Physiological aspects of date palm loading and alternate bearing under regulated deficit irrigation compared to cutting back of bunch
Introduction
The alternate bearing means the production of a large number of small fruits in one year (On-year) following the production of a small number of large fruit in the previous year (Sharma et al., 2019) because of abscission (Off-year) in some fruit trees due to hormone or nutrition (Khan et al., 2014; Rosenstock et al., 2010), which occurs in ‘Mazafati’ date (Phoenix dactylifera L.) trees because of nutritional imbalance between vegetative and reproductive organs (Pillay et al., 2005). The palm trees have three waves of fruit abscission. The first wave occurs a few weeks after pollination caused by the lack of fertilization and reduction of gibberellic acid produced by the seed (Nay and Perring, 2009). The second wave, which is more serious, is due to nutritional competition, occurs in mid-May at the end of the Kimri and the beginning of the Khalal stage, and eventually the third wave occurs before harvesting due to the high temperature-induced fruit wilting (Al-Qurashi et al., 2012; Othmani et al., 2019).
The alternate bearing is prevented by the old method of flowering and fruit thinning in On-year, which upgrades fruit dimensions, reducing misshapen fruit and abscission (by scattered fruits and bunch aeration) (Lauri et al., 2014), increasing maturity, improving fruit coloring, and controlling pests, but sometimes this practice, especially in wet areas, increases the percentage of black nose fruits (Chao and Krueger, 2007; Rosenstock et al., 2010).
Fruit thinning in the dates is done using the following methods: 1) Bunch removal (Ratio of Leaf/Bunch = 10) (Dennis, 2000; Zaid and De Wet, 1999), 2) Bunch thinning (Cutting back: Remove the tip of the stalk, Removing strands, Hand thinning) (Awad, 2006; Cohen et al., 2010), 3) Chemical thinning (2, 4-D, Ethephon, and NAA) (Iwahori and Oohata, 1976; Marashi et al., 2012). The most appropriate method of date palm ‘Mazafati’ thinning is 5 cm cutting back (Ehsani, 2007).
Deficit irrigation (DI) means eliminating part of irrigation water that has no significant effect on yield (Shackel, 2011). DI under the influence of the time of application is called regulated deficit irrigation (RDI), which is the use of water in amounts less than the crop evapotranspiration (ETc) in certain periods of the phonological stages of plant growth (Cuevas et al., 2007). This method, in contrast to the sustained deficit irrigation (SDI), focuses more on the crop physiology, and is applied at times when the fruit growth is relatively slow, and water stress will affect vegetative growth (Laribi et al., 2013).
The palm tree physiology shows that if the carbohydrate reserve is sufficient, its flowering is carried out in January at 10 °C (Chao and Krueger, 2007). At the development stage of the stalk (February and March), the fruit growth is slow, and the water requirement is low, but following the end of this stage (late April), a rapid increase in the growth and size of the fruit occurs when the water requirement increases and in the limitation of the water at this phenological stage will reduce the fruit growth and yield (Cohen et al., 2010; Gribaa et al., 2013).
Decreasing vegetative growth reduces the competition for photosynthetic compounds between fruits and vegetative organs, and thus increasing the size of fruits, improves penetration of light into the crown, and better fruit coloring (Faghih et al., 2019; Zhen et al., 2019). DI due to changes in the metabolism and the phloem supply, produces high levels of secondary metabolites, such as polyphenol compounds and antioxidants in fruits, and increasing the organoleptic characteristics of fruits. Moreover, DI during the flowering lowers the fertilization and fruit set, and the physiological thinning of immature fruits occurs in the first season (Khan and Prathapar, 2012; Mpelasoka et al., 2001).
On the other hand, DI in some fruits causes prematurity (Behboudian and Mills, 1997). Earlier maturation in these fruits can justify the no increase and even reduce the size of the fruit by DI, and theoretically, treatments that change the size of the fruit change the quality, firmness, juice dilution, soluble solids, and fruit acids (Leib et al., 2006). In case of any precocity and decrease in the size and quality of date fruit, irrigation schedule of trees is one of the most important factors that should be carefully considered and for palm trees, a history of DI is reported at 65–75 % ETc in the developmental stage of the stalk (Alikhani-Koupaei et al., 2018b; Gribaa et al., 2013).
All methods of fruit thinning require a lot of work and aren’t cost-effective. Therefore, it is necessary to investigate other physiological methods (Gonzalez-Rossia et al., 2006). In this regard, the control of alternate bearing dates ‘Mazafati’ was studied using RDI and compared with the running method by the palm owners, namely cutting back, for two consecutive years.
Section snippets
Plant materials
The present study was performed during two consecutive years of 2017 and 2018 on 10–12 years old ‘Mazafati’ date trees and 8 × 8 m spacing with a history of alternate bearing in Nikshahr city, southeast Iran with the properties of climate, soil and irrigation water provided in Table 1. Selected trees were pollinated with M003 genotype and subjected to uniform crop operation throughout the season. Under these conditions, the emergence of the first spathes and cracking them were respectively
Yield traits and biennial bearing index (BBI)
Considering the significant interaction between treatments × year for yield, bunch weight, and fruit weight, the mean comparison of these traits was done separately for each year. The factors including fruit loading, cutting back and RDI had a significant effect on yield (Fig. 1a), bunch weight (Fig. 1b), bunch number (Fig. 1c), and fruit weight (Fig. 1d). At the fruit load of On2017 and Off2017 trees, in the year when the bunch number was low, the bunch weight and the final yield also
Conclusion
The results of this study showed that the alternate bearing of date palm is related to the physiological factors including seasonal changes in the rate of internal carbohydrate metabolism, and the decomposition and the formation of starch deposits. In this study, the changes in reducing sugars up to the maximum formation content 37−48 g/100 g FW per fruit at the harvest time, and the maximum decomposition of the starch deposits in the late Kimri with the residual content 15−70 mg/g DW and the
Declaration of Competing Interest
The authors would like to express that they have no competing interests regarding this research.
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