce et al., 1993; Adams et al., 2001), though the very precocious control of final fruit size by cell division prior to anthesis has been well documented at the fruit level (Bohner and Bangerth, 1988; Ho, 1996) and also at the gene level (Frary et al., 2000). A reduction in the plant’s source: sink ratio reduces eventual fruit size by lowering both cell quantity and cell size (Bohner and Bangerth, 1988; Bertin et al., 2003). Many studies have described the effects of temperature on cell cycle duration, cell division rate and cell expansion rate in root or shoot meristems and in leaves (Brown and Rickless, 1949; Lopez-Saez et al., 1966; Francis and Barlow, 1998; Granier et al., 2000; Tardieu and Granier, 2000), but more rarely in fruit, though temperature is the primary climatic factor affecting tomato fruit growth rate (Walker and Ho, 1977; Pearce et al., 1993; Peet et al., 1997; Willits and Peet, 1998).

The reaction of tomato fruit development to temperature and plant fruit load was investigated in this research, as well as the cell number, cell size, and nucleus DNA endoreduplication in fruit pericarp.


Plant material and cultural conditions

The experiment was conducted out under controlled settings in an 875-m2 (21-m3) growth climatic chamber. Seeds of tomato ‘Raïssa’ were sown in sand, and 12 homogenous plants were pricked out at a developmental stage of about four or five visible leaves, in 10-dm3 pots filled with a balanced oxygenated nutrient solution, whose composition was checked every week and readjusted when necessary. Sowing was done in the growing climatic chamber itself, under comparable climatic circumstances to those observed after planting. Artificial illumination was provided by metal halide lamps. A 12-hour photoperiod with a photon flux of roughly 500 mol m2 s1 PAR above the canopy was used. The relative humidity of the air was kept at roughly 70%. During the light time, air was supplemented to 800 l CO2 l1 from the anthesis of the first truss. An electrical shaker was used to pollinate the flowers as they opened, and all side shoots were plucked as they emerged.

Experimental treatments

Three successive experiments were conducted under the same controlled conditions except the day/night air temperature regime which was successively set to 20/20 °C, 25/25 °C and 25/20 °C ± 0·5 °C. In all trials, the temperature of the nutritive solution was kept at 22 °C. In each experiment, inflorescences were pruned to five flowers (‘5F’ treatments) on six plants and to two flowers on the other six (‘2F’ treatments). Pruning was performed at 50% anthesis of the truss. Two leaves above the ninth truss, the plants were topped.

Observations and measurements

From appearance until fruit set, the developmental stage of each individual flower bud was documented twice a week (about 5 mm diameter). Anthesis was defined as complete flower opening. As fruits developed, the equatorial diameter of the first and second fruits (F1 and F2) on plants pruned to two flowers per truss, and the diameter of the first, third and fifth fruits (F1, F3 and F5) on plants pruned to five flowers, were recorded once a week with a caliper square.

When the first truss matured, fruits F1 and F2 in the 2F treatments and F1, F3, and F5 in the 5F treatments were sampled. The number of pericarp cells was counted on the first seven trusses of all of these fruits (excluding F2 in the 2F-25/20 °C treatment). Cell division had been finished in the first four trusses at this point. The ploidy level of pericarp cells was determined on ovaries and fruits of various ages collected from the first eight trusses at the first (F1) and fifth (F5) positions. By dividing the pericarp volume (as assessed by water displacement) by the total number of pericarp cells, the mean cell volume was calculated. Previous measurements of cell area on pericarp slices (using the method described in Bertin et al., 2003) confirmed that this ratio is a good predictor of cell size, especially given that the total intercellular space of tomato pericarp is relatively small (N. Bertin, unpubl. res.) and unlikely to be affected by treatments.

After tissue dissociation, the number of pericarp cells was determined using a technique derived from Bünger-Kibler and Bangerth (1983). Bertin et al. provide details on the approach used (2002). The ploidy level was determined using a PARTEC flow cytometer (PARTEC Ploidy Analyzer PA, GmbH, Germany), which was outfitted with an HBO lamp for UV illumination (Bertin et al., 2003). Because of the tiny amount of material, three duplicate measurements were taken in ripe fruit, but only one in flower buds and immature ovaries. The mean endoreduplication level was estimated as follows:

equation M1

where n is the number of DNA content peaks in the sample (max = 8), Ci is the C value in the nuclei of peak ni (C1 = 2, C2 = 4, C8 = 256), Ni is the number of nuclei in peak ni, and Ntot is the total number of nuclei in the sample.

Statistical analysis

The effects of temperature or plant fruit load on cell number were analysed in interaction with the truss position (first four trusses) by two-way ANOVA (Jandel Scientific Sigmastat), and F-tests were used to determine the statistical significance. When significant effects were found, a Tukey test was used to compare all pairwise mean response comparisons.

Fruit growth and mean ploidy curves were fitted to three-parameter Gompertz and sigmoid functions. The least squares approach was used to estimate parameters (Jandel Scientific SigmaPlot). The difference between two (or more) treatments was tested by comparing the sum of the residual sums of squares for the two (or more) individual fittings (ΣSSi) with the residual sum of squares for the common fitting to pooled treatments (SSc) considering that the statistic:

equation M2

Fisher’s law is followed with (n 1)k and (Ndata k) degrees of freedom. Ndata is the total number of data, n is the number of individual regression and k is the number of fitted parameters for each regression (three for Gompertz and sigmoid functions).

The Pearson product Moment test was used to examine correlations between fruit weight, cell number, cell size, and mean ploidy on fruits older than 30 days after anthesis (daa) (Jandel Scientific Sigmastat). Correlation coefficients are reported in the legends to Figs 2, ​4 and ​6 and statistical significance of these correlations are given in the text.

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Correlations between cell size and cell number (A), cell size and cell number (B), and fruit fresh weight and cell size (C). Lines denote linear changes. Each point is an individual fruit older than 30 daa sampled at the first or second position (F1 and F2) in the first four trusses of plants grown at 20/20 °C [open circles and dotted line R = −0·17 (A), 0·73 (B), 0·53 (C)], 25/25 °C [grey circles and dashed line R = −0·66 (A), 0·87 (B), −0·25 (C)] or 25/20 °C [black circles and continuous line R = −0·60 (A), 0·86 (B), −0·12 (C)] day/night temperature.

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Correlations between cell size and cell number (A), cell size and cell number (B), and fruit fresh weight and cell size (C). Lines denote linear changes. Each point is an individual fruit older than 30 daa sampled at the first position (F1) in the first four trusses of 2F [circles and full line R = −0·60 (A), 0·86 (B), −0·12 (C)] and 5F [triangles and dotted line R = −0·56 (A), 0·87 (B), −0·12 (C)] plants grown at 25/20 °C day/night temperature regime.

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Correlations between pericarp cell mean ploidy and fruit fresh weight (A and B), cell volume (C and D), and cell number (E and F). Lines denote linear changes. Each point is an individual fruit older than 30 daa sampled on the first four trusses at the F1 or F2 positions for the 2F treatments (circles) and at the F1 [open triangles and dotted line R = −0·29 (A), 0·22 (C), −0·29 (E)] or F5 [black triangles and dashed line R = 0·64 (A), 0·69 (C), −0·59 (E)] positions for the 5F treatment. Plants were grown at 20/20 °C [open circles and dotted line R = −0·25 (B), 0·007 (D), −0·33 (F)], 25/25 °C [grey circles and dashed line R = −0·58 (B), 0·54 (D), −0·72 (F)] or 25/20 °C [black circles and full line R = −0·11 (B), 0·66 (D), −0·43 (F) and triangles] Temperature throughout the day and at night.


The 2F treatments had the highest plant fruit load at all three temperatures. On the contrary for 5F treatments, numerous fruit abortions occurred in the 25/25 °C treatment and to a lesser extent in the 20/20 °C treatment, where four to five fruits actually set on each truss. To minimize misunderstanding caused by varying source-sink balances across treatments, the fruit load impact was only studied on the 25/20 °C treatment, while the temperature effect was studied only on the 2F treatments.

Effect of temperature on fruit growth in relation to cell number and cell size when carbon supply is non-limiting

At 20/20 °C, fruit growth (F1 + F2) was significantly decreased (P = 0·01) compared with 25/25 °C and 25/20 °C which were similar (Fig. 1A). The differences in the development pattern were not in the ultimate fruit size, but rather in the period of growth and the maximum growth rate.

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(A) Fruit size and growth rate were assessed on 2F plants cultivated at day/night temperatures of 20/20 °C (dashed line), 25/25 °C (dotted line), or 25/20 °C (continuous line). Fruit size measurements on the first and second fruits (F1 and F2) of the first four trusses were fitted using three-parameter Gompertz functions. The adjustment was done using pooled data from six plants (R2 > 095), and the vertical bars represent the standard error determined on individual plant adjustments. Derivative functions were used to calculate daily fruit growth rates. (B) Cell volume change throughout fruit maturation, calculated by dividing the pericarp volume by the total number of pericarp cells in 2F treatments. Each point is an individual fruit sampled at the first or second positions (F1 and F2) on the first eight trusses of four plants grown at 20/20 °C (open circles), 25/25 °C (grey circles) or 25/20 °C (black circles) day/night temperature.

For the three temperature regimes, the cell volume estimations on various ages of fruits and the total number of pericarp cells in the first four trusses of 2F plants are presented in Fig. 1B and Table 1. The onset of cell expansion was delayed by about 5 d at 20/20 °C compared with 25/25 °C and 25/20 °C (Fig. 1B), and similar rates of increase in cell volume (slopes of the linear part of the curves) at all temperatures led to lower final cell size at 20/20 °C. While the ultimate fruit sizes were identical throughout the three temperature regimes (Fig. 1A), the smaller cell size at 20/20 °C was offset by a greater number of cells. Moreover, the number of pericarp cells was larger at 20/20 °C in the first four trusses (P = 009), with no significant interaction with truss position (Table 1). The average number of cells on the first four trusses was 79% 106 and 77% 106 at 25/25 °C and 25/20 °C, respectively, compared to 90% 106 at 20/20 °C.

Table 1.

The number of pericarp cells (106), as well as the mean cell number in the first four trusses, were measured in the first (F1), third, and fifth fruits (5F plants).

Truss 1 Truss 2 Truss 3 Truss 4 Mean of four trusses
2F-20/20 °C F1 9·65 ± 2·79 9·74 ± 1·30 9·53 ± 2·79 7·09 ± 1·58 9·00 ± 1·25
2F-25/25 °C F1 6·91 ± 1·85 7·73 ± 1·98 8·87 ± 0·52 7·91 ± 1·25 7·86 ± 0·79
2F-25/20 °C F1 6·43 ± 2·08 8·65 ± 1·24 7·78 ± 1·16 7·94 ± 0·61 7·70 ± 0·91
5F-25/20 °C F1 5·28 ± 1·34 10·05 ± 1·40 9·02 ± 1·75 9·75 ± 1·49 8·52 ± 2·49
5F-25/20 °C F3 6·18 ± 2·84 5·15 ± 1·78 5·34 ± 2·14 5·96 ± 2·47 5·66 ± 0·50
5F-25/20 °C F5 4·69 ± 2·03 3·45 ± 1·26 5·25 ± 0·94 5·02 ± 0·15 4·60 ± 0·91

Fruits older than 30 daa and near to their ultimate size were investigated to determine the relationship between fruit size, cell number, and cell size. A negative connection between cell size and cell number revealed that temperature treatments compensate for cell size and cell quantity (Fig. 2A). The negative correlation also held within the 25/25 °C (P < 0·01) and 25/20 °C (P = 0·053) treatments, indicating that this compensation was not a specific response to temperature. Similarly the variations of fruit weight within each temperature treatment were positively correlated with the variations in cell number (P < 0·001 at the three temperature regimes), but not with those in cell size (Fig. 2B and C), except at 20/20 °C (P = 0·02) where the correlation was significant only due to one extreme point

Effects of plant fruit load on fruit growth in relation to cell number and cell size

The effects of plant fruit load on F1 in the 25/20 °C treatment were investigated. Because of the slower development rate, global F1 growth assessed on the first four trusses was considerably (P 0 05) lower in the 5F treatment compared to the 2F treatment (Fig. 3A). In the rise of cell volume during fruit ageing, a noticeable difference was detected between the two treatments (Fig. 3B). Unlike the temperature impact (Fig. 1B), the smaller cell volume in 5F fruits was more likely related to a decrease in cell growth rate beginning about 25 daa.

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(A) Fruit size and fruit growth rate of the first fruit (F1) of 2F (continuous line) and 5F (broken line) plants grown at 25/20 °C. For the first four trusses, three-parameter Gompertz functions were fitted to fruit size data. The adjustment was done using pooled data from six plants (R2 > 085), and the vertical bars represent the standard error determined on individual plant adjustments. Derivative functions were used to calculate daily fruit growth rates. (B) Cell volume evolution during fruit aging. Each point represents an individual fruit sampled at the first position (F1) on the first eight trusses of four 25/20 °C-grown 2F (circles) and 5F (triangles) plants.

The number of cells in the pericarp of F1 was not significantly different between the 2F and 5F treatments (Table 1), but it significantly varied among trusses (P < 0·01), the first truss containing fewer cells than the following ones (significant only in the 5F treatment). The amount of pericarp cells on the first four trusses was comparable in the two treatments (85 106 and 77 106 cells for 5F and 2F, respectively).

Analysis of fruits older than 30 daa showed that the negative correlation between cell volume and cell number was still present within each treatment (P = 0·053 and 0·059 for 2F and 5F, respectively), but no compensation occurred across the treatments (Fig. 4A), as was observed for temperature (Fig. 2A). Similarly, a substantial positive association (P 0 001) occurred within each treatment between fruit weight and cell number (Fig. 4B), but no significant link between fruit weight and cell volume was seen, either within each treatment or across treatments (Fig. 4C).

The plant fruit load had no effect on the number of cells in the pericarp of F1, but it had a substantial effect on the numbers of cells in the pericarp of F3 and F5 (Table 1). In the 5F-25/20 °C treatment, a two-way ANOVA was used to evaluate the truss and fruit impacts on cell number of the first four trusses. Except in the first truss, there was a large gradient in cell counts, with F1 having more cells than the other truss fruits. Differences among trusses were not significant and on the first four trusses the mean numbers of cells were 8·5 × 106, 5·7 × 106 and 4·6 × 106 in F1, F3 and F5, respectively. Compared with F1, the final fruit size was reduced by 12 % and 19 % in F3 and F5, respectively, and the fruit growth curves significantly (P < 0·05) decreased from F1 to F5, due to a decreasing gradient in fruit growth rate. This could not be explained by changes in estimated cell size, which did not differ significantly between fruits from the same inflorescence.

A positive association appeared between fruit weight and pericarp cell number in fruits older than 30 daa, but the compensatory relationship between cell number and cell size reported in F1 (Fig. 4A) did not hold for the third and fifth fruits of the truss.

Effect of temperature and plant fruit load on the ploidy level of pericarp cells and relationship with other fruit traits

Figure 5 depicts the dynamic of mean ploidy throughout fruit ageing as evaluated on the first eight trusses for fruit load and temperature treatments. Ploidy of fruit pericarp was similar in F1 and F5 fruits of the 5F treatment (not shown) and these data were pooled. The 5F-25/20 °C treatment initially delayed mean ploidy compared to the 2F-25/20 °C treatment, although ultimate results were near. Fitted curves differed considerably at the 5% error level, but not at the 1% error level.

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Mean C-value of pericarp cells assessed in F1 and F2 for 2F treatments (circles) and in F1 and F5 fruits for 5F treatments (squares) (triangles). Each point is the mean of three measurements performed on an individual fruit sampled on the first eight trusses of plants grown at 20/20 °C (open circles), 25/25 °C (grey circles) or 25/20 °C (black circles and triangles) day/night temperature. The lines reflect three-parameter sigmoid curves fitted to each treatment’s data (R2 > 084). The appropriate numbers of incomplete cell cycles are given on the right axis.

Temperature had a substantial effect on the dynamic of pericarp cell ploidy (P 1%). Mean ploidy was maximum at 25/25 °C, but surprisingly it was higher at 20/20 °C than at 25/20 °C. Other measurements made in the 5F treatmentsat 20/20 °C and 25/25 °C agreed with these patterns (not shown). Variations in DNA quantity between 25/20 °C and 25/25 °C correlated with less than one incomplete cycle.

Nuclear DNA content was quantified on some of the fruits used for cell counting, allowing for a correlative study of ploidy and other fruit attributes to be done on fruits older than 30 daa and almost at their ultimate size (Fig. 6). Fruit fresh weight was not substantially connected with the mean ploidy of pericarp cells in any of the treatments, and a negative association was seen, albeit not statistically significant, in the temperature treatments (Fig. 6A and B). Within each treatment, positive correlations between cell size and mean ploidy (significant only in the 2F-25/20 °C treatment P = 0·04; Fig. 6C and D) and negative correlations between cell number and mean ploidy (significant only in the 2F-25/25 °C treatment P = 0·013; Fig. 6E and F) could be noted. Only when comparing the 20/20 °C and 25/25 °C treatments did these associations hold across treatments, since this was the only situation where cell size and cell quantity really compensated for fruit weight. Only in such circumstance could the increase in cell size at 25/25 °C be linked to an increase in ploidy. The low ploidy level recorded at 25/20 °C for both the 2F and 5F treatments could not be linked to any fruit characteristics. At this temperature, the 5F treatment’s reduction in cell size was not related with low ploidy levels.


In accordance with the literature (Pearce et al., 1993; Adams et al., 2001) the maximum fruit growth rate was achieved at 25/25 °C (Fig. 1A). Actual fruit temperature was likely to be higher than 25 °C, since air temperature was monitored instead of fruit temperature (Adams and Valdès, 2002). High temperatures may have an indirect influence on fruit growth by affecting plant development, maintenance respiration, and assimilate availability. These indirect effects were avoided by limiting the investigation of temperature responsiveness to 2F plants.

Several publications observed minimal or negative impacts of increasing temperature on ultimate tomato size in the temperature range tested in this research, and ascribed this to balancing effects on the rates of fruit growth and fruit development (De Koning, 1994; Ho, 1996; Adams et al., 2001). The current study also found that this compensation is due to the inverse and compensatory effects of temperature on cell number and cell size (Fig. 2A). Temperature increases between 20 and 25 °C enhanced cell growth but slightly lowered cell number, therefore ultimate fruit size was unaffected (Fig. 1A). The prolonged length of cell division led in an increase in cell number at 20/20 °C (Fig. 1B). The differences in fruit growth patterns at 20/20 °C (slow) and 25/25 °C (accelerated) were caused by differences in the duration of the cell division period and the onset of cell expansion, which primarily shortened the period of cell expansion at 20/20 °C but did not significantly reduce the rate of cell expansion. This is consistent with Adams et al. (2001), who accelerated flower opening by heating flower buds to 25 °C. The shortening of the cell division period together with a reduction in the final number of cells at 25/25 °C, suggested that the proportion of cycling cells in the pericarp and its evolution during the division period were not affected by temperature between 20 and 25 °C. Indeed, given that the time required for a cell to divide decreases with increasing temperature, with a minimum duration of around 30 °C for many species (Francis and Barlow, 1988), a compensation between cell cycle duration and the proportion of cycling cells did not occur, as fruits contained fewer cells at 25/25 °C than at 20/20 °C.

In terms of fruit development, cell quantity and size, fruits grown at 25/20 °C were comparable to those grown at 25/25 °C. Tomato plants are known to integrate day/night temperatures in terms of fruit yield (Hurd and Graves, 1984; Peet et al., 1997) and thus the 25/20 °C treatment can be considered as a constant 22·5 °C temperature regime. However, cell number and cell size were similar in the 25/25 °C and 25/20 °C treatments, which is inconsistent with the absence of compensation between cell cycle duration and proportion of cycling cells, except if the cell cycle duration is already minimum at 22 °C in the tomato pericarp.

In contrast to what was observed in leaves of sunflower, tobacco and pea (Granier et al., 2000) cell division and tissue expansion did not have a common response to temperature in tomato, so that final fruit size and cell number were not correlated throughout the temperature treatments (Fig. 2B). Yet, within each treatment, changes in cell quantity were always closely and positively linked with changes in fruit size, although changes in cell size were not. Taking a given quantity of carbon and water availability for fruit development into account, cell division has a dominating role in determining intra-treatment variations of fruit size, which much outnumber inter-treatment variances.

Reduced plant fruit load (2F vs 5F) increased F1 fruit development rate and ultimate fruit size by boosting cell expansion without any inverse compensatory effects on cell quantity. Nevertheless, the lack of impacts of fruit load on cell number might be attributed to the fact that the comparison of 2F and 5F treatments only included F1. Likewise, F3 and F5 had much fewer cells than F1 (Table 1) and are much more vulnerable to assimilation competition than F1 (Bangerth and Ho, 1984; Bohner and Bangerth, 1988; Bertin et al., 2003). Plant defoliation, for example, causes a drop in both cell quantity and cell size that is more pronounced in distal fruits than in proximal fruits (Bohner and Bangerth, 1988). As observed by these authors on control trusses with six fruits, the cell size did not vary among fruits within the same truss in the 5F treatment, so that the gradient in fruit size could be totally attributed to the gradient in cell number. Results revealed that the competition for assimilates in a truss with six fruits, as is now being undertaken in a large-size tomato, is insufficient to alter cell growth in distal fruits. It is claimed that the many cells in F1 competed for assimilates, however the low number of cells in F3 and F5 may be adequately supplied to develop as much as the numerous cells in F1. In such circumstance, a negative connection between cell number and cell size was to be predicted, which was shown to be true only for F1. Because of their limited quantity, pericarp cells were unlikely to compete in F3 and F5. It is worth noting that for F1 and F2, the negative connection between cell size and cell number remained consistent throughout treatments (less at 20/20 °C). As a result, cells from the same tissue may be seen as a population of sinks competing for the given assimilates. Any therapy that changes cell number without reducing the global supply of assimilates is predicted to have inverse effects on cell number and cell size, and so have no influence on fruit size. This was seen in the temperature treatments. Contrary to popular belief, any treatment that alters the supply of assimilates to fruits, such as truss trimming, with or without effects on cell number, should likewise influence fruit size.

Numerous research have been conducted to characterize the endoreduplication dynamic throughout the development of different species and plant organs, but it is still unknown whether endoreduplication plays a role in the regulation of cell proliferation (Sugimoto-Shirasu and Roberts, 2003). In leaf epidermis (Melaragno et al., 1993), seed (Lemontey et al., 2000), and flowers, a relationship between cell size and average C-value was discovered (Kudo and Kimura, 2002; Lee et al., 2004). The ploidy-regulation of cell cycle progression in yeast cells might explain the influence of endoreduplication on cell size (Galitski et al., 1999). The fact that endoreduplication occurs before cell expansion lends weight to the concept that endoreduplication drives cell proliferation (Traas et al., 1998). Endoreduplication begins extremely early in tomato fruit during the most intense stage of cell division and ends with the termination of cell growth (Fig. 5). Nevertheless, connections identified in yeast or leaf epidermis may not exist at the level of fruit tissue, such as tomato pericarp, which has millions of cells in various stages, some of which are still proliferating and others which are heavily endoreduplicated. A common thread running across all ideas, regardless of the regulating mechanism, is that the sooner mitotic activity ends, the greater the ploidy level, since endoreduplication begins as mitosis is inhibited. In that case the increase of endoreduplication at 25/25 °C compared with 20/20 °C would result only from the earlier cessation of mitotic activity at 25/25 °C, which agrees with the delayed onset of cell expansion at 20/20 °C (Fig. 1). Schweizer et al. (1995) demonstrated a considerable influence of temperature on maize endosperm endoreduplication to explain inter-seasonal fluctuations. Why was endoreduplication at 25/20 °C the smallest (Fig. 5)? If the cell division duration was the same at 25/20 °C as it was at 25/25 °C, endoreduplication would be the same in both treatments. This might be an artifact of the experiment, however findings comparing 5F treatments at the three temperature regimes were validated (not shown). Endoreduplication may be affected by day/night temperature changes, however this has never been documented. Temperature may potentially influence the pace of advancement of nuclei from lower C-value to higher C-value, contributing to an increase in tissue mean ploidy, although this has never been studied, even though the rate of progression among C-values is not constant (Schweizer et al., 1995).

Endoreduplication is not impacted by competition among fruits inside a truss or within trusses, even when cell size is reduced (Bünger-Kinbler and Bangerth, 1983; Bertin et al., 2003). Traas et al. (1998) proposed that the increase in nuclear DNA content provides a given amount of DNA to support a given future increase in mass, but the exact final size is rather defined by the fruit environment, so that endoreduplication is expected to be more or less loosely correlated with a range of cell size, as observed in Fig. 6 in the present study. Endoreduplication, rather than being engaged in cell growth regulation, may first decide the potential size of the cell by directing the changeover from full to incomplete cell cycle, although actual cell size would mostly rely on carbon supply to individual cells. Figures 6C and D show that despite minimal competition (2F treatments), the potential cell size indicated by DNA endoreduplication was not attained since cell size was the same at 25/25 °C and 25/20 with high and low ploidy levels, respectively. Endoreduplication seems to be a poor predictor of actual cell size in tomato pericarp, since the two do not react similarly to the fruit environment.


The technical help of B. Brunel and J. C. L’Hôtel made this study feasible.


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