Introduction

Platanus (Platanus acerifolia (Aiton) Willd.) is an arboreal plant species with large, deciduous leaves and is the sole cultivated representative of Platanaceae family in Brazil. It occurs in Brazil and other South American countries, and used as an ornamental plant, for afforestation, for breaking wind in plantations, and in carpentry, for furniture and floor production owing to its high quality wood (^{Merino, 1991}; ^{Ono, de Barros & Rodrigues, 1994}).

This plant can also be used for shading pigpens, corrals, coops, stables, and other rural facilities, providing greater thermal comfort to animals during hot summer conditions. On the other hand, because is a deciduous plant, also allows sunlight into facilities during winter, providing heating and disinfection of the environment by sunlight action.

In Serra Gaucha region of the state of Rio Grande do Sul-Brazil, this plant is widely used by winemakers to support grapevines through its use as living fence posts, to break winds in orchards, and as a vegetable fuel in production of pastries and homemade jams. In this region, this species is propagated by winegrowers who use hardwood cuttings of approximately 2 m long, derived from pruning of mature plants, which is performed between fall and winter.

Use of plants as living fence posts for the support of wire fences that delimit land, for delimitation of roads, and for general delimitation in urban or rural areas has been described in some literature citations by various authors (^{Lamônica & Guerra, 2008}). Considering sustainable vision for the use of plants as living fences, platanus appears as an excellent candidate since this plant can be propagated through cuttings, living fences or hedge posts can be easily obtained due to its high propagation rate to form mature plants, and propagated plants are identical to mother plants (^{Lamônica & Guerra, 2008}).

Vegetative propagation of platanus is the most preferred method for winegrowers in Serra Gaucha region, with satisfactory results; root-inducing products or other methods that require greater attention during the seedling production process are not preferred.

Nevertheless, adult plants produce seeds that are dispersed randomly during winter and early spring; however, germination percentage is very low (^{Leonardis, 1977}). Therefore, vegetative propagation of this species by cuttings, an ancient practice, is widely used and recommended by several authors (^{Hartman & Kester, 1971}; ^{Dias, Franco & Dias, 1999}) and is the most efficient method for platanus seedlings production.

Plant rooting and development by cuttings can be influenced by genotype and time of collection of plant material (^{Silva, Oliveira, Pio, Zambon & Oliveira, 2012}). Physiological conditions of the mother plant and environment, are also influential, especially with respect to endogenous levels of auxin, size and number of nodes in cuttings, temperature, humidity of the environment, and substrate used in rooting (^{Oliveira, Pasqual, Chalfun, Regina & Rincón, 2003}).

However, for some species, use of cuttings can often be a slow and impractical method (^{Ono, de Barros & Rodrigues, 1994}) and is dependent on endogenous levels of tryptophan. This amino acid is the precursor of indole-3-acetic acid (IAA), the plant hormone belonging to the auxins, is most abundant in plants, and is responsible for the formation of adventitious roots and formation of plant organs such as leaves and stems in higher plants (^{Rossal, Kersten & Conter, 1997}; ^{Mercier, 2008}).

Lesions in cutting base may also influence the rooting ratio. Lesions favor water absorption and plant growth regulator when treated, although the use of this technique may also lead to unsatisfactory results (^{Trevisan & Boldrini, 2008}). Planting depth is another factor that may cause variation in rooting rates of cuttings and subsequent development in different species, as reported by ^{Ojima & Regitano (1969)}, ^{Pio, Ramos, Chalfun, Almeida, Carrijo, Mendonça, Alvarenga & Abrahão (2005)}, and ^{Marques, Moreira, Ramos, Araújo & Cruz (2012)}, in pitahaya tree, quince, and fig tree, respectively.

Considering numerous benefits, possibilities of use, and influence on platanus cuttings, studies on their adaptation to tropical and subtropical conditions, as well as the rhizogenic potential of this species are required for the development of simple and easy techniques to propagate this plant. Results of these studies may be beneficial for cultivation of this species, especially for small Brazilian farmers.

This study aimed to evaluate the behavior of platanus (Platanus acerifolia (Aiton) Willd.) cuttings, comprising injured and non-injured cuttings, planted at different depths in the municipality of Marechal Cândido Rondon-PR, Brazil.

Material and methods

Plant cuttings

With the aid of hacksaw, were planted on June 8, 2013. Each cutting section (approximately 50 cm long) had 3 vegetative shoots, with a diameter of 3.0-4.0 cm. Cuttings were cut close to lower shoots of the plant; cuttings were subsequently inserted into sand bed for rooting.

To perform the injury, a knife with a 30-cm flat blade was used to cut at 10 cm from basal part of the cutting, causing small cuts in the skin (Figure 1).

Figure 1 Injured P. acerifolia cuttings 

Subsequently, cuttings were planted 10 cm apart in a 100-cm-deep sand bed covered with 50% shade mesh.

Base of cuttings were abundantly watered to the top level of sand bed after planting to elimi nate possible spaces or air pockets between sand and cuttings, which have allowed an intimate contact between both components.

Throughout trial period, cuttings received regular irrigation sprays that were activated 20 times per day. Irrigation system was programmed via timer that turned on at 7:00 A.M. and off at Rooting Platanus (Platanus acerifolia (Aiton) Willd.) cuttings in Marechal Cândido Rondon - PR, Brazil: Influence of lesions at cutting bases and depth of planting 6:00 P.M. From 7:00 A.M. to 9:00 A.M. and 5:00 P.M to 6:00 P.M, system was activated every hour for 3 min. During other periods of the day, system was activated every 30 min for 3 min.

After 170 days of plant growth, plants were evaluated for rooting and sprouting percentage. Maximum root length and average length of stems were measured with a measuring tape and values were expressed in centimeters. Stem diameter was measured with a digital caliper, 1 cm above insertion between stem and shoot, values were expressed in millimeters. Number of leaves per stem was recorded. Dry mass of roots and shoots, after drying in an oven with air circulation at 65°C for 72 h, were weighed using a semi-analytical balance, and results were expressed in grams.

Statistic analysis

Experimental design was a 2 x 2 completely randomized factorial design, comprising a total of 100 injured and non-injured cuttings, staked 20 cm and 40 cm depth, with 5 replicates and 5 cuttings per replicate.

Once tabulated, data were subjected to analysis of variance (ANOVA) to identify significant difference among treatments and statistical significance for all comparisons was made at p<0.05. Tukey's multiple range test was used to compare the mean values of treatments.

Results

ANOVA results of significant differences among planting depths for all variables are shown in Table 1.

Table 1 Percentage of rooted cuttings (R) and sprouting (S), root length (CL), medium stem length (MSL), stem diameter (SD), number of leaves per stem (NLS), root dry mass (RDM) shoot dry weight (SDW) of P. acerifolia cuttings at two depth plantings (20 and 40 cm) 

* Averages followed by different lowercase letters in the column differ statistically by Tukey test at 5% probability of error.

Injuries made at base cuttings, in the present study did not activate significant physiological changes involving ethylene and auxin accumu lation; however, it appears that the current base cutting served as a physical barrier against ba-sipetal flow and auxin accumulation in the base cutting area, thereby promoting roots develop ment in this region (Figure 2).

Figure 2 P. acerifolia cuttings. In sand bed on planting day (a); sprouted cuttings (119 days after planting) (b). 

Figure 3, shows lower number of roots in cuttings planted at 40 cm depth compared to those buried at 20 cm depth.

Figure 3 P. acerifolia roots in cuttings at 20 and 40 cm planting depth. a) not injured cuttings subjected to a planting depth of 40 cm; b) cuttings subjected to a planting depth of 20 cm. 

In the present study, water was sprayed intermittently during experiment, providing good moisture soil conditions to platanus cuttings planted at evaluated planting depths (20 and 40 cm). However, those buried at greater depths, were damaged by excessive moisture contained in the substrate, thus reducing rooting percentage and consequently, damaging the vegetative growth of cuttings.

Discussion

For injured cuttings, there were no significant differences in the analyzed variables. Similarly, no significant differences between planting depth and injury in base cutting, were observed.

It was observed that percentage of rooted cuttings, results were not consistent with previous work carried out by ^{Dias, Franco & Dias (1999)}, who obtained rooting percentages ranged from 60 and 75% when compared platanus cuttings of 0.94-2.75 cm diameter. However, in the present study, cuttings were collected in August, 2013 which may explain differences in results between two studies. However, these results are comparable in variability to the report by ^{Ono, de Barros & Rodrigues (1994)}, and ^{Nicoloso, Lazzari & Fortunato (1999)}, who observed that best cuttings rates were obtained when cuttings were collected during fall season.

According to ^{Fachinello, Hoffmann, Nachtigal, Kersten & Fortes (1995)}, and ^{Kersten, Lucchesi & Gutierrez (1993)}, time of cuttings collection is very important and should be taken into account when collecting cuttings for vegetative propagation purposes. A possible explanation for this is that during certain periods of the year, formation and accumulation of rooting inhibitors can occur (^{Muñoz & Valenzuela, 1978}) or synthesis of phytohormones precursor, such as tryptophan, may occur (^{Rossal, Kersten & Conter, 1997)}. This provides more accurate and reliable estimates of injury at base cutting, which was expected that injured cuttings would have higher rooting percentages and rooting and occur in the injured area. According to ^{Mercier (2008)}, root formation process (for either inducing formation of lateral roots or for inducing root formation on stems) is regulated by ethylene (among other phytohormones) action, which can be synthesized by plant after suffering any injury. Ethylene is the hormone responsible for auxin accumulation (indolyl-3-acetic acid - EIA) in certain plant regions and, consequently, initiate the process of root formation.

In fact, in a previous work on planting depth, ^{Pio et al. (2005)}, using quince cuttings of 20 cm length, observed that cuttings totally buried presented lower rooting percentage than those buried to 2/3 of their length. They also observed that sprouting percentage and number of leaves per cutting were reduced when cuttings were completely buried, attributing these results to the presence of excess moisture in the substrate.

Results of the present study corroborate these previous studies; platanus cuttings planted at greater depths showed lower rooting percentage, fewer and smaller sprouted cuttings, and lower number of leaves produced on each stem (Table 1).

According to ^{Fachinello et al. (1995)}, appropriate moisture in the rooting substrate is essential to obtain satisfactory results in the rooting of different plant cuttings. Substrates with high moisture content may affect emergence and development of roots and lack of moisture can also damage plant development.

This can be related to planting depth; cuttings buried deeper tend to reach regions with higher moisture content, hindering emergence of roots and thus affecting the cuttings and their initial plant growth.

^{Ojima & Reginato (1969)} and ^{Munõz & Valenzuela (1978)}, using fig tree cuttings, verified that those staked to 4/5 of their length showed a higher rooting percentage than those staked to 1/2 of their length. These results justify the fact that cuttings buried deeper benefit from greater protection against dryness, thus favoring rooting.

Root and vegetative development in the cuttings planted at greater depths can also be related to aeration present at lesser depths (^{Munõz & Valenzuela, 1978}). ^{Pio et al. (2005)}, highlighted the need to keep rooting substrate fully aerated for best rooting rates and cuttings.

On the result basis from present study and those of previous studies, can be inferred that depending on the substrate used, a substrate can have a moisture gradient throughout its profile and water holding capacity, particle size, texture of each type of substrate and planting depth of cuttings, influence the optimum humidity required for rooting of platanus cuttings.

Conclusion

P. acerifolia cuttings buried to 20 cm deep show highest rates of rooting and vegetative development. The injury on base cutting not allowed an improvement in the rooting and vegetative development.