Photoassimilate Production and Distribution in Cherry J.A. Flore Department of Horticulture, Michigan State University, East Lansing, MI 48824 Desmond R. Layne Department of Horticulture, Clemson University, Clemson, SC 29634 The two commercially important fruiting cherries in North America of this type for cherry, except for the cultivar ‘Montmorency’ (Kappes, are Prunus cerasus L. (sour) and P. avium L. (sweet). Carbon partition- 1985). ing between individual organs, reproductive and vegetative, is depen- When the literature is compared for maximum rates of carboxyla- dent upon photoassimilate supply from photosynthesis and storage tion and electron transport for 109 different species, sour cherry had carbohydrate, as well as the ability of the translocation system to carboxylation and electron transport rates higher than those reported deliver these resources to sinks (Layne and Flore, 1995). Sink strength for apple, citrus, or grape (Wullschleger, 1993). Sour cherry had of individual organs varies with time of year, and age of the plant, as values of 21.5 Vc max for carboxylation and 78 Jmax for electron the sink demands and seasonal development pattern of the plant transport, while woody plants averaged 44 Vc max and 97 Jmax, changes with time. Herein we will review assimilate supply and respectively. storage, seasonal partitioning patterns, and the impact that biotic and In Prunus, DeJong (1983) found that A, mesophyll conductance abiotic stress has on supply, partitioning, and control. (gm), and leaf conductance (gl) to water vapor all were related linearly to leaf nitrogen content. Leaf intercellular CO2 (ci) concentrations ASSIMILATE SUPPLY decreased slightly with increasing CO2 assimilation and leaf N con- tent, indicating that CO2 assimilation was not being restricted by low µ –2 –1 Leaf. Photosynthetic (A) characteristics of sweet (17.9 mol·m ·s intercellular CO2 concentrations or stomatal conductance (gs) from µ –2 –1 CO2) and sour cherry (21.5 mol·m ·s CO2) have been described leaves with lower assimilation capacity. previously (De Jong and Doyle, 1985; Flore and Lakso, 1989; Layne Response to light. Cherry leaf A shows a typical asymptotic and Flore, 1995). These indicate that cherries rank high among Prunus. response to increasing light, similar to that reported for other C3 plants. There is little genetic variation of A for Prunus species. Flore and Sams Saturation to light occurs at 30% to 50% of full sun, light compensation (1986) reported differences between sweet and sour cherry and be- points are ≈75–80 µmol photosynthetic photon flux (PPF), with µ tween clones of sour cherry and noted that spur-type selections often reported photochemical efficiencies of 0.025-0.028 mol CO2 fixed/ had higher rates of A than the standard variety ‘Montmorency’. Small mol PPF (Layne and Flore, 1992). Whole-tree A shows a different similar differences have been reported for spur-type apples (Looney, response to increasing light. Compensation points are higher, quantum 1968). Genetic variation for respiration (R) has not been reported for efficiencies and maximum photosynthetic rates are lower, and satura- cherry. Respiration could influence the overall carbon balance of the tion does not occur under full-light conditions (Flore and Gucci, plant, especially if temperatures during the dormant period are high unpublished data). enough to increase metabolic activity. Genetic variation for R exists In general, as shade increases, leaves increase in leaf area, are and has been investigated in other species, but we know of no studies flatter, thinner, and have more chlorophyll per unit weight (Sams, 1980). Leaves preconditioned in this manner have lower light compen- sation points, similar quantum efficiencies, and lower maximum rates Received for publication 6 Oct. 1998. Accepted for publication 17 Nov. 1998. of A. Shoots are longer and thinner, and internodes are shorter, leaf The cost of publishing this paper was defrayed in part by the payment of page area and the number of lateral shoots increases, and whole-tree dry charges. Under postal regulations, this paper therefore must be hereby marked weight is decreased. The effect can be reversed by eliminating shade, advertisement solely to indicate this fact. but has the greatest effect on leaves that are in the bud stage, or are not HORTSCIENCE, VOL. 34(6), OCTOBER 1999 1015 COLLOQUIUM yet fully expanded. Levels below 20% full sunlight have a negative higher rate. Starch was the most common storage material, sucrose the impact on reproduction and cold hardiness (Flore, 1980; Flore and most predominant soluble carbohydrate during dormancy, and sorbi- Kesner, 1982; Sams, 1980). tol the most common soluble carbohydrate during spring budbreak. Temperature. The relationship between photosynthesis and tem- They speculated that if flower buds developed earlier than vegetative perature for Prunus sp. is parabolic. Cherry has a flat response between buds, as occurs in the Northwest United States, then flower develop- 17 and 30 °C (Sams and Flore, 1983). A true high temperature ment must rely upon TNC reserves until enough photosynthetic leaf threshold has not been well established for two reasons. Under field area was present to provide sufficient photosynthate. conditions, plants can adapt to higher temperatures (Caldwell et al., At budbreak, carbohydrate reserves provide the carbon needed for 1990; Lang et al., 1978), and unless vapor pressure of water is growth until the leaf area of the tree provides enough assimilation to controlled in assimilation cuvettes, vapor pressure deficits (VPD) meet sink demand. Johnson and Lakso (1986) found that spurs and between the leaf and air increase with increased temperature. Vapor shoots on apple did not begin to export carbon to the rest of the plant pressure deficits >1.5 kPa have been shown to cause stomatal closure until 15 to 25 d after budbreak, and that short shoots began earlier than and decreased photosynthesis in sour cherry (Beckman et al., 1992). long shoots. For sour cherry, Kappes (1985) found that net export from Leaf age. A increases with leaf expansion, reaches a peak just a terminal shoot did not occur until 17 d after leaf emergence when the before full development, then remains steady for two or more weeks shoot was about one-quarter (27%) of total dry-matter accumulation. before declining (Flore and Lakso, 1989; Sams and Flore, 1982). Roper and Kennedy (1986) found that chlorophyll content, gs, and SINK EFFECT quantum efficiency increased in sweet cherry as leaves expanded to a leaf plastochron index of 10, then remained constant until senescence. Sinks for photoassimilates, nutrients, water, and phytohormones Seasonal whole-tree profiles have been estimated from single-leaf or include reproductive (flowers and fruits) and vegetative (shoots or branch measurements conducted at predetermined intervals. Sams and roots) tissues. The sink effect is demonstrated in a number of ways: 1) Flore (1983) reported two very different patterns of A for ‘Mont- by either a higher net A or a delayed decline in A; 2) increased carbon morency’ on the same trees in two different years (1978, 1979) translocation to sinks; or 3) greater dry-matter production per unit leaf depending on crop load and leaf emergence rate. They concluded that area when comparing fruiting vs. nonfruiting trees. The capacity of there was a strong positive relationship between sink strength (vegeta- sink tissues to utilize photoassimilate is referred to as sink demand or tive and reproductive) that influenced leaf A, which could explain the sink strength, and it is the product of sink size and activity (Warren differences in A between the two different years. This observation Wilson, 1972). would indicate that factors other than leaf age greatly influence Fruit sink strength changes in cherry during the growing season and seasonal patterns of A. is greatest during stage III or final swell of fruit growth. Sams and Flore The initiation of gross and net export from sour cherry leaves of (1983) found no consistent effect of fruit on A of sour cherry. The sink different ages, a shoot, or fruit were investigated using carbon balance effect on A in cherry has not been consistent, nor is it demonstrable modeling by Kappes (1985) or by using radioisotopes (Kappes and under all conditions. Possible explanations for this include the require- Flore, 1989). The seventh leaf and the terminal leaf started net export ment for source-limiting conditions, the masking effect of competing at ≈17% and 51% expansion, respectively, which coincides with the sinks, or environmental conditions which favor maximum A. A source onset of gross export from the leaf. The whole shoot started exporting limitation occurs when A is limited by the capacity of the reactions that at 27% of expansion. This corresponded to 3, 4, and 17 d after leaf supply photoassimilate (Baysdorfer and Bassham, 1985). However, emergence for the seventh, terminal leaf, and whole shoot, respec- Flore (1985) found that the leaf : fruit ratio must be <2 for the sink tively. This study demonstrated that leaf age and position had a major effect to occur; Gucci et al. (1991a) noted a significant decline in A and influence on the initiation of export, but once export began it was an increase in leaf starch following harvest of sweet cherry, sour cherry indicative of greater CH2O production than the leaf could utilize. (Gucci, 1988), and plum (Gucci et al., 1991b). Roper et al. (1988) did Fruit. Fruit photosynthesis has been reported for sour cherry not observe a fruit effect of A when they compared fruiting and (Kappes, 1985; Kappes and Flore, 1986). Sour cherry exhibited a nonfruiting sweet cherry trees but leaf : fruit ratio was not reported and, positive net photosynthesis early in development, which was highly thus, it is difficult to know if the trees were source-limited. dependent upon fruit size, temperature, and light. Sour cherry fruit A Typically, the sink effect on A is not observed in the morning but contributes a significant portion of the carbon used by the fruit for rather in the afternoon.