Sink–source Transition in Peach Leaves during Shoot Development

Net photosynthesis, dark respiration, chlorophyll and carbohydrate content, and leaf and shoot growth of deciduous peach [Prunus persica (L.) Batsch] saplings, grown in greenhouse conditions, were measured to assess changes in carbon balance during leaf development. The 6th, 12th, and 16th leaf node were measured from the first flush at the base through expansion to maturity (the first node being the oldest). Shoot and leaves expanded following a sigmoid pattern in all nodes. The shape of the logistic curve did not vary between the 6th and the 16th leaf node, while the 12th leaf node showed a steeper response, suggesting that the latter reached 50% expansion relatively earlier. Photosynthesis varied with leaf development as young leaves had low CO2 assimilation rates that were reflected in their chlorophyll concentration. Net daily CO2 assimilation was negative in young expanding leaves. The sink–source transition, defined to be the time when the increase in daily carbohydrate exchange rate exceeded the daily increase in leaf carbohydrate content, occurred before full leaf expansion. The transition from import to export was attained 11–12 days after budbreak (corresponding to 41% to 45% of full leaf expansion) for the 6th leaf, about 7–9 days after (38% to 52% of full expansion) for the 12th leaf and after 9–10 days (32% to 38% of full expansion) for the 16th leaf. Below 30% to 50% of full expansion leaves might not respond to assimilate requirements from sinks, being sinks themselves. Received for publication 14 Mar. 2005. Accepted for publication 16 May 2005. Corresponding author: phone +39-050-883070; fax +39-050-883495; e-mail [email protected] Leaves are the main source of CO2 assimilation in plants. Photosynthetic rates are generally low in young expanding leaves and increase up to or slightly after full leaf area expansion (Turgeon and Webb, 1975). As carbohydrates accumulate, import stops and the export of soluble sugars begins. In early phases of leaf growth, a leaf is a net carbon importer (sink), its growth depending mainly on a supply of imported substances. Afterwards a leaf becomes a net carbon exporter (source), its growth depending primarily on its photosynthates. This sink stage of leaf development may take several days or weeks. Leaves of dicotyledonous plants change from sink to source when their area reaches 30% to 60% of the final area (Turgeon, 1989). The position and distribution of leaves along the shoot influences sink strength. The direction of translocation of assimilates varies with the developmental stage of single leaves. Fully developed apical leaves export carbohydrates toward younger leaves, basal leaves send carbohydrates to the shoot and roots, intermediate leaves in both directions (Dickson and Isebrand, 1991). Because large amounts of carbohydrates are used in respiration by sinks, absolute growth rate or rate of accumulation of dry matter does not accurately measure the capacity of a sink to receive assimilates. A more appropriate index of actual sink strength is the sum of net carbon gain minus respiratory carbon loss. During early stages of leaf growth, syntheses of chlorophyll, proteins and structural compounds are high, resulting in high catabolic rates to support the energy need. As the photosynthetic system matures, the requirement for respiratory energy rapidly decreases (Kozlowski, 1992). In temperate fruit trees new leaves expand during flowering and compete for resources with developing fruits, although eventually they are net contributors to the carbon economy of the tree (Kriedemann, 1968). In spring season, deciduous species make new leaves at a low cost of carbon per unit leaf area, but at a high cost in terms of carbohydrate reserves (Dickson, 1989). Kennedy and Johnson (1981) studied gas exchange in expanding leaves of apples (Malus ×domestica Borkh.) and found that net CO2 assimilation increased from 5 to 25 μmol·m–2·s–1 as the leaves expanded from 5 to 70% of final size. This increase in net CO2 assimilation was accompanied by increases in stomatal conductance. Escobar-Gutiérrez and Gaudillére (1996), comparing source and sink leaves, observed that the synthesis of soluble carbohydrates, in peach [Prunus persica (L.) Batsch.] was confined to source leaves, while the utilization of carbohydrates was limited to sink leaves. Sink–source relationships and the regulation of carbon allocation determine crop yield in fruit trees. The dynamic of the leaf development in source (assimilate availability) and sink (ability to utilize assimilates) limitations to crop yield in peach needs to be clarified in carbon balance models. Such models can incorporate and integrate growth functions, developmental patterns and environmental conditions, following the concept that plants grow as collections of semiautonomous but interacting organs (DeJong, 1999; Grossman and DeJong, 1994). There is a strong interdependency among leaf life-span, net assimilation, leaf nitrogen concentration, leaf mass to area ratio, and habitat resource availability that may result from the neces-