Moisture content

The stems of ‘Engagement’ rose of all treatments or times of exposure to the environment kept the moisture of their flower buds between 73 and 75%. In the leaves and stems, from treatments 0, 8, 16 or 24 h, the moisture decreased from 70 to 66%. For those of 24, 32, 40 and 48 h, moisture loss was greater in leaves (from 65.69 to 41.7%) than in stems (from 65.2 to 58.2%) (Figure 1 A).

In ‘Freedom’ rose, the moisture of the flower buds decreased from 75.4 to 61.2% in all times of exposure to the environment. In the stems and leaves of this cultivar, moisture loss was from 74.4 to 54.4 and from 67.3 to 22.4 respectively (Figure 1 B). In both cultivars, the leaves were the organs that lost the most moisture and the flower buds those that kept it the longest. It is worth mentioning that the flower buds, leaves and stems of ‘Freedom’ rose lost more moisture compared to those of ‘Engagement’ (Figure 1 A and 1B).

^{De la Cruz-Guzmán et al. (2018)} mention that the morphology of the flower buds and the compact arrangement of the petals allows them to maintain moisture for longer, compared to stems or leaves which have a larger surface area exposed to the environment and are therefore more susceptible to dehydration. At all times of exposure to the environment (0 to 48 h), dehydration was higher in the leaves of ‘Freedom’ rose (67.3 to 22.4%) than in ‘Engagement’ (from 69.5 to 41.7%).

Figure 1 Moisture content in leaves, stems and flower buds of ‘Engagement’ (A) and ‘Freedom’ (B) roses exposed to the environment from 0 to 48 h. Means with different letters, in each evaluation time, indicate significant differences (Tukey, p≤ 0.05). Each data represents the average of three repetitions ± standard error. 

Because both cultivars were exposed to the same water potential gradient (≈ -252 MPa), the lower moisture loss in ‘Engagement’ can be explained because its cuticle was two times bigger than that of ‘Freedom’ (Table 1). The cuticle alone acts as a barrier to the loss of water to the atmosphere, the thicker it is, the less foliar dehydration and the tissues will keep their moisture content for longer (^{Sperry, 2003}; ^{Tafolla-Arellano et al., 2013}). In the leaves of Rosa hybrida, epicuticular waxes that, together with the cuticle, increase the resistance to water loss and maintain the integrity of the cell membranes have been described (^{Taiz and Zeiger, 2010}).

Wax content can vary from cultivar to cultivar. For example, in ‘Lovely Red’ rose they are more abundant than in ‘Rouge Baiser’ rose (^{Cohen et al., 2012}). The above suggests that ‘Engagement’ rose, in addition to having thicker cuticle, could have a higher proportion of epicuticular waxes than ‘Freedom’. The width of the leaf of ‘Engagement’ rose was 26% greater than in ‘Freedom’ (180.9 μm), this because the thickness of the cuticle, length of the adaxial, abaxial epidermal cells and the width of the palisade parenchyma were 49, 41, 45.3 and 18.3% greater in ‘Engagement’ rose than in ‘Freedom’ (Table 1). These anatomical differences contributed to the longer retention of moisture in ‘Engagement’ rose, particularly because the wider mesophyll allowed it to store more water and its cuticle, thicker, lose it slowly compared to ‘Freedom’.

The reduction in leaf width indicates that the plants developed in xeromorphic environments or that they were exposed to a period of water stress during their cultivation (^{Ely et al. 2005}; ^{Moreno, 2009}). In contrast, thicker leaves are typical of well-irrigated plants, they have greater capacity to store water and lose it slowly if their cuticle is thicker (^{Moreno, 2009}). This could suggest that the flower stems of ‘Freedom’ rose remained in water stress at some stage of the preharvest, while those of ‘Engagement’ rose were well irrigated or had shorter periods of stress. Differences in anatomy are also attributed to the genetic characteristics of each cultivar, so the above statement would be valid for wild species and be used with greater caution in cultivars whose purpose is commercialization, since the producer will always seek to provide the best preharvest management.

Description of leaf anatomy

Both cultivars of Rosa hybrida had smooth cuticle, unistratified epidermis (Figure 2 A, 2B), paracytic-type stomata, with two cells annexed to the same level as the epidermal ones (^{Metcalfe and Chalk, 1979}) (Figure 2 C, 2D), similar to what was reported by ^{Hernández-Hernández (2009}) for ‘Vega’ and ‘Grand Gala’ roses. In ‘Engagement’ and ‘Freedom’ roses the distribution was hypostomatic, as in ‘Lovely Red’ and ‘Rouge Baiser’ roses (^{Cohen et al., 2012}).

Figure 2 Anatomy of the leaf of ‘Engagement’ (A, C) and ‘Freedom’ (B, D) roses. Cross section (A, B); paradermal view (C, D); e^*= adaxial epidermis; e= abaxial epidermis; pem= palisade parenchyma; pes= spongy parenchyma; es= stomata; scale= 50 μm. 

The location of the stomata on the abaxial surface of the leaves contributes to the decrease in the transpiratory rate since they do not directly receive sunlight (^{Willmer and Fricket, 1996}; ^{Raven, 2002}). Dark contents, possibly tannins, were observed on the leaves of ‘Freedom’ rose, which were not visible in ‘Engagement’.

Fresh weight (FW) and rehydration in the vase

In treatments with 8 or 16 h of exposure to the environment, the flower stems of ‘Engagement’ rose had an average FW of 118.5%, in that of 24 h, the FW increased to 138.6%, which can be attributed to the dehydration of the xylem. Until the 24 h treatment, all the flower stems rehydrated when placed in the vase. In those of 32, 40 or 48 hours, the fresh weight was 107.8, 99.6, 97.8% and the percentage of stems that rehydrated was 60, 40 and 0, respectively. For ‘Freedom’ rose, in treatments 8, 16 or 24 h of exposure to the environment, the FW of the flower stems fluctuated from 112.6 to 128.3% and 100% of the stems rehydrated. With those of 40 or 48 h, the FW decreased to 96% and no flower stem rehydrated when placed in the vase (Table 2).

The increase in fresh weight depends on the absorption rate that occurs in response to the water potential gradient (ΔΨ) between the vase solution and the xylem of the stem (^{Taiz and Zeiger, 2010}; ^{Van Doorn, 2012}). Exposure to the environment dehydrates the flower stems and decreases the moisture content of the xylem, which contributes to faster absorption when placed in the vase. ‘Engagement’ and ‘Freedom’ roses were exposed to the same environment at the laboratory table, so their dehydration was regulated by the thickness of the cuticle, width of the mesophyll and the number, area or diameter of the vessel elements.

The number of vessels per mm² was lower in the stem of ‘Engagement’ rose (331) than in that of ‘Freedom’ (402). In addition, the cuticle thickness, area and diameters of the vessel elements were 10.8, 16.6 and 9.7% higher in ‘Engagement’ than in ‘Freedom’, whose cuticle had a thickness of 9.1 μm, while the area and diameter of the vessel elements corresponded to 520.9 mm² and 25.2 μm, respectively (Table 3).

Short vessel elements with larger diameters are more efficient in conducting water, since shorter lengths give resistance to the collapse of the cell wall, by preventing its deformation (^{Carlquist, 1977}; ^{Sperry, 2003}).

In contrast, the reduction in the diameter of the vessel elements in ‘Freedom’ rose (Table 3) indicates a lower water conductivity, but ensures the flow of water, because they are less susceptible to cavitate (^{Hargrave et al., 1994}; ^{Nijsse et al., 2000}; ^{Martínez-Vilalta and Piñol, 2002}). The number and diameter of the vessel elements contribute directly to the hydration of the flower stems, since vessel elements with diameters greater than 75 μm have lower resistance to water flow (van Door and Reid, 1995; ^{Van Meeteren, 1999}). The diameter of the vessels of ‘Engagement’ or ‘Freedom’ roses were within the range that ^{Hernández-Hernández et al. (2009}) and ^{Cohen et al. (2012}) report for ‘Grand Gala’, ‘Vega’, ‘Lovely Red’ and ‘Rouge Baiser’ roses, which range between 13 and 67.7m.

Description of the anatomy of the stem

Both cultivars of Rosa hybrida had thick cuticle and unistratified epidermis. The parenchyma of the cortex is interrupted by packages of seven strata of sclereid cells in cell differentiation, in ‘Engagement’ rose and four strata without differentiation in ‘Freedom’ (Figure 3 C, 3D). Below the sclereids was the secondary phloem divided by rays from the pith. In xylem, porosity was diffuse (Figure 3 A, 3B).

The differentiation of tissues, which involves the development of cells to the thickening of the secondary walls, is related to vascular cambium activity and this with the radial and longitudinal growth of the stem (^{Jiménez-Noriega et al., 2019}), which suggests that the flower stems of ‘Engagement’ rose at the time of harvest were growing and therefore with higher metabolic activity than ‘Freedom’, which could also promote the absorption of water after exposing them to the environment and placing them in the vase.

Figure 3 Anatomy of the stem of ‘Engagement’ (A, C) and ‘Freedom’ (B, D) roses. Cross section (A, B) 100 (m; zone of differentiation of the sclereids (C, D) 20 (m. e= epidermis; p= parenchyma; es= sclereids; f= phloem; cv= vascular cambium; zd= zone in differentiation; x= xylem. 

Flower opening index (OI) and vase life (VL)

The flower stems of ‘Engagement’ rose, which rehydrated after 8 or 16 hours of being exposed to the environment, had a flower opening index of 0.68 ±0.21 and a vase life of 10.2 d, while the OI and VL of the flower stems of ‘Freedom’ rose were 0.78 ±0.15 and 8.2 ±1.96 d, respectively. The reduction in vase life of ‘Freedom’ was attributed to differences in stem and leaf anatomy. In this work, ‘Engagement’ rose had greater vase life and the diameters of its vessels were larger compared to ‘Freedom’.

This contrasts with the results obtained by ^{Hernández-Hernández et al. (2009}), since the ‘Vega’ cultivar that had the largest diameters exhibited on average 2.6 days less vase life than ‘Grand Gala’ and they attributed it to water stress due to its susceptibility to dehydration.

OI values less than 1.0 indicate that the flower buds were below their maximum opening and greater than or equal to 1.0 that the treatment promoted their flower opening. Without applying any treatment or postharvest management, the maximum flower opening of ‘Engagement’ and ‘Freedom’ roses is 7.9 ±1.2 and 6.7 ±0.5 cm, with a vase life of 8.9 ±1.07 and 7.6 ±1.06 d (^{De La Cruz-Guzmán et al., 2015}). This indicates that exposure to the environment for 8 or 16 h reduces the opening of the flower bud but increases the vase life of ‘Engagement’ and ‘Freedom’ roses by 1.3 and 0.6 d, respectively. Longer times of exposure to the environment reduce vase life, for example, in the 24 h treatment, all the flower stems were considered as rehydrated because the flower buds kept or restored their moisture content (Figure 1, Table 2). However, their vase life was less than or equal to 3 d for ‘Engagement’ rose and less than or equal to 2.5 d for ‘Freedom’ because the leaves dehydrated and withered more quickly.