Scientia Horticulturae, 20 (1983) 267--273 267 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. Trans- piration 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 condit ions is that of stomatal closure. Citrus is among the plants which can close their s tomata completely and have highly cutinized leaves with low cuticular conductance (Turner, 1979). Leaf water potential ($ leaf} of citrus can increase when s tomata close due to changes in climatic condit ions (Levy, 1980a; Levy and Syvertsen, 1983), or t reatment with growth regulators (Levy et al., 1979). Short-term after- effects of water stress on stomatal behavior are well documented; usually they involve changes in stomatal response to leaf water deficits (Fischer, 1970; Thomas et al., 1976) or to humidi ty 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 not 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 s tomata to remain open at a lower $1eaf and thus allows the plant to

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268

extract moisture from the soil at a lower ~ soil {Brown et al., 1976; Simmels- gaard, 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) sub- jected citrus trees to very severe water stress which caused most leaves to drop. Leaves that did not drop reached a maximum (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 ap- parently survive drought, although not wi thout 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-environ- ment greenhouse in 1-1 plastic pots with sandy loam. All seedlings were drip-irrigated daily with a commercial nutr ient 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 gravi- metrically 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 porom- eter (LI-COR Co., Nebraska) following precautions recommended by Elfving et al. (1972) and by Stigter and Visscher (1975). No water vapour diffu- sion could be detected when the porometer 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 cont inuously to assure that the s tomata were not oscillating (Levy and Kaufmann, 1976). These data, along with wet- and dry-bulb temperatures (Assmann psychrometer) , were used to calculate the absolute humidi ty difference from leaf to air (AH), assuming that the air in the leaf was saturated with water. The product of AH and gleaf was used to calculate transpirational flux density. Daily mean transpiration rate (g cm -2 s -1) was calculated from daily po t weight-loss and total one- leaf-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 mea- sured with a pressure chamber, and free proline accumulation, as an in- tegrated 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 porometer readings expressed in comparable units (Fig. 1). The intercept of abou 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 t reatments recovered or even surpassed the pre-stress rate after the first stress period (Fig. 2). This is

s imilar 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 non- stressed controls even after 20 days, and the reduction in transpiration ap- peared to be proportional to the amount of water stress that the plants received, as the moderate ly stressed plants were intermediate between the severely stressed and non-stressed control plants. The ~leaf of moderate ly stressed plants was significantly increased, but 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 pro- portional to the severity of water stress and not to ~ leaf (Fig. 4).

Elfving et al. (1972) rearranged the Van den Honert (1948) equation

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Fig . 1. R e l a t i o n s h i p b e t w e e n t r a n s p i r a t i o n rate e s t i m a t e d f r o m p o r o m e t e r r e a d i n g a n d m e a n da i l y t r a n s p i r a t i o n rate ( c a l c u l a t e d for c o m p a r i s o n o n a 12 -h d a y - l e n g t h bas i s ) . Y = 0 . 2 5 5 + 0 . 8 8 x ; r = 0 . 9 0 3 ( s o l i d r e g r e s s i o n l ine ) ; Q = c o n t r o l ; a = m o d e r a t e s tress ; • = s e v e r e s tress . B r o k e n l ine is 1 :1 rat io .

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Fig . 2. D a i l y t r a n s p i r a t i o n rate ( m e a s u r e d g r a v i m e t r i c a l l y ) , dur i ng the pre-s tress treat- m e n t a n d r e c o v e r y f r o m s tres s ; a r r o w i n d i c a t e s e n d o f water - s t re s s t r e a t m e n t s . S E is t h e s t a n d a r d error o f t h e d i f f e r e n c e b e t w e e n t r e a t m e n t s , d u r i n g t h e p e r i o d o f r e c o v e r y f r o m s tress , a = c o n t r o l ; ~ = m o d e r a t e s tress ; • = s e v e r e s tress .

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300

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Fig. 4. F r e e p r o l i n e a c c u m u l a t i o n a n d d i s a p p e a r a n c e in leaves . A r r o w d e n o t e s t i m e o f r e - w a t e r i n g . SE is t h e s t a n d a r d e r r o r o f t h e d i f f e r e n c e b e t w e e n t h e t r e a t m e n t s . A = m o d - e r a t e s t r e s s ; • = s eve re s t r e s s .

272

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

(rsoil-to-leaf)- ~leaf = ~soil - - F × (rsoil . to.leaf) (1 )

Ramos and Kaufmann {1979) found 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 roo 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 s tomata directly, and that under such con- ditions ~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 not increase root resistance and affected only stomata. Consequently, citrus trees which, under common 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 root 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 amount of water stress may en- able bet ter 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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