Plant material

Tuberose stems harvested in Morelos were collected from a commercial garden in Cuauchichinola, Mazatepec, Morelos; the locality has a warm sub-humid climate (Aw_o) and coordinates 18º 38’ 58.12’’ north latitude, 99º 22’ 49.62’’ west longitude, with an altitude of 910 m (^{Ornelas et al., 1990}). The stems were harvested in accordance to the harvest index of the producer, i.e. when at least two flowers were opened. The stems were harvested at 8:00 am and transported to the Agricultural Production Laboratory of the Faculty of Agricultural Sciences at the Universidad Autónoma del Estado de Morelos, which has a semi-warm climate A(C) and coordinates 18º 58’ 53.73’’ north latitude, 99º 13’ 58.43’’ west longitude, and an altitude of 1 874 m (^{Ornelas et al., 1990}). They were immediately placed in potable water and were left at room temperature (22 ± 2 ºC; 60% RH) for two hours.

Treatment design

Five groups were made each comprised of 18 tuberose stems cut to 60 cm; the groups were placed on the ground between two pieces of cardboard and had sandbags placed on top of them in order to produce a pressure of 70, 140, and 210 Pa, which simulates the compression caused when being transported from Morelos to Mexico City. In addition, there were two groups that did not have compression applied to them. The compression treatment was maintained for three hours, simulating the transport time between the place of harvest and the place of retail.

Following the compression treatment, the flower groups were placed in a 20% sucrose pulse solution in hermetic plastic containers for 24 hours, alongside four bags of Ethylbloc® [(2.5 g bag with 0.014% active ingredient, 1-methylcyclopropene (1-MCP)]. The pulse solution was not applied to one group of tuberose stems. The five groups were maintained as follows: 1) tuberose stems without compression and without pulse solution (control); 2) tuberose stems without compression and with pulse solution; 3) tuberose stems with compression of 70 Pa and pulse solution; 4) tuberose stems with compression of 140 Pa and pulse solution; and, 5) tuberose stems with compression of 210 Pa and pulse solution. After applying the treatments to the tuberose stems, they were kept in 1 L plastic test tubes and the ethylene and CO₂ production, water consumption, and relative fresh weight were evaluated on a daily basis. The evaluations were carried out at a room temperature of 22 ± 2 ºC, 60% RH.

Evaluated variables

Ethylene and CO₂ production. A tuberose inflorescence was placed daily in a plastic container of known volume (1.8 L) and sealed. After 1 hour, 6 mL of the head space were taken using a syringe and were injected in Vacuntainer® tubes of the same capacity where they were kept until their analysis. The concentration of gasses was determined using a gas chromatograph Varian® (Varian Star 3400CX, USA). 1 mL of the gas stored in the Vacuntainer® was taken, which subsequently was injected into the gas chromatograph. CO₂ (460 mg L-1) and ethylene (100 mg L-1) standards were used (INFRA®) as reference in order to calculate the ethylene and CO₂ production. The column temperatures of the injector and the detector were 80, 150, and 170 ºC, respectively. Helium was used as a carrier gas with a flow of 32.3 mL min-1. The production of CO₂ in mL kg-1h-1 and of ethylene in μL kg-1h-1 were reported per sprig.

Water consumption. 250 ml of potable water was placed in each test tube with the remaining water being measured on a daily basis. Each day, 250 mL were placed once more for subsequent evaluation.

Relative fresh weight. When daily evaluating the water consumption, the weight of the floral stem was quantified until its vase life was considered finished, which was when 50% of the flowers of each sprig were dead, approximately in a period of six days. It was reported as a weight percentage relative to the initial mass of the evaluation in the vase, which was considered 100%.

Data analysis

An entirely random experimental design was utilized. The experimental unit was a sprig and there were six copies. The data obtained was analyzed through a variance and comparison analysis of measurements by means of the least significant difference (p≤ 0.05) method when significant differences were detected with the software SAS® V.9.1 (^{Castillo, 2011}).

Respiration

The respiration of the sprigs used as a control at the start of the evaluation presented a phase similar to a pre-climacteric period with values of 31.3 mL kg-1h-1 of CO₂. Subsequently, a sharp decrease of up to 22.7 mL kg-1h-1 was observed, which later increased to a maximum of 46.3 mL kg-1h-1 (climacteric). Finally a post-climacteric phase was observed that was defined by decreasing respiration to 31.5 mL kg-1h-1 (Figure 1 A); this behavior is similar to what was reported in a previous study (^{Pérez-Arias et al., 2014}). The application of the pulse solution increased the respiration speed (Table 1, Figures 1 B, C, D, and E) due to the sugars provided, which were used as a substrate for respiration, preventing a rapid decline and senescence (^{Reid, 2009}).

Figure 1. Initial respiration speed in tuberose sprigs in vases after being subjected to various intensities of compression. A) control; B) without compression; C) compression of 70 Pa; D) compression of 140 Pa; and, E) compression of 210 Pa. Each point represents the measurement of 6 observations and its standard error. 

The respiration of the stems increased significantly when mechanical compression was applied (Table 1; Figure 1 A). Thus, in the flowers subjected to compression of 140 and 210 Pa, the respiration speed was kept between 54 and 57 mL kg-1h-1, without decreasing from 40 mL kg-1h-1 (Figure 1 D and E). The increase in respiration is attributed to the damage caused by compression, due to the fact that in damaged tissue oxygen is diffused more quickly in the interior of the cells and its metabolic activity increases (^{Watada et al., 1996}). Mechanical damage in flowers must be avoided at all costs, as it causes high respiration, which in turn reduces its useful life (^{Reid, 2009}).

Ethylene production

Ethylene production showed a decrease during the first three days and subsequently increased at a constant rate from the fourth to the sixth day of evaluation (Figure 2 A); similar behavior was observed in sprigs subject to pulse solution but without being subjected to any compression (Figure 2 B). In sprigs subjected to mechanical compression of 70, 140, and 210 Pa, the pre-climacteric, climacteric, and post-climacteric stages were observed (Figures 1 C, D, and E), in addition to a significantly greater ethylene production (Table 1).

Figure 2. Ethylene production in tuberose sprigs in vases after being subjected to various intensities of compression. A) control; B) without compression; C) compression of 70 Pa; D) compression of 140 Pa; and, E) compression of 210 Pa. Each point represents the measurement of 6 observations and its standard error. 

The results suggest that the climacteric behavior was not fully observed in the control sprigs and the sprigs subjected to pulse solution without compression (Figure 2 A and B); however, in the flowers subjected to mechanical damage, the climacteric was well defined and with greater intensity (Figure 2 C, D, and E). The mechanical damage induces a general response from the plant through the activation of the ACC synthase and ACC oxidase (^{Kakarut and Hubert, 2003}), and consequently a greater ethylene production is presented. ^{Waithaka et al. (2001)} indicates that the exogenous ethylene applied to the tuberose sprigs causes the abscission of open and closed flowers in approximately four days.

^{Naidu and Reid (1989)} reported that ethylene inhibits the opening of flowers on tuberose sprigs; however, it was concluded that in spite of these negative effects, ethylene is not important in the senescence of flowers. In this study, the control sprigs and those only subjected to pulse solution showed statistical similarities (Table 1), indicating that the application of Ethylbloc® did not prompt a decrease in ethylene production. This result is different from what was reported in orchids species, where the application of Ethylbloc® decreased ethylene production (^{Uthaichay et al., 2007}). The results obtained suggest that the compression increased ethylene production.

Relative weight

The flowers of the control treatment increased the relative weight by 115% by the third day of evaluation, subsequently decreasing at a constant rate until reaching values of 107% on the sixth day (Figure 3 A). The increase in the relative weight is due to the fact that the tuberose sprigs rehydrate during the post-harvest handling. The sprigs subjected to pulse solution but without mechanical compression reached a maximum of 119% (Figure 3 B), whereas the sprigs subjected to pulse solution and mechanical compression of 70, 140, and 210 Pa showed maximum values of 123, 128, and 126.4%, respectively, (Figure 2 C-E) and were the ones that had a greater increase in the relative weight (Table 1).

Figure 3. Initial weight percentage of tuberose sprigs in vases after being subjected to various intensities of compression. A) control; B) without compression; C) compression of 70 Pa; D) compression of 140 Pa; and E) compression of 210 Pa. Each point represents the measurement of 6 observations and its standard error. 

This suggests that the pulse solution favors a greater increase in the relative weight percentage, as indicated by ^{Var and Barad (2007)} who determined that the tuberose flowers subjected to pulse solution with sucrose maintain a greater water consumption and a lesser weight loss, which directly relates to the relative fresh weight.

Water consumption

The control sprigs were the ones that absorbed the least amount of water (Table 1), with the maximum water consumption being on the third evaluation day with values of 14 mL per sprig. The sprigs subjected to pulse solution with mechanical compression of 0, 70, and 140 Pa showed the maximum water absorption on the third evaluation day with 21, 19, and 24 mL per sprig, respectively (data not shown), whereas for sprigs subjected to 210 Pa of pressure, the maximum absorption was obtained on the fifth evaluation day with 17.3 mL per sprig (data not shown). This coincides with data by ^{Varu and Barad (2007)} where they mention that the tuberose flowers to which pulse solution with sucrose is applied maintain a greater water consumption and a lesser weight loss due to the fact that sugar prevents hydric stress and maintains the metabolic processes.

The greater water consumption and relative fresh weight in tuberose inflorescences on which greater mechanical compression was applied (Table 1; Figure 3 C-D) are probably due to having caused mechanical damage on the epidermis of the inflorescences and therefore having a greater water leak, which was replaced by the inflorescences with water from the vase. ^{Van Doorn (2012)} mentions that in cut flowers placed in a vase, the water consumption was less than the residual loss of the stomas and cuticular perspiration; this latter could be increased by compression treatments.