Journal of Experimental Botany, Vol. 54, No. 381, pp. 365-373,
January 2, 2003
© 2003 Oxford University Press
Increased growth of young citrus trees under reduced radiation load in a semi-arid climate1
Received 26 November 2001; Accepted 6 August 2002
2 Institute of Horticulture, ARO Volcani Center, POB 6, Bet Dagan 50250, Israel
3 Department of Environmental Physics and Irrigation, Institute of Soil, Water and Environmental Sciences, ARO Volcani Center, POB 6, Bet Dagan 50250, Israel
4 The Institute of Plant Sciences and Genetics in Agriculture, The Hebrew University of Jerusalem, Rehovot 76100, Israel
5 Department of Environmental Sciences, Weizmann Inst. of Science, Rehovot 76100, Israel
1 Contribution from the Institute of Horticulture, Agricultural Research Organization, The Volcani Center, Bet Dagan, Israel, No. 208/01.
6 To whom correspondence should be addressed. Fax: +972 3 9604017. E-mail: vwshep{at}agri.gov.il
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Abstract |
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This study investigated the effects of radiation heat-load reduction by shading on the growth and development of citrus trees in a warm subtropical region. The experiment was conducted from mid-June until late October when daily maximal air temperature averaged 29.3 °C. Two-year-old de-fruited Murcott tangor (Citrus reticulata BlancoxCitrus sinensis (L.) Osb.) trees were grown under 30% or 60% shade tunnels, or 60% flat shade (providing midday shade only), using highly reflective aluminized nets. Non-shaded trees were used as the control. Shading reduced direct more than diffuse radiation. Daily radiation was reduced by 35% for the 30% Tunnel and 60% Flat treatments, and by 55% for the 60% Tunnel. Two days of intensive measurement showed that shading increased average sunlit leaf conductance by 44% and photosynthesis by 29%. Shading did not significantly influence root and stem dry weight growth, but it increased the increment in leaf dry weight during the three month period by an average of 28% relative to the control, while final tree height in the 30% Tunnel treatment exceeded the control by 35%. Shoot to root and shoot mass ratios increased and root mass ratio decreased due to shading because of the increase in leaf dry weight. Shading increased starch concentration in leaves while the shadiest treatment, 60% Tunnel, decreased starch concentration in the roots. Carbon isotope ratio (
13C) of exposed leaves that developed under shading was significantly reduced by 1.9
in the 60% Tunnel, indicating that shading increased CO2 concentrations at the chloroplasts (Cc), as would be expected from increased conductance. Substomatal CO2 concentrations, Ci, computed from leaf net CO2 assimilation rate and conductance values, also indicate that shading increases internal CO2 concentrations. Based on tree dry mass, tree height, and total carbohydrates fractions, the 30% Tunnel and the 60% Flat were the optimal shade treatments.
Key words: 13C, carbohydrate allocation, carbon isotopes, leaf conductance, partitioning, photosynthesis, shade, screen, water relations.
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Introduction |
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Citrus, an important crop worldwide can be grown in various climatic conditions ranging from hot-humid equatorial climates through warm-subtropical and even cooler maritime climates (Spiegel-Roy and Goldschmidt, 1996). Its distribution is limited by its sensitivity to low temperatures (Bowman, 1956; Spiegel-Roy and Goldschmidt, 1996), and not by high temperatures. Nevertheless, it is well known that exposing citrus trees to high temperatures can lead to reductions in net CO2 assimilation rate as well as abscission of young fruit. Such is the case in warm-subtropical regions where the average maximal air temperatures during the summer are about 31 °C and leaf temperatures can reach 39–41 °C (8–10 °C above air temperature; Syvertsen and Lloyd, 1994). This range of temperatures is far above the temperature optimum for net CO2 assimilation rate (25–30 °C; Spiegel-Roy and Goldschmidt, 1996), and might be associated with stomatal closure and reductions in net CO2 assimilation rate, growth and yield.
Solar radiation levels common to semi-arid climates are sometimes high enough to cause photo-inhibition of photosynthesis. Measurements of chlorophyll fluorescence in citrus at radiation levels equivalent to those outdoors have not shown significant amounts of photo-inhibition (Gussakovsky et al., 1993), and changes in chlorophyll fluorescence during the day are reversible, since they recover by the following morning (Blanke, 2002; Gussakovsky and Shahak, 1995). There is, however, evidence that extensive long-term exposure to high radiation levels can lead to chlorophyll degradation and reductions in photosynthesis (Blanke, 2002).
In clear sky conditions solar irradiance is the main source of heat in crop systems. High radiation loads can be reduced by shading with screens, leading to reduced crop temperatures. In addition, the use of reflective screens can scatter some of the direct radiation intercepted by the screen downward toward the crop as diffuse radiation. The resulting modified radiation climate will affect the crop in several ways. Reduced leaf temperatures will bring net CO2 assimilation rate closer to its optimal temperature range, leaf to air vapour pressure difference will decrease, leading to increased leaf conductance at least at midday, and increased scattering of solar radiation will increase the radiation use efficiency as shown in theoretical analyses (Sinclair et al., 1992; Hammer and Wright, 1994). On the other hand, if the reduction in radiation is excessive, net CO2 assimilation rate will become light-limited and photosynthetic productivity will be reduced.
A recent study of shading lemon trees in a semi-arid climate with woven aluminized-plastic nets showed that a large increase in leaf conductance occurs as a result of a 30% reduction in radiation load (Cohen et al., 1997). The increased conductance was correlated with significant reductions in vapour pressure deficit (VPD), while water loss was only slightly affected. These responses, which were predicted by Syvertsen and Lloyd (1994), should also be followed by an increase in net CO2 assimilation rate and yield (Bustan et al., 1996; Cohen et al., 1997). Increases in productivity when radiation loads are moderately reduced have been reported for several crops and conditions, while for others productivity decreases when radiation is reduced to any degree (see Stanhill and Cohen, 2001, for a review; Bravdo, 1986; Raveh et al., 1998).
The effects of radiation reduction on plant growth and development are also dependent on plant growth stage. In some stages, plant development is limited by source (i.e. carbohydrate) availability, but in other stages development is limited by plant regulatory factors. Evidence that juvenile, rapidly growing citrus trees are source limited comes from trials with CO2 enrichment, where productivity increased along with photosynthetic rates (Idso and Kimball, 1991, 1992). So for young trees a reduction of radiative heat load by shading might lead to increases in net CO2 assimilation rate, total carbon, nitrogen, carbohydrate content, and overall growth parameters.
Shading may also lead to changes in plant morphology. In particular, shade plants may have greater leaf area than sun plants. In addition to influences on productivity, it is also important to document changes in allocation of photosynthates that occur following shading.
The objective of this study was to explore the possibility of increasing vegetative growth of rapidly growing, young, non-fruiting citrus trees by reducing radiative heat load (i.e. shading) in warm-subtropical regions. The possibility of increasing fruit yield of mature orchards by shading was explored in another experiment, which will be reported in the future. The intention here was to establish that the main response to shading is an increase in midday leaf conductance (i.e. reduction of ‘midday depression’), which increases net CO2 assimilation rate. There was particular interest in seeing if this overrode the decrease in net CO2 assimilation rate normally caused directly by reduced radiation. The experiment was conducted under Mediterranean conditions with defruited Murcott tangor (Citrus reticulata BlancoxCitrus sinensis (L.) Osb.) trees, where shading was applied during the hottest months of the year.
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Materials and methods |
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Shading trials were conducted during the summer (June through October) of 1999 in Rehovot, in Israel’s central coastal plain. Two-year-old Murcott tangor trees were grown in sandy soil in 10 l pots in the screenhouse nursery of Hebrew University’s Faculty of Agriculture. Trees were transferred to 28 l pails in mid-May. Of 40 trees, 35 were selected based on uniform appearance, these were divided randomly into five groups, and the few fruits on the juvenile trees were removed. Three screened structures were constructed in an adjoining open lot: two flat-topped tunnels for full day shade, and a third, with a flat roof screen for midday shade, all approximately 2.3 m high and 1 m wide, while tree canopy width did not exceed 0.8 m. The length of the structures was oriented North–South and trees were arranged in a single row in each structure. On 15 June four groups of trees were moved under the shading structures or to the open, non-shaded ‘control’ area. The fifth group of trees, which indicated the initial conditions of the trees, was dissected and stored in the freezer for analysis at the end of the experiment. Trees were watered and fertilized on a daily basis until drainage. Once a week, in order to leach the soil, the trees were supplied with water only. Trees were harvested 4 months later, during the second half of October.
Due to a number of constraints, it was not possible to replicate the experiment by building additional shade structures, or to run the experiment for a second year. So although the individual trees were treated as statistical replicates, differences that might have been detected by a more complete experimental design were not tested.
Shade treatments and radiation measurements
All shading was with woven aluminized-plastic nets (Aluminet, Polisak Ltd., Kibbutz Nir-Yitzchak, Israel). Radiometric properties of the screens have been reported elsewhere (Cohen and Fuchs, 1999). Additional measurements with a spectroradiometer (not reported) indicated that transmittance is spectrally neutral, i.e. wavelengths in the visible and near infrared spectrums (300–1100 nm) are not transmitted preferentially. The shade nets were hung horizontally above the rows (using 60% shade net; 60% Flat), or both from above and on the sides of the row, using 60% or 30% shade net: 60% Tunnel and 30% Tunnel, respectively). Global radiation was measured on several days after trees were removed from the structures with a thermopile pyranometer (type CM10, Kipp and Zonen, Delft, The Netherlands) and quantum meters (LI190, Li-Cor, Lincoln NE, USA), logged with an automatic data logger (Campbell Sci., Logan UT, USA).
Physiological parameters
Daily courses of leaf conductance and net CO2 assimilation rate were measured with a LI-1600 steady-state porometer and a LI-6200 photosynthesis system (Li-Cor Inc., Lincoln, NE, USA) 2, 6, and 8 weeks after commencement of shading. Conductance measurements were made hourly on two exposed (‘sunlit’) and one shaded mature leaves from each of the three middle trees in each treatment (i.e. nine leaves per treatment). Shaded leaves were selected from the outer part of the canopy, where they were exposed to much of the sky. Net CO2 assimilation rate measurements were made at 1 h intervals on two sunlit leaves from each of the five middle trees in each of the treatments. Measurements of midday assimilation rates of some shade leaves were made on one of the dates.
Six of the seven trees in each treatment were harvested late in October, dissected to leaves, stems (including the main stem and branches) and roots, and analysed for fresh and dry mass. Dry mass was determined after drying the tissues at 70 °C for 7 d. All leaves, stems and roots were dried and then ground. For carbohydrate analysis, 200 mg of the ground tissue (leaves, stems or roots) was extracted five times in 80% ethanol. After evaporation, glucose and sucrose were measured on the 80% ethanol-soluble fraction. Glucose was measured using dinitrosalicylic acid, according to the method of Miller (1959). Sucrose was measured using anthrone according to Van Handel (1968). Starch was measured in the insoluble fraction with anthrone after amyloglucosidase treatment, as described by Schaffer et al. (1985).
Carbon isotope ratios (13C) were measured by mass spectrometry as described by Israeli et al. (1996). Analyses were conducted on the dried leaf material of new growth leaves harvested at the end of the experiment. These leaves developed under the shading regimes. Twelve samples per treatment were finely ground and 2 mg subsamples were combusted in a CHN analyser (Carlo Erba 1108, Milan, Italy). CO2 produced in the combustion was used for mass spectrometric determination of 13C. Carbon dioxide was quantitatively separated on a gas chromatograph and measured, on-line, with a Micromass, Optima (UK) isotope ratio mass spectrometer, and referenced to PDB.
Morphological parameters and statistics
Stem circumference of all plants was measured at the beginning and end of shading. Stem cross-sectional area was calculated assuming that the stem cross-section was circular. Data were analysed by analysis of variance (ANOVA) and Duncan’s multiple range test (DMRT) at 
=0.05 using SAS.
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Results |
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Rehovot’s Mediterranean climate is characterized by rainless summers (May through October), with predominantly clear skies, especially in June, July and August. This is reflected in the high average maximum radiation values (Table 1) given together with other climate data for the experimental period. Except for the daily total global irradiance, day-to-day variations were small. The variations in daily irradiance reflect the decrease in day length and solar height during this season (a daily decrease of 0.1 MJ m–2 d–1 in total global irradiance). The effects of the different shading treatments on sunlit and shaded leaf irradiance (exposed and shaded leaf irradiance as measured with the porometer’s quantum sensor) are given in Fig. 1. All shade treatments reduced sunlit leaf-irradiance (Fig. 1A). For the 60% Flat treatment, however, the reduction was only during midday (approximately 10.00 h to 15.00 h). Total daily reduction in irradiance was about 35% for the 30% Tunnel and 60% Flat treatments, and 55% for the 60% Tunnel treatment. Average irradiance of shaded leaves, composed mostly of diffuse radiation, was similar for all treatments, and much lower than sunlit-leaf irradiance (Fig. 1B).
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The daily course of leaf conductance on a clear day (28 July) is shown in Fig. 2. Average conductance for sunlit leaves (Fig. 2A) was lowest for the control and highest for the 60% Tunnel treatment (92 and 155 mmol m–2 s–1, respectively; P <0.05). For the 30% Tunnel and 60% Flat treatments average conductance was 121 mmol m–2 s–1. Maximal leaf conductance was also lowest for the control and highest for the 60% Tunnel treatment (130 and 210 mmol m–2 s–1, respectively; P <0.05). For the 30% Tunnel and 60% Flat treatments maximal sunlit leaf conductances were 150 and 190 mmol m–2 s–1, respectively. For the middle of the day, 10.00 h to 14.00 h, sunlit leaf conductance of all shade treatments greatly exceeded that of the control (p <0.01). Late in the morning (from 09.00 h to 11.00 h) shade leaf conductance for the 60% Tunnel and 60% Flat treatments exceeded that of the control and 30% Tunnel treatments. But from midday onwards, shade-leaf conductance (Fig. 2B) was similar for all treatments and averaged 114 mmol m–2 s–1.
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The daily course of net CO2 uptake for sunlit leaves on a clear day (28 July) is shown in Fig. 3. For all treatments, net CO2 uptake increased during the morning and decreased during late afternoon. For the control, a sharp decrease in uptake occurred at noon. Average uptake was lowest for the control and highest for the 60% Tunnel treatment (2.9 and 4.2 µmol m–2 s–1, respectively; P <0.05). For both the 30% Tunnel and 60% Flat treatments average net CO2 uptake was 3.5 mmol m–2 s–1. Maximal uptake rate and integrated daily net CO2 uptake were also lowest for the control and highest for the 60% Tunnel treatment (4.0 versus 7.1 µmol m–2 s–1, and 96 versus 137 mmol m–2 d–1, respectively; P <0.05). For the 30% Tunnel and 60% Flat treatments maximal uptake rate and integrated daily net CO2 uptake averaged 5.1 µmol m–2 s–1 and 113 mmol m–2 d–1, respectively. Measurements made on other dates (30 June and 8 August) showed the same pattern of maximum values for the 60% Tunnel treatment and minimum for control. Shade leaves were not measured every hour, but midday measurements of shade leaves on 28 July gave very low values, 0.5–1.0 µmol m–2 s–1, and no differences were found between treatments.
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Initial and final leaf, branch and root dry weights for the different treatments are given in Table 2, and tissue mass ratios (i.e. leaf, stem and root mass ratios; LMR, SMR and RMR, respectively) are given in Fig. 4. In all cases total dry mass more than doubled during the 3 months, while that for leaves increased by factors of 3.8 to 4.9. This demonstrates the rapid vegetative growth of the young trees. Final LMRs were about 25%, SMRs
50%, and RMRs 25%. By contrast, for the initial plants LMR was 12% and RMR 31%. In all cases tissue mass ratio changed during the 3 months, except in the case of RMR, where for the control plants there was no significant change. Final leaf dry weight was higher for all shade treatments than for the control (P <0.01), but LMR was only significantly higher than the control in the 30% Tunnel treatment, and the other shading treatments had values intermediate between these. Differences in stem and root final mass (Table 2) were not statistically significant. RMR (Fig. 4c) was significantly less in the 60% Tunnel treatment than in the control, and the values for the less intense shading treatments were intermediate and significantly less than the initial plants.
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The increase in growth caused by shading was mainly due to an increase in above-ground (leaf and stem) dry weight, which changed the shoot to root ratio, i.e. the ratio of above-ground dry mass to root dry mass (Fig. 5). Shoot to root ratio at the end of the experiment was lowest for the control and highest for the 60% Tunnel treatment (36% above the control; P <0.05). Shoot to root ratios for the 30% Tunnel and 60% Flat treatments were intermediate between these, demonstrating the sensitivity of these ratios to the degree of shading.
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The increases in shoot growth in the shade treatments were partly reflected in the effect on tree height (Fig. 6A). At the end of the experiment the 30% Tunnel trees were tallest (34% taller than control; P <0.05). Differences in stem cross-sectional area (Fig. 6B) were not significant.
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The effects of shading on leaf, stem and root non-structural carbohydrate concentrations, on a dry weight basis, are given in Fig. 7. During the experimental period carbohydrate concentrations increased for all treatments, including the control. Reducing sugars (glucose) were the dominant carbohydrates in the leaves (Fig. 7A), starch was the dominant carbohydrate in the stems (Fig. 7B), while both reducing sugars and starch were prominent in the roots (Fig. 7C). Shading increased leaf carbohydrate concentrations (P <0.05), while root carbohydrate concentration was significantly reduced in the 60% Tunnel (P <0.05). Although carbohydrate concentrations were highest in leaves (P <0.05), stems constituted the main carbohydrate storage organs. The average final total non-structural carbohydrate content was 20.0, 24.9, and 11.4 g for leaves, stems, and root, respectively, calculated by multiplying the carbohydrate concentrations from Fig. 7 by the dry weight values from Table 2.
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Further analysis of the carbohydrate concentrations shown in Fig. 7 gives the increments in carbohydrate concentration over the 3 months (i.e. final concentration less initial concentration), expressed relative to the increments found in the control (Fig. 8). Values less than 100% indicate that the increase in carbohydrate concentration was less than that of the control. Carbohydrate concentration increment differed in the different tree parts (Fig. 8). This difference was most pronounced for starch. Shading approximately doubled the increment in starch concentration in the leaves, while it decreased that in the roots. For the shadiest treatment, the 60% Tunnel, the increment in root starch concentration was only 31% of that of the control.
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Carbon isotope ratios (
13C), which are influenced by CO2 concentrations at the chloroplasts, are given in Fig. 9. Measurements were on leaves that grew during the experimental period. 13C was lower in the 60% Tunnel treatment (1.9 below the control values; P <0.01), and similar in all other treatments.
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Discussion |
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A number of studies have found that crop productivity increases under moderate shading, while others have found that any kind of shading reduces productivity (see Stanhill and Cohen, 2001, for review). The current study found that biomass production of young de-fruited citrus trees in a semi-arid summer climate increased due to shading with reflective screens, which primarily reduced direct solar radiation. Two of the treatments reduced radiation during the whole day, while the third treatment lowered it during the midday hours only. The reasoning behind the experiment was that shading would reduce the midday depression of conductance (gl) and assimilation (A) typically observed in citrus in the summer under arid conditions, and this would lead to increased photosynthetic productivity. For young trees in a stage of rapid vegetative growth this should lead to increased growth rates. Reduced midday depression was observed directly in the shaded treatments on 2 d when daily courses of exposed (‘sunlit’) gl and A were measured. Since the reduced depression is related to the negative response of gl to vapour pressure deficit, a response which has been found in many or most plant species studied (Jones, 1992), it would be expected that these results have some widespread implications for the influence of shade on growth and productivity in conditions of high temperature and solar radiation common to arid zone summers.
The values of net CO2 assimilation rate (2–7 µmol m–2 s–1) and leaf conductance (50–200 mmol m–2 s–1) reported in this study are low relative to other crops, but normal for citrus (Spiegel-Roy and Goldschmidt, 1996; Sinclair and Allen, 1982), and the increased values obtained in the shading treatments were not exceptionally high. If shading is viewed as a way to cool the leaves and reduce the vapour pressure deficit, then the treatment is similar to moving the trees to a lower VPD climate. Levy and Syvertsen (1981) plotted data for leaf conductance as a function of VPD from humid Florida and semi-arid Israel, and found a continuous relationship for the negative influence of VPD on gl, thus explaining why net CO2 assimilation rate values observed in semi-arid Israel are usually lower that those observed in the more humid Florida. Syvertsen and Lloyd (1994) simulated net CO2 assimilation rate in different climates based on the response of gl to VPD, and predicted that if leaf conductance could be increased in the dry climates net CO2 assimilation rate would increase. The current results are in agreement with those model predictions and show that in the case of young trees in a vegetative stage of development this leads to increased growth rates.
The reflective screens also scatter some of the direct radiation downward, thus increasing the fraction of diffuse radiation below the screens. Increases in the diffuse fraction have been shown theoretically to increase plant radiation use efficiency (RUE; Sinclair et al., 1992; Hammer and Wright, 1994). The latter has been used to explain some experimental results showing increased productivity due to shading (Healey et al., 1998), and even indications of widespread increases in productivity due to reductions in solar radiation caused by increased aerosol loading of the atmosphere (Roderick et al., 2001). Unfortunately, experimental studies of productivity under shading and/or increasing diffuse fraction usually do not include measurements of gl, so in many cases it is hard to establish to what extent the two mechanisms, i.e. reduced midday depression of A and increased RUE due to increased diffuse fraction, are responsible for the increased productivity.
In the flat shade treatment, the amount of diffuse radiation was approximately equal to that in the control, so that the fraction of diffuse radiation increased significantly (unreported results). Although the RUE of the radiation arriving at the plant inevitably increased, there is no evidence that A of shaded leaves increased (Figs 1, 2), no differences between treatments were observed in the limited number of measurements of midday shade leaf photosynthesis, and the observed changes were only in the exposed leaves. This suggests that the main mechanism that increased productivity in this study was the reduction of midday depression.
The increased growth observed in the shaded trees was partitioned more to leaves (Fig. 4; Table 2), which increased shoot to root ratios (Fig. 5). It is possible that soil volume limited root growth and that this caused the increase in shoot to root ratios. However, the plants were in large pails and the relative increases in root and stem dry weight (Table 2) were similar, so this explanation for the differences in partitioning is unlikely.
Increases in shoot to root ratio in response to shade have also been reported for grasses (Samarakoon et al., 1990), wheat (Mitchell et al., 1996) and the shade plants Selenicereus megalanthus and Hylocereus undatus (Raveh et al. 1998). But 60% shading reduced growth and shoot to root ratios in apple trees (Chen et al., 1997). It is assumed that increasing shoot to root ratios is a strategy to enhance light interception. Evidence that this study’s results indicate an innate response to shading is the change in the distribution of the different types of sugars, and especially the large reduction in starch concentration increment in the roots that resulted from shading (Fig. 8), a reduction which became more severe with increasing shading intensity. The increments of sugar concentrations in the stem were not different for the different treatments. But in contrast to the roots, the carbohydrate concentration increments in the leaves were not especially influenced by the different shading treatments, and all shading treatments caused an approximate doubling of the leaf starch concentration increment. The latter may indicate that the trends in growth observed after four months would have continued, leading to larger differences in morphology between exposed and shaded trees.
13C of leaf organic matter is an integrative parameter that can indicate sustained changes in leaf function during the period that the leaves developed. 13C is negatively related to internal CO2 concentration (Ci) in the leaf during carbon uptake, so a decrease in 13C can indicate increased conductance during the time that the CO2 was fixed and/or decreased photosynthetic rates. The values for 13C of leaves that grew during the experiment (Fig. 9) are mid-range for those reported for other C3 plants (Ehleringer and Osmond, 1989) and shading reduced 13C. Israeli et al. (1996) reported similar decreases in 13C for banana leaves under 19%, 38%, and 68% shade in the hot Jordan Valley. No changes in leaf conductance were observed and net CO2 assimilation rate and productivity declined in all shading treatments in proportion to shading intensity (Israeli et al., 1996; Yakir and Israeli, 1995), while here, in citrus, leaf conductance and net CO2 assimilation rate increased in response to shading. These differences in behaviour of citrus and banana, that resulted in almost identical changes in 13C, demonstrate that care should be taken in inferring about productivity from changes in 13C.
Farquhar et al. (1982) proposed a model that can be used to calculate CO2 concentrations at the carboxylation sites in the chloroplast, Cc, from 13C using values of ambient and compensation CO2 concentrations. Calculations using a compensation concentration of 55 µl l–1 (following Bravdo, 1977) and ambient concentration of 365 µl l–1 give Cc values of 237 b, 246 b, 241 b, and 266 a µl l–1 (letters indicate DMRT classes; ANOVA P <0.01) for the control, 60% Flat, 30% Tunnel, and 60% Tunnel treatments, respectively. These high values indicate that stomatal constraint of net CO2 assimilation rate was low due to adequate water supply, as would be expected from daily irrigation. Israeli et al. (1996) reported for banana similar values of Cc, and found that Ci values, calculated with a diffusion model using values of net CO2 assimilation rate and leaf conductance measured on a series of days, were comparable but slightly higher than Cc. Ci values computed from the full day of net CO2 assimilation rate and leaf conductance measurements in August averaged 313 µl l–1, were not significantly different for the three shading treatments, but were significantly lower for the control. The differences between Cc and Ci result from the resistance to diffusion from the sub-stomatal cavity to the carboxylation sites (Evans and von Caemmerer, 1996). The larger differences observed in the current study as compared to the banana study, together with the lower rates of net CO2 assimilation rate indicate that this resistance is larger in citrus. High internal resistance to CO2 diffusion has been reported for citrus before (Laisk and Loreto, 1996; Lloyd et al., 1992), and has been shown to be related to the sclerophytic anatomy of citrus leaves (Syvertsen et al., 1995).
In summary, shading young de-fruited citrus trees in a subtropical clear sky climate during the hot summer months enhanced plant biomass and vegetative growth. The increased growth was significant in the leaves, which led to increased shoot to root ratios, and was associated with large increases in leaf conductance and net CO2 assimilation rate. Differences in 13C were consistent with these findings, indicating that internal CO2 concentrations increased with leaf conductance. However, although 13C was positively correlated with irradiance levels, in this case it was negatively correlated with plant photosynthetic productivity. The latter is contrary to a previous study of shading banana trees (Israeli et al., 1996), indicating that the relationship of 13C to productivity can be ambiguous. Trials on the effect of shading on growth and yield in a commercial fruit bearing citrus orchard are currently under way.
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Acknowledgements |
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We are grateful to H Blumenfeld, Yefet Cohen, D Galili, F Li, Y Li, and M Negreanu for technical assistance, and to Z Gal and Polysak Ltd for supplying the screens. This project was funded by Research Grant Award No. IS-2835-97R from BARD, The United States–Israel Binational Agricultural Research and Development Fund.
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References |
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