RESULTS AND DISCUSSION

12.3.1 FIRST EXPERIMENTAL PART

Based on the results of the first experimental part, the reduced irrigation frequency revealed that the 3-year-old Queens pubes certs seedlings could tolerate with no apparent negative impacts on their growth was one watering per week. Figure 12.2 provides a visual presentation of the seedlings for each irrigation frequency as indicated by the appearance of the leaves by the end of the experimental month. This was also supported by the water potential measurements, where the 3I/lw, 2I/lw, 11/lw, and ll/2w irrigation frequencies retained higher values when compared to the ll/4w. Although the moisture content had similar values for the 11/lw and ll/2w, the water potential was substantially different. This indicates that for the species of Quercus pubescens one watering every seven days was adequate to maintain its physiological growth status under field hot summer conditions. Similar results were also reported for the species of Myrtus cummunis L. (Iakovo- glou and Kokkinou, 2017).

Based on the seedlings’ characteristics, it seems that for the majority of the studied variables the irrigation treatments did not differ (Fig. 12.3). This means that the seedlings retained similar characteristics within the period of the experimental month under the irrigation treatments, indicating that those seedlings had the same potential in regards to their irrigation response. Nonetheless, some of the characteristics showed differences among treatments suggesting that those variables could be used as surrogates to address early the seedling respond under the irrigation treatments.

Specifically, the frequency of 31/1 w had the least number of lateral roots (Fig. 12.3), while their dry weights did not retain the smallest mean value (Fig. 12.4). Further, based on the dry weight of tap root, the total diy weight of the below and above ground seedling part was greater for the irrigation frequency of 21/lw and least for the ll/4w. Reduced seedling biomass was also observed for the species of Pistacia lentiscus L. under reduced irrigation frequency (Cortina et al., 2008). Similarly, for the leaf dry weight, the 2I/lw had the highest mean value with the ll/2w had the least. Although the irrigation treatment was only for the period of one month, the negative impacts of reduced water availability were apparent for those variables, particularly for the leaf characteristics (Figs. 12.1 and 12.3). Specifically, the seedlings lost part of their leavesto alleviate water stress conditions related to the increased transpiration demand under the hot summer field conditions. This indicates an avoidance mechanism for Quercuspubescens associated with leaf abscission to withstand water deficit conditions (McDowell et al., 2008).

The experimental results based on the specific leaf area

FIGURE 12.2 The experimental results based on the specific leaf area, the ratio of the root-to-shoot, as well as the characteristics of the soil moisture content and the physiological response of the seedlings as reflected by the water potential. Bars indicate the standard deviation of the mean value for each irrigation frequency, while letters indicate the level of statistical difference at thep < 0.05.

Seeding characteristics for Quercus pubescens that were grown under field conditions for each irrigation frequency

FIGURE 12.3 Seeding characteristics for Quercus pubescens that were grown under field conditions for each irrigation frequency. Specifically, seedling height, the diameter of the root collar, the number of leaves, branches, and lateral roots as well as the leaf area. Bars indicate the standard deviation of the mean value for each irrigation frequency, while letters indicate the level of statistical difference at the p < 0.05.

12.3.2 SECOND EXPERIMENTAL PART

Based on the results of the second experimental part, the seedling feature that indicated alterations on the growth of the seedlings were the leaves (Fig. 12.5). Specifically, the number of leaves as well as their leaf area of the seedlings decreased as the irrigation frequency reduced, with the ll/2w and ll/4w having the least mean values. Similarly, the leaf dry weight also decreased as the irrigation frequency reduced, suggesting a negative impact

Biomass characteristics for Quercus pubesceits seedlings that were grown under field conditions for each irrigation frequency

FIGURE 12.4 Biomass characteristics for Quercus pubesceits seedlings that were grown under field conditions for each irrigation frequency. Specifically, the dry weight of the leaves, stem, branches, tap, and lateral roots, as well as, the total dry weight of the upper and down part of the seedling height and the total seedling dry weight. Bars indicate the standard deviation of the mean value for each irrigation frequency, while letters indicate the level of statistical difference at the p < 0.05.

of irrigation frequency of ll/2w and ll/4w (Fig. 12.6). Specifically, the seedlings that were watered only once within the period of one month had no leaves (Fig. 12.7). Although the species of Quercuspubescens is characterized by its ability to retain its leaves, to tolerate water deficit conditions, leaf abscission is one of its physiological responses to tolerate those adverse growth conditions (Ryan, 2011). Similarly, the total dry weight of the above seedling parts reduced as the irrigation frequency reduced, while the number and diy weight of the branches and the total seedling dry weight did not show a specific pattern with reduction in the irrigation frequency.

Characteristics of pretreated seedlings

FIGURE 12.5 Characteristics of pretreated seedlings (seedling height, root collar diameter, the number of leaves, branches, and lateral roots as well as the leaf area) based on irrigation frequencies when grown for a month under controlled chamber growth conditions and frequent irrigation. Bars indicate the standard deviation of the mean for each irrigation frequency, while letters indicate the level of statistical difference at the p < 0.05.

Characteristics of pretreated seedlings based on irrigation frequencies when grown for a month under controlled chamber growth conditions and frequent irrigation

FIGURE 12.6 Characteristics of pretreated seedlings based on irrigation frequencies when grown for a month under controlled chamber growth conditions and frequent irrigation. Specifically, the dry weight of the leaves, stem, branches, tap, and lateral roots, the total dry weight of the upper and down part of the seedling height, and the total seedling dry weight. Bars indicate the standard deviation of the mean value for each irrigation frequency, while letters indicate the level of statistical difference at the p < 0.05.

Based on the root-to-shoot ratio, the irrigation frequencies of 21/lw and ll/lw had greater allocation to the root systems (Fig. 12.7). That was also supported by the greater ability of those irrigation treatments to grow new roots.

Characteristics of pretreated seedlings based on irrigation fr equencies when growth for a month under controlled chamber growth conditions and frequent irrigation

FIGURE 12.7 Characteristics of pretreated seedlings based on irrigation fr equencies when growth for a month under controlled chamber growth conditions and frequent irrigation. Specifically, the specific leaf area, the ratio of the root-to-shoot, as well as the characteristics of the soil moisture content, and the physiological response of the seedlings as reflected by the water potential and the root growth potential and their maximum length. Bars indicate the standard deviation of the mean value for each irrigation frequency, while letters indicate the level of statistical difference at the p < 0.05.

Consequently, reducing the frequency of water supply induced a shift in the carbon allocation toward greater root systems (Iakovoglou and Halivoloulos,

2016). So, based on the results on the development of new roots, the ll/'lw had the greater RGP that was followed by the other treatments. Similar results were also obtained for the species of Finns halepensis were watering once every two weeks resulted in greater root growth potential (Syropli et al., 2014). For the species of Quercus pubescens this suggests that, the irrigation pretreatment frequency of once per week for a month was adequate to trigger allocation for further root growth; a characteristic that promises greater transplanting success in the reforestation efforts while saving water (Chiantante et al., 2006).

This information is of vital importance particularly for ecosystems that experience water scarcity conditions. Although the majority of the species is morphologically and physiologically well equipped, future increased temperatures might pose greater difficulty in reforestation efforts. Nurseries could benefit from these irrigation preconditioning practices and produce seedlings ready to outgrow adverse field conditions by their increased ability to produce new roots that enhance their ability for further nutrients and water exploitation.

CONCLUSIONS

Based on the results, the species of Quercus pubescens, under field conditions (first experimental part) showed an increased ability to tolerate water scarcity conditions as low as irrigating only once every week. This indicates the increased ability of this species not only tolerate water scarcity conditions, but also being able to save water by the reduced water frequency. Further, those seedlings had increased ability to grow new roots (second experimental part). That is an important characteristic for the seedlings since it helps overcome transplanting stress. Nonetheless, further research on other species could further enlighten on species responses on the effects of irrigation frequency in accordance to their ability to grow new roots. This will potentially increase the transplanting success while saving water for ecosystems that experience growth periods of water scarcity conditions similar to those of the Mediterranean region.

ACKNOWLEDGMENTS

This research work was funded under the Project “Research & Technology Development Innovation Projects”-AgroETAK, MIS 453350 in the framework of the operational program “Human Resources Development.” It is со-funded by the European Social Fund through the National Strategic Reference Framework (Research Funding Program 2007-2013) coordinated by the Hellenic Agricultural Organization—DEMETER, Forest Research Institute.

KEYWORDS

  • climate change
  • ecophysiology
  • regeneration
  • preconditioning
  • seedling production
  • semi-arid

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