Hydraulic Relations and Water Use of Mediterranean Ornamental Shrubs in Containers

Agriculture is the largest consumer of water globally, accounting for 70% of total withdrawals (UNEP 2007) but is limited by shortfalls in water quality and quantity (Hoekstra & Chapagain 2007; Postel 2000). Limited water supplies, and continued population growth, demand more efficient water management (Sample 2009; IEEE 2010).

Demand for ornamental plants has been significantly increasing during the past decade (Incrocci et al. 2014) for purposes of urban decoration and gardening and also for environmental restoration and bioremediation (Denys et al. 2006; Sun et al. 2011). This fact, in combination with the abovementioned aspects, implies a double challenge for the plant nursery industry, which will have to increase its production (Savé et al. 1999) and, at the same time, reduce its water inputs through resource optimization.

Historically, one of the most important innovations in the nursery sector has been the switch from mineral soil to soilless cultivation in plastic containers filled with inert substrate that simplified the task of controlling and optimizing water inputs (Di Lorenzo et al. 2013). However, nursery stock in organic substrates are still over-irrigated and leach slow-release fertilizers (Ristvey et al. 2004), which wastes water and pollutes waterways (Stewart-Wade 2011).

Appropriate nursery irrigation requires scheduling in order to provide plants with the adequate amount of water at the correct time. Such scheduling can be based on (1) climate – the evaporative demand and its effects on soil–water balance; see Allen et al. (1998); (2) substrate – its water-holding properties, and the monitoring of substrate moisture (Θ) status; (3) plant water use traits – centered on the relationship between crop water stress and soil water deficit; see Jones (2004) or on their integration. In particular, mathematical modeling is a way to combine the abovementioned strategies with an integrated approach (Gu et al. 2020).

Although microclimate and substrate water-holding properties are quantifiable, plant water use traits are complex and vary among species and cultivars (Knox 1989; Schuch & Burger 1997; Mugnai et al. 1999). Few guidelines are nowadays available for the planning and management of irrigation, and in the reality of nursery production, these topics are addressed most of the times based on the empirical observations more than on modeling (Di Lorenzo et al. 2013).

The aim of the present work was to study the behavior of three ornamental species in containers under increasing water stress, through a continuous monitoring of their ecophysiological responses. The final aim is to highlight differences among the studied species and classify them based on their strategies of stress response, in order to optimize water investments and nursery practices according to the plant type.

N. oleander showed lower leaf area per plant than the two other species (Fig. 2). ETE_h was heavily influenced by the amount of solar radiation. Owing to this reason, water use was calculated as ET/RG, which is the ratio between the total evapotranspiration along the three hours of measurement and the total amount of incident radiation during the same time (Fig. 3). All species displayed a clear trend toward ET/RG reduction as soil water tension increases, with N. oleander always showing significantly higher values than the other two species. P. tobira showed intermediate values, and L. japonicum showed the lowest ones. N. oleander and P. tobira showed a gradual decrease along the 5 days, whereas L. japonicum remarkably dropped down from the second day, undergoing a gradual further decrease in the remaining days.

Fig. 2

Fig. 3

Stomatal conductance declined for all species as soil water availability diminished (Fig. 4), whereas control plants kept their values at 100% ± 5% (which means an average fluctuation of ± 5% around the first-day-optimal value) during the whole trial (data not shown in the graph). For all the days of experiment, N. oleander showed the highest gs values, followed by P. tobira and L. japonicum; gs for N. oleander drastically dropped down only at the last day of trial, whereas P. tobira showed a gradual reduction and L. japonicum dropped down already between day 2 and day 3. The differences in stomatal conductance among plants were significant at 95% for all species in all days, except some overlapping between P. tobira and L. japonicum during the first 2 days. On the last day, following the dramatic drop of N. oleander, its values became comparable with the ones of P. tobira (Fig. 4). The high level of correlation obtained between the measured gs values and the calculated ones confirms the reliability of the measured data. Three different correlation curves, one for each species, have been identified. The correlation curve for L. japonicum meets the origin of axes, with a slope close to one; N. oleander has a similar slope but all the measured values are higher than expected; P. tobira displays a markedly lower slope (Fig. 5).

Fig. 4

Fig. 5

ΔT (T_leaf − T_atm) increased for all species as water stress got more severe (Fig. 6), whereas control plants kept stable values (data not shown in the graph). N. oleander always showed the significantly lowest values, whereas the other two species had higher and similar values, with L. japonicum showing slightly higher ones. The difference between N. oleander and the other two species was significant at 95% during the first 4 days, whereas P. tobira and L. japonicum had values of ΔT that were more closely associated, even if those of L. japonicum always exceeded the ones of P. tobira.

Fig. 6

The results point out N. oleander as the Mediterranean shrub displaying the greatest tolerance to conditions of increasing water stress. The values of evapotranspiration for N. oleander stay at the highest levels during all the stress period, possibly because of the lower leaf area and its different foliar architecture (Badger et al. 1982; Reinert et al. 2013). In fact, N. oleander shows a more markedly vertical leaf habit than the other two species, resulting in a lower mutual shading of its leaves, partially counterbalanced by a sharper angle of solar incidence. Moreover, the lower leaf area index of N. oleander is likely to be associated with a higher contribution of evaporation from the substrate to the total evapotranspiration (Allen 1998; Yan et al. 2012).

This phenomenon is also explained by the capacity of N. oleander of maintaining higher levels of stomatal gas exchange despite the increasing drought conditions. In fact, absolute gs values of N. oleander were the highest during the whole trial, even on the last day when they decreased sharply, consistent with the observations of Mugnai et al. (1999). Relative gs values show an even stronger trend, because, at the end of the trial, N. oleander stays at a 40% of its maximum gas exchange capacity, whereas the other two species drop down to a 20% and earlier than N. oleander. Concurrent higher surface evaporation from N. oleander substrate, likely due to less shading from lower leaf area, apparently led to more negative substrate water tension increasing more rapidly over the study period, despite regular re-watering, than the other two species. Finally, linear correlation between measured and calculated gs clearly shows that N. oleander systematically gives higher scores of stomatal conductance than the ones predicted by the equation.

The similar leaf areas for P. tobira and L. japonicum exclude that differences in evapotranspiration between the two may be due to significant differences in evaporation from the substrate. The higher ETE/RG for P. tobira, therefore, suggests that it maintains higher levels of foliar transpiration than L. japonicum in conditions of water stress, which is precisely confirmed by the evolution of their gs values along time. The greater sensitivity of L. japonicum to water stress may be due to two reasons: gas exchange levels starting initially from lower values, which decreases rapidly with water stress onset. These observations suggest a more isohydric behavior of L. japonicum toward drought: the stomata close as soon as the water stress starts to manifest. This behavior is associated with lower risks of hydraulic failure and also reduced gas exchange and carbon uptake and is typical of plants that are defined “drought avoiders” (Tardieu & Simonneau 1998).

On the other side, the hydraulic behavior of N. oleander suggests anisohydric behavior of ostensibly drought-tolerant plants, which maintain gas exchange at the cost of more negative internal water potentials and risk of hydraulic failure and embolisms (Sperry & Tyree 1988; Sade et al. 2012). For this reason, anisohydric plants usually have stronger vascular structures and better protoplasmic and biochemical stability (Demmig et al. 1988; Vilagrosa et al. 2010).

The increase in ΔT from day 1 to day 5 is a typical consequence of reduced foliar transpiration because of stomatal closure from water stress, which reduces evaporative cooling and so leaf temperature rises (Mugnai 2004; Zuccarini et al. 2011). Low T_leaf − T_atm in N. oleander during the study indicates greater evaporative cooling from transpiration, which directly reflects its higher rates of stomatal transpiration in comparison with the other two species.

Few studies have reported on hydraulic behavior of N. oleander, P. tobira, and/or L. japonicum under water stress, and they were not focused on interspecific comparisons. In this sense, this work is among the few ones reporting on differences in hydraulic behavior among Mediterranean shrubs that can improve their water management in nursery production.

Björkman et al. (1980) already showed gas exchange rates of N. oleander decreased with water stress and suggested that effects of water stress and high irradiation may be interrelated, leading to inactivation of PSII (Björkman & Powles 1984). Later, Demmig et al. (1988) demonstrated in N. oleander, high capacity of its leaves to create foliar zeaxanthin under water and light stress, which protects photo-systems from damage through promotion of radiation less dissipation of excess energy, possibly explaining greater drought tolerance in N. oleander and also from a biochemical point of view.

This study suggests that N. oleander tolerates greater drought stress through anisohydric behavior, which maintains gas exchange at lower substrate water potentials. L. japonicum conversely appears to show isohydric behavior in rapidly reducing gas exchange at the onset of water stress, whereas P. tobira has an intermediate response.

The present data point out N. oleander as the most efficient choice for landscape use in environments subject to recurring situations of water scarcity and, at the same time, they provide useful information for a more focused irrigation management of these Mediterranean shrubs for the plant nursery industry.