EFFECTS OF ROOTSTOCKS ON YIELD AND FRUIT QUALITY OF PARENT WASHINGTON NAVEL TREES

Transcription

1 Florida Department of Agriculture and Consumer Services Bur. of Citrus Budwood Registration Annu. Rpt. Jul 1, 1997-June p Florida Department of Agriculture and Consumer Services Bur. of Citrus Budwood Registration Annu. Rpt. Jul 1, 1998-June p Florida Department of Agriculture and Consumer Services Bur. of Citrus Budwood Registration Annu. Rpt. Jul 1, 1999-June p. 14. Florida Department of Agriculture and Consumer Services Bur. of Citrus Budwood Registration Annu. Rpt. Jul 1, 2000-June p Florida Department of Agriculture and Consumer Services. 2002a. Bur. of Citrus Budwood Registration Annu. Rpt. Jul 1, 2001-June p Florida Department of Agriculture and Consumer Services. 2002b. Citrus Summar Florida Agr. Stat. Servs. p Foguet, J. L., C. A. Oste, J. T. Jubes, and S. Alvare Influencia de tres portainjertos sobre la pigmentacion antocianica de la naranja Rub Blood. Revista Industrial Y Agricola de Tucuman. 47: Harding, P. L., J. R. Winston, and D. F. Fisher Seasonal changes in Florida oranges. USDA Tech. Bul Kimball, D. A Citrus Processing-A Complete Guide. Aspen Publishers, Gaithersburg, MD. pp , Miller, E. V., J. R. Winston, and D. F. Fisher A phsiological stud of carotenoid pigments and other constituents in the juice of Florida oranges. USDA Tech. Bul. 780:1-31. SAS Institute SAS user s guide: statistics. SAS Inst., Car, N.C. Rouse, R. E Citrus fruit qualit and ield of si Valencia clones on 16 rootstocks in the Immokalee Foundation Grove. Proc. Fla. State Hort. Soc. 113: Rouse, R. E. and C. O. Youtse Juice qualit from oung trees of 6 Valencia clones on 16 rootstocks in the Immokalee Foundation Grove. Proc. Fla. State Hort. Soc. 106: Stewart, I Color as related to qualit in citrus, p In S. Nag and J. A. Attawa (eds.). Citrus nutrition and qualit. ACS Smp. Series 143. Amer. Chem. Soc., Wash., D.C. Stewart, I., G. D. Bridges, A. P. Pieringer, and T. A. Wheaton Rohde Red Valencia, an orange selection with improved juice color. Proc. Fla. State Hort. Soc. 88: Wutscher, H. K. and F. W. Bistline influences juice color of Hamlin orange. HortScience 23: Proc. Fla. State Hort. Soc. 116: EFFECTS OF ROOTSTOCKS ON YIELD AND FRUIT QUALITY OF PARENT WASHINGTON NAVEL TREES ALI AL-JALEEL Najran Horticulture Development Research Center Ministr of Agriculture, Food & Agriculture Organiation Najran, Kingdom of Saudi Arabia MONGI ZEKRI 1 Universit of Florida, IFAS Hendr Count Etension Office P.O. Bo 68 LaBelle, FL Additional inde words. citrus, fruit production, fruit sie, soluble solids Abstract. The horticultural adaptabilit and performance of Parent Washington Navel (Citrus sinensis (L.) Osbeck) orange trees were evaluated for seven ears on nine commercial rootstocks in the Najran area of Saudi Arabia. Fruit ield, fruit sie, peel thickness, percentage juice, soluble solids and acid were measured. Trees on Volkamer lemon (VL), Citrus macrophlla (CM) and rough lemon (RL) were the most productive, whereas trees on Swingle citrumelo (SC) and Cleopatra mandarin (Cleo) were the least productive. Trees on sour orange (SO), Carrio citrange (CC), Citrus Taiwanica (CT), and Amblcarpa (Amb) were intermediate in fruit production. The largest fruit were from trees on VL, CM and RL, while the smallest fruit were on trees budded on Cleo and SO. Peel thickness was the highest in fruit collected from trees on RL and the lowest in fruit collected from those on Cleo and SO. Fruit from trees on Cleo had the highest juice content while those from trees on RL had the lowest juice content. Fruit from trees on CC and SO Florida Agricultural Eperiment Station Journal Series No. N Corresponding author. accumulated the highest soluble solids and fruit from trees on CM, CT, RL, and VL accumulated the lowest soluble solids. Overall, trees on vigorous rootstocks (VL, CM, RL) performed better and were more productive and more profitable than trees on other rootstocks. Trees on SC and Cleo performed the poorest. Citrus acreage in the Kingdom of Saudi Arabia is over 15 thousand ha (37.5 thousand acres). The Najran area, located in southwest Saudi Arabia, comprises 25% of the total citrus acreage. There are over one million trees and over 1,000 citrus orchards in the Najran area. Orchard sie ranges from 1 to 50 ha (125 acres). In the Kingdom of Saudi Arabia, fruit is not sold based on soluble solids. All fruit is marketed fresh b weight and consumed locall (Zekri and Al-Jaleel, 2000). Clementine mandarin and Parent Washington Navel orange were among the most popular citrus cultivars grown in the Najran area. Fifteen ears ago, the most popular rootstocks were sour orange, Troer and Carrio. Because of increasing problems with high ph and salinit, rootstocks have become a more critical issue than in previous ears. Volkamer lemon, Macrophlla, and Cleopatra mandarin have been gaining ground and are becoming ver popular (Al-Jaleel and Zekri, 2002). s have had a substantial role in the development of the citrus industr in the world. The effect of rootstocks on citrus fruit production and fruit qualit has been intensivel studied in man citrus producing areas (Continella et al., 1988; Economides and Gregoriou, 1993; Fallahi and Rodne, 1992; Fallahi et al., 1989; Gardner and Horanic, 1961, 1966; Grisoni et al., 1989; Monteverde et al., 1988; Roose et al., 1989; Rouse and Mawell, 1979; Wheaton et al., 1991; Zekri, 1996, 1997, 1999, 2000a, b). Findings from these studies have revealed different results and inconsistent conclusions, which 270 Proc. Fla. State Hort. Soc. 116: 2003.

2 were attributed to several factors including climatic conditions and soil characteristics. Therefore, it is unwise to adopt rootstock recommendations from one part of the world to another without a thorough evaluation locall. Since environmental conditions and cultural practices are unique and var considerabl from one area to another, a long term stud was carried out to determine the horticultural adaptabilit and performance of Parent Washington Navel orange (Citrus sinensis (L.) Osbeck) trees on nine commercial rootstocks grown on a tpical soil where the Saudi Arabian citrus industr is flourishing. On-site field evaluation of rootstocks benefits local growers in selecting the most suitable rootstocks for their citrus cultivars under their specific climatic and edaphic conditions. Materials and Methods s used were sour orange (Citrus aurantium L.) (SO), Carrio citrange [(Citrus sinensis (L.) Osbeck Poncirus trifoliata (L.) Raf.] (CC), Cleopatra mandarin (Citrus reshni Hort. e Tan.) (Cleo), rough lemon (Citrus limon) (RL), Swingle citrumelo [(Citrus paradisi (L.) Poncirus trifoliata (L.) Raf.] (SC), Taiwanica (Citrus Taiwanica) (CT), Amblcarpa (Citrus limonellus var. Amblcarpa Hassk) (Amb), Volkamer lemon (Citrus volkameriana Ten and Pasq.) (VL), and Macrophlla (Alemow) (Citrus macrophlla Wester) (CM). Fruit parameters measured were fruit ield, fruit sie, individual fruit weight, peel thickness, and percentage juice, soluble solids, and acid of Parent Washington Navel orange trees. The budwood source and budded trees were free of all known virus and viroids. The trees were planted in 1987 with a 6.0 m (20 ft) b 6.0 m (20 ft) spacing and a tree densit of 278 trees/ hectare (111 trees per acre). The soil teture was loam sand (85% sand, 11% silt, and 4% cla) throughout the root one having a 2.9% CaCO 3 content and a ph of 8.2. The trees were managed according to standard local commercial practices, pruned annuall, and watered as needed using a drip irrigation sstem delivering 120 (30 gal) to 180 L (45 gal)/d. The irrigation water had a ph of 8.0 and an electrical conductivit of 1.02 ds m -1. In earl November, each tree was fertilied with 1.0 kg of (N-P-K-Mg) and 25 kg (55 lbs) of composted manure. One kg (2.2 lbs) of N from urea was also applied 3 times (1/2 in Januar, 1/4 in March, and 1/4 in Ma). In the spring, foliar spras of manganese and inc were also applied. The soil was kept free of weeds using post-emergence herbicides. Pest populations were managed using a local pest management program. Fruit ield of each tree was taken at harvest. Fruit samples from each eperimental plot were collected for fruit qualit measurements and evaluations. Individual fruit weight, fruit diameter, peel thickness, juice weight, total soluble solids or Bri, and titratable acid concentrations were determined in the laborator using standard procedures. Juice was etracted from the fruit samples, weighed, and tested for Bri and acid. The Bri content (mostl soluble sugars) was determined using a hdrometer that measured the specific gravit, which was converted to degrees Bri. The percentage acid was determined b titration using sodium hdroide and a phenolphthalein indicator. The eperiment was a complete randomied block design and consisted of nine treatments (rootstocks) with four replications of 4-tree plots. Statistical analsis was conducted using analsis of variance and means separation b Duncan s multiple range test at 5% level. Results and Discussion Fruit ield. Over the seven-ear production period, trees on Volkamer lemon (VL), Citrus macrophlla (CM) and rough lemon (RL) were the most productive. Trees on Swingle citrumelo (SC) and Cleopatra mandarin (Cleo) were the least productive. Trees on sour orange (SO), Carrio citrange (CC), Citrus Taiwanica (CT), and Amblcarpa (Amb) were intermediate in fruit production (Table 1). The poor crop for trees on Cleo was partl attributed to Phtophthora infestation, which also reduced growth and tree sie. Trees on Cleo grew slowl and fruited poorl during the first few ears. Trees on SC had dieback, were relativel small, and consistentl produced fewer fruit. Similar results were obtained with Olinda Valencia trees grown in the same area (Al-Jaleel and Zekri, 2002). This was also consistent with Gardner and Horanic (1961) who concluded that scions on Cleo were not precocious. Similar results of ield problems for trees on Cleo have been found from man citrus areas in the world. Cleo is considered a la rootstock because trees grafted to it fruit relativel poorl until the are 10 to 15 ears of age (Castle et al., 1993). The ield results of this stud were consistent with results of several studies conducted in different citrus growing regions where cumulative ields were higher on trees budded on VL and RL than on those budded on SC and Cleo (Castle et al., 1988; Fallahi et al., 1989; Monteverde et al., 1988; Wutscher and Shull, 1973; Zekri, 2000b). However in other studies, no significant differences in cumulative ields were found Table 1. Fruit ield (kg per tree) of Parent Washington Navel trees on nine rootstocks Average bc bc cd ab d c bc a a Mean of twent-eight measurements (four replications b seven ears). Superscripts indicate mean separation within the last row (among rootstocks) b Duncan s multiple range test, 0.05 level. Proc. Fla. State Hort. Soc. 116:

3 among rootstocks including Cleo, SO, CC, VL, RL, CT, and CM (Fallahi et al., 1991; Hearn, 1989). Trees on SC produced the most fruit ield (Rouse and Mawell, 1979; Wheaton et al., 1991; Wutscher et al., 1975; Wutscher and Shull, 1976a, 1976b). All these results indicated the inconsistenc in ield differences as affected b rootstocks, which could be attributed to differences in scion cultivars, tree age, climatic conditions, and soil characteristics. Fruit sie. The largest fruit were found from trees on VL, CM and RL, while the smallest fruit were found on trees budded on Cleo and SO (Table 2). Similar results were found b several other workers where fruit were smaller or lighter from trees on SO and Cleo and larger or heavier from trees on VL, RL, and CM (Al-Jaleel and Zekri, 2002; Continella et al., 1988; Economides and Gregoriou, 1993; Monteverde et al., 1988; Rouse and Mawell, 1979; Wutscher and Shull, 1976a; Zekri, 2000b). On the other hand, no significant differences were found in Valencia orange fruit sie and weight among trees growing on RL, Cleo, SO, and CT (Wutscher and Shull, 1973) and Fairchild fruit weight was higher from trees on CT than fruit from trees on RL, CC, and CM (Fallahi and Rodne, 1992). Furthermore, Ambersweet orange fruit from trees on Cleo were found larger and heavier than those from trees on SC, which could be attributed to the low number of fruit per tree on Cleo (Zekri, 1996). In general, fruit sie is correlated with fruit number per tree. The fewer the fruit on the tree, the larger and heavier are the fruit. Moreover, in a particular ear beside fruit load, the ultimate sie a citrus fruit achieves is the result of man comple factors including nutrition and irrigation programs, rainfall distribution, pruning, and the rootstock/scion combination. Large fruit sie is most often preferred in the fresh fruit market and brings higher prices earl in the season. Peel thickness. Another determinant of citrus fresh qualit is peel thickness, firmness or teture. Both etremes in peel thickness are not desirable. Fruit with thick peel are usuall low in juice, while those with thin peel are prone to splitting and are sensitive to postharvest problems that can occur during shipping and storage. Peel thickness was also affected b the rootstock. Peel thickness was the highest in fruit collected from trees on RL and the lowest in fruit collected from those on Cleo and SO (Table 3). Differences in fruit peel thickness as affected b rootstocks were also reported in some previous studies. Peel thickness was the highest in fruit collected from Olinda Valencia trees on CT and CM and the lowest in fruit collected from those on Cleo, SC, and Amb (Al-Jaleel and Zekri, 2002). Peel thickness of Orlando tangelo was higher for trees on RL (Fallahi et al., 1991) and that of grapefruit was higher on CT (Fallahi et al., 1989) as compared with CC. Fruit rind thickness was found to be the highest for Marsh grapefruit trees on CT and Amb and the lowest for trees on SC, CC, and Estes RL (Economides and Gregoriou, 1993). Trees on SO and VL produced fruit with the thickest rind (Monteverde et al., 1988; Wutscher and Bistline, 1988). However, for Marrs oranges, rind thickness was the highest from trees on SC (Wutscher and Shull, 1976b) and grapefruit rind thickness was higher for fruit from trees on Cleo than CT and RL (Wutscher et al., 1975). From all those studies, there is no consistent trend that the more vigorous rootstocks promoted thicker peel. Furthermore, not all rootstock studies showed differences in peel or rind thickness among rootstocks. Wutscher and Shull (1976a) did not find a significant difference in rind thickness of Orlando tangelo fruit from trees grown on all the four rootstocks studied, SC, SO, Cleo, and CT. Juice content. Overall, b being less than 45%, juice content was relativel low. The juicier the fruit, the better is its acceptance not onl for the juice market but also as a fresh fruit. Like other fruit qualit variables, juice content was affected b the rootstock and varied through the ears. Fruit from trees on Cleo had the highest juice content while those from trees on RL had the lowest juice content (Table 4). Similar results were reported from some earlier studies. Fruit from trees on SC had the highest juice content while those from trees on SO, RL, and CT had the lowest juice content (Al-Jaleel and Zekri, 2002). Fruit from trees on SC and Cleo had the best juice percentage compared with trees on VL, CC, and SO (Monteverde et al., 1988). Fruit from trees on CT had the lowest percent juice compared with RL, VL, CM, and CC (Fallahi et al., 1989). Fruit from trees on CC and RL contained more juice than those from trees on CM (Fallahi et al., 1991). However, juice content of Marrs orange was the highest for trees on SO and the lowest for trees on CT (Wutscher and Shull, 1976b). In general, the larger the fruit and the thicker the peel or rind, the lower is the juice content. Juice content was the highest for SO and the lowest for Estes RL, VL, Amb and Cleo (Economides and Gregoriou, 1993), and Hamlin fruit from trees on RL and VL had the lowest percentage juice (Wutscher and Bistline, 1988). Not all rootstock studies demonstrated that rootstocks had an influence on juice content. No significant difference in juice content of Orlando tangelo, Comune Clementine, Fairchild mandarin and grapefruit was found from trees grown on all the studied rootstocks (Continella et al., 1988; Fallahi and Rodne, 1992; Wutscher et al., 1975; Wutscher and Shull, 1976a). Table 2. Fruit diameter (cm) of Parent Washington Navel trees on nine rootstocks Average 7.73 d 7.83 cd 7.67 d 8.01 abc 7.86 bcd 7.88 bcd 7.82 cd 8.09 a 8.05 ab Mean of twent-four measurements (four replications b si ears). Superscripts indicate mean separation within the last row (among rootstocks) b Duncan s multiple range test, 0.05 level. 272 Proc. Fla. State Hort. Soc. 116: 2003.

4 Table 3. Peel thickness (mm) of fruit from Parent Washington Navel trees on nine rootstocks Average 5.77 b 5.92 ab 5.71 b 6.28 a 6.19 ab 5.97 ab 5.81 ab 6.08 ab 6.05 ab Mean of twent-four measurements (four replications b si ears). Superscripts indicate mean separation within the last row (among rootstocks) b Duncan s multiple range test, 0.05 level. Soluble solids. The flavor and palatabilit of citrus fruits is a function of relative levels of soluble solids, acids, and presence or absence of various aromatic or bitter juice constituents. Although fruit qualit standards, which determine minimum levels of acceptabilit, have not been established in Saudi Arabia, soluble solids concentration in the juice has not been completel ignored as an important parameter for fresh fruit. s were found to affect soluble solids concentration in fruit juice. Soluble solids concentration in fruit from trees on CC and SO was the highest while it was the lowest for fruit from those on CM, CT, RL, and VL (Table 5). Similar results were obtained with Olinda Valencia trees grown in the same area (Al-Jaleel and Zekri, 2002). Other workers also found similar results. Total soluble solids were the lowest for RL and the highest for Cleo and SO (Hearn, 1989; Wutscher et al., 1975). Total soluble solids were among the highest from fruit on SO (Monteverde et al., 1988; Wutscher and Shull, 1973; Wutscher and Shull, 1976b). Fruit from trees on SO, CC and/or Cleo had the highest soluble solids concentration, while those on VL and RL had the lowest soluble solids concentration (Castle et al., 1988; Continella et al., 1988; Fallahi et al., 1989; Fallahi and Rodne, 1992; Wutscher and Bistline, 1988). Total soluble solids were among the highest for fruit from trees on SC and the lowest for fruit from trees on RL, VL and Milam (Economides and Gregoriou, 1993; Zekri, 2000b). The results on soluble solids of all these studies are consistent showing poorer internal fruit qualit for trees grown on relativel vigorous rootstocks such as RL and VL compared with trees grown on less vigorous rootstocks such SC and SO. Acid content. Total acidit of citrus juices is an important factor in overall juice qualit and in determining time of harvest in several citrus producing countries. In this stud, acid content in the juice was not affected b the rootstocks (Table 6). However, in other studies, acid content in the juice differed among rootstocks. Acid content in the juice of fruit from Olinda Valencia trees on SC and Amb was higher than that from trees on SO, Cleo, RL, CT, and VL (Al-Jaleel and Zekri, 2002). The lowest total acids in the fruit juice were from trees on CT and the highest were from trees on SC (Wutscher and Shull, 1976a, 1976b). Acid content was the highest for trees grown on SO and the lowest for trees on VL and RL (Continella, 1988). Total acid was among the highest in the juice from trees on CC, SC and Cleo and the lowest from trees on RL and VL (Fallahi et al., 1989, 1991; Fallahi and Rodne, 1992; Wutscher and Bistline, 1988; Wutscher et al., 1975). Although internal fruit qualit including acid content can be affected b the scion cultivar, tree age and other factors, the results on acid content from most of these studies are consistent showing relativel lower acid content for trees grown on lemon rootstocks. Conclusions s can affect the success and profitabilit of virtuall an commercial citrus culture. use is considered essential in citriculture because of its influence on how and where citrus can be grown successfull, and its influence on scion fruit qualit and quantit. In this stud, it was quite obvious that rootstocks had a significant effect on fruit ield Table 4. Juice content (% b wt) of fruit from Parent Washington Navel trees on nine rootstocks Average ab ab a b ab ab ab ab ab Mean of twent-four measurements (four replications b si ears). Superscripts indicate mean separation within the last row (among rootstocks) b Duncan s multiple range test, 0.05 level. Proc. Fla. State Hort. Soc. 116:

5 Table 5. Total soluble solids concentration (%) of juice from Parent Washington Navel trees on nine rootstocks Average a a b c b c b c c Mean of twent-four measurements (four replications b si ears). Superscripts indicate mean separation within the last row (among rootstocks) b Duncan s multiple range test, 0.05 level. Table 6. Acid content (%) of juice from Parent Washington Navel trees on nine rootstocks Average 0.79 a 0.79 a 0.75 a 0.76 a 0.80 a 0.77 a 0.79 a 0.80 a 0.79 a Mean of twent-four measurements (four replications b si ears). Superscripts indicate mean separation within the last row (among rootstocks) b Duncan s multiple range test, 0.05 level. and qualit. Failure to assess accuratel the impact of climate, soils, and rootstocks on economic profitabilit of citrus can be a major reason for crop losses or reduced income because of reduced ield and qualit potential. Trees on CM, VL, and RL rootstocks were more vigorous, precocious and more productive than those on the other rootstocks. Cleo and SC rootstocks are not recommended for the Najran area of Saudi Arabia because of Cleo s high susceptibilit to Phtophthora particularl in poorl drained situations and because of SC poor adaptabilit to high ph soil, calcareous soils, and/or relativel saline environment. Based on this stud, CM, VL, and RL are good choices as rootstocks for Parent Washington Navel orange in the Najran area of Saudi Arabia. Literature Cited Al-Jaleel, A. and M. Zekri Yield and fruit qualit of Olinda Valencia trees grown on nine rootstocks in Saudi Arabia. Proc. Fla. State Hort. Soc. 115: Castle, W. S., D. P. H. Tucker, A. H. Kredorn, and C. O. Youtse s for Florida citrus. Fla. Coop. Et. Serv. Bull. SP 42. Castle, W. S., H. K. Wutscher, C. O. Youtse, and R. R. Pelosi Citrumelos as rootstocks for Florida citrus. Proc. Fla. State Hort. Soc. 101: Continella, G., C. Germana, G. La Rosa, and E. Tribulato Performance and phsiological parameters of Comune Clementine influenced b four rootstocks, p In Proc. Sith Inter. Citrus Congr. Economides, C. V. and C. Gregoriou Growth, ield, and fruit qualit of nucellar frost Marsh grapefruit on fifteen rootstocks in Cprus. J. Amer. Soc. Hort. Sci. 118: Fallahi, E., Z. Mousavi, and D. R. Rodne Performance of Orlando trees on ten rootstocks in Ariona. J. Amer. Soc. Hort. Sci. 116:2-5. Fallahi, E. and D. R. Rodne Tree sie, ield, fruit qualit, and leaf mineral nutrient concentration of Fairchild mandarin on si rootstocks. J. Amer. Soc. Hort. Sci. 117: Fallahi, E., J. W. Moon, Jr., and D. R. Rodne Yield and qualit of Redblush grapefruit on twelve rootstocks. J. Amer. Soc. Hort. Sci. 114: Gardner, F. E. and G. E. Horanic A comparative evaluation of rootstocks for Valencia and Parson Brown oranges on Lakeland fine sand. Proc. Fla. State Hort. Soc. 74: Gardner, F. E. and G. E. Horanic Growth, ield, and fruit qualit of Marsh grapefruit on various rootstocks on the Florida East Coast A preliminar report. Proc. Fla. State Hort. Soc. 79: Grisoni, M., P. Cabeu, and B. Aubert Resultats de doue annees d un essai de comportement de cinq porte-greffe en association avec quatre cultivars d agrumes a l ile de la Reunion. Fruits 44: Hearn, C. J Yield and fruit qualit of Ambersweet orange hbrid on different rootstocks. Proc. Fla. State Hort. Soc. 102: Monteverde, E. E., F. J. Rees, G. Laborem, and J. R. Rui Citrus rootstocks in Veneuela: Behavior of Valencia orange on ten rootstocks, p In Proc. Sith Inter. Citrus Congr. Roose, M. L., D. A. Cole, D. Atkin, and R. S. Kupper Yield and tree sie of four citrus cultivars on 21 rootstocks in California. J. Amer. Soc. Hort. Sci. 114: Rouse, R. E. and N. P. Mawell Performance of mature nucellar Redblush grapefruit on 22 rootstocks in Teas. J. Amer. Soc. Hort. Sci. 104: Wheaton, T. A., W. S. Castle, J. D. Whitne, and D. P. H. Tucker Performance of citrus scion cultivars and rootstocks in a high-densit planting. HortScience 26: Wutscher, H. K. and F. W. Bistline Performance of Hamlin orange on 30 citrus rootstocks in southern Florida. J. Amer. Soc. Hort. Sci. 113: Wutscher, H. K., N. P. Mawell, and A. V. Shull Performance of nucellar grapefruit, Citrus paradisi Macf., on 13 rootstocks in south Teas. J. Amer. Soc. Hort. Sci. 100: Wutscher, H. K. and A. V. Shull The performance of Valencia orange trees on 16 rootstocks in south Teas. Proc. Trop. Reg. Amer. Soc. Hort. Sci. 17: Wutscher, H. K. and A. V. Shull. 1976a. Performance of Orlando tangelo on 16 rootstocks. J. Amer. Soc. Hort. Sci. 101: Wutscher, H. K. and A. V. Shull. 1976b. Performance of Marrs earl orange on eleven rootstocks in south Teas. J. Amer. Soc. Hort. Sci. 101: Proc. Fla. State Hort. Soc. 116: 2003.

6 Zekri, M Leaf mineral concentration, growth, ield, fruit qualit, and economics of Ambersweet orange trees on two rootstocks. Proc. Fla. State Hort. Soc. 109: Zekri, M Performance of Ambersweet, a new citrus hbrid cultivar, on two rootstocks in Florida. Fruits 52: Zekri, M Performance of Valencia orange trees on four rootstocks in a high-densit planting. Proc. Interamerican Soc. for Tropical Hort. 43: Zekri, M. 2000a. Citrus rootstocks affect scion nutrition, fruit qualit, growth, ield and economical return. Fruits 55: Zekri, M. 2000b. Evaluation of orange trees budded on several rootstocks and planted at high densit on flatwoods soil. Proc. Fla. State Hort. Soc. 113: Zekri, M. and A. Al-Jaleel The citrus industr in Saudi Arabia. The Citrus Industr Mag. 81: Proc. Fla. State Hort. Soc. 116: LATE-SUMMER TOPPING INCREASES FRUIT SIZE IN INDIAN RIVER MURCOTT WITH LITTLE REDUCTION IN YIELD ED STOVER 1 AND SCOTT CILIENTO Universit of Florida, IFAS Indian River Research and Education Center 2199 S. Rock Road Ft. Pierce, FL TRAVIS MURPHY River Countr Citrus, Inc W. Midwa Road Ft. Pierce, FL Additional inde words. Citrus reticulata, tangor, thinning, water relations Abstract. Fruit sie is a significant factor in the value of most citrus varieties grown for sale as fresh fruit. Production of fruit too small to market is most common in heavil over-cropped trees, a condition often observed in Murcott (Citrus reticulata C. sinensis) and other tangerine-tpes. Greatest improvements in fruit sie result from cropload reduction relativel earl in fruit development, but usuall result in decreased ield, with increased fruit sie not full compensating for decreased numbers of fruit. In 2001, we conducted a trial on increasing fruit sie of Murcott with ecessive fruit set. A randomied complete block factorial trial was established in which topping 0.5 m from a 4.6 m tall tree and application of 28 kg ha -1 of KNO 3 were compared to non-treated controls. All fruit were harvested and sied from each of the treatment trees. KNO 3 treatments did not significantl affect an parameter measured. Topping decreased fruit per tree b 15% but increased mean fruit weight b 13%, resulting in no significant reduction in total ield. Cartons of fruit in the count sies were increased from 0.83 to 1.56 carton/tree but cartons count sies were not significantl increased. Peak production was at the 176 sie class for all treatments. Since topping was conducted four months after bloom and et virtuall no ield reduction was apparent, reduced competition between fruit for photosnthates does not appear to full eplain the fruit sie increase. It is proposed that reduced water stress, from reducing leaf area and associated transpiration, ma be the principal mechanism for increasing fruit sie in this eperiment. This research was supported b the Florida Agricultural Eperiment Station, and approved for publication as Journal Series No. N Corresponding author. Ecessive cropping is a frequent occurrence in mandarins and mandarin-like hbrids, resulting in man fruit that are too small to market, alternate bearing, and sometimes substantial tree damage from branch breakage. Although hand thinning is known to avoid these problems and increase fruit sie, labor costs in Florida make this practice prohibitivel epensive. Plant growth regulator (PGR) treatments have been widel studied to reduce cropload through increasing phsiological drop in heavil cropped trees (Stover et al., 2002a). In Florida, PGRs have received little use for this purpose, even though naphthaleneacetic acid (NAA) has long been registered for use on tangerines, tangors, tangelos, and oranges. Instead, Florida citrus growers have relied on hedging, topping, and skirting to decrease cropload on blocks with ecessive fruit load. When problems with small fruit sie become apparent in the summer, there has been some success increasing fruit sie through use of foliar potassium spras (Boman, 1997). This paper reports results of a trial in which a Murcott block with ecessive fruit set was subjected to topping and KNO 3 applications in an effort to increase fruit sie. Materials and Methods Trees used in this stud. A mature grove of Murcott on Cleopatra mandarin (C. reticulata) rootstock, located west of Ft. Pierce, Fla., was used in this stud. Trees were planted at 4.6 m b 7.6 m (286 trees per ha) in double row beds on Oldsmar sand (hperthermic Alfic Arenic Haplaquods). Trees were microsprinkler irrigated and received routine care for commercial production of Murcott. The eperiment was conducted using two topping treatments (topped or not) and two KNO 3 rates (0 or 28 kg ha -1 ) as a factorial randomied complete block design. As a result, half rows were the eperimental units and there were four eperimental units per treatment combination. Buffer rows were used to prevent KNO 3 overspra. Data were collected on three trees in each eperimental unit, with a tpical tree selected in the middle and toward each end of each half row. Tree selection for average sie and cropload was made prior to topping to reduce potential bias. Topping treatments. An average of 0.5 m per tree was removed from trees that averaged 4.6 m in height. Flat topping was conducted on 8 Aug using a commercial topper with entire rows topped. Proc. Fla. State Hort. Soc. 116: