Grape Berry Translocation Of Mineral Nutrients

The grape berry is supplied through the berry stem or pedicel by a vascular system composed of xylem and phloem elements. The xylem is responsible for transporting water, minerals, growth regulators, and nutrients from the root system to the vine. The berry is also supplied by the phloem, which is the vas-

culature involved in photosynthate (sucrose) transport from the leaves to other plant parts. It has reduced function with regard to berry nutrient transport early in berry development, but becomes the primary route of entry after veraison.

The translocation of solutes in the grape berry is not well understood (Ol-lat and Gaudillere 1996). Mineral nutrients are transported by the xylem and phloem vascular systems. Potassium, magnesium and sodium are phloem-mobile, whereas calcium is phloem-immobile in the vine, although xylem and phloem transport of elements is largely dependent on the type of plant (Welch 1986, Welch and Rengel 1999).

Nutrients that have low phloem mobility may also be transported by the xylem. Based on the hypothesis that K+ is transported by xylem and phloem (Mengel 1976) and that Ca++ is transported only by the xylem (Hanger 1979), changes in the K/Ca ratio in berries have been used as an indicator of changes in the relative berry K+ influx via xylem and phloem (Hrazdina et al. 1984, Ollat and Gaudillere, 1996, Rogiers et al. 2000, 2001).

Studies carried out on the water and mineral supply to the grape berry showed that before veraison, the fruit is supplied by the peripheral and central vascular system extending from the pedicel (Lang and Thorpe 1989, Greenspan et al. 1994, 1996). Several studies have shown that from veraison onwards there is an interruption in the function of the berry xylem (Findlay et al. 1987, Düring et al., 1987, Creasy et al. 1993, Rogiers et al. 2001) and therefore during ripening the fruit is mainly supplied via phloem transport (Ollat et al. 2002). The vascular system and the parameters involved in its transporting of mineral elements is therefore an interesting indicator in conductance studies pre- and post-veraison (Rogiers et al. 2006a, b). This raises the question regarding the influence of water supply on plant mineral supply. Root absorption depends on soil water levels, soil type, the availability of mineral elements in the soil, and the operation of the root system. Moreover, once a mineral element is absorbed, it is distributed according to the plant's needs (sink-source relationships). Several studies have reported a relationship between cultural practices, i.e. tilling and planting of cover-crops, and grape composition (Schaller 1999, Klein et al. 2000, Mpelasoka et al. 2003). On the other hand, there are few reports on the effects of water supply on fruit mineral supply (Dundon and Smart 1984, Esteban et al. 1999).

This study was carried out under controlled conditions in terms of total leaf area, primary shoot length and grape load (1 bunch per primary shoot). It was demonstrated that vine water status is more important compared to the ratio of leaf area/fruit regarding the accumulation of mineral nutrients into berry. The question of availability of these cations in the soil and the relationship between their uptake by the roots and distribution in the different plant organs (sink-source competition) warrants further studies. At any case, these results now need to confirm a continuing accumulation of Ca++ in whole berry during ripening under favourable water conditions. Such steady accumulation by berry suggests a continuous influx of Ca+ from the parent grapevine into the enlarging berry, suggesting that partial functioning of the xylem post-véraison is possible. In both, the irrigated and non-irrigated vines, the quantity of Ca++ in the skin increased during ripening. This could suggest Ca++ transport from other compartments especially from the flesh to the skin.

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