Physiological Responses of Avocado Leaves to ...

Physiological F.

v.

SANCES,3

Responses of Avocado Leaves to Avocado Brown Mite1 Feeding Injury2 N. C. TOSCANO,4

M. P. HOFFMANN,4 J. B. BAILEy4

L. F. LAPRE,s

M. W. JOHNSON,4

AND

Department of Entomology,Universityof California, Riverside, California92521 ABSTRACT

Environ.Emomol.II: 516-518(1982) Effects of various levels of feeding injury caused by the avocado brown mite, Oligonychus punicae (Hirst) (Acari: Tetranychidae), on avocado leaves were examined. Mite feeding from the upper leaf surface reduced chlorophyll content of the upper mcsophyll layer of avocado leaves, whereas chlorophyll content remained unchanged in the lower spongy parenchyma layer. Stomatal and mesophyll conductance values, photosynthesis, and transpiration were found to be negatively correlated with increasing levels of spider mite injury. Avocado photosynthesis rates were reduced 56% when 100% feeding injury was sustained on the upper leaf surface.

ical processes responsible for such reductions was not elucidated. Sances et al. (1979a) suggested that an understanding of a plant's physiological capacity to withstand feeding injury by spider mites was necessary before economic injury levels could accurately be established. Reductions in photosynthesis rates from plants with heavy infestations of tetranychid mites have been reported for various crops (Hall and Ferree 1975, Poskuta et a!. 1975, Wedding et al. 1958, and Andrews and LaPre 1979). Sances et al. (1981, 1982) determined the sequence of reductions of physiological processes of strawberry in correlation with increasing Tetranychus urtieae Koch feeding injury. Tetranychids investigated in previous studies feed primarily from the under surface of the leaf. On strawberries, T. urtieae feeding injury results in the destruction of the spongy mesophyll and parts of the lower palisade layer. O. punieae feeding injury differs somewhat from T. urtieae and other studied tetranychids because its feeding is primarily from the upper leaf surface. Resulting internal tissue injury, therefore, is predominantly to the upper palisade layers of the leaf. The work reported here was conducted to determine the effects of various levels of O. punicac injury on stomatal and mesophyll conductance, photosynthesis, transpiration, and chlorophyll content of mature avocado leaves. Materials and Methods

'Oligonychus punicae (Hirst)(Acarina: Tctranychidae). 2Received forpublication 23June1981. 3Presentaddress:PacificAgricultural Laboratories, Inc.,P.O.Box439. SanLuisRey.CA 92054. 4Coopcrali\'e Extension. sPresentaddress:P.O.Box5051,Riverside, CA 92517.

Investigations were conducted on 5-year-old avocado trees (Hass variety) in a commercial orchard at Highland Valley, San Diego County, Calif., on 15 October 1980. Four mature leaves of about the same age, but exhibiting different levels of O. punicae feeding injury (no injury, light, medium, and heavy), were selected from each of four trees (16 leaves total) as study blocks. Leaves were washed and dried before measurements were conducted. Stomatal and mesophyll conductance, photosynh' d .. d t eSls, an transptratlOn rates were measure on each attached leaf by using a dual isotope porometer as described by Sances et al. (1979a) and Johnson et

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The avocado brown mite, Oligonyehus punieae (Hirst), is a pest of avocados in southern California, where it reaches high population densities in the coastal and intermediate climatic areas (McMurtry and Johnson 1966). Jeppson et al. (1975) state that 0. punieac feeding on avocado leaves is initially confined to the area surrounding the midrib on the upper surface. Later, feeding extends along the smaller veins and may eventually cover the entire leaf if spider mite densities become high. McMurtry (1970) observed that the upper surfaces of mature avocado leaves were the most favorable for adult survival, population increase, oviposition, and development of immatures, as compared with new leaves and the undersurface of mature leaves. He reported that avocado brown mite population growth can be limited by the accumulation of webbing and mite exuviae on the leaves. O. punieac feeding results in a brownish discoloration of the leaves which is referred to as bronzing. In extremely heavy infestations, complete bronzing of the upper leaf surface and small portions of the lower leaf surface may result. This sometimes leads to partial defoliation of the tree (McMurtry and Johnson 1966). Leaf drop can occur when population densities average 70 adult females per leaf during short time periods or when densities are maintained at 50 adult females per leaf for several weeks. However, it has not been determined if partial defoliation of the tree affects growth or fruit yields during the season in which the injury occurs. Previous studies have not examined the physiological effects of O. punieae feeding on avocado leaves. Earlier studies of the effects of tetranychid feeding injury on various crops concentrated on gross reductions in plant growth and productivity (Lienk et al. 1956, Avery and Briggs 1968, van de Vrie 1972, Barnes and Andrews 1978). However, the effect of tetranychid feeding injury on physiolog-

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Results and Discussion Preestablished injury levels of no inJury, light, medium, and heavy injury corresponded with actual percent bronzing of 5, 46, 91, and 100, respectively, of the upper leaf surfaces. Mean chlorophyll content, expressed in units of absorbance, was consistantly higher in the mesophyll cells adjacent to the upper leaf surface except in those leaves where 100% injury was sustained (Fig. I). In the upper cell layer, chlorophyll content was decreased by feeding injury levels greater than 65%. Chlorophyll content in the lower cell layer remained the same at all injury levels. This would be expected because avocado brown mite feeding is normally confined to the upper surface of the leaf until spider mite populations become extremely high. Reductions in leaf chlorophyll content (chlorosis) have been associated with spider mite in-

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jury on such crops as apple (Hall and Ferree 1975), almonds (Andrews and LaPre 1979), citrus (Wedding et al. 1958), and strawberry (Sances et al. 1979b). Both stomatal conductance (Cs) and mesophyll conductance (Cm) rates, expressed as a mean of 20 readings, were negatively correlated with tetranychid feeding injury (P < 0.10) (Fig. 2). Diffusion of water vapor through stomata is due primarily to the relative degree of stomatal opening. This value, therefore, is positively correlated with Cs and is used to express the degree of stomatal opening. Mesophyll conductance is a measure of the degree of diffusion and transport of CO2 after entering the leaf. It expresses the functional rate of biochemical processes associated with CO2 assimilation. Reductions in Cs are thus interpreted as decreases in stomatal opening, whereas decreases in Cm indicate internal tissue damage expressed as decreased movement of CO2 and fixation within the leaf. These results indicate that tetranychid feeding damage exceeding 45% of the upper leaf surface can affect stomatal behavior on the leaf's undersurface. The linear decrease in Cm indicates a direct relationship between O. punicae feeding damage and the functional integrity of the cellular layers of the leaf. Photosynthesis, a function of Cs and Cm, was negatively correlated with mite damage (P < 0.05) (Fig. 3). Leaves sustaining 46% leaf damage on the upper leaf surface exhibited 30% reductions in photosynthesis rates, as compared with leaves with 91% damage where rates decreased 41%. Similarly, transpiration, which is directly proportional to Cs, was negatively correlated with mite damage (Fig. 3). Reduced transpiration not only indicates a decreased loss of water from the leaf, but demonstrates the significance of leaf stomata to gaseous diffusion. This limitation in leaf porosity also reduces uptake of CO2 through stomata, which in turn limits photosynthesis. Because the photosynthesis

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PERCENT LEAF INJURY FiG. 2.-Effects of O. punicae feeding Injury on stomatal conductance (Cs• that due to degree of stomatal opening) and mesophyll conductance (Cm, that due to functional aspects of leaf tissue) within avocado leaves. Each data point represents the mean of 20 readings.


al. (I 979). The average of five readings was recorded for each leaf. After physiological rates were measured, leaves were taken to the laboratory, where other data were collected. Chlorphyll content was estimated in leaves by a portable chlorophyll reflectance meter with a spectral sensitivity of 610 to 700 nm (Wallihan 1973). Eight readings were taken from the upper and lower leaf surfaces of each of the leaves examined. Percent leaf area bronzed was estimated by observing the leaf tissue on the upper leaf surface through a 2-cm2 transparent grid with I-mm2 divisions with a binocular microscope (Sances et al. 1979b). Ten readings were made per leaf, and means were calculated for each leaf. Data were analyzed by an analysis of variance for a nested design with four single trees as four main plots, each composed of four leaf subplots. Linear regression analysis was conducted on pertinent relationships.

RESPONSES TO

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FIG. 3.-Effects of O. punicae feeding injury on photosynthesis and transpiration of avocado leaves. Each data point represents a mean of 20 readings. rate also is dependent upon biochemical fixation once inside leaf tissue, it is additionally affected by significant destruction of mesophyll cells resulting from spider mite injury. The effect of injury to the second portion of the CO2 pathway was measured by reductions in Cm and chlorophyll content. These data indicate that O. punicae feeding on avocado may severely restrict the gas exchange capacity of infested leaves and thereby reduce important physiological processes. The specific density of mite populations or intensity of feeding (mite levels over time) necessary to cause this injury has not yet been fully quantified. Fixed-effect experiments, with well-defined mite densities for specified periods of time, are necessary to adequately establish an economic threshold for this tetranychid pest. The physiological injury to leaves reported herein may provide a framework for these experiments, although the relationship between the extent of physiological injury to leaves and the accompanying effect on yield must ultimately be demonstrated.

Acknowledgment We thank S. Streeter the data and C. Adams

for assistance in tabulating for statistical assistance.

REFERENCES

CITED

Andrews, K. L., and L. LaPre. 1979. Effect of Pacific

spider mite on physiological processes of almond foliage. J. Econ. Entomol. 72: 651-{j54. Avery, D. J., and J. B. Briggs. 1968. The aetiology and development of damage in young fruit trees infested with fruit tree red spider mite, Panonychus ulmi (Koch). Ann. Appl. BioI. 61: 277-288 . Barnes, M. M., and K. L. Andrews. 1978. Effects of spider mites on almond tree growth and productivity. J. Econ. Entomol. 71: 555-558. Hall, F. R., and D. C. Ferree. 1975. Influence of two-spotted spider mite populations on photosynthesis of apple leaves. J. Econ. Entomol. 68: 517-520. Jeppson, L. R., H. H. Keifer, and E. W. Baker. 1975. Mites injurious to economic plants. University of California Press, Berkeley. 614 pp. Johnson, H. B., P. G. Rowlands, and I. P. Ting. 1979. Tritium and carbon 14 double isotope porometer for simultaneous measurements of transpiration and photosynthesis. Photosynthetica 13: 409-418. Lienk, S. E., P. J. Chapman, and O. F. Curtis. 1956. Response of apple trees to mite infestation. II. J. Econ. Entomol. 49: 350-355. McMurtry, J. A. 1970. Some factors of foliage condition limiting population growth of Oligonychus punicae (Acarina: Tetranychidae). Ann. Entomol. Soc. Am. 63: 406-412. McMurtry, J. A., and H. G. Johnson. 1966. An ecological study of the spider mite Oligonychus punicae (Hirst) and its natural enemies. Hilgardia 37: 363-402. Poskuta, J., A. Kotodziej, and D. Kropczynska. 1975. Photosynthesis, photorespiration, and respiration of strawberry plants as influenced by the infestation with Tetranychus urticae (Koch). Fruit Sci. Rep. 2: 1-11. Sances, F. V., N. C. Toscano, E. R. Oatman, L. F. LaPre, 1\1. W. Johnson, and V. Voth. 1982. Reductions in plant processes by Tetranychus urticae feeding on strawberry. Environ. Entomol. (in press) Sances, F. V., J. A. Wyman, and I. P. Ting. 1979a. Physiological responses to spider mite infestation on strawberries. Environ. Entomol. 8: 711-714. 1979b. Morphological responses of strawberry leaves to infestations of twospotted spider mite. J. Econ. Entomol. 72: 710-713. Sances, F. V., J. A. Wyman, I. P. Ting, R. A. Van Steenwyk, and E. R. Oatman. 1981. Spider mite interactions with photosynthesis, transpiration, and productivity of strawberry. Environ. Entomol. J(I: 442-44X. van de Vrie, M., J. A. McMurtry, and C. B. Huffaker. 1972. The ecology of tetranychid mites and their natural enemies. 1. The biology, ecology, pest status, and host plant relations of tetranychids. Hilgardia 41: 343432. Wallihan, E. F. 1973. Portable reflectance meter for estimating chlorophyll concentration in leaves. Agron. J. 65: 659-Q62. Wedding, R. T., L. A. Riehl, and L. R. Jeppson. 1958. Red mite on citrus. Calif. Agric. 12: 9-12.


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Vol. 11, no. 2