Winter Drought Stress Can Delay Flowering and Avoid Immature Fruit Loss during Late-season Mechanical Harvesting of ‘Valencia’ Oranges

We determined if winter drought stress could delay flowering and fruit development of immature ‘Valencia’ sweet oranges to avoid young fruit loss during late-season mechanical harvesting. Beginning in December over three consecutive seasons (2007–2009), Tyvek water-resistive barrier material was used as a rain shield groundcover under 13to 15-year-old trees. There were three treatments: 1) drought = no irrigation and covered soil; 2) rain only = no irrigation, no cover; and 3) normal irrigation with rain and no cover. Covers were removed in February or March and normal irrigation and fertilization were resumed. The drought stress did not affect fruit yield, size, percentage juice, or juice quality of the current crop harvested in May and June relative to continuously irrigated trees. Drought stress delayed flowering by 2 to 4 weeks so that the immature fruit for next season’s crop were smaller than on continuously irrigated trees during June but fruit growth caught up by September. During mechanical harvesting, previously drought-stressed trees lost fewer young fruit than continuously irrigated trees. Thus, winter drought stress effectively delayed flowering and avoided young fruit loss during late-season mechanical harvesting without negative impacts on yield or fruit quality of ‘Valencia’ orange trees. Successful mechanical harvesting of perennial fruit crops requires efficient, economical harvesting systems that do not shorten a tree’s productive life or diminish fruit quality relative to hand harvesting (Roka et al., 2000). Although most of the world’s citrus is harvested manually, adoption of mechanical harvesting using trunk shakers or canopy shakers and catch frames is expected to increase in the next few years as a consequence of its higher efficiency and lower costs than conventional hand harvesting (Brown, 2005), especially in the large citrus plantations of processed fruit in Florida and Brazil (Roka, 2004). No negative physiological, growth, or yield responses of mechanically harvested trees have been reported in early or midseason orange cultivars (Li and Syvertsen, 2005). In addition, mechanical harvesting during peak bloom (approximately March) in late-season ‘Valencia’ orange trees does not remove any more flowers than manual harvesting so mechanical harvesting has little effect on subsequent fruit set (Li et al., 2005). In lateseason cultivars like ‘Valencia’, which have immature green fruit for next year’s crop and mature harvestable fruit on the tree at the same time, no yield losses have been reported if ‘Valencia’ trees are mechanically harvested until immature fruit reach 2.5 cm in diameter (Li and Syvertsen, 2004). However, demand from the citrus industry has increased pressure to extend the harvest season past June when next year’s crop becomes large enough to be susceptible to mechanical harvesting. Several studies (Hedden et al., 1984; Roka et al., 2005; Whitney et al., 1975) have reported yield losses in the next year after trees had been mechanically harvested late in the season. Techniques for improving lateseason harvesting so that little or no impact on the next year’s yield occurs are needed. Citrus flowering in Florida can occur after a low temperature (less than 20 C) or drought induction period (Davenport, 1990; Valiente and Albrigo, 2004) during the winter rest period when vegetative growth is minimum (Moss, 1969; Reuther et al., 1973). In warm tropical citrus, flowering follows rain or irrigation after a dry period (Cassin et al., 1969) and summer drought stress also can regulate flowering in subtropical Mediterranean-like climates (Barbera et al., 1985). Florida has a subtropical humid climate with 70% of the average annual precipitation (1100 to 1300 mm) falling between June and September resulting in relatively dry winter seasons (Obreza and Pitts, 2002). Flowering occurs in spring when soil moisture is adequate, but the start of differentiation and budbreak begins as early as late December or the first week of January (Albrigo, 1997). Thus, drought stress could delay flowering in citrus. We hypothesized that if the Florida ‘Valencia’ bloom period could be delayed by a few weeks using winter drought stress without negative effects on the quality of the current season’s crop, the fruitlets from delayed flowering would be too small to be affected by mechanical harvesting late in the current harvest season and thus safely extend the mechanical harvesting period. This would require a winter drought of 3 months duration to perhaps delay flowering 3 to 4 weeks. To be successful, mature fruit yield and juice quality would have to be maintained and subsequent fruit growth of the delayed crop would have to catch up to that of well-irrigated trees so as not to impact the yield or fruit quality of next year’s crop. Because ‘Valencia’ trees commonly display alternate bearing cycles of high and low annual yields, investigations of drought treatment effects on the timing of flowering and yield require 3 consecutive years to conclusively separate treatment effects from year– to-year variations. Materials and Methods Tree growth conditions. The study was conducted at the University of Florida/IFAS, Citrus Research and Education Center, Lake Alfred, FL (long. 28.09 N, lat. 81.73 W; elevation 51 m). A well-managed grove with uniform, 13-year-old ‘Valencia’ sweet orange [Citrus sinensis (L.) Osb.] trees budded on ‘Swingle’ citrumelo [Citrus paradisi Macfad. · Poncirus trifoliata (L.) Raf.] was used. The soil was well-drained Candler fine sand with less than 1% organic matter (Li et al., 2006). The tree rows were northto southoriented and spaced 7 m apart. Trees were planted in pairs such that spacing between adjacent trees was 2 m with 5 m between pairs of trees in the row. Beginning in Dec. 2006, three irrigation treatments were arranged in a randomized block design with three replicate blocks of 10 trees each (five sets of two trees). Thus, the experimental design was three irrigation treatments · three blocks with 30 replicate trees in each treatment. Irrigation treatments were 1) drought stress, no irrigation with covered soil to shed rainfall; 2) rain only; and 3) well-irrigated controls, normal rainfall plus supplemental microsprinkler irrigation (rain + irrigation). Supplemental microsprinkler irrigation was applied during winter as needed up to two times per week for up to 1.5 to 4 h duration as recommended (Morgan et al., 2006) to maintain trees in a well-irrigated condition. For the rain-excluding soil covers, continuous sheets of Tyvek home wrap (Dupont , Wilmington, DE) were used to cover the entire soil surface under the canopies. The treatments were maintained from 4 Dec. 2006 to 15 Mar. 2007 (100 d) when the covers were removed and previously droughted trees were well irrigated. Received for publication 14 Oct. 2009. Accepted for publication 22 Dec. 2009. This work was supported by UF/IFAS. To whom reprint requests should be addressed; e-mail [email protected]. HORTSCIENCE VOL. 45(2) FEBRUARY 2010 271 To determine any possible treatment carryover effects into 2007–2008 from the 2006–2007 winter treatments, both previous control (rain + irrigation) and drought treatments were split in two in Dec. 2007 with half of the trees becoming well watered without soil cover (control) and the other half covered beneath canopies and subjected to drought stress (drought). Treatments were maintained from 13 Dec. 2007 to 21 Mar. 2008 (100 d) after which the Tyvek covers were removed and all trees were well irrigated. On Dec. 2008, treatments were switched again back to the initial distribution in 2007 except for removing all covers at the same time on Mar. 2009. Consequently, some trees (n = 15) were under the same winter treatments for 3 years (e.g., drought:drought:drought), whereas other trees were under alternate treatments of drought:rain + irrigation:drought or rain + irrigation:drought:rain + irrigation, as can be seen in Table 1. The intermediate rain-only treatment was maintained during the winter season in the same trees throughout the 3 years. Stem water potential. To evaluate tree water status, midday stem water potential (SWP; McCutchan and Shackel, 1992) was measured near 1200 HR in three previously covered mature leaves on one tree in each treatment in each of the three blocks (n = 27). All leaves were from the previous summer flushes that were located in the west side of the canopy. Leaves to be measured were first placed in aluminum foil-covered plastic bags for at least 1 h before measurement. SWP was measured periodically during the winter treatments using a Scholander-type pressure chamber (PMS Instrument, Corvallis, OR; Scholander et al., 1965) until 2 weeks after irrigation was resumed, but data are only reported for approximately minimum SWP in Mar. 2007–2009 just before resuming normal irrigation. Flowering intensity and fruit set. Flowering intensity was estimated each year at the estimated peak of flowering time (March to April) by counting the number of open flowers within a 0.3 · 0.6-m frame placed against the branches in two canopy positions, east and west, 1.5 m from the ground (Ribeiro et al., 2008) in six replicate trees in each treatment, two trees per treatment in each of three blocks. Counts were made on 30 shoots per tree, 15 shoots on the east side, and 15 on the west side of the canopy within the limits of the frame. In July after May–June physiological drop, frame counts were used to estimate fruit set in each treatment as described previously. The total number of flowers (at full bloom) and green fruit (in July) per tree were estimated by extrapolating the organs counted within the area of frame to the whole canopy surface area (m). Canopy surface areas were estimated from a dimension analysis as a prolate spheroid surface with a flat bottom (Albrigo et al., 1975). Juice analysis. Fifty mature fruit were annually collected in April just before harvest from six trees in each treatment, two trees from each block. These fruit were weighed and values added to the yield per tree at harvest. Samples were juiced in a processing pilot plant using a commercial extractor and percentage juice content (%), total soluble solids ( Brix), acidity (%), and brix:acidity ratio were determined using standard methods for Florida orange juice (Kimball, 1999; Wardowski et al., 1995). Harvesting. The efficiencies of fruit removal from mechanical and hand harvesting (percent of total fruit) were compared at the end of May and at the middle of June 2007. A trunk shaker with a frequency of 4 Hz and amplitude of 0.1 m was used for mechanical harvesting (Futch and Roka, 2005a). In 2008 and 2009, mechanical and hand harvesting were compared at the beginning of June. An Oxbo 3210 pull-behind canopy shaker (Oxbo International Corp., Clear Lake, WI) at 242 cpm and 0.5 mph was used for mechanical harvesting (Futch and Roka, 2005b) in these 2 years. After mechanical harvesting, fruit left in trees were removed (gleaned) and total yield (kg) per tree was calculated. Number of fruit was estimated using the weight per fruit obtained from the samples for juice analysis. Shaking efficiency (percentage of fruit removal) was estimated every year as: mechanically harvested fruit (kg) · 100: total fruit (kg), in which total fruit weight was the sum of mechanically harvested plus gleaned fruit. Fruitlet abscission and green fruit growth measurements. The green fruitlets that dropped on the ground at mechanical harvesting were collected and weighed. Fruitlet size (diameter) and fresh weight were immediately measured on a random subsample of 50 green fruit. In the field, the size of 60 fruitlets per tree on nine trees per treatment (three trees per treatment in each of three blocks) were measured biweekly from May to January to compare the growth of next year’s crop on previously drought-stressed trees and well-irrigated trees. Statistical analysis. Treatment effects were tested using analysis of variance and means were separated by Duncan’s multiple range test (P < 0.05) from the SAS statistical package (SAS Institute, Cary, NC). To test for any year-to-year carryover effects from irrigation treatments, additional analyses of variance of juice quality parameters were run using only trees that received the same treatment in consecutive years.