Horticulture

A total of 137 accessions from 18 wild almond species were collected from Iran and leaf and fruit traits were characterized. Also evaluated were flowering and ripening date, self-incompatibility and kernel bitterness. An extensive phenotypic diversity was found both among and within species. Differences in average leaf dimensions among and within species were associated with average rainfall but not altitude of collection site. Adjacent accessions located in drier areas had smaller leaf dimensions than those located in semi-humid or humid regions. No relation was found between average fruit dimensions and collection site conditions. Principal component analysis revealed that the nut weight and width, and the kernel weight had highest loading in the first component accounting for 45.8% of total variation. In contrast, leaf traits in the second component accounted for 22.3% of total variation. No significant correlations were detected between leaf dimensions and fruit traits in all species evaluated. Results document a rich source of new germplasm for almond improvement programs. Small fruit size, pollen-pistil self-incompatibility, and bitter kernel flavour are the most common obstacles to the utilization of this wild germplasm in breeding.

A total 23 morphological traits, 19 AFLP-primer combinations, 80 RAPD primers and 32 SSR primer pair were used to compare the informativeness and efficiency of random amplified polymorphic DNA (RAPD), amplified fragment length polymorphism (AFLP) and simple sequence repeat (SSR) markers in establishing genetic relationships among 29 almond cultivars and three related wild species. SSRs presented a high level of polymorphism and greater information content, as assessed by the expected hetrozygosity, compared to AFLPs and RAPDs. The lowest values of expected hetrozygosity were obtained for AFLPs; however AFLPs showed the highest efficiency, owing to their capacity to reveal large numbers of bands per reaction, which led to high values for various types of indices of diversity. All the three techniques discriminated almond genotypes very effectively, except that SSRs failed to discriminate between ‘Monagha’ and ‘Sefied’ almond genotypes. The correlation coefficients of similarity were statistically significant for all the three marker systems, but were lower for the SSR data than for RAPDs and AFLPs. For all the markers, high similarity in dendrogram topologies was obtained, although some differences were observed. All the dendrograms, including that obtained by the combined use of all the marker data, reflect relationships for most of cultivars according to their geographic diffusion. AMOVA detected more variation among cultivated and related wild species of almond within each geographic group. Bootstrap analysis revealed that the number of markers used was sufficient for reliable estimation of genetic similarity and for meaningful comparisons of marker types.

The present work describes the changes in the activities of key antioxidant enzymes and the levels of some metabolites in relation to salt tolerance in eight wild almond species. All the species were exposed to four levels of NaCl (control, 40, 80 and 120 mM). Plant fresh biomass, α-, γ- and δ-tocopherol, total soluble proteins, malondialdehyde (MDAeq), H2O2, total phenolics, and the activities of superoxide dismutase (SOD), peroxidase (POD) and catalase (CAT) were analyzed in leaves of salt-stressed and non-stressed plants of the eight almond species. In all the species, salt stress significantly enhanced the activities of SOD and POD, levels of total phenolics and γ- and δ-tocopherols. High levels of salt stress significantly depressed the levels of total soluble proteins, MDA and CAT activity, while salt stress did not significantly affect leaf H2O2 contents. Regression analysis showed that the relationship between salt levels and total soluble proteins, CAT, γ-tocopherol, MDAeq, SOD and POD were statistically significant. Principal component analysis discriminated the almond species based on their degree of tolerance/sensitivity to saline conditions: Prunus reuteri and P. glauca were ranked as salt tolerant, P. lycioides and P. scoparia as moderately tolerant, and P. communis, P. eleagnifolia, P. arabica and P. orientalis as salt sensitive. The results could be used for selecting salt tolerant genotypes to be used as rootstocks for almond cultivation.

Pollen germination and pollen tube growth was quantified among various native Iranian wild almonds (P. dulcis (Mill.) D. A. Webb, P. eleaegnifolia Mill., P. orientalis Mill., P. lycioides Spach, P. reuteri Bioss. et Bushe, P. arabica Olivier, P. glauca Browick and P. scoparia Spach in order to identify differences in the tolerance of pollen to temperature variations. Pollen germination and pollen tube growth were observed after incubation in darkness in a germination medium for 24 h at 10–50°C at 5°C intervals. Maximum pollen germination of the wild almond species and specify that 60% was obtained for P. orientalis pollen and 98% for P. scoparia. Pollen tube length ranged from 860 μm was obtained in P. lycioides and 1490 μm in P. scoparia. A modified bilinear model best described the response to temperature of pollen germination and pollen tube length. Almond species variation was found for cardinal temperatures (T min, T opt and T max) of pollen germination percentage and pollen tube growth. Mean cardinal temperatures averaged over eight almond species were 14.7, 24.2, and 43.7°C for maximum percentage pollen germination and 14.48, 25.3, and 44.4 °C for maximum pollen tube length. The principal component analysis (PCA) identified maximum percentage pollen germination and pollen tube length of the species, and T max for the two processes as the most important pollen parameters in describing a species tolerance to high temperature. PCA also classified Prunus L. spp. into four groups according to the tolerance of pollen to temperature variations. The T min and T opt for pollen germination and tube growth, rate of pollen tube growth were less predictive in discriminating species for high temperature tolerance.

In wild species of almond (Prunus spp.), the activities of ascorbate peroxidase (APX), monodehydroascorbate reductase (MDHAR), dehydroascorbate reductase (DHAR), and glutathione reductase (GR), as well as the levels of ascorbate/glutathione pools and H2O2 were subjected to water deficit and shade conditions. After 60 days of water shortage, the species were subjected to a rewatering treatment. During water recovery, leaves exposed to sunlight and leaves under shade conditions of about 20–35% of environmental irradiance were sampled. After 70 days without irrigation, mean predawn leaf water potential of all the species fell from −0.32 to −2.30 MPa and marked decreases in CO2 uptake and transpiration occurred. The activities of APX, MDHAR, DHAR, and GR increased in relation to the severity of drought stress in all the wild species studied. Generally, APX, MDHAR, DHAR, and GR were down-regulated during the rewatering phase and their activities decreased faster in shaded leaves than in sun-exposed leaves. The levels in total ascorbate, glutathione, and H2O2 were directly related to the increase in drought stress and subsequently decreased during rewatering. The antioxidant response of wild almond species to drought stress limits cellular damage caused by reactive oxygen species during periods of water deficit and may be of key importance for the selection of drought-resistant rootstocks for cultivated almond.

Eight native Iranian almond species from three sections, ‘Euamygdalus’ (Prunus communis; Prunus eleagnifolia and Prunus orientalis); ‘Lycioides’ (Prunus lycioides and Prunus reuteri) and ‘Spartioides’ (Prunus arabica, Prunus glauca and Prunus scoparia) were in vitro screened for drought tolerance using sorbitol and polyethylene glycol (PEG) as an osmoticum. Different levels of water stress were induced using five concentrations of either sorbitol or polyethylene glycol in Woody Plant Medium (WPM). Water potential of various media ranged from −0.80 to −2.05 MPa and water stress in culture medium adversely affected plantlet growth. Wild species from ‘Spartioides’ were less affected than ‘Lycioides’ and ‘Euamygdalus’. At the same level of water potential, sorbitol had lower adverse effects than PEG; the latter being severe. Prunus × sorbitol and Prunus × PEG interactions were significant. At 0.2 M sorbitol and 0.003 M PEG, ‘Spartioides’ produced significantly more roots with higher total root length and root volume, as well as root-dry weight than those of ‘Lycioides’ and ‘Euamygdalus.’ It is concluded that in vitro screening of native Iranian almond species under specific and limited water-stress conditions may provide a system for effectively differentiating the wild species of almond for their expected root mass production under field conditions.

Prunus dulcis L. 'Mamaei' is grown widely in souhtwest of Iran. It blooms in early spring when temperatures are still low. Based on our knowledge there are no reports in the literature regarding pollen behavior of this cultivar under specified condition. Thus, the possible factors for low germination percentage in this cultivar have not been reported. The effect of three different temperatures (10, 25, or 35 C), polyamines (putrescine, spermidine, and spermine) and polyamine synthesis inhibitor, methylglyoxals-bis (guanylhydrazone) (MGBG) on in vitro pollen germination and pollen tube growth were investigated in P. dulcis L. 'Mamaei'. All temperatures and chemicals significantly affected both pollen germination percentage and pollen tube growth. In general, different polyamines stimulated the pollen germination percentage compared to the control at all temperatures, but increasing the temperature, particularly to 35 C, had demonstrated inhibitory effects on pollen germination. At a concentration of 0.05 mM putrescine and spermidine and 0.005 and 0.025 mM spermine revealed longer pollen tube growth than that of the control at 10 C, while higher concentrations tended to inhibit pollen tube growth. At 25 C, most of the treatments had an inhibitory effect on pollen tube growth except for 0.25 mM putrescine and 0.005 mM spermine, which slightly stimulated pollen tube growth. Pollen germination and pollen tube growth were inhibited by MGBG at all temperatures and in all concentrations.

Three different drought stress levels (water potential of the nutrient solution, Ψ s = −0.6, −1.2 and −1.8 MPa, respectively), and a control treatment (Ψ s = −0.1 MPa), were applied during 2 weeks to three almond species, followed by 3 weeks of recovery. The selected test species were Prunus dulcis (Miller) D.Webb (bitter almond) and two wild almond species, P. lycioides (Spach) C.K. Schneider and P. scoparia (Spach) C.K. Schneider. All three are species native to Iran, and can be used as rootstock, but only P. dulcis is actually used for commercial almond production.