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RESEARCH PRODUCT

Partitioning of Sorbitol and Sucrose Catabolism within Peach Fruit

subject

description

The peach (Prunus persica (L.) Batsch (Peach Group)) fruit is a sink organ comprised of different types of tissue, which undergoes three distinct developmental stages during the growth season. The objective of this study was to characterize the activity and partitioning of sorbitol and sucrose catabolism within 'Encore' peach fruit to determine whether the two forms of translocated carbon play different roles in the various fruit tissues and/or stages of development. Sorbitol catabolic activity was defined as the sum of NAD-dependent sorbitol dehydrogenase (SDH) and sorbitol oxidase (SOX) activities, whereas sucrose catabolic activity was defined as the sum of sucrose synthase (SS), soluble acid invertase (AI), and neutral invertase (NI) activities. Partitioning of sorbitol and sucrose catabolism in each tissue was calculated as percentage of total sorbitol or sucrose catabolic activity in the entire fruit. At cell division, sorbitol catabolic activit y was similar in the endocarp and mesocarp, but lower in the seed. However, sorbitol catabolism was mostly partitioned into the mesocarp, due to its large size compared to that of other tissues. SDH was more active in the mesocarp, while SOX was more active in the endocarp. Sucrose catabolism was most active and partitioned mainly into the endocarp. At endocarp hardening, both sorbitol and sucrose catabolic activities were highest in the seed, but despite this, sucrose catabolism was partitioned mostly in the mesocarp. At cell expansion, sorbitol and sucrose catabolic activities were still higher in the seed only when expressed on a weight basis and similar in mesocarp and seed when expressed on a protein basis. Both sorbitol and sucrose catabolism were partitioned mostly into the mesocarp. Sorbitol and sucrose contents were generally higher in the tissues that exhibited lower catabolic activities. All carbohydrates were always partitioned mostly into the mesocarp. Our results show that, at the cell division and endocarp hardening stages, sorbitol and sucrose catabolism are partitioned differently in the fruit and that SDH activity may play an important role in mesocarp cell division and final fruit size determination. The peach (Prunus persica (L.) Batsch (Peach Group)) and other stone fruit are characterized by three stages of growth: an early period of rapid cell division, an intermediate stage of relatively slow growth, and a final swell due to cell expansion. In mid- and late-maturing cultivars, this growth pattern is repre- sented by a double-sigmoid curve (Westwood, 1993), although early-maturing cultivars do not show an intermediate or endocarp hardening stage. Already in the cell division stage, the fruit tissues start differentiating into the mesocarp (flesh), endocarp (pit), and seed. In this first stage, the mesocarp and endocarp are very active and account for nearly all the fruit growth. During the second stage, the mesocarp undergoes a relatively quiescent period and the endocarp lignifies. The seed, on the other hand, undergoes an active stage of growth due to compositional changes and accumulation of storage compounds. The final period of fruit growth is characterized by cell expansion and carbohydrate accumulation in the mesocarp, and maturation in the seed. Thus, the fruit sink is comprised of separate parts, which act themselves as sinks, experiencing changes in activity and demand for photo- synthate during the season. Sorbitol and sucrose are both synthesized in mature leaves of peach trees and are present at ratios of about 4:1, respectively (Escobar-Gutierrez and Gaudillere, 1994). This ratio decreases to 2:1 in the translocating tissues (leaf petiole, stem bark, and fruit peduncle) and down to 1:4 and 1:50 late in the season in the fruit mesocarp and seed, respectively (R. Lo Bianco, unpublished data). In the fruit mesocarp, sucrose accumulates more than sorbitol late in the season (Lo Bianco et al., 1999a), suggesting