Sergio Tonetto de Freitas and Elizabeth J. MitchamDepartment of Plant Sciences, University of California, Davis, CA 95616 USAEmail: stfreitas@ucdavis.edu

Abscisic acid increases fruit Ca++ uptake and controls blossom-end rot in tomato

Introduction

Blossom-end rot (BER) is a brown discoloration at the blossom end region of tomato fruit believed to be the result of plasma membrane breakdown in response to low levels of Ca++ in the tissue. Treating tomato plants with ABA can potentially increase fruit Ca++ uptake by decreasing leaf transpiration and favoring the xylemic movement of water and Ca++ into the fruit. Alternatively, ABA treatment may increase the number of functional xylem vessels that reach the blossom end region of the fruit, facilitating the movement of Ca++ into that region. Both cases will result in higher fruit Ca++ content and reduce the probability of BER incidence. ABA treated plants could also have higher water content, which may favor cell expansion and fruit growth, resulting in higher yield and plant water use efficiency.

Materials and Methods

The tomato cultivar Ace 55 (Vf) was grown in a greenhouse environment and manually pollinated at full bloom. One day after pollination (DAP), plants were weekly sprayed with water (control), or 500 µg L-1 of ABA. There were four single plant replications per treatment. Before initiating the treatments, the plants were irrigated every day with a fertilizing solution containing all essential nutrients. On the day of manual pollination, 20 g of slow release fertilizer without Ca++ was added to each plant and irrigation was only with deionized water thereafter. Plants and fruit were analyzed three days after each treatment was applied. Tomato fruit were evaluated for blossom-end rot incidence, Ca++ concentration, number of functional xylem vessels, expression of genes involved in Ca++ movement into storage organelles, and fruit yield. Tomato plants were evaluated for stem water potential, and stomatal conductance.

General hypothesis

Our hypothesis is that tomato plants treated with ABA have higher Ca++ uptake into the fruit, which controls BER and increases fruit size and yield.

Conclusions

These results indicate that ABA treatment is a potential tool for tomato production, increasing fruit Ca++ uptake and controlling BER development, as well as improving plant water use efficiency.

Acknowledgements

* CAPES, Brazil, Fulbright Organization, and Valent BioSciences.

Figure 1. Blossom end rot incidence per plant (A). Calcium concentration in the blossom end tissue of tomato fruit 45 DAP (B) ± Standard Deviation.

Figure 2. Stomatal conductance (A) and tomato fruit with xylem vessels stained with 1% Safranin-O 12 DAP (B) ± Standard Deviation.

Results and Discussion

The ABA treatment increased fruit Ca++ uptake and controlled 100% BER development in tomato fruit (Figure 1A and 1B). The results obtained point to possible mechanisms by which ABA treatment increases Ca++ accumulation in the blossom end tissue of tomato fruit. One mechanism could be related to the reduction in leaf transpiration (Figure 2A), which maintained higher stem water potential (data not shown) and possibly xylemic water and Ca++ movement into the fruit. Another mechanism could be the higher abundance of functional xylem vessels that reached the blossom end tissue of the tomato fruit (Figure 2B), allowing more Ca++ to be translocated to this region. The data also suggests that ABA triggers a very specific mechanism for fruit Ca++ uptake, by increasing Ca++ movement into storage organelles of fruit tissue (Figure 3A) which may maintain a Ca++ gradient in the apoplast, favoring Ca++ movement into the blossom end region of the fruit. The higher Ca++ accumulation in the blossom end tissue contributed to the lower membrane leakage observed (Figure 3B), maintaining proper membrane function and control of BER.

A B

≈ 31.0 mm ≈ 31.0 mm

Water ABA B

Figure 3. Expression of putative vacuolar Ca++/H+ transporters (A) and membrane leakage of tomato fruit tissue (B) ± Standard Deviation.

A

A B