Data were analyzed using linear mixed models and generalized linear mixed models with the GLIMMIX procedure of SAS . The normal or log‐normal distribution and identity link function were used for continuous response data, the Poisson distribution and log link function for count data, and the binary distribution and logit link function for binary data. Inverse link functions were used to transform predicted means back to the original units of measure for presentation. P-values for multiple mean comparisons were adjusted with the Holm‐Tukey method. Potential relationships between vegetative and rooting attributes were analyzed using linear regression . The cultivars with high rooting percentage were Desertnyi, Eversweet, Golden Globe, Haku Botan, Ki Zakuro, Loffani, Nochi Shibori, Parfianka, Phoenicia, and Wonderful. Ambrosia and Green Globe had lower rooting percentages compared to other cultivars in both experiments. These data are consistent with other studies on rooting of pomegranate cuttings. Karimi et al. reported significant differences among cultivars for several growth attributes. Highest rooting percentages previously reported were 90% by Singh et al. , 86.7% by Polat and Caliskan , and 96.8% by Owais . Hussain et al. reported a mean percentage of 75.6% for hardwood cuttings propagated under different environmental conditions. Overall, research results indicated that pomegranate is readily propagated by hardwood cuttings,blueberry container size but there are exceptions, with certain genotypes more difficult to root than others.
Green Globe rooted poorly in both experiments, with rooting approximately 25% that of most other cultivars evaluated. According to Kennedy , Mae is a cultivar in the USDA NCGR collection that also roots poorly using hardwood cuttings, but no published data confirm this report. Dry root mass data are generally in agreement with Owais , who reported a maximum dry root mass of 0.223 g for hardwood cuttings. In our experiments, a gel formulation of IBA was used to stimulate rooting. Gel formulations are used less commonly in commercial propagation than liquid or powder formulations, but acceptable rooting was obtained using 3 g×L -1 IBA for all cultivars except Green Globe. In contrast, Owais utilized very high concentrations of IBA, up to 12 g×L -1 , and found that cuttings treated with 6 or 9 g×L -1 IBA had the highest rooting percentages compared to control cuttings or cuttings treated with 3 or 12 g×L -1 IBA. Karimi et al. reported that rooting percentages of pomegranate were lower for cuttings treated with 1 g×L -1 IBA when compared to non-treated cuttings. Some of these concentrations of IBA are higher than what is typically used for pomegranate in commercial practice. Our subsequent experiments with higher concentrations of IBA did not increase rooting percentages of ‘Ambrosia’ and ‘Green Globe,’ with ‘Ambrosia’ and ‘Green Globe’ having rooting percentages of 38.5% and 0%, respectively, for dormant hardwood cuttings treated with 9 g×L -1 IBA. If difficult-to-root cultivars are chosen for nursery propagation, further research on propagation methods will be needed.The factors causing one pomegranate cultivar to be more successful at rooting than another have not been confirmed.
Some research suggests that endogenous auxins explain cultivar differences in propagation. Auxins can regulate gene expression in plant tissues and endogenous auxin concentrations are under genetic control . According to Levin , ‘Wonderful’ and ‘Desertnyi’ are related, with ‘Wonderful’ being one of the parents used to hybridize ‘Desertnyi,’ with the parentage being x ‘Wonderful.’ Cuttings of both ‘Wonderful’ and ‘Desertnyi’ rooted at 96.9%. Whether rooting success is a heritable trait in pomegranate is unknown. There were significant differences in vegetative growth attributes between Wonderful and other cultivars in both experiments. Owais recommended hardwood cuttings of pomegranate be made 15 to 20 cm long and 6 to 7 mm in diameter. Our 10- cm cuttings rooted better than the 20-cm hardwood cuttings used by Owais , suggesting that dividing the longer cuttings in half not only produced twice as many cuttings, but also perhaps better rooting. In both of our experiments, stem diameter had no effect on any attribute measured and many cuttings were less than 6 mm in diameter. This indicated that more cuttings of smaller diameter may be used, but with the same propagation success. Wonderful, the industry standard, and nine other fruiting and ornamental cultivars of pomegranate rooted very well using hardwood cuttings treated with a gel formulation of 3 gL -1 IBA. Also, differences in certain vegetative growth attributes, such as plant height and branching, could be detected early in the production of some cultivars. This is the first known study to evaluate rooting and vegetative growth of dormant hardwood cuttings of Wonderful and other important cultivars from the USDA NCGR pomegranate germplasm collection. Further research is needed to discover methods to enhance the propagation success of the cultivars with poor rooting.
Global food security is threatened by climate change, which includes increasing temperatures and other unpredictable changes in weather patterns potentially damaging to agricultural systems . For example, long-term drought in California has caused significant economic losses to farmers . Drought in California has driven farmers to engage in crop abandonment, stress irrigation, the replacement of water-intensive crops to alternative crops , and most recently, using secondary water resources often available from urban or suburban wastewater systems. To lessen the impacts of climate change, it has been proposed that crop diversification may reduce food system vulnerability . Challenges posed by drought and water scarcity issues have led physiologists and breeders to focus on water-use efficiency in agriculture . Improving production efficiency and drought tolerance through cultivar selection has been proposed in citrus Prunus species , dates , and coffee , among others. Because tree crops have some degree of variability of physiological traits among genotypes in the same species, it is useful to study diversity within crop species to determine if there are cultivars that use water more efficiently or are as productive in cropping system conditions of high abiotic stress. Pomegranate is touted as a drought tolerant crop, especially once established , and it is being evaluated in California as an alternative tree crop species to replace or supplement more water-intensive species, such as avocado, citrus and almond , all three of which face pest and disease threats of their own. Historically, pomegranate has been grown in California for hundreds of years, even before statehood, starting with the Spanish missionaries who came over from Spain in the 1700s and planted mongrel seed from Spain in their gardens. Pomegranate was a minor crop in California until the 21st Century, when planting area increased by tenfold within twenty years and its value reached over $200 million annually. The body of knowledge regarding differences in pomegranate physiology, establishment, and site/climate effects among cultivars conserved in germplasm is limited. This limitation is a barrier to commercial growers and nurseries adopting new or forgotten cultivars that have traits with the potential to decrease water use and increase consumer demand for the fruit. Breeders also benefit from having germplasm phenotyped so that efforts to utilize molecular tools to map quantitative trait loci within breeding populations can be pursued for crop improvement. The pomegranate variety collection located at the United States Department of Agriculture – Agricultural Research Service National Clonal Germplasm Repository in Davis, CA, conserves about 200 genotypes of pomegranate in Winters, CA,growing raspberries in containers many of which have consumer-friendly phenotypic traits not seen in ‘Wonderful,’ the industry standard . Experiments have demonstrated differences in morphology and vegetative growth traits during the propagation phase of the present study . Studies have shown that there can be differences among cultivars for many agriculturally-relevant physiological traits of pomegranate in other collections, including transpiration rate, stomatal conductance, water-use efficiency, photosynthetic rate and chlorophyll content . The objectives of this research were: 1) to evaluate eleven unique pomegranate cultivars for field performance in semi-arid and coastal Mediterranean climates to determine plant establishment rates and site effects on reproductive biology; and 2) to evaluate four pomegranate cultivars for field performance in coastal versus inland agroecosystems to determine if there are differences among genotypes for physiological traits that would be conducive to commercial crop production in drought conditions.
The sites were located at the Agricultural Experiment Station of the University of California, Riverside, CA and on private land in Somis, CA . Riverside, CA is a semi-arid climate with hot, dry summers and cool winters and Somis, CA is a coastal Mediterranean climate with warm, dry summers and wet, cool winters. At Riverside, the mean annual precipitation of the area is 262 mm and mean high temperatures are 28⁰ and 36⁰ C for June and August, respectively. Mean temperature lows are 13⁰ and 18⁰ C for June and August, respectively and 5⁰, 6⁰, and 7⁰ C for December, January and February, respectively. The soil is a sandy loam with good drainage and was previously an established lemon grove. At Somis, mean annual precipitation of the area is 468 mm and mean high temperatures are 26⁰ and 28⁰ C for June and August, respectively. Mean temperature lows are 13⁰ and 14⁰ C for June and August, respectively and 5⁰, 5⁰, and 6⁰ C for December, January and February, respectively. The soil at Somis is a heavy sandy clay that was previously an established avocado grove. All trees were growing under natural light, outside in field conditions with drip irrigation three times per week to replace water lost to soil evapotranspiration as determined by California Irrigation Management Information System weather stations in Riverside and Ventura counties. Trees were grown under conventional commercial management practices and fertilized in spring with urea and potassium sulfate, totaling 31.75 kg N, and 34 kg K per year, respectively, over approximately 0.81 ha. All experimental trees were followed during their first four years of development and were located on the inside of the grove, with at least one border tree acting as a buffer to reduce the edge effect.Two pomegranate cultivar trials were utilized for this study. The cultivars utilized in the study were Ambrosia, Desertnyi, Eversweet, Golden Globe, Green Globe, Haku Botan, Loffani, Parfianka, Phoenicia, a proprietary cultivar and Wonderful . Cultivars were either cultivars bred for coastal sites and those that are typically grown in inland sites. Coastal cultivars included Ambrosia, Eversweet, Golden Globe, Green Globe, and Loffani. Inland cultivars included Desertnyi, Haku Botan, Parfianka, Phoenicia, and Wonderful. For the proprietary cultivar included, it is unknown whether it is a coastal or inland cultivar. All plants were sourced from the National Clonal Germplasm Repository, Davis, CA, and propagated via dormant hardwood cutting at the same time in winter of 2012 and. All trees included were mature and most cultivars were bearing commercial loads of fruit by the fourth year. For physiology trait evaluations, the healthiest tree in each of three blocks were selected among 15 trees per cultivar in the trial, which was planted in a randomized complete block design. Wonderful is the industry standard in many countries and was chosen as a control in the cultivar field trial. Wonderful is a widely-grown commercial cultivar that originated in Florida, it accounts for approximately 90-95% of production in the USA. It is a highly vigorous, thorny tree that has high yield with red fruit that have red arils with moderate seed hardness and a sweet-tart flavor. The growth habit is willowy, with a tendency to sucker at the base of the tree. There exists limited quantitative field establishment or physiology data for Wonderful and all other cultivars in this study. Trunk diameter was measured in cm with a digital caliper in spring of each year, with the measurement taken approximately 15 cm above the soil level. Tree canopy diameter was measured in-row and between rows to determine how quickly trees were growing into each other and into the rows. Tree height and in- and between row spacing measurements were taken with a folding rule. Tree canopy area was calculated with an equation reported by Serfontein et al. , where “S” is canopy area, “a” is tree height, “b” is skirt radius and “e” is eccentricity of an ellipse.During fruit development , an infrared gas analyzer was used to measure maximum rates of net CO2 assimilation , stomatal conductance , and transpiration during the morning hours. Morning light availability ranged from 1500-1600 µmol m-2 ·s-1 photosynthetic photon flux density . Photosynthesis measurements for three days were pooled for the four cultivars, which occurred on 29 July 2016, 4 August 2016, and 7 September 2016 for Riverside and 25 July 2016, 31 July 2016, and 31 August 2016 for Somis.