Acclimation of citrus to water stress

Scientia Horticulturae, 20 (1983) 267--273

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Elsevier Science Publishers B.V., Amsterdam -- Printed in The Netherlands

ACCLIMATION OF CITRUS TO WATER STRESS

YOSEPH LEVY

Agricultural Research Organization, Gilat Regional Experiment Station, Mobile Post Negev, 85-280 (Israel) Contribution No. 198-E from the ARO, The Volcani Center, Bet Dagan, Israel (Accepted for publication 8 November 1982)

ABSTRACT Levy, Y., 1983. Acclimation of citrus to water stress. Scientia Hortic., 20: 267--273. Alemow (Citrus macrophylla Wester) seedlings were subjected to moderate or severe water stress by watering them at different intervals for several irrigation cycles. Transpiration rate was measured after irrigation was resumed. Severe water stress reduced transpiration but increased leaf water potential (~leaf), while moderate water stress reduced transpiration less and did not affect ~leaf" This suggests that moderate water stress influences only stomatal conductance and not root and shoot resistance. Keywords: alemow; Citrus macrophyUa Wester; irrigation; leaf water potential; stomatal conductance; transpiration; water stress. INTRODUCTION

It has long been recognized that plants adapt to their environment, and especially to any lack of water. The main physiological mechanism by which plants limit water-loss under drought conditions is that of stomatal closure. Citrus is among the plants which can close their stomata completely and have highly cutinized leaves with low cuticular conductance (Turner, 1979). Leaf water potential ($ leaf} of citrus can increase when stomata close due to changes in climatic conditions (Levy, 1980a; Levy and Syvertsen, 1983), or treatment with growth regulators (Levy et al., 1979). Short-term aftereffects of water stress on stomatal behavior are well d o c u m e n t e d ; usually they involve changes in stomatal response to leaf water deficits (Fischer, 1970; Thomas et al., 1976) or to humidity and temperature (Kaufmann and Levy, 1976). The after-effect of water stress on stomatal conductance can usually be detected for 1--5 days after plants are re-watered and its cause, when n o t due to lingering leaf water deficit (Fischer, 1970), remains unclear. However, for some plants there are reports of what was termed by Fischer et al. (1970) as "apparent over-recovery of stomatal opening" after a period of water stress. Drought hardening in such plants (Levitt, 1980) is apparently due to pre-conditioning moisture stress, which causes the stomata to remain open at a lower $1eaf and thus allows the plant to 0304-4238/83/$03.00

© 1983 Elsevier Science Publishers B.V.

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extract moisture from the soil at a lower ~ soil {Brown et al., 1976; Simmelsgaard, 1976). Water stress can reduce yields of citrus, especially in cultivars with high yield potential, such as grapefruit (Levy et al., 1978). This may be due to lower rates of photosynthesis caused by lower leaf diffusive conductance to water vapor and CO2 (gleaf) (Elfving et al., 1972; Kaufmann and Levy, 1976; Levy et al., 1978; Syvertsen et al., 1981). Fereres et al. (1979) subjected citrus trees to very severe water stress which caused most leaves to drop. Leaves that did not drop reached a m a x i m u m (pre-dawn) ~leaf of --62 bars. These very severely stressed, surviving leaves still suffered from the after-effect of water stress 60 days following resumption of irrigation. Such extremely low ~ leaf values are similar to, or even lower than, those observed in xerophytic plants (Bunce et al., 1979). Citrus leaves can apparently survive drought, although not without physiological damage. Such extreme drought conditions, however, are not encountered in groves that receive proper horticultural care, and will not be discussed in this work. Modern irrigation methods, such as drip or micro-sprinkler systems, allow the maintenance of citrus trees without significant water deficit by the application of very frequent irrigations, sometimes even daily. This paper reports studies of the acclimation potential of frequently watered citrus seedlings to several short-term, successive drought cycles, and their recovery when returned to a frequent irrigation schedule. MATERIALS AND METHODS

Citrus macrophylla (Wester) seedlings were raised in a controlled-environm e n t greenhouse in 1-1 plastic pots with sandy loam. All seedlings were drip-irrigated daily with a commercial nutrient solution of a 20--20--20 (N:P:K) fertilizer which also contained micro-elements. Pots were leached with an excess of tap water once a week to prevent salt accumulation. Experiments were performed on 10-month-old seedlings in the same controUed~environment greenhouse under 30% shade, and day and night temperatures of 30--32°C and 18--20°C, respectively. Plants were divided into 6 experimental blocks, stratified according to their size. The pots were enclosed in polyethylene bags, total evapotranspiration was recorded gravimetrically dally, and deionized water was added as needed to replace losses due to transpiration. Three irrigation regimes were maintained: (i) no water stress, daily watering, identical to the drip-irrigation the seedlings had received before; (ii) moderate water stress, watering every 2--3 days; (iii) severe water stress, watering every 4--5 days, after severe wilting was noted. New growth was prevented by pinching developing buds daily. After 24 days of differential irrigation treatments, plants were watered twice with a fertilizer solution, allowed to drain, enclosed again in polyethylene bags, and irrigated daily according to their evapotranspiration. Leaf diffusive conductance to water vapor (gleaf) was measured at mid-

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day on the abaxial side of the leaf with a modified LI-20 S diffusion porometer (LI-COR Co., Nebraska) following precautions r e c o m m e n d e d by Elfving et al. (1972) and by Stigter and Visscher (1975). No water vapour diffusion could be detected when the p o r o m e t e r was placed on the adaxial side of C. macrophylla leaves, as was also reported for C. limon leaves (Levy, 1980a). Leaf-to-air temperature difference, measured with a T-shaped thermocouple, was recorded continuously to assure that the stomata were not oscillating (Levy and Kaufmann, 1976). These data, along with wetand dry-bulb temperatures (Assmann psychrometer), were used to calculate the absolute humidity difference from leaf to air (AH), assuming that the air in the leaf was saturated with water. The p r o d u c t of A H and gleaf was used to calculate transpirational flux density. Daily mean transpiration rate (g cm -2 s -1) was calculated from daily p o t weight-loss and total oneleaf-side surface area of the whole plant, assuming a 12-h light period. Leaf width and length were measured at the beginning and at the end of the experiment and did not change significantly. Leaf area was measured at the termination of the experiment with a Model 3000 leaf-area-meter (LI-COR Co., Nebraska) equipped with a transparent belt conveyor. ~leaf was measured with a pressure chamber, and free proline accumulation, as an integrated indication of plant water stress (Levy, 1980b), was measured in the same leaves. RESULTS AND DISCUSSION

Daily whole plant transpiration rates correlated very well (r = 0.903) with transpiration rates estimated from p o r o m e t e r readings expressed in comparable units (Fig. 1). The intercept of a b o u t 0.26 pg cm -2 s -~ represents the estimated transpiration when gleaf = 0, and may indicate the rate of water loss from green stems. The transpiration rate of both water-stress treatments recovered or even surpassed the pre-stress rate after the first stress period (Fig. 2). This is similar to the response described by Fischer (1970) and by Fischer et al. (1970). Subsequent water stress periods resulted in reduced transpiration rates during the recovery period. When daffy irrigation was resumed, the transpiration rate of pres-stressed plants remained below that of the nonstressed controls even after 20 days, and the reduction in transpiration appeared to be proportional to the a m o u n t of water stress that the plants received, as the moderately stressed plants were intermediate between the severely stressed and non-stressed control plants. The ~leaf of moderately stressed plants was significantly increased, b u t the ~ leaf of severely stressed plants was either similar to that of control plants or even less negative than control (Fig. 3). Proline accumulation, on the other hand, was proportional to the severity of water stress and n o t to ~ leaf (Fig. 4). Elfving et al. (1972) rearranged the Van den Honert (1948) equation

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as a function of Csoil, flux (F) and resistance to flow in the liquid phase

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Ramos and Kaufmann {1979) f o u n d that water stress increased the hydraulic resistance of citrus roots, and this has been confirmed by Levy et al. (1983). According to eqn. (1), as r increased in pre-stressed plants, ¢ leaf should have decreased if everything else was constant. However, only a large reduction in r o o t resistance can change the total flux (Weatherly, 1976), and data in Fig. 3 indicate that this may be true only for severely stressed plants. The fact that ~ leaf was actually increased after plants were moderately stressed indicates that the increase in rsoil.to.leaf of such plants is related mainly to processes in the leaves, affecting stomata directly, and that u n d e r such conditions ~leaf is mainly a function of gleaf and not vice versa (Levy, 1980a). These processes may play an important role in drought-hardening of citrus. The fact that a moderate stress reduced transpiration and caused a higher ~leaf for a period of at least 20 days indicates that such moderate stress did n o t increase root resistance and affected only stomata. Consequently, citrus trees which, under c o m m o n cultural practices, are exposed to periods of moderate water stress between irrigations may transpire at a lower rate than trees which are kept constantly well-watered. A severe water stress, however, may cause increased r o o t resistance, possibly due to irreversible damage to the roots, as described by Ramos and Kaufmann (1979), Fereres et al. (1979) and Levy et al. (1983), and impair the tree's ability to extract soil moisture and withstand water stress. The possibility of manipulating transpiration rates through control of the a m o u n t of water stress may enable better control of tree water balance and water-use efficiency of citrus trees. ACKNOWLEDGEMENTS

The author is grateful to Dr. J.P. Syvertsen of the University of Florida for his helpful criticism of the manuscript, and for the skilled technical assistance of Ruth Regev.

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273 Fereres, E., Cruz-Romero, G., Hoffman, G.J. and Rawlins, S.L., 1979. Recovery of orange trees following severe water stress. J. Appl. Ecol., 16: 833--842. Fischer, R.A., 1970. After-effect of water stress on stomatal opening potential. II. Possible causes. J. Exp. Bot., 21: 386--404. Fischer, R.A., Hsiao, T.C. and Hagan, R.M., 1970. After-effect of water stress on stomatal opening potential. I. Techniques and magnitudes. J. Exp. Bot., 21: 371--385. Honert, T.H. van den, 1948. Water transport in plants as a catenary process. Discuss. Faraday Soc., 3: 146--161. Kaufmann, M.R. and Levy, Y., 1976. Stomatal response of Citrus jambhiri to water stress and humidity. Physiol. Plant., 38: 105--108. Levitt, J., 1980. Responses of Plants to Environmental Stress, Vol. 2. Academic Press, New York, 606 pp. Levy, Y., 1980a. Effect of evaporative demand on water relations of Citrus limon. Ann. Bot., 46: 695--700. Levy, Y., 1980b. Field determination of free proline accumulation and water-stress in lemon trees. HortScience, 15: 302--303. Levy, Y. and Kaufmann, M.R., 1976. Cycling of leaf conductance in citrus exposed to natural and controlled environments. Can. J. Bot., 54: 2215--2218. Levy, Y. and Syvertsen, J.P., 1983. Water relations of citrus in climates with different evaporative demand. Proc. Int. Soc. Citriculture, Tokyo, 1981, in press. Levy, Y., Bielorai, H. and Shalhevet, J., 1978. Long-term effects of different irrigation regimes on grapefruit tree development and yield. J. Am. Soc. Hortic. Sci., 103: 680---683. Levy, Y., Greenberg, J. and Ben-Anat, S., 1979. Effect of ethylene-releasing compounds on oleocellosis in 'Washington' navel oranges. Scientia Hortic., 11: 61--68. Levy, Y., Syvertsen, J.P. and Nemec, S., 1983. Effect of drought stress and vesicular--arbuscular mycorrhiza on citrus transpiration and hydraulic conductivity of roots. New Phytol., 93: 61--66. Ramos, C. and Kaufmann, M.R., 1979. Hydraulic resistance of rough lemon roots. Physiol. Plant., 45: 311--314. Simmelsgaard, S.E., 1976. Adaptation to water stress in wheat. Physiol. Plant., 37: 167--174. Stigter, C.J. and Visscher, C.J.W., 1975. Application of a new calibration method to an unventilated dynamic diffusion porometer. Neth. J. Agric. Sci., 23: 303--307. Syvertsen, J.P., Smith, M.L., Jr. and Allen, J.C., 1981. Growth rate and water relations of citrus leaf flushes. Ann. Bot., 47: 97--105. Thomas, J.C., Brown, K.W. and Jordan, W.R., 1976. Stomatal response to leaf water potential as affected by preconditioning water stress in the field. Agron. J., 68: 706-708. Turner, N.C., 1979. Drought resistance and adaptation to water deficits in crop plants. In: H. Mussell and R.C. Staples (Editors), Stress Physiology in Crop Plants. Wiley, New York, pp. 343--372. Weatherly, P.E., 1976. Introduction: Water movement through plants. Trans. R. Soc., London, Sect. B, 273: 435--444.