Tomato (Solanum lycopersicum L.) is a highly nutritious vegetable and remains in high demand round the year throughout the world. In Japan, tomatoes are consumed in fresh as well as processed forms, but demand for high quality fresh produce is very high. Like other fleshy fruits water is the major component of tomato, while the solid portion is chiefly composed of sugar, organic acids, amino acids, vitamins and minerals. Consumers prefer fruit with high contents of sugars, organic acids, and amino acids owing to good taste and health-benefits concerns. For processing, fruits with low water content are preferable regarding processing efficiency and cost. Solid contents of fruit, therefore, not only define its nutritional value but indicate consumer preference and processing efficiency as well.

Solid content of fruit may be manipulated through modifying growing conditions, changing leaf to fruit ratios, or introgression of desirable traits from the wild relatives into the cultivated species. On tomato plants, except for the first truss, each fruit truss is preceded by usually 3 leaves which supply assimilates for the fruit growth on that truss. Fresh fruit growers usually keep 4-5 fruits on a single truss, but, to improve fruit solid content and quality, source-sink balance must be properly maintained. Plants assimilate carbohydrate in leaves, and then translocate them as sucrose in the phloem vessels to sink organs along with water. Resultantly, any changes in sucrose concentration of phloem sap may affect the final solid content of fruits. Therefore, this relationship between phloem sucrose concentration and fruit solid content is of high importance.

Soil salinity and water deficit are serious problems in many tomato growing countries. Exposure of plants to saline- water-deficit-stress has long been shown to increase fruit solid contents. However, problem of saline-alkali soil or bicarbonate-rich irrigation water is more widespread than salinity, especially in Asia, Pacific and Australia. These soils and waters have high pH along with high EC. The exclusive influence of high EC on fruit solid contents have been studied extensively, but the combined effects of high pH and EC on fruit solid contents have never been studied before. If saline-alkali stress also improves solid accumulation in tomato fruit, whether soil pH besides EC also play a role in this improvement will be very interesting to know.

In this study, we firstly focused on the relationship between sucrose concentration of the phloem sap and fruit final solid contents under different leaf / fruit ratios. Secondly, we attempted to understand the impact of saline-alkali stress on the solid accumulation in fruits and the possible role of soil pH in the response.

1.Relationship between fruit solid contents and the sucrose concentration of the phloem sap at different source-sink ratios in tomato

Regarding the previous observations that more than 90% of the water is translocated via the phloem into a fruit in tomato, soluble solid content of fruits may be strongly influenced by the sucrose concentration of the phloem. In this experiment, we evaluated the relationship of fruit soluble solid contents and phloem sap sucrose concentration in tomato at various source-sink ratios (0.2, 0.4, 0.6, 0.75, 1, 2 and 3 leaf to fruit ratios [LFR). Fruit fresh weight was 40 g at the lowest LFR and increased to 80 g at LFR of 1 (80 g), but no obvious change was observed at higher LFR. Fruit dry weight increased linearly (r=0.930) from 3.5 to 7 g when LFR was increased from 0.2 to 1, indicating source dependent regulation of dry matter accumulation. However, it did not change when the LFR was beyond 1, indicating that dry matter accumulation was not affected by the source at high LFR.

By contrast, contents of fruit dry matter, total soluble sugars, and organic acids increased linearly within the whole range of LFR from 0.2 to 3 (r= 0.972, 0.890, and 0.943, respectively). Using these plants, phloem sap was collected from the cut end of the pedicle in EDTA solution. The sucrose concentration of the collected phloem sap showed a positive correlation to fruit dry matter contents (r=0.930). Contents of fruit total sugars also correlated positively with phloem sucrose concentration (r=0.900). These data suggested that, while dry matter accumulation per fruit is not affected by the source above LFR of 1, the content of soluble sugars and dry matter on a fresh weight basis is determined source dependently via the sucrose concentration of the phloem sap.

2.Impact of saline-alkaline stress on the accumulation of solids in tomato fruit

Growing of tomato plants in saline conditions is often reported with high solid content in fruit, probably due to high EC in the rooting medium. Since saline-alkaline soil, unlike saline soil, also has high pH besides high electric conductivity (EC), whether it can influence fruit solid contents has not been investigated so far. Therefore, this experiment was performed to investigate the role of saline-alkaline stress (0-120mM) in solid accumulation in tomato fruits. The pH and EC of soil leachate indicated that addition of 90 and 120mM sodium bicarbonate (NaHCO3) to plants increased pH to above 8 two-week after the initiation of alkaline-salt application, while the pH of the control remaind at 7. Similarly, EC of soil leachate was increased to 5 and 6 dS・m-1 at 90 and 120mM respectively, but, in control (0mM) plants, EC gradually decreased from 4 to 1.5 dS・m-1.

Fruit size and fresh weight was similar in all treatments within the range of 0-90mM, but only 120mM treatment decreased fruit weight. Likewise, salt application had no effect on fruit dry weight but its dry matter content was increased significantly from 6.8% (0mM treatment) to 8.5% (90 and120mM treatment). Total soluble sugar (TSS) content increased in 90mM treatment (3%) in comparison to the control (2%), but starch content remained unchanged. The increase in TSS in salt treatments was due to significant accumulation of hexose as well as sucrose in the ripe fruits. In addition to carbohydrates, saline-alkaline stress influenced organic acids accumulation as well. Citric acid, being the major acid, was significantly higher (21-32mg g-1 FW) at NaHCO3 concentrations higher than 30mM (18mg g-1 FW). These results show that saline-alkaline stress (0-90mM) can increase solid contents of fruits without reducing fruit weight, as is observed in saline stress. These results suggest that the high dry matter and sugar contents in saline-alkali treatments is because of metabolic alteration in fruit rather than condensation effects.

3.Comparative study on the accumulation of solid content in tomato fruit under saline-alkaline and saline stresses during different growing seasons

Increasing number of evidence support the hypothesis that high rhizospheric EC enhances solid contents of tomato fruits. The results in previous experiment showed that saline-alkaline stress, retaining high pH in addition to high EC in the medium, can also improve solid contents in tomato. To elucidate the role of soil pH in influencing solid content of fruit, we compared the effects of saline-alkaline (NaHCO3), saline (NaCl) and mix-salt (NaHCO3+NaCl) conditions in different growing seasons.

Results indicated that saline-alkaline treatment increased mean soil-leachate pH (pH 8), while, in saline treatment, pH was similar to the control (pH 6). On the contrary, mean soil-leachate EC was 2-time lower (5-6 dS・m-1) in saline-alkaline treatment as compared with saline treatment (11-13 dS・m-1), while EC was 3 dS・m-1 in the control. Fruit fresh weight and dry weight decreased in all salt treatments in similar fashion. Fruit dry matter content increased significantly from 7 to 14% and TSS from 3 to 5% in saline-alkaline and other stress treatments. Accumulation of sucrose, on fresh as well as dry weight basis, was increased in all stress treatments. Organic acids accumulated in stressed fruit higher than the control but their content was higher, particularly in saline-alkaline treatment. All the stress treatments increased citric acid content but the maximum content (8mg g-1 FW) was recorded in saline-alkaline treatment in comparison to control plants (3mg g-1 FW). The contents of fruit dry weight, TSS, and organic acids in all stress treatments were higher in winter crop than summer crop.

These results show that in spite of lower EC in saline-alkaline treatment, soluble sugar and organic acids contents were equal to that of high EC treatments, in both growing seasons. This data, therefore, suggest that, in saline-alkaline conditions, soil pH in association with soil EC also have a role in influencing solid content in tomato.

In conclusion, this study suggests that enhanced contents of fruit dry matter and soluble solids at high leaf to fruit ratio in tomato are attributable to increased sucrose concentration of the phloem sap. In addition, saline-alkali stress conditions can improve soluble solid content in tomato fruits either by decreasing fruit water content or by metabolic alteration.

トマトは世界的に最も需要が高い果菜類であり,生食用,加工用いずれにおいても固形分含量の高い品種が求められる.果実の固形分含量は,遺伝的に,あるいは栽培環境を最適化することによって高めることができる.栽培環境として,ソース・シンク比やストレス環境の影響が強いことが知られている.また,世界的には塩類土壌とならびアルカリ性塩類土壌は,広範囲に分布し,作物生産に影響を与えていると推測されるが,アルカリ性土壌の特徴である高pHと高ECが果実の生長や品質に与える影響について,評価した報告は見あたらない.この研究の目的は,ソース・シンク比やアルカリ性塩類土壌が果実の固形分含量に与える影響を評価すること,およびその生理学的な機構について調べることである.

1. 異なるソース・シンク比における果実の固形分含量と師部液糖濃度との関係

植物は炭水化物を葉で同化し,師部を通ってスクロースとして果実へ輸送する.スクロースの輸送は水の輸送を伴い,トマトでは果実へ流入する水の90%が師部経由で流入すると考えられている.したがって,果実が蓄積する水と糖の比率は,師部液の糖濃度をそのまま反映すると予想され,師部液の糖濃度の変化は果実の固形分含量に影響すると考えられる.この実験では,ソース・シンク比の変化にともなう果実固形分含量の変化を調べるとともに,その調節における師部液糖濃度の役割を明らかにしようとした.

トマト'ハウス桃太郎'を温室で栽培し,第2果房とその直下の葉からなるソース・シンクユニットについて,葉果比を0.2,0.4,0.6,0.75,1, 2,3となるよう,摘葉あるいは摘果した.果実重は,葉果比0.2から1の範囲では,葉果比が高まるについて増加したが,それ以上葉果比を高めても果実重は増加しなかった.これに対して,果実乾物率,糖濃度,有機酸濃度は,葉果比0.2から3の全範囲において直線的に増加した.師部液を採取して糖濃度を測定したところ,師部液糖濃度も同様に増加し,師部液糖濃度とこれらの含量との間には正の相関がみられた.以上のように,葉果比1以上に高めても果実重は高まらず,これまでの報告と一致した.しかし,固形分含量は葉果比をさらに高めることによって上昇し,その上昇には師部液糖濃度の上昇が関係していることが示された.

2. アルカリ性塩類ストレスが固形分含量に与える影響

トマト'ハウス桃太郎'をポットでストレスを与えない栽培条件で栽培し,開花後から週2回ずつ0-120mMのNaHCO3養液を十分量与えた.ポット下部から流出した溶液のpHは120mM区では8以上,対照区では約6であった.ECは120mM区では,6 dS・m-1,対照区でははじめ4 dS・m-1であったが、しだいに1.5 dS・m-1まで低下した.

果実重は処理間に差は認められなかったが,果実乾物率は対照区で6.8%であったのに対して90-120mM区では8.5%にまで上昇した.全糖濃度も,対照区の2%から90mM区で3%にまで上昇した.主要な有機酸であったクエン酸の濃度にも処理による上昇がみられた.

つぎに,NaCl処理とNaHCO3とを組み合わせた比較を行った.すなわち,90mM NaCl,60mM NaCl + 30mM NaHCO3, 30mM NaCl + 60mM NaHCO3, 90mM NaHCO3の4区で比較を行った.NaHCO3の濃度が高まるほどポット流出液のpHは上昇した.それに対して,ECは,NaCl濃度が高まるほど上昇した.果実重はすべての処理区において対照区より有意に低下したが,処理間に差は認められなかった.また,乾物率,糖濃度は処理によって上昇したが,処理間には差は認められなかった.有機酸濃度も各処理によって上昇したが,NaHCO3処理のときに特に濃度の上昇が大きかった.

本実験でみられたECの範囲では,ECの上昇に伴って果実固形分含量が上昇することがトマトで報告されている.NaHCO3処理では, NaCl処理区と比べてECがかなり低かったにもかかわらず,NaCl処理と同程度の果実重抑制および固形分含量上昇の効果が見られた.このことは,高pHによってこのような処理効果が現れた可能性があった.

以上のように,本研究は、トマトにおける糖濃度の調節において師部液糖濃度が重要な役割を担っていること,および,アルカリ性塩類処理によって果実の糖濃度が上昇することを初めて示した.本研究で得られた知見は,トマトの品質向上のための生理学的な基礎データとして有益であり,応用研究として,および基礎研究として意義のあるものとして認められる.以上により,審査委員一同は,本論文が博士(農学)の学位論文として,価値あるものとして認めた.