Concerns about the availability and rising cost of harvest labor have led Florida's citrus industry toward the adoption of mechanical harvesting (Brown, 2005). Apparent visible mechanical injuries during harvest have proven harmless to the citrus tree's overall physiological performance (Li and Syvertsen, 2005; Li et al., 2005, 2006). The acreage of sweet orange (Citrus sinensis) in Florida harvested with a trunk or canopy shaker has continuously grown in recent years. However, mechanical harvest of late-maturing ‘Valencia’ oranges in May has been problematic because mature and immature fruit are present on the tree at the same time. Mechanical harvesting is abandoned after young ‘Valencia’ orange fruit reach a certain size to avoid removal of young fruit representing the next year's crop during harvesting. Reducing the intensity of the harvest machine to prevent green fruit removal during late season harvesting has shown to be impractical due to decreased efficiency of the harvesting operation. However, removal of mature and young green fruit could be differentiated by using fruit abscission compounds (Burns et al., 2006), provided the compounds are selective for mature fruit and nonphytotoxic.
Several abscission compounds that accelerate abscission by decreasing the detachment force of mature citrus fruit have been identified (Burns et al., 2003; Hartmond et al., 2000a, 2000b; Kender et al., 1998; Yuan et al., 2002). Among these compounds, 5-chloro-3-methyl-4-nitro-1H-pyrazole has shown promise in terms of selectivity and uniformity of fruit loosening (Burns, 2002); furthermore, research has demonstrated increased mechanical harvesting efficiency when CMNP is used (Burns et al., 2005). Application of CMNP to citrus canopies did not decrease yield in multiyear trials (Whitney, 2003). Despite this fact, the paucity of information on short- and intermediate-term physiological effects of CMNP has been one of the major concerns of growers who must understand its impact and interaction with established citrus management strategies when CMNP registration is completed.
The mode of action of CMNP is currently unknown, but previous work has provided some information on reactions of leaves and fruit to CMNP and fundamental physiological responses. Temperatures below 15 °C for the first 24 h after application prevent fruit loosening (Yuan and Burns, 2004). When applied by soil drenching, CMNP can cause typical herbicide-like phytotoxic symptoms on leaves of various plants, including citrus (Alferez et al., 2005, 2006). Using foliar CMNP application, citrus leaves and immature fruit had no visual phytotoxic symptoms and did not abscise when used at a concentration that caused mature fruit abscission. At this CMNP concentration, limited physiological measurements showed marked effects in mature fruit, but not in leaves (Alferez et al., 2005; Yuan and Burns, 2004). Higher application rates, however, caused excessive leaf abscission (Burns, 2002), suggesting that CMNP was entering the leaf, but supraoptimal concentrations were necessary to overcome an unknown physiological impediment to efficacy.
Based on this physiological evidence, we hypothesized that citrus leaves and young fruit are capable of recovering from any potential phytotoxicity induced by a range of concentrations of foliar-applied CMNP. We measured the photosynthetic ability and water status of leaves of fruiting citrus trees to determine the effects of CMNP in comparison with ethephon, a commercially available abscission compound that often causes excessive defoliation and young fruit drop (Kender et al., 2000; Morton et al., 1978). We also determined the effect of CMNP and ethephon on young fruit retention and growth rate. Results provide useful information that may assist the ongoing commercial registration effort of CMNP as an abscission agent for citrus.
Alferez, F., Pozo, L. & Burns, J.K. 2006 Physiological changes associated with senescence and abscission in mature citrus fruit induced by 5-chloro-3-methyl-4-nitro-1H-pyrazole and ethephon application Physiol. Plant. 127 66 73
Alferez, F., Singh, S., Umbach, A.L., Hockema, B. & Burns, J.K. 2005 Citrus abscission and Arabidopsis plant decline in response to 5-chloro-3-methyl-4-nitro-1H-pyrazole are mediated by lipid signaling Plant Cell Environ. 28 1436 1449
Burns, J.K., Buker, R.S. & Roka, F.M. 2005 Mechanical harvesting capacity in sweet orange is increased with an abscission agent HortTechnology 15 758 765
Burns, J.K., Pozo, L.V., Yuan, R. & Hockema, B. 2003 Guanfacine and clonidine reduce defoliation and phytotoxicity associated with abscission agents J. Amer. Soc. Hort. Sci. 128 1 42 47
Burns, J.K., Roka, F.M., Li, K.-T., Pozo, L. & Buker, R.S. 2006 Late-season ‘Valencia’ orange mechanical harvesting with an abscission agent and low-frequency harvesting HortScience 41 3 660 663
Chow, W.S., Osmond, C.B. & Huang, L.K. 1989 Photosystem II function and herbicide binding sites during photoinhibition of spinach chloroplasts in vivo and in vitro Photosyn. Res. 21 1573 1579
Coleman, J.O.D., Blake-Kalff, M.M.A. & Davies, T.G.E. 1997 Detoxification of xenobiotics by plants: Chemical modification and vacuolar compartmentation Trends Plant Sci. 2 144 151
Desikan, R., Last, K., Harrett-Williams, R., Tagliavia, C., Harter, K., Hooley, R., Hancock, J.T. & Neill, S.J. 2006 Ethylene-induced stomatal closure in Arabidopsis occurs via AtrbohF-mediated hydrogen peroxide synthesis Plant J. 47 907 916
Hartmond, U., Whitney, J.D., Burns, J.K. & Kender, W.J. 2000a Seasonal variation in the response of ‘Valencia’ orange to two abscission compounds HortScience 35 2 226 229
Hartmond, U., Yuan, R., Burns, J.K., Grant, A. & Kender, W.J. 2000b Citrus fruit abscission induced by methyl-jasmonate J. Amer. Soc. Hort. Sci. 125 5 547 552
Inskeep, W.P. & Bloom, P.R. 1985 Extinction coefficients of chlorophyll a and b in N, N-dimethylformamide and 80% acetone Plant Physiol. 77 483 485
Kender, W.J., Hartmond, U., Salyani, M., Burns, J.K. & Whitney, J.D. 1998 Effects of concentration and application time of ‘Prosulfuron’ on the abscission of ‘Hamlin’ and ‘Valencia’ oranges Proc. Florida State Hort. Soc. 111 108 112
Kender, W.J., Hartmond, U., Yuan, R., Pozo, L. & Grant, A. 2000 Factors influencing the effectiveness of ethephon as a citrus fruit abscission agent Proc. Florida State Hort. Soc. 113 88 92
Li, K.-T. & Syvertsen, J.P. 2005 Mechanical harvesting has little effect on water potential and leaf gas exchange in citrus trees J. Amer. Soc. Hort. Sci. 130 5 661 666
Li, K.-T., Syvertsen, J.P. & Burns, J.K. 2005 Mechanical harvesting of Florida citrus trees has little effect on leaf water relation or return bloom Proc. Florida State Hort. Sci. 118 22 24
Li, K.-T., Syvertsen, J.P. & Dunlop, J. 2006 Defoliation after harvest with a trunk shaker does not affect canopy light interception in citrus trees Proc. Florida State Hort. Sci. 119 187 189
Morton, C.C., Jahn, O.L., Young, R.H. & Biggs, R.H. 1978 Ethephon-induced defoliation patterns and subsequent yields in citrus J. Amer. Soc. Hort. Sci. 103 5 670 673
Roháček, K. & Barták, M. 1999 Technique of the modulated chlorophyll fluorescence: Basic concepts, useful parameters, and some applications Phytosynthetica 37 339 363
Scholander, P.F., Bradstreet, E.D., Hemmingsen, E.A. & Hammel, H.T. 1965 Negative hydrostatic pressure can be measured in plants Science 148 339 346
Untiedt, R. & Blanke, M. 2001 Effects of fruit thinning agents on apple tree canopy photosynthesis and dark respiration Plant Growth Regulat. 35 1 9
van Kooten, O. & Snel, J.F.H. 1990 The use of chlorophyll fluorescence nomenclature in plant stress physiology Photosyn. Res. 25 147 150
Whitney, J.D. 2003 Trunk shaker and abscission chemical effects on yields, fruit removal, and growth of orange trees Proc. Florida State Hort. Soc. 116 230 235
Yuan, R. & Burns, J.K. 2004 Temperature factor affecting the abscission response of mature fruit and leaves to CMN-pyrazole and ethephon in ‘Hamlin’ oranges J. Amer. Soc. Hort. Sci. 129 287 293
Yuan, R., Hartmond, U. & Kender, W.J. 2002 Naphthalene acetic acid and 2,3,5-triiodobenzoic acid affect the response of mature orange fruit to abscission chemical HortScience 37 2 348 352