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Growth, gas exchange, and mineral relations of black sapote (Diospyros digyna Jacq.) as influenced by salinity
Scientia Horticulturae 72 Ž1998. 103–110
Growth, gas exchange, and mineral relations of black sapote žDiospyros digyna Jacq. / as influenced by salinity Michael V. Mickelbart ) , Thomas E. Marler
1
Fairchild Tropical Garden, 11935 Old Cutler Road, Miami, FL 33156, USA Accepted 11 July 1997
Abstract The influence of salinity on growth, leaf gas exchange, and ion absorption of black sapote Ž Diospyros digyna Jacq.. plants was studied under glasshouse conditions. Sea salt was added to a complete nutrient solution to obtain salinity levels of 1 Žcontrol., 2, 4, 6, or 8 dS my1 in one study, and 1, 12, or 16 dS my1 in a second study. Plants were irrigated with these solutions for 19 weeks Žexperiment 1. or 17 weeks Žexperiment 2.. Salinity in the nutrient solution reduced growth, measured as the increase in leaf area, trunk cross-sectional area, and plant dry weight. Stomatal conductance and transpiration were also reduced by salinity. The time required for leaf gas exchange to begin declining and the ultimate level of gas exchange relative to the control plants were reduced with increased salinity. Leaf, stem, and root Naq and Cly content increased with salinity in experiment 1. Leaf, stem, and root Kq and Kq:Naq decreased with salinity. Leaf Naq was maintained below that of stems or roots. Few toxicity symptoms were visible only on the plants receiving the highest salinity level in experiment 2, and no defoliation occurred throughout both experiments. Sequestration of Naq in woody organs and the resulting higher leaf Kq:Naq are primary mechanisms of moderate tolerance of black sapote seedlings to salinity. q 1998 Elsevier Science B.V. Keywords: Salinity; Diospyros digyna; Kq:Naq
) Corresponding author. Present address: Horticulture Department, Purdue University, 1165 Horticulture Building, West Lafayette, IN 47907, USA. 1 Present address: University of Guam, College of Agriculture and Life Sciences, Mangilao, Guam 96923.
0304-4238r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 4 2 3 8 Ž 9 7 . 0 0 0 9 5 - 2
104
M.V. Mickelbart, T.E. Marlerr Scientia Horticulturae 72 (1998) 103–110
1. Introduction Black sapote is a popular fruit species in some subtropical and tropical regions, especially in native lowland regions of Central America ŽMartin et al., 1987; Morton, 1987.. The black sapote tree is handsome, tolerant of a wide range of soils, and usually grows well with little or no cultural inputs ŽMorton, 1987.. Martin et al. Ž1987. believe that black sapote should be considered for further distribution and planting. Saline conditions are widespread and salinity has long been a threat to crop production for large portions of the world’s lands ŽChapman, 1975; Carter, 1975.. Salinity is becoming more of a problem in humid coastal areas due to greater salt water intrusion into the fresh ground water ŽZekri and Parsons, 1989.. Ogden et al. Ž1981. classified black sapote as fairly tolerant with respect to salinity. This classification was based on general horticultural observations, not on experimental results. Most fruit species from humid tropical and subtropical regions are sensitive to salinity stress. Thus, we conducted this study to verify the anecdotal ranking of fairly tolerant, because of the recommendation for further planting of the species, and because a database on any aspect of managing the species is lacking. The objectives were to determine quantitatively the effect of substrate salinity on growth, leaf gas exchange, and mineral relations of black sapote seedlings. 2. Materials and methods 2.1. Experiment 1 Nine-month-old seedlings of ‘Merida’ black sapote were bare-rooted and transplanted to silica sand in 5.1 l containers on 28 May 1990. Thirty-six plants were placed on raised benches in a glasshouse with maximum photosynthetic photon flux of ca. 1200 m mol my2 sy1 . The plants were watered daily, alternating rainwater and complete nutrient solution. The nutrient solution was made with 0.68 g ly1 Hydro-sol ŽW.R. Grace, Fogelsville, PA. and 0.45 g ly1 calcium nitrate, and diluted to 1 dS my1. This solution approximated one-half Hoagland solution ŽHoagland and Arnon, 1950.. Salinity treatments were applied beginning on 10 July . The salinity solutions were increased 1 dS my1 dayy1 until the ultimate level was reached to reduce the chance of osmotic shock. Treatments consisted of the nutrient solution as control, and 2, 4, 6, or 8 dS my1 made by the addition of sea salt to the control solution. The respective osmotic potentials of the solutions were y0.04, y0.08, y0.17, y0.25, or y0.34 MPa. The respective approximations of percentage ocean water were 2%, 5%, 9%, 14%, or 19%. Solutions were delivered from elevated tanks to dribble ring emitters via polyethylenetubing and micro-tubing. This delivery system ensured that the entire volume of substrate received the solution, and the solution was delivered in excess to ensure a minimum of 25% leachate. Thirty plants were arranged in a randomized complete block design with six replications per salinity level. The remaining six plants from the original population were used to estimate initial dry mass and leaf area of the experimental plants. Leaves were removed and the area measured ŽLI 3000 area meter, LI-COR, Lincoln, NE., then
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the roots were cleaned by gentle water pressure, and the entire plants were dried at 608C prior to weighing. Initial trunk diameter was measured on the experimental plants on 10 July 1990. Leaf water vapor exchange was determined with a steady state porometer ŽLI-1600, LI-COR. beginning on 9 July 1990 Žday 1., and every 2 to 14 days until 16 November 1990 Žday 129.. One young, fully-expanded leaf per plant was chosen at random for each measurement. On several days during the week prior to initiating the treatments, measurements were made throughout the photoperiod, and stomatal conductance was maximum during the middle portion of the photoperiod. As a result, measurements were made during midday throughout the study. Final growth measurements were made on 19 November 1990. Trunk diameter was measured, then leaves were removed and area was measured as previously described. Roots were cleaned with gentle water pressure, and all tissue was dried at 608C prior to weighing. Trunk diameter was converted to trunk cross-sectional area. Growth of the plants during the experimental period was calculated by subtracting the initial value for each variable from the final value. Tissue from the plants receiving salinity of 1, 4, or 8 dS my1 was retained after dry mass was determined. Tissue was consolidated into three replications of two trees each. Root, stem, and leaf tissue was milled to pass through a 60-mesh screen. Analysis of tissue Naq, Cly, and Kq was conducted according to Wolf Ž1982.. Mean high and low temperature during the experimental period was 338C and 238C, respectively, and mean high and low relative humidity was 91% and 58%, respectively, as determined by hygrothermograph ŽWeathertronics Model 5020, Qualimetrics, Sacramento, CA.. 2.2. Experiment 2 A second study was conducted in 1990 and 1991. Nine-month-old plants grown from seed of ‘Merida’ black sapote were transplanted to silica sand in 5.1 l containers on 24 October 1990 and watered daily with the control nutrient solution. Twenty-four plants were maintained under these conditions until the treatments were begun on 12 December 1990. Salinity levels consisted of 1, 12, or 16 dS my1 . The respective osmotic potentials of the solutions were y0.04, y0.51, or y0.69 MPa. The respective approximations of percentage ocean water were 2%, 28%, or 37%. Solutions were delivered as previously described. Eighteen plants were arranged in a randomized complete block design with six replications per salinity level. The remaining six plants from the group were used as in experiment 1 to estimate initial dry mass and leaf area of the experimental plants. Gas exchange measurements were begun on 13 December Žday 1., and were taken every 7 to 14 days until 11 April Žday 118.. Final growth measurements were conducted on 11 April 1991. Growing conditions during this study were 328C and 208C Žmean high and low temperature. and 91% and 50% Žmean high and low relative humidity.. 2.3. Statistics The growth variables, final gas exchange, and tissue mineral content data were subjected to analysis of variance. Significant comparisons were defined by determining
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M.V. Mickelbart, T.E. Marlerr Scientia Horticulturae 72 (1998) 103–110
the linear and quadratic models with salinity level as the independent variable. The developmental response of gas exchange to salinity was calculated for each measurement day as a value relative to the mean of the control plants on that day. This allowed data from the two experiments to be combined for presentation. 3. Results 3.1. DeÕelopmental leaf gas exchange Leaf stomatal conductance Ž g s . of black sapote plants exposed to salinity began to decline relative to control plants after the initiation of salinity treatments ŽFig. 1.. The number of days until this decline started was inversely related to the salinity level, with more than 60 days required for the dose levels up to 4 dS my1 and as little as 30 days required for the 8, 12, and 16 dS my1 levels. Moreover, water vapor exchange declined to a lower minimum value for the plants exposed to the higher salinity levels when compared with plants exposed to the lower salinity levels. The pattern of g s and transpiration responses over time and to salinity level were similar, so transpiration data are not shown. 3.2. Growth and final gas exchange The three growth variables, leaf area, dry mass, and trunk cross-sectional area were reduced by salinity in a linear fashion ŽTables 1 and 2.. We observed no threshold in the
Fig. 1. Relative stomatal conductance of Diospyros digyna trees grown in sand culture as a function of time. Plants were irrigated with complete nutrient solution of 1 dS my1 ŽI. or with this solution amended with sea salt to 4 Ž'., 8 Žv ., or 16 ŽB. dS my1 . Data for plants exposed to 2, 6, or 12 dS my1 were not included in order to simplify the figure. Each point represents the mean relative to control of six replications"standard error. The overall mean for control plants was 248 mmol my2 sy1 .
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Table 1 The increase in leaf area, dry weight, and trunk cross-sectional area, and final stomatal conductance of D. digyna plants as influenced by 19 weeks of exposure to solutions of 1, 2, 4, 6, or 8 dS my1 Ž ns6. Salinity ŽdS my1 .
Leaf area Žcm2 .
Dry mass Žg.
Trunk cross-sectional area Žmm2 .
Stomatal conductance Žmmol my2 sy1 .
1 2 4 6 8 Significance
8024 6358 5932 5616 5372 L) )
212.6 169.9 165.9 124.4 139.0 L)
218 176 188 132 142 L) )
196 185 171 133 112 L) )
) ,) )
Linear models are significant at P F 0.05 or 0.01, respectively.
growth response pattern as influenced by salinity, rather, the lowest level of salinity Ž2 dS my1 . reduced growth 19% to 21% below that of the control plants ŽTable 1.. Growth of the plants exposed to 16 dS my1 relative to that of the control plants ranged from 31% for trunk cross-sectional area to 47% for dry mass accumulation ŽTable 2.. By the end of each experiment, leaf water vapor exchange was fairly stable within each salinity level ŽFig. 1.. Stomatal conductance was reduced to 57% of the control plants at 8 dS my1 ŽTable 1., and to 27% of the control plants at 16 dS my1 ŽTable 2.. No visible leaf burn symptoms of salinity stress appeared in any leaves except a few from the plants receiving 16 dS my1 . These few leaves exhibited marginal necrosis, and were located randomly throughout the canopy with no consistent pattern being observed with respect to leaf age or position within the canopy. Leaf abscission did not occur in any plants. 3.3. Mineral analysis Increased root zone salinity increased Naq and Cly concentrations in leaves, stems, and roots of black sapote ŽTable 3.. The absolute concentration of Cly was greater than that of Naq for all salinity levels and organs. However, salinity increased Naq concentration relative to that of control plants to a greater degree than for Cly. Stem tissue of salt-stressed plants increased in Naq and Cly concentration more than did leaf or root tissue relative to that of control plants. Concentration of Kq in all plant organs
Table 2 The increase in leaf area, dry weight, and trunk cross-sectional area, and final stomatal conductance of D. digyna plants as influenced by 17 weeks of exposure to 1, 12, or 16 dS my1 Ž ns6. Salinity ŽdS my1 .
Leaf area Žcm2 .
Dry mass Žg.
Trunk cross-sectional area Žmm2 .
Stomatal conductance Žmmol my2 sy1 .
1 12 16 Significance
7458 4395 2886 L) )
156.4 98.3 73.6 L) )
159 90 49 L) )
216 106 58 L) )
))
Linear models are significant at P F 0.01.
M.V. Mickelbart, T.E. Marlerr Scientia Horticulturae 72 (1998) 103–110
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Table 3 Influence of 19 weeks of salinity on Naq, Cly, and Kq, and the ratio Kq:Naq in root, stem, and leaf tissue of D. digyna plants. Data are means of three replications presented as percentage of dry weight, each consisting of tissue from two plants Salinity ŽdS my1 .
Naq
Cly
Kq
Kq:Naq
LeaÕes 1 4 8 Significance
0.04 0.16 0.38 L) )
1.29 3.69 5.86 L) )
1.27 1.20 0.97 L)
29.6 9.2 2.5 L) ) Q )
Stems 1 4 8 Significance
0.02 0.25 0.60 L) )
0.19 0.41 1.61 L) )
1.47 1.28 0.88 L) )
66.4 5.4 1.6 L) ) Q ) )
Roots 1 4 8 Significance
0.40 1.11 1.74 L) )
1.75 2.16 2.39 L) )
2.93 1.25 1.37 Q) )
7.8 1.1 0.8 L) Q )
) ,) )
Linear ŽL. or quadratic ŽQ. models are significant at P F 0.05 or 0.01, respectively.
was decreased by salinity. The marked increase of stem Naq concentration, a 30-fold increase at the 8 dS my1 level, resulted in stems exhibiting the largest relative decrease in the ratio Kq:Naq Ž2% for stem tissue compared with 10% for root tissue relative to that of the control plants.. Sodium and chloride elemental contents were calculated from absolute dry mass for each organ Ždata not shown. and the percentage data is presented in Table 3. These calculations revealed that stems of salt-stressed plants accumulated Naq to a greater degree than leaf or root tissue, and to a greater degree than Cly was accumulated in all three organs. Leaf Naq increased 8-fold, root Naq increased 3-fold, and stem Naq increased 24-fold for plants receiving 8 dS my1 as compared with the control plants. The relative increase in Cly for plants receiving 8 dS my1 was below four-fold for all three organs.
4. Discussion Visible toxicity symptoms of leaf burn and defoliation are common for salt-sensitive woody perennial species ŽMaas and Hoffman, 1977.. The black sapote plants in this study maintained a healthy appearance throughout both experiments, with almost no visible toxicity symptoms. Moreover, no defoliation occurred for any salt-stressed plants. The plants receiving 16 dS my1 , the highest level of salinity in this study, withstood 17 weeks of exposure and maintained reduced but sustained growth through-
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out the study. Biomass accumulation, determined by the increase in dry mass, was reduced by only 47% below that of the control plants. This appreciable maintenance of growth and the lack of defoliation and foliar toxicity symptoms indicate at least some level of salt tolerance for black sapote. The decrease in g s with longer exposure to salinity that was exhibited by black sapote plants in this study is common for many species exposed to high substrate salinity levels ŽPlaut et al., 1990; Warne et al., 1990; Zekri and Parsons, 1990.. This conservation of water within the plant may aid in decreasing translocation of salts to the leaves ŽGreenway and Munns, 1980., and may thus decrease leaf mortality ŽFlanagan and Jeffries, 1988.. Woody perennial species tend to accumulate toxic levels of Naq andror Cly when exposed to substrate salinity ŽMaas, 1994.. Adverse effects of salinity have been attributed to accumulation of either ion, for instance Naq for Citrus sinensis ŽL.. Osbeck ŽWalker et al., 1983. and Cly for a range of tree species ŽTownsend, 1980.. In this study with black sapote, the relative increase in leaf Naq Žcompared with that of control plants. was greater than the relative increase in leaf Cly for the salt-stressed plants. However, the plants allowed Cly transport freely into leaves, since leaves developed a higher concentration than stems or roots when the plants were exposed to salinity ŽTable 3.. In contrast, leaf Naq concentration was lower than stem or root Naq concentrations when the plants were exposed to salinity. This pattern has been reported for other woody perennial tree species ŽTownsend, 1980.. Thus, even though the relative increase in leaf Naq was more than the relative increase in leaf Cly, the salt-stressed plants exhibited stem accumulation of Naq which decreased leaf Naq relative to the other organs. Similarly, the initial increase in leaf Naq of salt-stressed AÕerrhoa carambola L. seedlings was slow relative to that of leaf Cly due to root accumulation of Naq ŽMarler and Zozor, 1994.. Moreover, while Naq did increase in leaves with increased salinity, the Kq:Naq ratio was always higher in leaves than in roots, which has also been noted in other tropical fruit species such as Casimiroa edulis La Llave and Lex. ŽNerd et al., 1992. and Persea americana Mill. ŽMickelbart, unpublished data.. The maintenance of high Kq:Naq ratios may alleviate Naq toxicity in leaves of black sapote.
5. Conclusion Increased substrate salt resulted in decreased water vapor exchange and growth of black sapote plants. Although measured variables decreased, growth continued and there were few visible toxicity symptoms in leaves after 17 to 19 weeks of salinity exposure. There were isolated cases of necrotic leaf symptoms on plants receiving 16 dS my1 , but no defoliation was observed. Leaf Naq was below that of stems or roots for plants receiving 4 or 8 dS my1 , but leaf Cly increased above that of stems or roots. Therefore, Naq is at least partially sequestered in woody tissues of black sapote, resulting in few visible signs of salt toxicity in foliage. The results indicate a moderate tolerance of black sapote to salinity primarily through the sequestration of Naq in woody organs and the maintenance of favorable Kq:Naq ratios.
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References Carter, D.L., 1975. Problems of salinity in agriculture. In: Poljakoff-Mayber, A., Gale, J. ŽEds.., Plants in Saline Environments. Springer-Verlag, New York, pp. 25–35. Chapman, V.J., 1975. The salinity problem in general, its importance, and distribution with special reference to natural halophytes. In: Poljakoff-Mayber, A., Gale, J. ŽEds.., Plants in Saline Environments. SpringerVerlag, New York, pp. 7–24. Flanagan, F.B., Jeffries, R.L., 1988. Stomatal limitation of photosynthesis and reduced growth of the halophyte, Plantago maritima L., at high salinity. Plant Cell Environ. 11, 239–246. Greenway, H., Munns, R., 1980. Mechanisms of salt tolerance in nonhalophytes. Ann. Rev. Plant Physiol. 31, 149–190. Hoagland, D.R., Arnon, D.I., 1950. The water culture method for growing plants without soil. Circ. Calif. Agric. Exp. Sta. No. 347., Berkeley, USA, 39 pp. Maas, E.V., 1994. Tolerance and responses of tropical crops to salinity stress. In: Davenport, T.L., Harrington, H.M. ŽEds.., Proc. Plant Stress in the Tropical Environment, 20–25 Sept. 1992 at Kailua–Kona, Hawaii, USA, pp. 47–56. Maas, E.V., Hoffman, G.J., 1977. Crop salt tolerance—current assessment. J. Irrig. Drain. Div., Am. Soc. Chem. Eng. 103, 115–134. Marler, T.E., Zozor, Y., 1994. Salinity, growth, nutrient content, and leaf gas-exchange of carambola. In: Davenport, T.L., Harrington, H.M. ŽEds.., Proc. Plant Stress in the Tropical Environment, 20–25 Sept. 1992 at Kailua–Kona, Hawaii, USA, pp. 83–85. Martin, F.W., Campbell, C.W., Ruberte, ´ R.M., 1987. Perennial edible fruit of the tropics, an inventory. USDA Agriculture Handbook No. 642, 247 pp. Morton, J.F., 1987. Fruits of Warm Climates. Creative Resource Systems, Winterville, NC, USA, 505 pp. Nerd, A., Lapidot, M., Mizrahi, Y., 1992. White sapote Ž Casimiroa edulis .: performance under various culture salinities and environmental stress conditions in field studies. Scientia Hort. 51, 213–222. Ogden, M.A.H., Jackson, L.K., Campbell, C.W., 1981. Florida tropical fruit culture via master gardener. Proc. Fla. State Hort. Soc. 94, 222–225. Plaut, Z., Grieve, C.M., Maas, E.V., 1990. Salinity effects on CO 2 assimilation and diffusive conductance of cowpea leaves. Physiol. Plant 79 Ž1., 31–38. Townsend, A.M., 1980. Response of selected tree species to sodium chloride. J. Am. Soc. Hort. Sci. 105, 878–883. Walker, R.R., Torokfalvy, E., Grieve, A.M., Prior, L.D., 1983. Water relations and ion concentrations of leaves on salt-stressed citrus plants. Aust. J. Plant Physiol. 10, 265–277. Warne, P., Guy, R.D., Rollins, L., Reid, D.M., 1990. The effects of sodium sulphate and sodium chloride on growth, morphology, photosynthesis and water use efficiency of Chenopodium rubrum. Can. J. Bot. 68 Ž5., 999–1006. Wolf, B., 1982. A comprehensive system of leaf analyses and its use for diagnosing crop nutrient status. Commun. Soil Sci. Plant Anal. 13 Ž12., 1035–1059. Zekri, M., Parsons, L.R., 1989. Growth and root hydraulic conductivity of several citrus rootstocks under salt and polyethylene glycol stresses. Physiol. Plant. 77, 99–106. Zekri, M., Parsons, L.R., 1990. Comparative effects of NaCl and polyethylene glycol on root distribution, growth, and stomatal conductance of sour orange seedlings. Plant Soil 129, 137–143.
Growth, gas exchange, and mineral relations of black sapote žDiospyros digyna Jacq. / as influenced by salinity Michael V. Mickelbart ) , Thomas E. Marler
1
Fairchild Tropical Garden, 11935 Old Cutler Road, Miami, FL 33156, USA Accepted 11 July 1997
Abstract The influence of salinity on growth, leaf gas exchange, and ion absorption of black sapote Ž Diospyros digyna Jacq.. plants was studied under glasshouse conditions. Sea salt was added to a complete nutrient solution to obtain salinity levels of 1 Žcontrol., 2, 4, 6, or 8 dS my1 in one study, and 1, 12, or 16 dS my1 in a second study. Plants were irrigated with these solutions for 19 weeks Žexperiment 1. or 17 weeks Žexperiment 2.. Salinity in the nutrient solution reduced growth, measured as the increase in leaf area, trunk cross-sectional area, and plant dry weight. Stomatal conductance and transpiration were also reduced by salinity. The time required for leaf gas exchange to begin declining and the ultimate level of gas exchange relative to the control plants were reduced with increased salinity. Leaf, stem, and root Naq and Cly content increased with salinity in experiment 1. Leaf, stem, and root Kq and Kq:Naq decreased with salinity. Leaf Naq was maintained below that of stems or roots. Few toxicity symptoms were visible only on the plants receiving the highest salinity level in experiment 2, and no defoliation occurred throughout both experiments. Sequestration of Naq in woody organs and the resulting higher leaf Kq:Naq are primary mechanisms of moderate tolerance of black sapote seedlings to salinity. q 1998 Elsevier Science B.V. Keywords: Salinity; Diospyros digyna; Kq:Naq
) Corresponding author. Present address: Horticulture Department, Purdue University, 1165 Horticulture Building, West Lafayette, IN 47907, USA. 1 Present address: University of Guam, College of Agriculture and Life Sciences, Mangilao, Guam 96923.
0304-4238r98r$19.00 q 1998 Elsevier Science B.V. All rights reserved. PII S 0 3 0 4 - 4 2 3 8 Ž 9 7 . 0 0 0 9 5 - 2
104
M.V. Mickelbart, T.E. Marlerr Scientia Horticulturae 72 (1998) 103–110
1. Introduction Black sapote is a popular fruit species in some subtropical and tropical regions, especially in native lowland regions of Central America ŽMartin et al., 1987; Morton, 1987.. The black sapote tree is handsome, tolerant of a wide range of soils, and usually grows well with little or no cultural inputs ŽMorton, 1987.. Martin et al. Ž1987. believe that black sapote should be considered for further distribution and planting. Saline conditions are widespread and salinity has long been a threat to crop production for large portions of the world’s lands ŽChapman, 1975; Carter, 1975.. Salinity is becoming more of a problem in humid coastal areas due to greater salt water intrusion into the fresh ground water ŽZekri and Parsons, 1989.. Ogden et al. Ž1981. classified black sapote as fairly tolerant with respect to salinity. This classification was based on general horticultural observations, not on experimental results. Most fruit species from humid tropical and subtropical regions are sensitive to salinity stress. Thus, we conducted this study to verify the anecdotal ranking of fairly tolerant, because of the recommendation for further planting of the species, and because a database on any aspect of managing the species is lacking. The objectives were to determine quantitatively the effect of substrate salinity on growth, leaf gas exchange, and mineral relations of black sapote seedlings. 2. Materials and methods 2.1. Experiment 1 Nine-month-old seedlings of ‘Merida’ black sapote were bare-rooted and transplanted to silica sand in 5.1 l containers on 28 May 1990. Thirty-six plants were placed on raised benches in a glasshouse with maximum photosynthetic photon flux of ca. 1200 m mol my2 sy1 . The plants were watered daily, alternating rainwater and complete nutrient solution. The nutrient solution was made with 0.68 g ly1 Hydro-sol ŽW.R. Grace, Fogelsville, PA. and 0.45 g ly1 calcium nitrate, and diluted to 1 dS my1. This solution approximated one-half Hoagland solution ŽHoagland and Arnon, 1950.. Salinity treatments were applied beginning on 10 July . The salinity solutions were increased 1 dS my1 dayy1 until the ultimate level was reached to reduce the chance of osmotic shock. Treatments consisted of the nutrient solution as control, and 2, 4, 6, or 8 dS my1 made by the addition of sea salt to the control solution. The respective osmotic potentials of the solutions were y0.04, y0.08, y0.17, y0.25, or y0.34 MPa. The respective approximations of percentage ocean water were 2%, 5%, 9%, 14%, or 19%. Solutions were delivered from elevated tanks to dribble ring emitters via polyethylenetubing and micro-tubing. This delivery system ensured that the entire volume of substrate received the solution, and the solution was delivered in excess to ensure a minimum of 25% leachate. Thirty plants were arranged in a randomized complete block design with six replications per salinity level. The remaining six plants from the original population were used to estimate initial dry mass and leaf area of the experimental plants. Leaves were removed and the area measured ŽLI 3000 area meter, LI-COR, Lincoln, NE., then
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the roots were cleaned by gentle water pressure, and the entire plants were dried at 608C prior to weighing. Initial trunk diameter was measured on the experimental plants on 10 July 1990. Leaf water vapor exchange was determined with a steady state porometer ŽLI-1600, LI-COR. beginning on 9 July 1990 Žday 1., and every 2 to 14 days until 16 November 1990 Žday 129.. One young, fully-expanded leaf per plant was chosen at random for each measurement. On several days during the week prior to initiating the treatments, measurements were made throughout the photoperiod, and stomatal conductance was maximum during the middle portion of the photoperiod. As a result, measurements were made during midday throughout the study. Final growth measurements were made on 19 November 1990. Trunk diameter was measured, then leaves were removed and area was measured as previously described. Roots were cleaned with gentle water pressure, and all tissue was dried at 608C prior to weighing. Trunk diameter was converted to trunk cross-sectional area. Growth of the plants during the experimental period was calculated by subtracting the initial value for each variable from the final value. Tissue from the plants receiving salinity of 1, 4, or 8 dS my1 was retained after dry mass was determined. Tissue was consolidated into three replications of two trees each. Root, stem, and leaf tissue was milled to pass through a 60-mesh screen. Analysis of tissue Naq, Cly, and Kq was conducted according to Wolf Ž1982.. Mean high and low temperature during the experimental period was 338C and 238C, respectively, and mean high and low relative humidity was 91% and 58%, respectively, as determined by hygrothermograph ŽWeathertronics Model 5020, Qualimetrics, Sacramento, CA.. 2.2. Experiment 2 A second study was conducted in 1990 and 1991. Nine-month-old plants grown from seed of ‘Merida’ black sapote were transplanted to silica sand in 5.1 l containers on 24 October 1990 and watered daily with the control nutrient solution. Twenty-four plants were maintained under these conditions until the treatments were begun on 12 December 1990. Salinity levels consisted of 1, 12, or 16 dS my1 . The respective osmotic potentials of the solutions were y0.04, y0.51, or y0.69 MPa. The respective approximations of percentage ocean water were 2%, 28%, or 37%. Solutions were delivered as previously described. Eighteen plants were arranged in a randomized complete block design with six replications per salinity level. The remaining six plants from the group were used as in experiment 1 to estimate initial dry mass and leaf area of the experimental plants. Gas exchange measurements were begun on 13 December Žday 1., and were taken every 7 to 14 days until 11 April Žday 118.. Final growth measurements were conducted on 11 April 1991. Growing conditions during this study were 328C and 208C Žmean high and low temperature. and 91% and 50% Žmean high and low relative humidity.. 2.3. Statistics The growth variables, final gas exchange, and tissue mineral content data were subjected to analysis of variance. Significant comparisons were defined by determining
106
M.V. Mickelbart, T.E. Marlerr Scientia Horticulturae 72 (1998) 103–110
the linear and quadratic models with salinity level as the independent variable. The developmental response of gas exchange to salinity was calculated for each measurement day as a value relative to the mean of the control plants on that day. This allowed data from the two experiments to be combined for presentation. 3. Results 3.1. DeÕelopmental leaf gas exchange Leaf stomatal conductance Ž g s . of black sapote plants exposed to salinity began to decline relative to control plants after the initiation of salinity treatments ŽFig. 1.. The number of days until this decline started was inversely related to the salinity level, with more than 60 days required for the dose levels up to 4 dS my1 and as little as 30 days required for the 8, 12, and 16 dS my1 levels. Moreover, water vapor exchange declined to a lower minimum value for the plants exposed to the higher salinity levels when compared with plants exposed to the lower salinity levels. The pattern of g s and transpiration responses over time and to salinity level were similar, so transpiration data are not shown. 3.2. Growth and final gas exchange The three growth variables, leaf area, dry mass, and trunk cross-sectional area were reduced by salinity in a linear fashion ŽTables 1 and 2.. We observed no threshold in the
Fig. 1. Relative stomatal conductance of Diospyros digyna trees grown in sand culture as a function of time. Plants were irrigated with complete nutrient solution of 1 dS my1 ŽI. or with this solution amended with sea salt to 4 Ž'., 8 Žv ., or 16 ŽB. dS my1 . Data for plants exposed to 2, 6, or 12 dS my1 were not included in order to simplify the figure. Each point represents the mean relative to control of six replications"standard error. The overall mean for control plants was 248 mmol my2 sy1 .
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Table 1 The increase in leaf area, dry weight, and trunk cross-sectional area, and final stomatal conductance of D. digyna plants as influenced by 19 weeks of exposure to solutions of 1, 2, 4, 6, or 8 dS my1 Ž ns6. Salinity ŽdS my1 .
Leaf area Žcm2 .
Dry mass Žg.
Trunk cross-sectional area Žmm2 .
Stomatal conductance Žmmol my2 sy1 .
1 2 4 6 8 Significance
8024 6358 5932 5616 5372 L) )
212.6 169.9 165.9 124.4 139.0 L)
218 176 188 132 142 L) )
196 185 171 133 112 L) )
) ,) )
Linear models are significant at P F 0.05 or 0.01, respectively.
growth response pattern as influenced by salinity, rather, the lowest level of salinity Ž2 dS my1 . reduced growth 19% to 21% below that of the control plants ŽTable 1.. Growth of the plants exposed to 16 dS my1 relative to that of the control plants ranged from 31% for trunk cross-sectional area to 47% for dry mass accumulation ŽTable 2.. By the end of each experiment, leaf water vapor exchange was fairly stable within each salinity level ŽFig. 1.. Stomatal conductance was reduced to 57% of the control plants at 8 dS my1 ŽTable 1., and to 27% of the control plants at 16 dS my1 ŽTable 2.. No visible leaf burn symptoms of salinity stress appeared in any leaves except a few from the plants receiving 16 dS my1 . These few leaves exhibited marginal necrosis, and were located randomly throughout the canopy with no consistent pattern being observed with respect to leaf age or position within the canopy. Leaf abscission did not occur in any plants. 3.3. Mineral analysis Increased root zone salinity increased Naq and Cly concentrations in leaves, stems, and roots of black sapote ŽTable 3.. The absolute concentration of Cly was greater than that of Naq for all salinity levels and organs. However, salinity increased Naq concentration relative to that of control plants to a greater degree than for Cly. Stem tissue of salt-stressed plants increased in Naq and Cly concentration more than did leaf or root tissue relative to that of control plants. Concentration of Kq in all plant organs
Table 2 The increase in leaf area, dry weight, and trunk cross-sectional area, and final stomatal conductance of D. digyna plants as influenced by 17 weeks of exposure to 1, 12, or 16 dS my1 Ž ns6. Salinity ŽdS my1 .
Leaf area Žcm2 .
Dry mass Žg.
Trunk cross-sectional area Žmm2 .
Stomatal conductance Žmmol my2 sy1 .
1 12 16 Significance
7458 4395 2886 L) )
156.4 98.3 73.6 L) )
159 90 49 L) )
216 106 58 L) )
))
Linear models are significant at P F 0.01.
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Table 3 Influence of 19 weeks of salinity on Naq, Cly, and Kq, and the ratio Kq:Naq in root, stem, and leaf tissue of D. digyna plants. Data are means of three replications presented as percentage of dry weight, each consisting of tissue from two plants Salinity ŽdS my1 .
Naq
Cly
Kq
Kq:Naq
LeaÕes 1 4 8 Significance
0.04 0.16 0.38 L) )
1.29 3.69 5.86 L) )
1.27 1.20 0.97 L)
29.6 9.2 2.5 L) ) Q )
Stems 1 4 8 Significance
0.02 0.25 0.60 L) )
0.19 0.41 1.61 L) )
1.47 1.28 0.88 L) )
66.4 5.4 1.6 L) ) Q ) )
Roots 1 4 8 Significance
0.40 1.11 1.74 L) )
1.75 2.16 2.39 L) )
2.93 1.25 1.37 Q) )
7.8 1.1 0.8 L) Q )
) ,) )
Linear ŽL. or quadratic ŽQ. models are significant at P F 0.05 or 0.01, respectively.
was decreased by salinity. The marked increase of stem Naq concentration, a 30-fold increase at the 8 dS my1 level, resulted in stems exhibiting the largest relative decrease in the ratio Kq:Naq Ž2% for stem tissue compared with 10% for root tissue relative to that of the control plants.. Sodium and chloride elemental contents were calculated from absolute dry mass for each organ Ždata not shown. and the percentage data is presented in Table 3. These calculations revealed that stems of salt-stressed plants accumulated Naq to a greater degree than leaf or root tissue, and to a greater degree than Cly was accumulated in all three organs. Leaf Naq increased 8-fold, root Naq increased 3-fold, and stem Naq increased 24-fold for plants receiving 8 dS my1 as compared with the control plants. The relative increase in Cly for plants receiving 8 dS my1 was below four-fold for all three organs.
4. Discussion Visible toxicity symptoms of leaf burn and defoliation are common for salt-sensitive woody perennial species ŽMaas and Hoffman, 1977.. The black sapote plants in this study maintained a healthy appearance throughout both experiments, with almost no visible toxicity symptoms. Moreover, no defoliation occurred for any salt-stressed plants. The plants receiving 16 dS my1 , the highest level of salinity in this study, withstood 17 weeks of exposure and maintained reduced but sustained growth through-
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out the study. Biomass accumulation, determined by the increase in dry mass, was reduced by only 47% below that of the control plants. This appreciable maintenance of growth and the lack of defoliation and foliar toxicity symptoms indicate at least some level of salt tolerance for black sapote. The decrease in g s with longer exposure to salinity that was exhibited by black sapote plants in this study is common for many species exposed to high substrate salinity levels ŽPlaut et al., 1990; Warne et al., 1990; Zekri and Parsons, 1990.. This conservation of water within the plant may aid in decreasing translocation of salts to the leaves ŽGreenway and Munns, 1980., and may thus decrease leaf mortality ŽFlanagan and Jeffries, 1988.. Woody perennial species tend to accumulate toxic levels of Naq andror Cly when exposed to substrate salinity ŽMaas, 1994.. Adverse effects of salinity have been attributed to accumulation of either ion, for instance Naq for Citrus sinensis ŽL.. Osbeck ŽWalker et al., 1983. and Cly for a range of tree species ŽTownsend, 1980.. In this study with black sapote, the relative increase in leaf Naq Žcompared with that of control plants. was greater than the relative increase in leaf Cly for the salt-stressed plants. However, the plants allowed Cly transport freely into leaves, since leaves developed a higher concentration than stems or roots when the plants were exposed to salinity ŽTable 3.. In contrast, leaf Naq concentration was lower than stem or root Naq concentrations when the plants were exposed to salinity. This pattern has been reported for other woody perennial tree species ŽTownsend, 1980.. Thus, even though the relative increase in leaf Naq was more than the relative increase in leaf Cly, the salt-stressed plants exhibited stem accumulation of Naq which decreased leaf Naq relative to the other organs. Similarly, the initial increase in leaf Naq of salt-stressed AÕerrhoa carambola L. seedlings was slow relative to that of leaf Cly due to root accumulation of Naq ŽMarler and Zozor, 1994.. Moreover, while Naq did increase in leaves with increased salinity, the Kq:Naq ratio was always higher in leaves than in roots, which has also been noted in other tropical fruit species such as Casimiroa edulis La Llave and Lex. ŽNerd et al., 1992. and Persea americana Mill. ŽMickelbart, unpublished data.. The maintenance of high Kq:Naq ratios may alleviate Naq toxicity in leaves of black sapote.
5. Conclusion Increased substrate salt resulted in decreased water vapor exchange and growth of black sapote plants. Although measured variables decreased, growth continued and there were few visible toxicity symptoms in leaves after 17 to 19 weeks of salinity exposure. There were isolated cases of necrotic leaf symptoms on plants receiving 16 dS my1 , but no defoliation was observed. Leaf Naq was below that of stems or roots for plants receiving 4 or 8 dS my1 , but leaf Cly increased above that of stems or roots. Therefore, Naq is at least partially sequestered in woody tissues of black sapote, resulting in few visible signs of salt toxicity in foliage. The results indicate a moderate tolerance of black sapote to salinity primarily through the sequestration of Naq in woody organs and the maintenance of favorable Kq:Naq ratios.
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