Lulo is native to the highlands of the Andes mountains, specifically Colombia, Ecuador and Peru. It thrives in mild climates with low light, intercropped with coffee, at an elevation between 1 000 and 2 500 m (^{Medina et al., 2009}). The fruit of the lulo is similar to the tomato, has a bittersweet flavour and an orange colour with bright green flesh and has a mild flavour (^{Mejia et al., 2012}), features that enhance it in international markets.

The propagation of lulo is primarily by seed. Good-quality seedlings exhibit morphological characteristics such as thick stems, leaves dark green and large and soft roots (^{Oda, 2007}) and to increase quality, it is possible to ensure the production of healthy and vigorous plants that can also increasing yield and quality.

This study analyses the growth of seedlings in organic substrates, based on the calculation of growth rates. Growth analysis is a useful set of quantitative methods to describe and interpret the functioning of plants (^{Hunt, 2003}).

The experiment was conducted in a greenhouse located in Córdoba Campus of the Postgraduate College in Agricultural Sciences, 650 m, 18° 50' north latitude and 96° 51' west longitude. The local climate is mild-humid with rains in summer and average temperature of 20 °C, maximum 35 °C and minimum 10 °C, with an average annual rainfall of 1 807 mm (Esparza-Soto, 1986). The tests for this study were conducted during the months of October and November 2012, in a rectangular greenhouse with two roof covered with shadecloth waters, which allows a luminous transmittance of 70%. In this test period the average temperature was 18 °C, with maximum temperatures of 30 °C and 12 °C minimum.

The substrates tested were composed of peat and compost in different proportions (v / v, v / v): 60/40, 40/60 and 20/80, respectively, which were the treatments. Lulo seeds were germinated on these substrates, each replicated four times. The experiment was completely randomized, with four replications. The experimental unit consisted of 32 seedlings for each treatment (substrate).

The Table 1 shows the physical and chemical characteristics of the substrates used in the study. This parameters of pH and bulk density, reported by ^{Gómez-Merino et al. (2013)}, were taken into account, and analysed important complementary parameters related to salinity as electrical conductivity (EC), sodium, chloride and bicarbonate soluble.

Leaf area and dry biomass of leaves and roots: sixty days after sowing direct parameters such as growth were evaluated. Leaf area was measured with a LI-COR LI3100C integrator; from this were calculated according to that described by ^{Poorter et al. (2012)} the following growth rates: leaf area ratio (RAF), the total leaf area of the plant dry weight; specific leaf area (SLA), leaf area between the dry weight of leaves; ph leaf ratio, proportion of the total dry weight of biomass having leaves); and root ratio (PR), a proportion of total dry weight of biomass having the root). With the results, an analysis of variance and Tukey comparisons were made (α= 0.05) using the Statistical Analysis System (^{SAS, 2011}) software.

The easiest and most direct way to assess the effect a given environmental factor in the development of plants is to apply this factor to two or more levels in different subgroups over a period of time. With this, the detailed measurements can contribute to the understanding of how photosynthesis, respiration, specific leaf area and distribution of dry biomass are affected by the treatments ^{Poorter (2012)}. In this study, the levels of the compost (40, 60 and 80%) in the substrate were varied and the growth rate was calculated to evaluate the effects of these variations.

The RAF is an indicator of the balance between the potential photosynthetic capacity and respiratory cost; i.e. it is a measure of the photosynthetic capacity per unit of plant biomass (^{Amanullah et al. 2007}). This research shows that the increase in the proportion of compost in the substrate (80% v / v), significantly reduced the rate of leaf area; conversely, increasing the substrate peat increases this parameter as well. Numerically obtained 450 cm²g^-1 of RAF and 20/80 treatment to 650 and 730 to 60/40 and 40/60 respectively. It is worth noting that treatments 1 and 2 showed no significant difference between them (p> 0.05). These results suggest that the high concentration compost reduces the size of the leaves (Figure 1).

Figure 1 Ratio of leaf area (RAF) of lulo seedlings established in organic substrates made of peat and compost in different proportions. Mean ± SD with different letters indicate statistically signif icant differences (Tukey, p≤ 0.05) between treatments. 

The AFE (Figure 2) is an indicator of the relative thickness of the leaf and functionally and ecologically is a morphological feature of great importance, since it can account for up to 80% of the variation in growth rates (^{Villar et al., 2004}). This index showed the same trend observed in RAF. The highest average was recorded with the ratio 40/60 peat/compost in the substrate, followed by treatment with 60/40 peat/compost, with no statistical difference between these two treatments (60/40 and 40/60 peat/compost, v/v). These statistically exceed the processing with the highest proportion of compost (20/80 peat/compost) on average 32.93%.

Figure 2 Specific leaf area (SLA) lulo seedlings established in organic substrates made of peat and compost in different proportions. Mean ± SD with different letters indicate statistically significant differences (Tukey, p≤ 0.05) between treatments. 

The results of the PH index (Figure 3) show statistically significant differences between the three treatments, with the same general trend in RAF and AFE. The largest proportion of leaves was observed in plants under treatment 40/60 (peat/ compost), followed by treatment plants subject to 60/40, while the smaller proportion is observed in 20/80 treatment. The pH is an indicator of the proportion of total plant biomass distributed blades, which represents a measure of investment in photosynthetic organs (^{Villar et al., 2004}).

Figure 3 Leaf ratio (PH) lulo seedlings established in organic substrates made ​of peat and compost in different proportions. Mean ± SD with different letters indicate statistically significant differences (Tukey, p≤ 0.05) between treatments. 

The treatments significantly affected the root ratio (PR). Most PR were reported in the treatment with a higher proportion of compost in the substrate (80%) (Figure 4) while the lower PR was observed in plants treated with 40/60 (peat/compost). PR variations caused by abiotic factors are variable. For example, seedlings of lemon (Citrus limon) the PR was reduced as the salinity of the growth medium increased (House et al., 2003), while seedlings orange (Citrus sinensis), this increased to salinity (House et al., 2004). Furthermore, it is reported that high levels of PR may be advantageous in drought or nutrient limitation (^{Villar et al., 2004}).

Figure 4 Root ratio (PR) lulo seedlings established in organic substrates made ​of peat and compost in different proportions. Mean ± SD with different letters indicate statistically significant differences (Tukey, p≤ 0.05) between treatments. 

In this study, the lowest proportions of compost in the substrate (40 and 60%) resulted in lower value of PR, which, according to ^{De Grazia et al. (2006)} indicates a more efficient absorption of water and nutrients to meet the demand of seedlings.

The high proportion (80%) of compost in the substrate significantly reduced growth rates, mainly due to elevated EC, Na⁺, Cl^- and HCO₃ ^- (Table 1). ^{Zubillaga and Lavado (2001)} reported that, the use of compost, while bringing nutrients has as main disadvantage the presence of high concentrations of soluble salts, which limits the extent to which mixtures can be used for substrates. Particularly in this species, ^{Gómez-Merino et al. (2013)} indicate which composted proportions 80% on the substrate in combination with perlite, although increase seed germination, cause slow growth and consequently inferior seedlings for transplantation.