Pollen grain (equatorial and polar view) in sweet cherry (Prunus avium L.) cultivars: a-'Karina'; b'Kordia'; c-'Regina' and d-'Summit'

... are reports in literature on the occurrence of variation in in vitro pollen germination in sweet cherry cultivars by years [25]. Regularity in microsporogenesis is directly associated with vitality and in vitro pollen germination [26], therefore the different factors that affect pollen formation and their varying impact on certain cultivars can induce non-uniform germination. In addition, the tendencies above may also be due to the experimental conditions affecting in vitro pollen germination, such as incubation temperature and composition of the medium, along with the different, genotype-induced requirements of each cultivar studied in relation to the specific conditions in the trial. It is well known that the temperatures within the range of 15-20°C are generally optimal for in vitro pollen germination in apricot [27] and sour cherry [28] cultivars. Optimal temperature for in vitro pollen germination may depend on flowering time of the cultivars (early or late) [6]. Beyhan & Karakaş [29] report on the best in vitro pollen germination in seven sweet cherry cultivars on medium containing 20% sucrose concentration, whereas the results obtained on media supplemented with 15% and 10% sucrose content were poorer, and in agreement with our results. The results obtained for in vitro pollen germination of 'Kordia' and 'Summit' are in agreement with some earlier results relative to in vitro pollen germination done in the same cultivars under climate conditions of Western Serbia [30]. Pollen grain morphological characterization All the sweet cherry cultivars studied were described as having monad pollen, elliptical, with iso-polar symmetric and three ectoapertures (tricolporate) (Figure 1). Generally, the pollen morphology was similar among the cultivars (Table 2). Polar diameter (length) varied from 51.08 μm ('Regina') to 53.14 μm ('Summit'), as well as equatorial diameter (width) from 24.09 μm ('Regina') to 26.06 μm ('Summit'). The comparison of pollen grain diameter among 'Summit' and the other cultivars revealed significant differences. Differences in L/W ratio were greater, ranging from 2.039 ('Summit') to 2.126 ('Karina'). L/W ratio did not distinguish all the cultivars, as 'Karina' was not significantly different from 'Regina'. Pollen shape in all the studied sweet cherry cultivars was identified as perprolate (L/W ratio >2) [31]. Germinal furrows extended the length of the grain. Germinal furrow length ranged from 43.08 μm ('Regina') to 45.38 μm ('Summit'), while germinal furrow width varied only slightly, ranging from 1.33 μm ('Regina') to 1.45 μm ('Summit') ( Table 3). The distance between furrows was the smallest in 'Summit' (13.94 μm) and greatest in 'Regina' (14.83 μm). Non-significant results were obtained by analysis of germinal furrow width. As for germinal furrow length and distance between furrows, 'Karina' was not significantly different from 'Regina' and 'Kordia' respectively. The exine ridging was generally longitudinal (Figure 2), with more parallel ('Karina') or less parallel ridges ('Summit'). The ornamentation of pollen surface can be classified as striate. Number of ridges crossing 100 μm 2 of exine equatorial region ranged from 18.67 ('Regina') to 20.33 ('Summit') ( Table 4). Mean ridge widths varied slightly, ranging from 0.36 μm ('Summit') to 0.40 μm ('Karina' and 'Kordia'). Mean striae widths were found to be close to the mean of ridge, width being the smallest (0.45 μm) in 'Summit' and the greatest (0.56 μm) in 'Karina'. Not significant results were obtained by ridge width analysis. 'Kordia' did not significantly differ from 'Karina' in number of ridges in 100 μm 2 of exine, as well as from 'Summit' with regard to striae width. The pits (perforations less than 1 μm in diameter) are more pronounced at exine surface in 'Kordia' than in other three cultivars, particularly 'Regina', which is almost without pits (Figure 2). The size of pollen grain is generally less than 0.1 mm in diameter, but there is wide variation among plant species [32]. Although pollen shape of different species varies, a great majority of pollen is basically spherical or oval. The pollen shape of Prunus species is described as truncate to elliptical in almond [24]; cylindrical in peach and plum [8]; elliptical or obtuse-triangular in apricot [20]. According to L/W ratio, the pollen shape of studied sweet cherry cultivars is more elongated than in other Prunus species -apricot [8,20], peach and plum [8], almond [24] and sour cherry [21]. Analysis of measured parameters indicated that morphological variability was lower in sweet cherry than in other Rosaceae species such as apple [9] and strawberry [11], which may be due to its lower ploidy level (only 16 chromosomes). Sorkheh & al. [24] obtained similar results in diploid almond cultivars. Previous results obtained in apple, plum, peach, strawberry and raspberry cultivars suggested direct relationship between pollen size and ploidy level [17,11]. Mulas & al. [23] showed stability of pollen traits; ultrastructural morphology of almond pollen does not vary according to rootstock used, irrigation or geographical ...