2022 Volume 91 Issue 4 Pages 467-475
Citrus nursery trees in California must be grown in insect exclusion facilities to protect them against Huanglongbing (HLB), a deadly disease caused by Candidatus liberibacter spp. and spread by the Asian citrus psyllid (Diaphorina citri). Faster year-round propagation is critical for citrus nurseries to offset their investment in new exclusion facilities, but nurseries currently face serious problems in terms of poor bud push and slow scion growth in fall-budded, container-grown trees. The purpose of this study was to explore the effect of supplemental LED lighting techniques on the photosynthesis and total non-structural carbohydrate (TNC) partitioning within citrus trees that is responsible for growth cessation during the fall. A total of 72 trees of Carrizo citrange rootstock with and without Clementine Mandarin scions, were placed in growth chambers during four photoperiods: T1, 10 h LED + 4 h extension of day length (EoD, 10 μmol·m−2·s−1); T2, 10 h LED with 1 h supplemental night interruption (NI); T3, 10 h LED with 10 h supplemental far-red (FR) lighting; and T4, controls (10 h LED). The LED light spectrum was adjusted to 90 red and 10 blue ratios. The trees were grown in the growth chambers at 21/13°C day/night temperatures and 80% RH for 12 weeks. Trees were harvested and separated into leaves, stems, and roots at the end of the experiment. Different tissues were oven dried, ground and analyzed for TNC. The results showed that NI and EoD resulted in significantly higher plant growth. Both NI and EoD allowed trees to accumulate lower levels of carbohydrates in the root system, thereby decreasing the root: shoot ratio. NI and EoD trees had lower root: shoot ratios for TNC than control and FR trees in both budded and unbudded trees, suggesting the storage of reserves in roots can be enhanced during shorter day length. Results also showed that NI and EoD photoperiods triggered phytochrome with low light intensity, which in turn induced a long day effect and further translocation of reserves from roots to aboveground parts to improve vegetative growth.
The 2.3 billion USD commercial citrus industry in California accounts for the majority of fresh citrus in the United States (USDA NASS, 2021). This industry faces one of its greatest challenges to date in Huanglongbing (HLB), a deadly disease spread by the Asian Citrus Psyllid, Diaphorina citri. Due to devastation by this disease in Florida, propagation of citrus in California requires insect exclusion facilities that are similar to greenhouses. The increased cost of these regulations/ACP control regulations has resulted in a 20% reduction of California citrus acreage (Babcock, 2018). The problem is further exacerbated due to problems of poor bud push, and slow scion growth in fall-budded container-grown trees due to lower temperatures and shorter photoperiods during fall. Therefore, the dormant season results in considerable loss to nurseries. Shorter day length resulted in increased starch accumulation, decreased translocation and concentration of foliar sucrose in soybean (Glycine max) (Chatterton and Silvius, 1979) and tobacco (Nicotiana tabacum) (Huber et al., 1984). Iglesias et al. (2002) reported that the accumulation of sugar in the leaves results in an inhibition of photosynthesis through feedback mechanisms. Overaccumulation of starch can damage chloroplast functioning and an excess of soluble sugars may cause osmotic stress (Krapp et al., 1991; Webber et al., 1994). In addition, a shorter photoperiod led to increased total nonstructural carbohydrates in the leaves of white clover (Trifolium repens) (Boller and Nösberger, 1983) and blueberry (Vaccinium spp. section cyanococcus) (Darnell, 1991). It has been reported that photoperiodic changes alter the growth and carbohydrate partitioning in Dioscorea spp. (Chu and Ribeiro, 2002), Aloe vera (Aloe barbedensis Miller) (Paez et al., 2000), and many deciduous tree species like poplar (Populus spp.) (Rohde et al., 2011). Low temperature and short-days have additive effects on the growth, survival, and distribution of woody plants, whereas high temperatures and long days promote vegetative growth (Kozlowski and Pallardy, 2002). Similarly, Bowman and Albrecht (2020) reported that citrus nursery trees could overcome the restricted growth in winter when the day length was supplemented with LED lighting.
Citrus is known to store carbohydrates in winter and translocate them the following spring for an early season growth flush (Zamski and Schaffer, 1996), a characteristic of deciduous fruit trees (Schaffer et al., 1986). Therefore, translocating reserves for vegetative growth may increase the vegetative growth in winter and also lead to higher assimilation rates. The effect of photoperiods on the accumulation and distribution of reserves on vegetative growth and photosynthesis of young container-grown citrus trees needs to be better understood. In perennial plants, the role of stored total non-structural carbohydrates (TNC) becomes more important during shade, drought, or under conditions that reduce assimilation compared to the photosynthate demand of the tree (Piper and Fajardo, 2014). The amount of stored carbohydrates is determined by the concentrations of starch and sugar. Starch is solely a storage compound for use in the future, whereas soluble sugars are involved in various intermediate functions other than supporting new growth and respiration demands. In conjunction with other low molecular weight carbon compounds, soluble sugars have been found to have key roles in cold tolerance and acclimation (Graham and Patterson, 1982; Tixier et al., 2019) and in signaling (Gibson, 2005). However, interconversions of starch and soluble sugars at different times makes their functions difficult to distinguish (Dietze et al., 2014). Supplemental lighting with different photoperiod regimes has been shown to increase vegetative growth in oranges (Brar and Spann, 2014). The red to far-red ratio (R:FR) regulates germination, flowering, photosynthesis, stem elongation and bud outgrowth in different plant species (Demotes-Mainard et al., 2016). FR lighting resulted in stem elongation, leaf expansion and dry mass accumulation in Petunia (Petunia hybrida) and Coleus (Solenostemon scutellariodes) (Park and Runkle, 2018). In the above-cited studies, longer photoperiods indicate a greater daily light integral (DLI) than the shorter photoperiods, which does not explain the true phytochrome-mediated control of vegetative growth. In this study, low intensity white and FR lighting (below the light compensation point) produced with smart LED was used during a brief night interruption (NI) and extension of day length (EoD) period to induce a long day effect during an otherwise short-day photoperiod (Reference is needed). LED can be adjusted to control the wavelength and intensity, allowing control of photosynthesis and photomorphogenesis in plants (Mitchell and Sheibani, 2020; Wu et al., 2020). It is hypothesized that this additional lighting, at low intensity, signals phytochrome to override growth cessation during short day conditions in trees without any change in the DLI among photoperiods. This will trigger the trees to use their assimilates for vegetative growth, rather than storing them as TNC. The role of LED lighting in plant production/physiology is species and cultivar-specific (Sipos et al., 2020). Therefore, the purpose of this study was to investigate the plant growth and TNC partitioning within citrus nursery trees in response to different photoperiods and also to clarify the phytochrome-mediated control of growth by light or photoperiod.
Seventy-two 24-week-old trees obtained from a commercial citrus nursery (Tree Source Nursery, Woodlake, CA) were used in this experiment. All seedlings were ‘Carrizo’ citrange rootstock (Citrus sinensis L. × Poncirus trifoliata). However, 36 out of the 72 were budded with ‘Clementines’ (C. clementina hort. ex Tanaka) when they were 16-week-old seedlings. The other 36 remained unbudded. The reason for selecting two types of plant material (budded and unbudded) was to examine how these different plants respond to photoperiods. Chip budding was performed at a height of 10 cm above the stem base. After bud push, trees were transferred to a greenhouse where they were potted into 1-gallon pots (30 cm × 10 cm) using a soilless medium based on coconut coir, perlite and vermiculite. Daily water use of the trees was determined by weighing the containers periodically before and after watering. Trees were acclimated in the greenhouse for two weeks before being placed in the growth chambers for the experiment. To supply macronutrients, N-P-K (commercial formula Triple 21; Brandt, Inc., IL, USA) was applied through fertigation every two weeks as per industry standard rates suggested for tree age. Micronutrients Mg, S, Mn, and Zn (Hye-Green Micro-Mix; Gar Tootelian, Inc., CA, USA) were applied as a foliar spray (6 g·L−1) every two weeks. The study was conducted for two consecutive years, 2019 and 2020. This study is based on the 2020 dataset.
Experimental conditionsGrowth chambers (Model A-1000; Conviron Inc., MB, Canada) were equipped with LED lights (PRO 650e; Lumigrow, CA, USA) controlled by a cloud-based Lumigrow smart-PAR system. Nine budded and nine unbudded trees were placed in each of the four different growth chambers. The following four different photoperiods were assigned randomly to the growth chambers: 1. EoD, which consisted of 10 h LED (500 μmol·m−2·s−1) + 4 h EoD (10 μmol·m−2·s−1); 2. NI, 10 h LED (500 μmol·m−2·s−1 + 1 h NI (10 μmol·m−2·s−1); 3. FR, 10 h LED (500 μmol·m−2·s−1) + 10 h FR, and 4. Controls, 10 h LED (500 μmol·m−2·s−1) as a short day photoperiod. The light intensity numbers in parentheses refer to the photosynthetic photon flux density maintained at the canopy level. Photosynthetic photon flux was maintained at 500 μmol·m−2·s−1 with a red to blue light ratio of 9:1 during the 10 h LED period in all photoperiods (Fig. 1). The light spectrum was monitored periodically using a spectroradiometer (Spectrum Technologies, Inc., IL, USA). Light intensity was determined by running light response curves with an LI-6800 (LI-COR Biosciences, NE, USA). To minimize photosynthetic activity during the NI and EoD periods, a light intensity below the light compensation point (10 μmol·m−2·s−1) was set during these photoperiods. A DLI of 18 mols·m−2·day−1 was maintained during all photoperiods. NI was for one hour from 12:30 AM to 1:30 AM to interrupt the dark period and the EoD extended the day length from 10 h to 14 h by providing two hours of additional light at the beginning and two hours at the end of the daily 10-hour cycle. FR was provided as supplemental lighting throughout the 10 h LED period. The temperature in all of the growth chambers was set to 21/13°C for day/night, and humidity was set to 80%, both monitored by Watchdog Micro Station 1000 series data loggers (Spectrum Technologies, Inc.). Temperature and RH conditions were mimicked for the minimal Industry Standard Protocols for Greenhouse Conditions in the winter months. Nurseries in California utilize some heating systems to avoid very low ambient temperatures. Trees were shuffled every three days within the growth chambers. Plants were kept for 12 weeks under the experimental conditions.
A typical light spectrum from, a) 10-h LED and b) EoD and NI, with a 9:1 red:blue lighting ratio.
Growth data were collected before the start of the experiment and bi-weekly throughout the experiment for each tree. Total shoot length for each of the nine replicate trees was recorded by adding the length of all branches and stems of the tree. The total numbers of nodes (leaf number) and branches were recorded for each tree (n = 9) every two weeks. Stem diameter was recorded from the marked position on the stem two inches above the soil line from each tree with digital Vernier calipers every two weeks. Leaf area was measured every two weeks from five representative leaves from each tree using a portable leaf area meter (LI-3000C; LI-COR Biosciences).
Instantaneous net photosynthesisThe CO2 assimilation rate was measured on fully expanded leaves every week from three budded and three unbudded trees during each photoperiod with a portable photosynthesis system (LI-COR 6800; LI-COR Biosciences). Three leaves on each of the selected trees were tagged and photosynthesis measurements were taken between the hours of 10 AM and 11 AM each time. A fluorometer chamber with a 6-cm2 round aperture was used by matching the light intensity (500 μmol·m−2·s−1) and spectrum (9 red:1 blue) in the growth chambers. Total photoassimilates were calculated by multiplying the total leaf area of the tree by the instant photosynthetic rate of the photoperiod.
Total fresh and dry weightTrees were harvested destructively at the end of the experiment and separated into roots, shoots/stem and leaves. The fresh weight of each fraction was recorded, and then each fraction was oven-dried at 65°C for 24 hours to achieve consistent dry weight.
Total chlorophyll contentTotal chlorophyll content was determined using a colorimetric method (Inskeep and Bloom, 1985) before the start and at the end of the experiment. Three trees were randomly selected from each photoperiod and two leaf discs (1/4-inch diameter) were excised from each tree using a hole punch. Leaf discs were then placed in clean test tubes and 2 mL of N, N-dimethylformamide was added. Tubes were then placed in the dark for 72 hours, after which tubes were vortexed, and 1.5 mL of the solution was transferred to a cuvette to read absorbance at 647 and 664 nm in a spectrophotometer. Chlorophyll concentration (mg·L−1) was then calculated from absorbance readings using the following formula:
After harvesting the trees and dry weight determination, trees were separated into root, stem and leaf tissues and ground with a Thomas Wiley mill (Wiley® Mill 4 1/2 HorsePower), passed through a 2 mm sieve, then further ground with a CT 293 CyclotecTM (FOSS Analytical, Denmark) and passed through a 0.5 mm sieve. TNC were quantified in the ground material. Soluble sugar and starch content were quantified using the protocol described by Bailey (1958) with a few modifications. Soluble sugars were extracted by mixing 25 mg of dried ground tissue in 1 mL of ultrapure water and incubating it at 70°C for 15 minutes. Extracts were then centrifuged (10 minutes at 13,000 rpm), and the supernatant was decanted, diluted, vortexed and loaded into a 96-well tray along with the glucose standards. An anthrone reagent (0.1% by vol. of H2SO4) was mixed vigorously with the samples that had been loaded onto the tray. The plate was then incubated at 85°C for 20 minutes, allowed to stay at room temperature for 10 minutes and then read with a microplate reader (Epoch; BioTek) by measuring absorbance at 620 nm.
Starch was analyzed from the same sample by removing the supernatant, washing the pellet with ethanol followed by ultrapure water to remove any remaining soluble sugars. Starch degradation was done by boiling the pellet saved from the soluble sugars analysis at 100°C for 10 minutes, followed by addition of 100 μL of 70 units·mL−1 amyloglucosidase from Aspergillus niger (Sigma-Aldrich) and 100 μL 7 units·mL−1 of alpha amylase from Aspergillus oryzae (Sigma-Aldrich). The pH was controlled by adding 500 μL of 0.2 M acetate buffer, pH 5.5. Digests were then incubated at 37°C for 4 hours. After incubation, the samples were processed similarly to the steps explained above for soluble sugars. Total starch content and soluble sugars were calculated by multiplying the concentrations by the total dry weights of leaves, shoots and roots.
Data analysisThe experiment was set up as a completely randomized design. Photoperiods were randomly assigned to the growth chambers. Data were analyzed separately for budded and unbudded trees using one-way analysis of variance (ANOVA). Post-hoc separation of means was conducted using Tukey’s HSD test (P < 0.05). Statistical analysis and graphical presentation of data were done with Prism statistical software, version 8.0 (GraphPad, La Jolla, CA, USA).
Table 1 shows the increases in growth parameters in each photoperiod during the experimental period. In both budded and unbudded trees, the EoD and NI photoperiods showed significantly higher increases in the numbers of leaves than FR and control trees. Similarly, increases in shoot growth were significantly higher in EoD and NI than in the control for both budded and unbudded trees; however, unlike unbudded trees, FR showed significantly higher shoot growth than control trees in budded trees. There were no differences among the photoperiods for the increase in the number of shoots in budded trees, while NI and FR in unbudded trees had significantly higher and lower numbers of shoots than controls, respectively (Table 1). Stem diameter was not significant among photoperiods for budded trees, but NI had significantly higher stem diameter than FR and controls in unbudded trees. Leaf area was not significantly affected by the photoperiods in either budded or unbudded trees (Table 1). Interestingly, the total daily photoassimilates were higher in both EoD and NI in unbudded trees, indicating a growth response of unbudded trees of rootstocks with trifoliate parentage to photoperiodic changes. Leaf number was a better contributor to total daily photoassimilates than the leaf area. Photoperiods promoted shoot growth and increased the number of leaves by overcoming the short-day effect, as reported in previous studies (Brar and Spann, 2014; Inoue, 1989; Warner et al., 1979). It has been documented that vegetative growth increases in response to increasing day length in woody plants (Garner and Allard, 1920; Kozlowski and Pallardy, 2002). Also, previous studies on different crops have reported the effects of FR on shoot elongation (Davis and Simmons, 1994; Ito et al., 2014; Miyashita et al., 1995), but there were no significant differences in the number of leaves. FR is known to increase shoot growth by internodal elongation without any increase in node count (Casal, 2013).
Effects of different photoperiods on increases in growth parameters of budded (‘Carrizo’ citrange (Citrus sinensis L. × Poncirus trifoliata) budded with ‘Clementine’ (C. clementina hort. ex Tanaka), and unbudded ‘Carrizo’ citrange container-grown citrus nursery trees (n = 9).
The root-to-shoot ratio on a dry weight basis was calculated to assess the assimilate partitioning between above- (leaves and stems) and below-ground portions. EoD and NI had significantly lower root:shoot ratios than the control in both budded and unbudded trees. Shoot growth in citrus is known to be inversely correlated with root growth (Bevington and Castle, 1985) which is in accordance with our findings. No differences in the root:shoot ratio were observed between FR and controls, suggesting no translocation of reserves from roots to stems occurred in response to short day-length (Fig. 2). Instead, stem elongation may be due to an increase in auxin synthesis at the apex and young leaves that results in production of abscisic acid in axillary buds and restricts bud outgrowth, as found in tomato (Tucker, 1976). This hypothesis may explain why the FR failed to elongate stems in unbudded trees, as those were topped to a uniform height before putting them in the growth chambers, which removed the apical region where auxin synthesis would have occurred.
Root:shoot ratio of citrus nursery trees during different photoperiods (n = 9). The shoot includes stem + leaves. z EoD, 10 h LED + 4 h extension of day length; NI, 10 h LED + 1 h night interruption; FR, 10 h LED with 10 h supplemental far-red lighting; Control, 10 h LED short day control. Columns are means separated by Tukey’s test, P < 0.05. Vertical bars on each column represent SE, letters denote significant differences.
a) Representative unbudded trees (‘Carrizo’ citrange (Citrus sinensis L. × Poncirus trifoliata) and b) representative budded trees (C. clementina hort. ex Tanaka) during different photoperiods as listed below from left to right. EoD, extension of day length; NI, night interruption; FR, supplemental far-red lighting; Control, short day control.
There was a significant effect of photoperiods on root, shoot and leaf dry weights. In budded trees, NI and EoD had significantly lower root dry weight than control and FR trees. In unbudded trees, EoD was the only photoperiod that significantly lowered the root dry weight. There were no differences in shoot and leaf dry weights among the photoperiods in budded trees, except that NI had significantly lower shoot dry weight than FR (Table 2). Unbudded trees during NI and EoD photoperiods had significantly higher dry weight in shoots than FR and control trees. There were no significant differences among photoperiods for leaf dry weights in unbudded trees (Table 2). Dry weight partitioning between roots, shoots and leaves among the photoperiods in budded and unbudded trees suggested that budded trees during 10-hour day length (control) stored reserves in the roots, whereas unbudded trees 10-hour day length stored reserves in shoots. This result further suggests that trifoliate rootstocks inhibit photoperiodic sensitivity to budded scions. Inoue (1989) reported similar results in Satsuma mandarin on trifoliate orange rootstock, in which the shoot growth slowed significantly during short days and was higher during long days. Piringer et al. (1961) reported that growth of trifoliate orange slowed markedly under short day conditions (8 h photoperiod) while the growth rate of budded grapefruit was also affected by photoperiod.
Tissue dry weights of budded (‘Carrizo’ citrange (Citrus sinensis L. × Poncirus trifoliata) budded with ‘Clementine’ (C. clementina hort. ex Tanaka), and unbudded ‘Carrizo’ citrange trees during four different photoperiods (n = 9).
The supplemental light intensities used in the NI and EoD photoperiods were below the light compensation point, and FR is not photosynthetically active. Therefore, minimal added photosynthetic activity was possible during these photoperiods. Even so, plants during NI and EoD nevertheless grew significantly more than control plants. This pattern is explained by the root: shoot ratio on a dry weight basis, as the trees that had higher vegetative growth had lower dry weight roots, as opposed to control trees, which had saved the same photoassimilates in their roots and increased their root mass in response to 10-hour day length (Goldschmidt, 1982). These results suggest low intensity (10 μmol·m−2·sec−1) lighting during these photoperiods triggered phytochrome signaling by altering the phytochrome forms by changing the red-light absorbing phytochrome form to FR-light absorbing to overcome short-day growth cessation (Smith, 1995).
Leaf physiological parametersThere were no significant differences in instantaneous net photosynthesis or total chlorophyll content among different photoperiods in both budded and unbudded trees in either replication of the study. This was expected as all photoperiods received the same amount of DLI. Total daily photoassimilates were calculated by multiplying instantaneous daily photosynthesis by total leaf number and leaf area for each photoperiod. In budded trees, total photoassimilates per day were significantly lower during NI than during all other photoperiods, whereas for unbudded trees, both EoD and NI had significantly higher photoassimilates than FR and control trees (Table 3).
Effects of different photoperiods on physiological parameters of budded (‘Carrizo’ citrange (Citrus sinensis L. × Poncirus trifoliata) budded with ‘Clementine’ (C. clementina hort. ex Tanaka), and unbudded ‘Carrizo’ citrange citrus nursery trees (n = 9).
NI showed significantly lower root starch content than controls in budded trees. In unbudded trees root starch content during EoD and NI was significantly lower than during the control. FR had no significant effect on root starch content. None of the photoperiods had significant effects on starch content of leaves or shoots in comparison with control trees (Table 4). Soluble sugar content of either leaves, shoots or roots was not affected by the photoperiod compared to the controls in both budded and unbudded trees. It is worth mentioning that in unbudded trees, the amount of soluble sugars in shoots of trees treated with NI was significantly higher than those during FR (Table 4). The TNC content in all plant parts trended lower during NI and EoD than in control trees in general although this was not always significant. This may explain their significantly higher vegetative growth TNC converted to structural carbohydrates) than the control trees. From the TNC content in the above and below ground parts, we could see the short-day control on assimilate storage in roots when comparing the photoperiods. This agrees with the results of previous studies in citrus that found the storage of assimilates in roots in response to dormancy or short days (Salisbury and Ross, 1992; Zamski and Schaffer, 1996). NI and EoD induced lower starch content in the leaves of budded trees than in the control trees, although this was not significant. Previous studies by Jones et al. (1974) and Jones and Steinacker (1951) in citrus also found that carbohydrate levels decreased during periods of growth flushes and increase during periods of dormancy. Citrus leaves serve the early season growth needs as they remain evergreen and photosynthetic throughout the season (Goldschmidt, 1999).
Effects of different photoperiods on total non-structural carbohydrates TNC in leaves, shoots, and roots of budded (‘Carrizo’ citrange (Citrus sinensis L. × Poncirus trifoliata) budded with ‘Clementine’ (C. clementina hort. ex Tanaka), and unbudded ‘Carrizo’ citrange citrus nursery trees (n = 9).
Regardless of the photoperiod, roots were found to have significantly higher total starch content than the shoots and leaves for budded trees. A similar trend was seen in unbudded trees where again roots were found to have significantly higher total starch content than the shoots and the leaves. Leaves had the least storage of starch compared to shoots and roots for both budding types. In budded trees, the difference between total soluble sugars in root and shoots was not significant, but the amount of TSS in leaves was lower. Shoots had the highest and leaves the lowest portion of total soluble sugars in unbudded trees.
Different species and genotypes have different seasonal carbohydrate storage and mobilization patterns depending on their growth habits, presence of fruit and other such factors (Spann et al., 2008). There was a significant amount of starch storage in the roots of control trees compared to the NI trees, suggesting the effect of short-day length on control trees resulted in starch storage in roots rather than using it for aboveground growth. This agrees with the results of previous studies documenting enhanced storage of assimilates in roots of citrus (Zamski and Schaffer, 1996), potato (Solanum tuberosum) (Golovko and Tabalenkova, 2019), and radish (Raphanus sativus) (Sirtautas et al., 2011) in response to dormancy or short days. These studies also found an increased dry weight accumulation with increasing day length mainly due to photosynthetic response. Also, NI and EoD shoots had lower starch content than control trees, suggesting that trees during these photoperiods were using photoassimilates for vegetative growth instead of storing it in the form of starch. A previous study by Monerri et al. (2011) in citrus also found that carbohydrate levels decreased during periods of growth flushes and increased during periods of slow growth. Higher soluble sugar levels in the shoots and lower starch levels in roots of unbudded trees suggests the conversion of starch into soluble sugars for use in vegetative growth in response to the photoperiods. Kramer and Kozlowski (1979) and Tixier et al. (2019) reported that woody perennials depend on storage of photoassimilates in different storage organs that they remobilize and redistribute to the sink tissues at various stages of growth throughout the season. This has also been well documented by labeling studies in Apple (Malus domestica) (Quinlan, 1969), peach (Prunus persica) (Da Silva et al., 2014) and pistachios (Pistacia vera) (Schaffer et al., 1986). Short day length in the controls restricted the vegetative growth due to storage of non-structural carbohydrates in roots, whereas during NI and EoD, translocation of reserves from roots resulted in increased vegetative growth. McCamant (1988) described a similar response in which decreases in root starch at low temperatures were correlated with aboveground growth that ceased when trees were de-budded or topped. Rootstock-scion interaction can also have a significant effect on carbohydrate mobilization patterns, which has been shown in many orchard crops such as cherries (Gonçalves et al., 2006; Olmstead et al., 2010) apples (Avery, 1968) and peach (Caruso et al., 1997; Yano et al., 2002). In the current study, the different carbohydrate mobilization patterns in budded trees may be attributed to the effect of a grafted scion cultivar (Clementine) on photoassimilate distribution.
Results showed that total photoassimilate accumulation was not a limitation of vegetative growth in the control trees. This can be explained by the root:shoot ratio data showing TNC storage in roots compared to its investment in aboveground vegetative growth in control trees. Similarly, unbudded trees during control and FR had significantly lower photoassimilates than NI and EoD trees, possibly due to the significantly higher leaf number in NI and EoD, leading to their higher accumulation and subsequent partitioning in growth. Indeed, control and FR trees had higher root biomass than NI and EoD trees for both budding types. Results of starch storage suggested the role of roots as a major storage organ for starch in containerized citrus trees in winter, in response to low temperatures and shorter photoperiods. NI and EoD trees were able to stimulate translocation of these starch reserves to aboveground parts, which may be the reason for their increased vegetative growth compared to control trees.
ConclusionsNI and EoD photoperiods at low light intensity increased vegetative growth of budded and unbudded containerized citrus nursery trees at winter temperatures. Photoperiods caused the translocation of reserves between above- and below-ground parts of containerized citrus trees grown in growth chambers under winter conditions. The lower root:shoot ratio of NI and EoD trees compared to the control trees confirmed the phytochrome-mediated control of citrus growth because DLI was the same across during the photoperiods. Therefore, LED lighting can be used to promote vegetative growth of citrus trees during winter months. NI was more consistent than EoD in terms of vegetative growth and may be more economical. Future work on long-term photoperiods under commercial greenhouse conditions is recommended to observe how continued tree growth is affected by photoperiods.
