The ABA-sprayed plants had an average fruit phloem sap uptake of 0.778ml fruit–1 d–1 and an average phloem sap solute concentration of 301mg ml–1 . Therefore, the solute accumulation per day was 1.04×144.3=150.07mg fruit–1 d–1 in non-ABA-sprayed plants and 0.778×301=234.18mg fruit–1 d–1 in ABA-sprayed plants. Therefore, the results also showed that ABA-sprayed plants had higher phloem solute accumulation per fruit than non-ABA-sprayed plants from 15 to 30 DAP. According to the present data and other studies, ABA could also be acting at the whole-plant level as a signal triggering carbohydrate accumulation and osmotic adjustment in sink organs . In addition, spraying peach fruit with ABA has been shown to increase the activity of sorbitol oxidase, a predominant enzyme in the metabolism of the translocated sugar alcohol sorbitol, which was followed by an enhanced sugar accumulation in the fruit . The higher phloem sap solute concentration in ABA sprayed plants can decrease fruit apoplastic solute potential, which is then equilibrated by a parallel decline in fruit total water potential . Under these conditions, plastic growing bag higher fruit solute accumulation can increase the water potential gradient between the fruit and stem, favouring fruit xylem sap uptake .
Accordingly, the present results show that whole-plant ABA spray treatment not only increased phloem solute accumulation per fruit, but also decreased leaf transpiration, maintaining a higher stem water potential and higher total fruit water uptake, compared with non-ABA sprayed plants.Following irrigation in the morning, tomato plants treated with ABA had a smaller increase in xylem sap flow rate intothe leaves compared with non-treated plants, presumably due to suppression of stomatal opening . In all treatments, the increase in xylem sap flow after the time of irrigation in the morning probably reflected the combined effects of leaf rehydration as well as increasing light intensity stimulating stomatal opening, and increasing VPD from increased air temperatures and decreased relative humidity in the greenhouse environment. The reduction in leaf xylem sap flow after 15:30h to 16:30h was presumably a result of the reverse changes in the environmental conditions observe dearly in the day after irrigation. These results and other studies also show a direct relationship between high leaf transpiration and higher leaf Ca2+ uptake , suggesting that leaf Ca2+ accumulation is dependent only on leaf xylem sap uptake triggered by leaf transpiration rates. In addition, the data also show that Ca2+ concentration in the leaf xylem sap extracted by pressurizing the leaves or by inducing leaf guttation were similar, suggesting that there is no significant Ca2+ contamination of leaf xylem sap when the leaves are cut and pressurized in the pressure chamber for xylem sap extraction.
In that case, ABA treatment may have also reduced the hydraulic resistance within the fruit, favouring xylemic water movement in the fruit towards the blossom-end tissue, provided a hydrostatic gradient responsible for xylem sap flow was present in the fruit . Since Ca2+ is believed to be mobile in the plant exclusively through the xylem vessels , the observed increase in xylem sap flow towards the fruit in the pedicle, and a reduced hydraulic resistance within the fruit, may explain the observed higher fruit Ca2+ accumulation in ABA-treated plants. Neither effect was observed in ABA-dipped fruit, suggesting that changes in Ca2+ partitioning in the plant are responsive only to whole plant ABA treatment. The pattern of fruit xylem sap uptake followed an increase after irrigation in all treatments until 15:30h to 16:30h, decreasing thereafter at both 15 and 30 DAP. Similar to the leaves, this pattern could be explained by the combined effects of an increase in plant water content right after irrigation and an increase in VPD early during the day that increased the evaporative demand due to increasing air temperatures and decreasing relative humidity durig the day time. Late in the day, decreasing SWP due to plant water loss and decrease in the VPD and consequently the evaporative demand due to decreasing air temperatures and increasing relative humidity could limit xylem sap flow into the fruit, resulting in the observed decrease in fruit xylem sap uptake. At night, the VPD was low but not zero, and continued plant water loss under these conditions may have been associated with the observed reverse flow of fruit xylem sap at 15 DAP for non-ABA-treated plants. Although a reverse xylem sap flow was observed later in the irrigation cycle, the fruit growth rate was always positive, indicating that phloem sap uptake maintained the positive growth rates even under reverse xylem sap flow conditions. The reverse flow of fruit xylem sap was not observed at 30 DAP, possibly because of higher fruit solute content compared with 15 DAP. The higher solute content decreased fruit water potential , which possibly increased the strength of the fruit as a sink for xylemic sap uptake under limited xylem conductivity conditions. The present results showed that fruit xylem sap uptake decreased from 15 to 30 DAP in tomato.
Previous studies have shown that phloem may represent 76–83% and xylem may represent 17–24% of fruit peduncle water uptake at early stages of growth and development . Consistent with the data presented, other studies have also shown that at later stages of growth and development, the xylem contribution to fruit water uptake decreases due to loss of xylem functionality and/or reduction in the hydrostatic gradient responsible for xylemic sap uptake and movement in the fruit . However, other studies have shown that xylem transport into trusses of tomato fruit cultivar Gourmet remained functional throughout the first 8 weeks of growth. In addition, these studies showed that ~75% of water net influx into the fruit occurred through the external xylem and ~25% via the perimedullary region, which contains both phloem and xylem . Differences in the phloem/xylem ratio of fruit sap uptake presented in the literature could be attributed to different genotypes and/or the growing conditions of each study. In future studies, direct measurements of phloem sap uptake into the fruit using nuclear magnetic resonance should be carried out for the same tomato cultivar and growing conditions as used in the present study to compare precisely the methods and the results obtained .Although no statistically significant changes in Ca2+ concentrations in stem xylem sap were observed among the treatments, spraying plants with ABA increased the Ca2+ concentration in the xylem sap moving into the fruit. The movement of Ca2+ in the xylem vessels depends on adsorption and desorption of Ca2+ from active exchange sites within the cell walls . In that case, fruit of ABA-sprayed plants possibly had exchange sites within the xylem cell walls that were more saturated with Ca2+, wholesale grow bags maintaining higher levels of soluble Ca2+ in the xylem sap stream. In addition, evidence suggests that special nutrient transport systems exist at the interface between living cells and xylem vessels . The higher Ca2+ concentration observed in the xylem sap of the peduncle of fruit from ABA-sprayed plants could be the result of the higher flow rate of xylem sap into the fruit leading to a higher saturation of Ca2+ binding sites in the xylem vessels and cell uptake requirements that reduced Ca2+ binding to active exchange sites in the cell walls as well as the Ca2+ uptake into living cells at the interface with the xylem vessels.Spraying tomato plants with ABA increased the Ca2+ concentration and Ca2+ accumulation in the pericarp tissue at the fruit peduncle end by increasing fruit xylem sap uptake, decreasing fruit phloem sap uptake, increasing Ca2+ concentration in the xylem sap moving into the fruit, and possibly by increasing phloem Ca2+ transport into the fruit. The results show that ABA spray treatment increased fruit xylem sap uptake4.72-fold, fruit xylem sap Ca2+ concentration 1.28-fold, and fruit growth 1.41-fold, compared with water spray treatment, respectively. These results suggest that the increase in fruit xylem sap uptake was the most important effect of ABA spray treatment leading to the observed higher fruit Ca2+ accumulation from 15 to 30 DAP. The Ca2+ accumulation in fruit tissue estimated by multiplying the xylem sap Ca2+ concentration in the fruit peduncle by its respective flow rate into the fruit from 15 to 30 DAP was ~84% of the Ca2+ accumulation quantified by the difference in total fruit Ca2+ content observed at 30 DAP minus the total fruit Ca2+ content observed at 15 DAP.
Considering that fruit water uptake is via the xylem and phloem, the results suggest that the phloem may have also contributed to fruit Ca2+ uptake under the experimental conditions described in this study. The results also show a greater difference between the quantified and estimated Ca2+ accumulation in the fruit of ABA-sprayed plants than in the fruit of other treatments , suggesting that spraying plants with ABA also enhanced fruit phloem Ca2+ uptake. Considering that spraying tomato plants with ABA decreased fruit phloem sap uptake, it is possible that this treatment increased Ca2+ concentration in the phloem sap to increase fruit Ca2+ uptake to compensate for the reduction of phloem sap uptake. These results agree with previous studies suggesting that phloem can also have an important contribution to fruit Ca2+ uptake depending on the phloem sap Ca2+ concentration and phloem sap flow rate into the fruit . In the present study, it was assumed that fruit transpiration rates were similar among all treatments. Future studies related to the effect of ABA on xylem and phloem fruit water uptake should include direct measurements of fruit transpiration rates. In the xylem vessels, after reaching the peduncle end of the fruit, Ca2+ can be taken up by the cells, bind to active exchange sites within the cell walls, or remain soluble in the xylem vessels to be translocated towards the blossom-end tissues of the fruit . Accordingly, the present results show that higher xylem sap and tissue Ca2+ content at the fruit peduncle end resulted in higher fruit Ca2+ translocation to and Ca2+ accumulation in the blossom-end tissues in response to whole-plant ABA treatment. Dipping the fruit in ABA did not affect xylem sap or tissue Ca2+ content at the fruit peduncle end, but resulted in higher Ca2+ accumulation and higher Ca2+ in the apoplast in the blossom-end tissue at 15 DAP, suggesting that ABA also triggered a fruit-specific mechanism that favoured Ca2+ translocation from the peduncle end towards the blossom-end region of the fruit. This latter effect was not observed at 30 DAP. According to the present data, spraying the whole plant with ABA or dipping the fruit in ABA maintained a higher number of functional xylem vessels that reduced the resistance to xylemic water and Ca2+ movement into the blossom-end tissue, which could help to explain the observed higher Ca2+ content in the distal end of the fruit. In ABA-dipped fruit, the increase in Ca2+ concentration in the blossom-end tissue was only observed at 15 DAP, possibly due to the reduction in any ABA effect on maintaining a higher number of functional xylem vessels at late stages of fruit growth and development. It is possible that ABA could also increase the number of functional xylem vessels connecting the fruit to the plant, which should be determined in future studies. In addition, higher cuticular wax content in epidermal cells at 30 DAP compared with fruit at 15 DAP could limit fruit ABA uptake during the later dip treatments.At the whole-plant level, ABA treatment triggered stomatal closure, decreasing xylemic water and Ca2+ flow to the leaves, which maintained higher stem water potential. Under such conditions, whole-plant ABA treatment favoured xylemic water and Ca2+ movement into the rapidly expanding fruit, resulting in higher Ca2+ content reaching the fruit peduncle end. However, the data suggest that xylem sap uptake could not fully explain fruit Ca2+ accumulation due to the difference between the observed total fruit Ca2+ accumulation and the estimated fruit Ca2+ accumulation based on the Ca2+ concentration in the xylem sap and xylem sap flow rate into the fruit. These results suggest that phloem could have acted as a source of Ca2+ to the fruit under the experimental conditions described in this study. More detailed studies should include direct measurements of fruit transpiration rates to better characterize the role of phloem in fruit Ca2+ uptake. In addition, a better understanding of phloem contributions to fruit Ca2+ uptake can be accomplished by developing efficient methods to extract and quantify Ca2+ in the phloem sap moving into the fruit.