Introduction

Roots are a fundamental component for crops, but being an underground organ, it is less studied than the aboveground structures. The study of roots requires more time and effort than that of the shoots so that the evaluated species are few and most of the studies have focused on monocots, particularly grasses, because of their economic importance (^{Martino, 2001}). This family of plants has thin roots that proliferate through small diameter pores in the soil (^{Valentine et al., 2012}; ^{Glaba and Szewczyk, 2014}; ²⁰¹⁵). Crop yield is associated with the level of exploration reached by the root system (^{Albino Garduño et al., 2015}).

To evaluate the soil-plant system behavior it is necessary to characterize the root system and the medium’s porous structural pattern (^{Tsutsumi et al., 2003}). The pore network geometry includes the size distribution, the porous space topology and the form of the interconnection of spaces (^{Vogel and Roth, 2001}). Small pores in soils of agricultural fields restrict the permeability and root systems growth (^{Williams and Weil, 2004}). Radical hairs even decrease their length when soil pores diameter is reduced (^{Haling et al., 2014}). Therefore, it is important to evaluate the roots’ effectiveness to penetrate the soil, as a medium means that represent a physical barrier. When the root advances through the soil and faces smaller diameter pores, it has to expand them to continue its longitudinal growth. In clay soils the root elongation rate is reduced if the exerted pressure is insufficient to create larger pores; that is, mechanical resistance will limit its growth (^{Dexter, 2004}, Williams and Weil, 2004). Thus, roots may affect the pores size. ^{Hallett et al. (2009)} found an 11 % increase in the average pore diameter in soils with vegetation when compared with bare soils. ^{Martin et al. (2012)} reported a positive effect on the size of the pores by the roots of plants but not by mycorrhizal hyphae.

Soil impedance inversely affects the elongation rate and directly the average root thickness (^{Bennie, 1996}). The increase in impedance and decrease in the elongation rate are often accompanied by a radial expansion of the root’s axes (^{Bengough and Mullins, 1991}; ^{Croser et al., 1999}; ^{Gregory, 2006}). Apparently, thickening allows the root to develop higher pressure and overcome soil resistance (^{Clark et al., 1999}). The maximum pressure that roots can exert to deform pores is 0.5 to 0.6 MPa, slightly higher in monocotyledons than in dicotyledons, and is the result of cell turgor near 0.8 MPa in roots (^{Clark et al., 2003}). This behavior occurs in soils with some plasticity, such as clays; however, in soils with rigid pores, such as the sandy ones, the roots ability to penetrate is not related to the pushing pressure but to its diameter (^{Bengough et al., 2011}; ^{Mikkelsen, 2015}).

Jalapeño pepper, so called because its traditional center of production is the Mexican city of Xalapa, is a spicy variety of Capsicum annuum cultivated and consumed in America. The root of the adult plant is deep and voluminous; besides it has numerous lateral roots with small diameter (^{Nuez et al., 2003}).

The objective of this study was to evaluate the thickness of the lateral roots of Jalapeño peppers in relation to the size of the pores of the soil, its effect on the growth of the soot, and determine the minimum diameter of the galleries that the roots can penetrate in a medium with rigid pores without affecting the biomass production. Our hypothesis was that there is a minimum pores size in the soil, below which, the roots limit their elongation and the growth of the plant is restricted.

Materials and Methods

The study was conducted in the Universidad Centroccidental Lisandro Alvarado, Barquisimeto, Venezuela, in an open shed structure with translucent roof, which intercepts 40 % of the solar radiation. The diurnal temperature within the structure ranged between 24 and 32 °C and relative humidity between 70 and 40 %.

Results and Discussion

The average roots diameter ranged between 0.292 and 0.327 mm (Table 3). The difference was noticeable in the morphological pattern of the root system between treatments (Figure 1).

Figure 1 Root morphology of Jalapeño pepper (Capsicum annuum L.) cultivated in sand with different granulometry (circles correspond to T1: 1.700 mm; T2: 1.125 mm; T3: 0.635 mm; T4: 0.335 mm; T5: 0.200 mm). 

The roots in T1, T2 and T3 showed extensive and uniform growth, while in T4 and T5 the tap root showed lower growth; the lateral roots extended horizontally close to the substrate surface. Once these roots reached the container walls, their growth continued vertically downward, thus originating a tubular configuration, with total roots absence at the substrate interior (Figure 1, Table 3). This indicated that the roots did not penetrate the finer sand with small pores. The above shows that the root thickness made it impossible to access through pores between particles whose diameter was between 0.25 and 0.42 mm. Limitations imposed by soil porosity on roots growth in maize plants were reported by ^{Bushamuka and Zobel (1998)} and ^{Osuna-Ceja et al. (2006)}.

The average roots diameter in the T4 and T5 treatments was similar to that of the roots in T1 and T2 (Table 3), so that the roots that did not penetrate the sand pores grew out of it and their thickness was not modified. However, these proliferate less.

The diameters of the roots that entered substrates T1 and T2 were not different, but in T3 they were thinner (Table 3). This supposes that radial growth decreased and allowed access to pores between particles with diameters from 0.42 to 0.85 mm. That is, the thickness reduction in T3 roots was 9 %. This constriction is smaller than that reported by ^{Scholefield and Hall (1985)}, of 2.8 times constriction in monocotyledons (Lolium perenne). Jalapeño pepper is a dicotyledonous that showed a certain degree of contraction to allow roots entering smaller pores than its nominal diameter.

^{Scholefield and Hall (1985)} showed that roots of L. perenne could grow through smaller rigid pores, although at slower rate, when they are sufficiently aerated. These authors observed that roots whose diameter was of 0.88 mm could penetrate rigid pores of 0.315 mm; in addition, in graminean this capacity of reduction of the external diameter was limited by the thickness of the caliptra and stele of their roots. In contrast, ^{Bengough and Mullins (1991)} reported that even in maize the stele of the roots narrow in response to radial constriction.

Most soils with small pores, such as clayey soils, allow the apices of roots to penetrate the middle cracks and continue to grow. In contrast, in our study, the size of the sand particles in each treatment, was established in narrow intervals, and relatively homogeneous pores without cracks.

^{Tsegaye and Mullins (1994)} observed that mechanical impediments promote the production of lateral roots thinner than those of the main axis and, therefore, were able to penetrate smaller pores. This would optimize the overall exploration capacity of the root system and compensate for the smaller growth of the thicker roots. ^{Siegel et al. (2005)} noted that compact soils with small pores affected root growth in C. annuum. ^{Bosland and Votaya (2000)} pointed out that the roots of adult plants of this species have a main axis from which numerous laterals develop. It seems the case in roots from T3 treatment that grew through pores of particles with diameters between 0.42 to 0.85 mm. ^{Zwieniecki and Newton (1995)} reported woody shrubs whose roots entered rocks fissures by changing their morphology, in which the outer cortex flattened, and the stele remained cylindrical. They concluded that the smallest pore size the roots penetrate could be estimated by the diameter of the stele (approximately 100 μm in those studied bushes).

In sandy soils with rigid pores, growth occurs through large pores; however, when roots found small size rigid pores, some species can decrease their diameter and cross the pore (^{Kolb et al., 2012}). In our study, plant roots reduced their diameter when the particles had an average diameter of 0.635 mm (T3); but, they were unable to enter sand with particle diameters of less than 0.420 mm (T4 and T5).

The aboveground biomass of T3 (with medium pores) exceeded T4 and T5 (with small pores) and T1 (with large pores) (Table 3). In the first case, roots could not penetrate the small pores and grew in the annular space between the sand and the container; this restricted the water and nutrients absorption; and plant growth was restricted. In T1, the response is attributable to the limited water retention of the coarse sand, even with the highly frequent irrigation. This is evident with the downward trend on the biomass from T3 to T1 treatments. This allowed to confirm that the soil pore size of is a factor that influences plants growth because it would affects oxygen, water, and nutrients supply (^{Vaz et al., 2001}). The adequate amount of continuous macropores in which the roots can freely penetrate is a requirement for their growth (^{De Freitas et al., 1999}; ^{Dexter, 2004}).

Conclusions

The roots of Jalapeño peppers do not penetrate the pores formed by particles smaller than 0.420 mm, which affects the production of aboveground plant biomass. Some roots enter pores smaller than their average diameter by reducing their thickness.