Cucumber (Cucumis sativus L.) flowers are classified into three types, i.e, staminate (male), pistillate (female) and hermaphrodite (perfect) flowers. Staminate and pistillate flowers are borne on different nodes. Although pistillate flowers are solitary at occasional nodes in many cultivars, the number of pistillate flowers per node varies from one to more than three, depending on the cultivar, i.e., mono-pistillate, di-pistillate and multi-pistillate type cultivars (one, two and three or more pistillate flowers per node, respectively). On the other hand, the several staminate flowers are borne on one node. Generally, cucumber plants are monoecious cultivars which produce both staminate and pistillate flowers separately on the same plant. Some cultivars bear only pistillate flowers (gynoecious) or staminate and hermaphrodite flowers on the same plant (andromonoecious). Furthermore, some cucumber cultivars are able to set fruit without pollination and fertilization (parthenocarpy). Parthenocarpy is a valuable trait, especially in gynoecious cultivars that produce only a few staminate flowers.

In Japan, cucumber growers invest much labor towards vine training and pest control because they grow monoecious cultivars with single pistillate flower per node for a long period. Production cost can be reduced, if a large number of cucumber fruits could be harvested within a short period, using multi-pistillate cultivars or gynoecious, parthenocarpic cultivars. However, in both multi-pistillate and gynoecious cultivars, fruits at the middle nodes stop growing after anthesis and some fruits abort possibly because of intense competition for assimilates among fruits.

Pollination is known to increase the concentration of endogenous phytohormones and stimulate cell division, resulting in increases in fruit set and fruit growth. However, the effects of pollination and crop load on fruit retention have not been well studied in relation to endogenous hormones and sink strength (cell number and cell size) in cucumber cultivars bearing many pistillate flowers simultaneously. Therefore, in this study, fruit growth patterns were compared among pollinated and non-pollinated fruits, in relation to endogenous hormones, cell division, cell enlargement as well as sugar metabolizing enzyme activity in gynoecious, parthenocarpic cultivars.

1. Effects of pollination and growing season on fruit abortion in multi-pistillate and gynoecious cultivars

The effects of pollination and the growing season on the number of pistillate flowers and the number of fruits with commercial size were investigated in '028' (multi-pistillate, monoecious type) and 'NK × AN8' (mono- or di-pistillate, gynoecious, parthenocarpic type) cultivars, in summer 2004 and spring 2005. The ratio of the number of nodes with pistillate flowers (PF nodes) to that of total nodes was not different between spring and summer in either cultivar, but the number of pistillate flowers per PF nodes was lower in summer than in spring, especially in '028'. The fruit abortion rate was significantly reduced by pollination and was higher in summer than in spring in both cultivars. The number of harvested fruits in gynoecious, parthenocarpic cultivar, 'NK × AN8', was slightly lower in non-pollinated group than in pollinated group because it can bear much more pistillate flowers per plant in summer than in spring. On the other hand, the number of harvested fruits was much lower in summer in '028' because it can bear almost the same number of pistillate flowers per plant in both seasons.

2. Effect of pollination on endogenous hormones, cell division and cell enlargement during fruit development

The number of cells, cell size, mitotic index, histone H4 gene expression, and concentrations of endogenous cytokinins and auxin were compared during the development of fruitlets among pollinated and non-pollinated groups, in order to clarify whether the effect of pollination on fruit retention in a gynoecious, parthenocarpic cucumber is related to an increase in endogenous phytohormones. Fresh weight was greater in the pollinated group than in non-pollinated group at 4-12 days after anthesis (DAA) in both winter and spring, and at 2-6 DAA in summer. Mitotic index increased from anthesis to 2 DAA and then decreased gradually in the pollinated group, but immediately decreased after anthesis in the non-pollinated group. Histone H4 gene in the pericarp zone was expressed more strongly during the period from pre-anthesis to 2 DAA in the pollinated group. Concentrations of zeatin (Z), isopentenyladenine (iP), and indole-3-acetic acid (IAA) were higher in the non-pollinated group than in pollinated group at 2 and 4 DAA. They peaked at 4 DAA in both pollinated and non-pollinated groups, whereas iP and IAA showed no distinct peaks in the pollinated group. These results indicate that pollination stimulates cell division and hastens the start of cell elongation, but do not provide any evidence to support the idea that pollination activates cell division by stimulating the synthesis of cytokinins and auxin in cucumber fruits.

3. Effect of crop load on fruit growth and endogenous phytohormones

The effects of fruit load (fruits from nodes 9 to 14) and removal of these fruits at 4 or 8 DAA on the growth, cell number, cell size of fruit at node 15 as well as endogenous IAA, Z, zeatin riboside (ZR), iP and isopentenyladenosine (iPR) concentrations were investigated in a gynoecious, parthenocarpic cucumber cv. NK × AN8. Fruit fresh weight increased rapidly from anthesis to 4 DAA and reached commercial size (ca.100 g) at 6 DAA when only one fruit was allowed to grow (no fruit-load treatment), while fruits showed little growth and finally aborted when fruits at nodes 9 - 14 were retained (fruit load treatment). The fruit at node 15 restored growth and reached commercial size at 12 DAA when fruit load was removed at 4 DAA. IAA concentrations increased and reached its peak at 8 DAA, then decreased slightly at 12 DAA. Fruit removal decreased IAA concentrations. Z concentrations decreased close to zero in the fruit load treatment, but maintained rather high when fruit load was removed at 4 or 8 DAA. These results suggest that the high concentrations of IAA and low concentrations of Z cause fruit abortion in fruits which have ceased to grow for more than 10 days.

4. Sugar metabolizing enzyme activity in fruit and carbohydrate translocation

In order to clarify whether the activities of sucrose metabolizing enzymes were higher in active growing fruits than in fruits which stopped growing, acid invertase (AI), neutral invertase (NI) and sucrose synthase (SS) activities in fruits were monitored during fruit development in defoliated and non-defoliated 'NK × AN8' cucumbers. The fruits attained commercial size at 14 DAA in the control plants (non-defoliation), whereas in the defoliated plants, the fruits failed to reach commercial size. When defoliation was carried out at 0, 2 and 4 DAA, fruit growth was markedly restricted and 40, 20 and 20% of the fruits aborted, respectively. Fruit abortion did not occur in the defoliation treatments at 6 and 8 DAA and in the non-defoliation treatment. In the non-defoliated plants, AI, NI and SS activities were high at 4 DAA at which fruit cells began to elongate, while no significant differences were observed among other dates. AI activity in the defoliated plants was slightly increased at 8 DAA, i.e. 2 days after defoliation at 6 DAA. Sucrose and oligosaccharide concentrations in exudates from cut surface of fruit peduncles increased from 2 to 6 DAA in the non-defoliated plants, while sucrose increased at 8 DAA, i.e. 2 days after defoliation at 6 DAA in the defoliated ones. These results suggest that an increase in AI stimulates sucrose import into fruits around 4 DAA, resulting in the prevention of fruit abortion in parthenocarpic cucumber.

In conclusion, gynoecious, parthenocarpic type cucumbers can be cultivated in order to achieve high yield within a short period. However, pollination is necessary to reduce fruit abortion in multi-pistillate or gynoecious cultivars, especially in summer. Pollination stimulates cell division and hastens cell enlargement, but does not necessarily increase endogenous auxin and cytokinins concentrations. It is possible that high IAA and low Z concentrations cause fruit abortion in fruits which have ceased to grow for more than 10 days.

多くのキュウリは雌花と雄花が別の節に着生する雌雄同株系統であるが,品種によっては全ての節に雌花が着生する品種がある。これを雌性系統という。雌性系統の品種は雌花がほとんど着生しないので,この系統だけを栽培したのでは受粉できないが,雌性品種の多くは単為結果性が強く,受粉しなくても着果する。雌花は通常1節に1つが着生するが(単雌花性),品種によっては2またはそれ以上雌花を着生するものがある(複雌花性,多雌花性という)。わが国では一般に単雌花性の雌雄同株系統の品種を栽培し,長期間にわたって収穫を続けるので,栽培管理に多くの労力を必要とする。そこで,生産費を軽減するため,短期間に多くの果実を生産することのできる多雌花系統や雌性系統の利用が試みられている。しかし,これらの系統は落果が多いので,実際の収量は雌花数から予想されるものよりも少ない。

受粉によって内生植物ホルモン濃度が上昇し,その結果,細胞分裂が促進さて落果が抑えられると考えられている。しかし,多雌花性系統や雌性系統において受粉や着果量の多少が着果に及ぼす影響についてはよく調べられていない。そこで,本研究では,植物ホルモン,シンク強度と着果や果実成長の関連について調査した。

受粉と栽培季節が多雌花性系統と雌性系統キュウリの落果に及ぼす影響

受粉と栽培季節が雌花数と収穫果実数に及ぼす影響について多雌花性系統と雌性系統を用いて春と夏に調査した。雌花着生節の割合はどちらの系統も季節間で差は認められなかった。雌花節当たりの雌花数は春よりも夏の方が少なく,特に多雌花性系統でその差が顕著であった。雌性系統の未受粉区では,夏に着果率が低下したが,1雌花節当たりの雌花数が増加したため,収穫果実数は受粉区に比べてやや低下する程度であった。多雌花性系統の未受粉区では,受粉区に比べ,収穫果実数は夏に大きく減少した。

受粉処理が発育中の果実の内生ホルモン,細胞分裂,細胞伸長に及ぼす影響

受粉が細胞数,細胞の大きさ,分裂頻度,ヒストンH4遺伝子の発現,サイトカイニンとオーキシン濃度に及ぼす影響を雌性系統キュウリで調査した。細胞分裂頻度は,受粉区では開花から2日目まで上昇し,その後低下したが,未受粉区では開花後低下を続けた。開花後2-4日目のゼアチン(Z),イソペンテニルアデニン(iP),インドール酢酸(IAA)濃度は未受粉区の方が受粉区よりも高かった。受粉区,未受粉区とも,Z濃度は開花後4日目にピークを示したが,iPとIAA濃度は受粉区ではピークを示さなかった。これらの結果から,受粉は細胞分裂を促進し,細胞肥大の開始を早めるが,受粉によってサイトカイニンやオーキシン濃度は上昇しないことが明らかになった。

着果負担が果実成長と内生植物ホルモンに及ぼす影響

雌性系統キュウリを用いて,9-14節の果実の着果,摘果による着果負担の解除が15節の果実の成長,細胞数,細胞の大きさ,サイトカイニンとオーキシン濃度に及ぼす影響を調査した。摘果区の果実重は開花後4日目まで急増し,6日目には100gに達した。摘果をしなかった区では果実はほとんど成長せず,落果した。開花4日目に摘果した区では12日目に果実重は100gになった。摘果区のIAA濃度は8日目にピークとなり,その後低下した。摘果しなかった区ではIAA濃度は12日目にも高く維持され,Z濃度はほぼ0となった。一方,4,8日目に摘果した区のZ濃度は,摘果区に比べ,高く維持された。これらの結果から,10日以上成長を停止した果実の落果は高IAA,低Z濃度が原因と考えられた。

糖代謝酵素活性と炭水化物の転流

スクロース代謝関連酵素活性と果実成長との関連を明らかにするため,摘葉が果実の酸性(AI)並びに中性インベルターゼ(NI),スクロースシンターゼ(SS)活性に及ぼす影響を調査した。

対照区の果実は開花後14日目に100gに発達した。0,2,4日目に摘葉した区では40,20,20%の果実が落果したが,6,8日目に摘葉した区では落果は認められなかった。対照区のAI,NI,SS活性は細胞が伸長を開始した開花後4日目にピークを示したが,その他の日には差がなかった。摘葉区ではAI濃度は8日目にわずかに上昇した。これらの結果は,対照区では開花後4日目のAI活性の上昇がスクロースの流入を促し,その結果,落果が抑制されることを示唆している。

以上要するに,本研究では(1)雌性系統キュウリを利用することによって,短期間に多収をはかることが可能であるが,その場合,夏には受粉が必要なこと,(2)受粉は細胞分裂と伸長を促進して落果を抑制するが,サイトカイニンやオーキシン生成を促進しないことを明らかにしたもので,学術上,応用上寄与するところが多い。よって,審査委員一同は,本論文が博士(農学)を授与されるに相応しいと認めた。