Comparison of ultrastructure, pollen tube growth pattern and starch content in developing and abortive ovaries during the progamic phase in hazel

Abstract

HIGHLIGHTSIn an abortive ovary of hazel, an integument seldom differentiated and a mature embryo sac never developed.In an abortive ovary of hazel, pollen tube growth was arrested at the style base about 40 days after blooming. Thus, fertilization of the ovule was precluded.Ovary abortion in the four hybrid cultivars was indicated to be associated with insufficient resource availability to support fruit set by all flowers, whereas ovary abortion in C. heterophylla was at least partly determined by pollen availability. In Northeast China, a high frequency of ovary abortion contributes to serious losses in yield of hazelnut. The development of pistillate inflorescences and fruit clusters of four large-fruited hybrid hazel cultivars and the small-fruited Corylus heterophylla were used to study ovary abortion and its possible causes during the progamic phase in hazel. The average number of pistillate (ANP) flowers per inflorescence and average number of fruit (ANF) per cluster of four hybrid hazel cultivars were 7.6-8.5 and 2.4-3.0 respectively; in C. heterophylla, its ANP and ANF was 5.8-6.2 and 3.5, respectively. The total drop varied from 50 to 67%. Ovary abortion in hazel initiated from about 30 days after blooming. The percentage of abortive ovaries (PAO) in the four hybrid hazel cultivars ranged from 63 to 72%, and was significantly higher than that of C. heterophylla (29-42%). Only the abortive ovary ratio of C. heterophylla was significantly reduced after artificial pollination. Fruit number per cluster was positively and negatively correlated with yield and nut mass, respectively. In abortive ovaries, the diameter remained less than 2 mm during the entire fruit development, an integument seldom differentiated and a mature embryo sac never developed. In addition, pollen tube growth was arrested at the style base about 40 days after blooming. Thus, fertilization of the ovule was precluded. Compared with abortive ovary, starch content in developing ovary of four hybrid hazel cultivars and C. heterophylla were significantly higher. This study suggests that abortive ovary was incapable to finish fertilization process due to the absence of mature embryo sac and arrested pollen tubes, and this is likely associate with insufficient resource availability to support fruit set by all flowers in four hybrid hazel cultivars, whereas ovary abortion in C. heterophylla is at least partly determined by pollen availability.

Keywords: abortive ovary; delayed fertilization; fruit cluster; hazelnut; pistillate inflorescence.

Figure 1

Differences in external morphology and pollen tube growth in developing and abortive ovaries during the progamic phase. (A) External morphology of abortive ovary (indicated by arrow) and developing ovary, 30 days after blooming. (B) Normally developing ovary (left and center) and abortive ovary (right), 40 days after blooming. (C) Normally developing fruit and abortive fruit in a fruit cluster, 65 days after pollination. Arrows indicate abortive ovaries. (D) Young pollen tubes in the style, 5 days after blooming. (E) Pollen tubes curled and arrested at the style base, showed by arrows, 20 days after blooming. (F) Pollen tubes growing into developing ovary, arrows showed pollen tubes, 40 days after blooming. (G) Pollen tubes (showed by arrow) growing into abortive ovary, 40 days after pollination. (H) Pollen tubes (showed by arrow) growing toward ovule, 45 days after pollination. (I) Pollen tubes arrested in abortive ovary (showed by arrow), 45 days after pollination. (J) Pollen tubes in style (showed by arrow) changed direction and grew toward ovule, 50 days after pollination. (K) Pollen tubes arrested in abortive ovary, 50 days after pollination. (L) Pollen tubes starting to penetrate ovule of developing ovary, 55 days after pollination. (M) Abortive ovary with Curled and arrested pollen tubes, 55 days after pollination. (N) Pollen tubes in developing ovary release two sperm cells in embryo sac, 60 days after blooming. (O) Abortive ovary with curled and arrested pollen tubes, 60 days after blooming. Key: Ao, abortive ovary; Do, developing ovary; Es, embryo sacs; M, micropyle; O, ovary; Ov, ovule; P, parenchyma; St, style; Scale bars: A,B = 1 mm; C = 400 μm; D = 300 μm; E = 100 μm; F–O = 300 μm.

Figure 2

Anatomical differences in abortive and young developing fruit during the progamic phase. (A) Early ovary primordium, 20 days after blooming. (B) Early rudimentary ovary, 25 days after blooming. (C) Developing ovary, 30 days after blooming. (D) Abortive ovary, 30 days after blooming. (E) Developing ovary with early ovule, 40 days after blooming. (F) Abortive ovary with early ovule primordia indicated by arrows, 40 days after blooming. (G) Ovule in developing ovary, 50 days after blooming. (H) Ovules in abortive ovary, 50 days after blooming. (I) Ovule with mature embryo sac in developing ovary, 55 days after blooming. (J) Ovule lacking differentiated embryo sac and nucellus in abortive ovary, 55 days after blooming. (K) Embryo in fertilized ovule of developing ovary, 65 days after blooming. Key: Bp, bract primordium; E, embryo; En, endosperm; Es, embryo sac; F, funiculus; Int, integument; M, micropyle; N, nucellus; Op, ovary primordium; O, ovary; Ov, ovule; P, parenchyma; Pl, placenta; St, style. Scale bars: A,B = 100 μm; C–E = 100 μm; F = 30 μm; G = 100 μm; H = 30 μm; I–K = 100 μm.