J. AMER. SOC. HORT. SCI. 128(1):113–119. 2003. Changes in Photosynthesis and Carbohydrate Metabolism in Mature Apple Leaves in Response to Whole Plant Source–Sink Manipulation Rui Zhou1 and Bruno Quebedeaux2 Department of Natural Resource Sciences and Landscape Architecture, University of Maryland, College Park, MD 20742 ADDITIONAL INDEX WORDS. Malus sylvestris var. domestica, Sorbitol, sucrose, starch, aldose-6-phosphate reductase, sucrose- phosphate synthase, ADPGlucose pyrophosphorylase ABSTRACT. Photosynthesis and carbohydrate metabolism in apple [Malus sylvestris (L.) Mill. var. domestica (Borkh.) Mansf.] source leaves were monitored during a 7-day period after source–sink manipulations by girdling or partial defoliation treatments. In the girdling treatment, sorbitol, sucrose, glucose, and starch accumulated in leaves, and net –1 –1 photosynthetic rates (Pn) at 350 µL·L CO2 decreased during a 7-day period. Pn measured at 1000 µL·L [CO2] was also decreased but the changes were less. Stomatal conductance and intracellular CO2 concentration decreased markedly in leaves of girdled shoots. When shoots were partially defoliated, starch and glucose concentrations in remaining source leaves declined steadily during the 7-day study period. Sorbitol and sucrose concentrations decreased during the first 2 days after defoliation, then increased the following 5 days. Pn of the remaining leaves measured at ambient and elevated CO2 levels were enhanced markedly. Aldose-6-phosphate reductase activity in source leaves increased markedly from 27.5 to 39.2 µmol·h–1·g–1 fresh weight (FW) after partial defoliation but remained unchanged in leaves after girdling. Selective and maximum sucrose phosphate synthase (SPS) activities increased following partial defoliation and decreased following girdling. ADP-glucose pyrophosphorylase activity remained relatively unchanged in the partial defoliation treatments but increased markedly in the girdled-shoot leaves. These results suggested that girdling-induced photosyn- thetic inhibition is mainly due to stomatal limitation, however, the photosynthesis enhancement by partial defoliation may be due primarily to acceleration of photosynthetic capacity per se. These studies showed that the metabolism of sorbitol, sucrose and starch, three photosynthetic end products in mature apple leaves, was coordinately regulated in source leaves in response to source–sink manipulations. Photosynthetically fixed carbon in source leaves can be con- (Buchanan, 1998). Sink regulation of carbohydrate metabolism verted to starch and sucrose in most plant species. Starch is in source leaves involves changes in metabolite levels, in the synthesized and stored within chloroplasts, and in most plants, it activities of key enzymes for sucrose and starch biosynthesis, and is the primary storage form mobilized for translocation, usually in the transcription of some specific photosynthetic and carbohy- during the night. Sucrose is formed in the cytoplasm and acts as drate metabolism genes (Krapp and Stitt, 1995). One model was the transport compound for export from source leaves to sink proposed with respect to this sink regulation. Higher sink demand tissues. In some plants, sucrose may also be a storage compound. removes sucrose and favors sucrose-phosphate synthesis cata- Carbon allocation between these two pathways not only deter- lyzed by sucrose-phosphate synthase (SPS) and the hydrolysis of mines the availability of carbon for export and storage in the sucrose-phosphate catalyzed by sucrose phosphatase. The result- leaves, but also may control the rate of photosynthesis (Stitt and ing increase in phosphate release in the source cytoplasm leads to Sonnewald, 1995). Reduced sucrose synthesis in spinach leaves increased efflux of triose phosphate from the chloroplast via the was shown to decrease the rate of photosynthesis (Battistelli et al., triose-phosphate transporter. Glucose-6-phosphate, which is 1991). formed from triose phosphate, stimulates the activity of SPS, thus Photosynthesis and carbohydrate metabolism in source leaves providing sucrose to meet the increased sink demand (Dunford, respond to sink activity (Aczon-Bieto, 1983; Goldschmidt and 1998). Huber, 1992; Krapp and Stitt, 1995; Paul and Driscoll, 1997; Paul Previously, most investigations on carbon allocation and car- and Foyer, 2001; Plaut et al., 1987; Rufty and Huber, 1983). In bohydrate metabolism have been conducted on plants with su- general, when sink demand increases, photosynthesis is stimu- crose and starch as final photosynthetic end products. In the lated and more carbon is partitioned to sucrose instead of starch. Rosaceae family, including apple, peach and many other tree This process facilitates carbon translocation. On the other hand, fruits, in addition to sucrose and starch, sorbitol is a primary when photosynthate transport of source leaves is blocked, photo- photosynthetic end product. Sorbitol is also the main translocatable synthesis is inhibited, sucrose synthesis is reduced, and starch carbohydrate in these species (Bieleski, 1982; Loescher and synthesis is stimulated (Dunford, 1998). Everard, 1996). The allocation of newly fixed carbon in these The regulation of sucrose and starch metabolism has been species is influenced by the leaf developmental stage (Bieleski extensively studied during the last 2 decades (Preiss and Sivak, and Redgwell, 1985), and environmental conditions, such as 1996; Quick and Schaffer, 1996). Sucrose and starch synthesis water stress (Wang and Stutte, 1992), light intensity (Escober- are competing reactions and are highly coordinated in vivo Gutierrez and Gaudillere, 1997), photoperiod (Wang et al., 1997) and CO2 concentration (Pan et al., 1998). Sink strength may also Received for publication 28 Feb. 2002. Accepted for publication 24 Oct. 2002. regulate carbohydrate metabolism in the source leaves. Layne 1Current address: Department of Horticulture, Cornell University, Ithaca, NY and Flore (1995) reported that carbohydrate metabolism in the 14853. source leaves of sour cherry changed significantly in response to 2To whom reprint requests should be addressed; e-mail [email protected]. the source manipulation. As a strong sink organ in some develop- J. AMER. SOC. HORT. SCI. 128(1):113–119. 2003. 113 9211-Photo 113 11/25/02, 1:45 AM mental stages, fruit was also shown to alter the carbohydrate were generated as Pn–Ci curves. Three leaves per treatment were concentrations of source leaves (Nii, 1997; Wibbe and Blanke, measured for Pn–Ci curves and the typical curve was presented. 1995). However, the biochemical mechanisms are still unknown CARBOHYDRATE ANALYSIS. After the photosynthetic rates were with respect to the regulation of carbon allocation in the leaves of measured, the leaves were harvested and the midrib of each leaf these sorbitol-producing plant species. was removed. One half of the leaf was used for carbohydrate In the present study, girdling and partial defoliation treatments analysis and the other half for enzyme extraction and biochemical were applied to young potted apple (Malus sylvestris var. analyses. Leaf samples for carbohydrate analysis were weighed domestica) trees. Girdling, which blocks the photoassimilate and immediately frozen in liquid nitrogen, lyophilized and kept transport pathway, resulted in reduced sink demand for carbohy- at –80 oC until analysis. Carbohydrate analysis was conducted as drates from the source leaves. Partial defoliation treatments, with described by Wang et al. (1997). One hundred milligrams of the the removal of most source leaves and alteration of the source sink dried sample was extracted three times, each 20 min, with 2 mL ratio, can cause more carbohydrate demand upon the remaining of 80% (v/v) ethanol at 80 oC. The homogenates were centrifuged source leaves by the sink tissues and may allow more carbohy- at 15,000 gn for 10 min to give ethanol-soluble and ethanol- drate export out of the source leaves. The objective of the current insoluble fractions. The pellets were evaporated to dryness and studies was to investigate the effects of sink demand on photosyn- used for starch determination. The ethanol soluble fractions were thesis and the activities of key enzymes related to carbohydrate pooled and evaporated to dryness with a concentrator (SpeedVac; metabolism in apple leaves. Savant Instruments, Inc., Farmingdale, N.Y.), and resolublized in 1.5 mL of deionized water. The soluble fraction was then passed Materials and Methods through a Dowex 50H+ and a Dowex 1Cl– resin column. The soluble neutral fraction eluted from the two columns with dis- PLANT MATERIALS. Young apple trees (‘Gala’ on M9 rootstock) tilled water provided the soluble carbohydrate fraction. Sorbitol, were cut 10 cm above the bud union in early spring and grown sucrose, glucose and fructose were separated with a high-perfor- outside of the University of Maryland greenhouse at College mance liquid chromatograph (HPLC) (Shimadzu Corp., Colum- Park, Md., under a natural environment from April to June in 20- bia, Md.) on a carbohydrate column (HPX-87C; Bio-Rad, Rich- L pots filled with peat-based Professional Growing Medium 300- mond, Calif.) (Wang et al., 1997). Degassed, distilled, deionized S (Pro-Gro Products, Elizabeth City, N.C.). The trees were then water at 0.6 mL·min–1 at 80 oC was used as the mobile phase. A pruned to two shoots per plant with ≈30 leaves on each. The trees refractive index detector (RID-10A; Shimadzu) at 30 oC was used were watered twice each day and fertilized