JXB Advance Access originally published online on July 13, 2007
Journal of Experimental Botany 2007 58(11):2917-2928; doi:10.1093/jxb/erm149
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© 2007 The Author(s).
This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License (http://creativecommons.org/licenses/by-nc/2.0/uk/) which permits unrestricted non-commercial use, distribution, and reproduction in any medium, provided the original work is properly cited.
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RESEARCH PAPER |
Depression of sink activity precedes the inhibition of biomass production in tomato plants subjected to potassium deficiency stress
1Graduate School of Biosphere Science, Hiroshima University, 1-4-4 Kagamiyama, Higashi-hiroshima, 739-8528, Japan
2Faculty of Environmental and Information Sciences, Yokkaichi University, 1200 Kayoucho, Yokkaichi, 512-8512, Japan
3International Atomic Energy Agency (IAEA), Wagramer Strasse 5, A-400, Vienna, Austria
4School of Life Science, Sambalpur University, Jyoti vihar, Sambalpur 768019, India
* To whom correspondence should be addressed. E-mail: fujiko{at}hiroshima-u.ac.jp
Received 20 February 2007; Revised 30 May 2007 Accepted 1 June 2007
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Abstract |
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Tomato [Solanum lycopersicum (formerly Lycopersicon esculentum) L. cv. Momotarou] plants were grown hydroponically inside the greenhouse of Hiroshima University, Japan. The adverse effects of potassium (K) deficiency stress on the source–sink relationship during the early reproductive period was examined by withdrawing K from the rooting medium for a period of 21 d. Fruits and stem were the major sink organs for the carbon assimilates from the source. A simple non-destructive micro-morphometric technique was used to measure growth of these organs. The effect of K deficiency was studied on the apparent photosynthesis (source activity), leaf area, partitioning 13C, sugar concentration, K content, and fruit and stem diameters of the plant. Compared with the control, K deficiency treatment severely decreased biomass of all organs. The treatment also depressed leaf photosynthesis and transport of 13C assimilates, but the impact of stress on these activities became evident only after fruit and stem diameter expansions were down-regulated. These results suggested that K deficiency diminished sink activity in tomato plants prior to its effect on the source activity because of a direct effect on the water status of the former. The lack of demand in growth led to the accumulation of sugars in leaves and concomitant fall in photosynthetic activity. Since accumulation of K and sugars in the fruit was not affected, low K levels of the growing medium might not have affected the fruit quality. The micro-morphometric technique can be used as a reliable tool for monitoring K deficiency during fruiting of tomato. K deficiency directly hindered assimilate partitioning, and the symptoms were considered more detrimental compared with P deficiency.
Key words: Fruit and stem diameter, partitioning, potassium, tomato, micro-morphometry
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Introduction |
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The potassium (K) requirement of greenhouse tomatoes is high for vegetative growth (Wall, 1940; Lucas, 1968), fruit production (Besford and Maw, 1975), and fruit quality (Winsor, 1968; Trudel and Ozbun, 1971). Low K levels of the nutrient medium limit assimilation of the element into plant parts and retard plant growth, flower development, and fruit set (Besford and Maw, 1975). There is evidence that K exerts a direct effect on the partitioning of dry matter to the fruits and roots, and the growth of these organs is inhibited at low K (Haeder and Mengel, 1972). Also, the quality of the fruit changes according to the availability of K in the growth medium (Davies and Winsor, 1967; Winsor, 1968). During the reproductive phase of growth, fruits are the strongest sink for both carbon assimilates and K. In some determinate tomatoes, the demand for K during rapid fruit growth is above the uptake capacity such that leaf K is remobilized, resulting in foliar deficiency of the element (Widders and Lorenz, 1979). Because of remobilization and recycling from old parts to new organs (Pujos and Morard, 1997; Peuke et al., 2002), visible symptoms of injury do not appear on the growing sinks immediately in K-deficient nutrient medium (Mengel and Kirkby, 1987). Visual symptoms such as leaf necrosis and chlorosis appear much later when plants are exposed to a long period of the stress (Besford, 1978a, b; Pujos and Morard, 1997). It is important that farmers recognize the incidence of K deficiency during the early production stage and restore normal growth of the crop by supplementation of K in the culture medium.
Plant cells have a very high requirement for K for photosynthesis, enzyme activation, protein synthesis, cell turgor, and ion homeostasis (Marschner, 1995). Low K levels in the growing medium can disturb these processes and destabilize the source–sink relationship. In tomato, the adverse effect of K deficiency was documented on leaf photosynthesis (Zhao et al., 2001), but the effect was not due to impairment of turgor-induced regulation of stomatal conductance (Behboudian and Anderson, 1990). Some other authors have shown that mild K deficiency suppresses assimilate translocation without affecting photosynthesis at the source (Hart, 1969, 1970; Mengel and Viro, 1974). On the other hand, the effects of K deficiency on the sink activity are mired in controversies. It is reported that K deficiency reduces photosynthate translocation (Huber, 1984) without affecting the metabolism at the sink side (Mengel, 1980; Beringer and Haeder, 1981). Hart (1969) observed that the reduction in assimilate translocation by K deficiency in sugarcane is caused neither by death of the phloem nor by diminished sink growth. Conversely, Tsuno and Fujise (1965) found that K deficiency affects photosynthate translocation through reduced growth of storage roots, which is a predominant sink in sweet potato. Geiger and Conti (1983) reported that translocation and partitioning of dry matter remain independent over a wide range of K supply in the growth medium. These contradictions led to the suggestion that the effect of K deficiency on plant growth remains elusive, and it is necessary to re-examine the source–sink relationship of the plant under the influence of the stress (Roitsch, 1999). Biomass production is reduced due to impairment of sink activity by phosphorus (P) deficiency in tomato (Fujita et al., 2003b) and by salt stress in Japanese persimmon (Fujita et al., 2003a) and tobacco (Moghaieb et al., 2006). Also, there are examples where source activity is depressed earlier than that of the sink under the influence of stresses such as drought (Berman and Dejong, 1996; Sowder et al., 1997; Van den Boogaard et al., 1997; Escobar-Gutierrez et al., 1998) and nitrogen deficiency (Fujita et al., 2004). However, the effects of K deficiency on the source–sink relationship of tomato plants have not been studied so far. Additionally, the incidence of K deficiency at the early production stage in tomato is detrimental to fruit growth (Pujos and Morard, 1997). Therefore, a reliable technique is necessary to assess the ability of the plant to encounter nutritional disorders and replenish the stock solution. In the present experiment, the objective was to monitor the effect of K deficiency on the diurnal changes in stem and fruit diameters of tomato plants while simultaneously recording the impact of the stress on activities such as leaf photosynthesis and transport of 13C assimilates, and the carbohydrate status of the plant. It is necessary to identify the resource management techniques on the part of the plant at suboptimal K levels for the benefit of farmers.
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Materials and methods |
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Experiment 1
Plant material and culture:
Tomato (Solanum lycopersicum (formerly Lycopersicon esculentum) L. cv. Momotarou] plants were grown in pots inside the greenhouse of the Graduate School of Biosphere Science, Hiroshima University, Japan. Each pot (70.0 l) was filled with nutrient solutions, consisting of N [Ca(NO3)2.4H2O] 3.57 mM, P (NaH2PO4.2H2O) 0.32 mM, K (K2SO4/KCl 1:1) 1.02 mM, Ca (CaCl2.H2O) 0.75 mM, Mg (MgSO4.7H2O) 0.82 mM, Fe (Fe+3-EDTA) 0.02 mM, Mn (MnSO4.4H2O) 3.64 µM, B (H3BO3) 0.05 mM, Zn (ZnSO4.7H2O) 0.15 µM, Cu (CuSO4.5H2O) 0.16 µM, Mo (NaMoO4.2H2O) 0.1 µM, Co (CoSO4.7H2O) 0.17 µM. There were seven pots; each pot contained six plants. At the fruiting stage (74 d old), K2SO4/KCl was totally omitted from the nutrient medium in three pots, and this treatment was continued for 21 d. The plants in the control condition received full nutrition. The plants were grown under a daily light period of 14 h (05.00 h to 19.00 h) and maximum irradiance of 800 µmol m–2 h–1. The maximum and minimum temperatures were 32 °C and 20 °C, respectively.
Measurement of biomass production:
Plants from both control and K deficiency conditions were harvested at 0, 7, 14, and 21 d after treatment (DAT) in three replicates. The plant was separated into leaves, stem, fruits, and roots. The plant parts were dried in an air-forced oven at 70 °C for 7 d for the estimation of dry weight. The dry plant parts were ground into a powder with a vibrating sample mill (Model T1-100, Heiko Co. Ltd, Fukushima, Japan) and aliquots were taken for analysis of K, sugar concentration, and 13C.
Measurement of minerals:
An aliquot of the plant material was digested with an acid mixture (HNO3:HClO4, 3:1, v/v). The K concentration in the digest was determined by flame photometry (ANA-135, Tokyo, Japan).
Measurement of photosynthesis, transpiration, and stomatal conductance:
Photosynthetic rate, transpiration, and stomatal conductance of the leaves just below the first fruiting truss were measured with a portable infrared gas analyser (Model L1-6400, Li-Cor, Lincoln, NE, USA). The leaf chamber was the open type and measurements were made at 11:00 h each day in both control and K deficiency treatment plants. The photosynthetically active radiation during measurement was >1000 µmol m–2 s–1, and observations were recorded after the plant reached steady-state photosynthesis. All measurements were recorded three times and averaged on each occasion of sampling.
Measurement of stem and fruit diameter:
Changes in stem and fruit diameter were continuously recorded in both control and K-deficient plants during the period of treatment with a shrinkage-type micro-displacement detector (Fujita et al., 2003b). The sensors were fastened to the stem at 15 cm above the basal end of the stem or a growing fruit, and connected to the power system and data logger. The sensors were connected to a computerized data acquisition system (NEC, Sanei Kogyo Co. Ltd, Tokyo, Japan). All measurements were recorded twice, and the pattern of response was found to be similar in all.
13CO2 feeding:
13CO2 feedings were given to the leaf immediately below the first fruiting node on days 7 and 14 after K deficiency treatment in both control and treated conditions. The leaf was enclosed in a transparent plastic bag containing barium carbonate (Ba13CO3), packed in a plastic cylinder. The bag was filled with 13CO2 from barium carbonate. 13CO2 was generated by addition of 40% lactic acid (v/v). The leaf was allowed to assimilate 13CO2 for 1.5 h on days 7 and 14. The plants were harvested 36 h after feeding by separating into 13CO2-fed leaf, other leaves, fruits (first, second, and third truss), stem, and roots. The freeze-dried plant parts were ground into a powder for measurement of 13C abundance.
13C analyses:
The 13C abundance in the powdered plant sample was determined with a mass spectrometer (model Delta plus, Finnigan Co., San Jose, CA, USA) (Nobuyasu et al., 2003). The 13C atom% excess in the plant sample was calculated as the difference in 13C atom% between 13C-fed and unfed samples. The amount of labelled C in the plant sample was calculated using the equation shown below.
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The element analyser facilitated in the mass spectrometer determined the concentration of C. The concentration obtained was multiplied by the weight of the plant part to determine the amount of total C in the sample. The partitioning of isotope was calculated as the percentage of the total label in a plant part relative to that in the plant part of the fed leaf.
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The export rate of the isotope was calculated by the equation given below.
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Measurement of sugar content:
Aliquots of the powdered plant parts were boiled with 80% (v/v) aqueous ethanol three times for the extraction of sugars. The extracts were pooled in a volumetric flask and the flask was filled up to the mark with distilled water. The sugar content in the ethanol-soluble extract was determined using the anthrone reagent, according to the method of Yemm and Willis (1954).
Experiment 2
Plant material and biomass production:
Tomato (S. lycopersicum L. cv. Momotarou) plants were grown according to Experiment 1. At the flowering stage (53 d), K was withdrawn from the nutrient medium, and this treatment was continued for 9 d. The plants in the control received full nutrition. The plants were grown under a natural light period of 14 h (05.00 h to 19.00 h) and maximum irradiance of 800 µmol m–2 h–1. The maximum and minimum temperatures were 30 °C and 18 °C, respectively. Biomass production of plants was determined according to Experiment 1. Tomato was harvested at 0, 3, and 7 DAT in eight replicates.
Measurement of stem diameter:
Changes in stem diameter were continuously recorded in both control and K-deficient plants during the period of treatment according to Experiment 1. All measurements were recorded four times, and the pattern of response was found to be similar in all.
14CO2 feeding and analyses:
14CO2 feedings were given to the fifth leaf from the bottom of a plant on days 3 and 7 after K deficiency treatment in both control and treated conditions. The leaf was enclosed in a transparent plastic bag with a plastic cylinder including NaH14CO3 solution (185 kBq per treatment). 14CO2 was generated by addition of 40% lactic acid (v/v). The leaf was allowed to assimilate 14CO2 for 1.5 h under natural light conditions. The plants were harvested 24 h after feeding. These plants were separated into 14CO2-fed leaf, other leaves, stem, roots, and shoot apex. The plant parts were placed in an air-forced drought oven at 70 °C for 7 d. All samples were ground into a powder by sample mill (model T1-100, Heiko Co. Ltd., Fukushima, Japan). The specific activity of 14C in each plant part was determined using a liquid scintillation counter (model LSC-5100, Aloka Co., Tokyo, Japan).
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Results |
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Experiment 1
Dry mass accumulation:
Careful examination of the data seems to indicate that there may be a statistically significant difference between the dry weight of the plants from the two treatments at either 7 d or 14 d (Fig. 1). The K-deficient plants are lower in weight at these points and they appeared to be statistically different. However, during the last 7 d, dry weights in the leaves, stem, and roots decreased to 53, 66, and 72%, respectively, relative to the control.
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Stem diameter:
The diameter of the stem in the control exhibited daytime shrinkage and night-time expansion, and increased temporally up to 21 DAT (Fig. 2). Similar rhythmic shrinkage and expansion in the stem were observed in spite of K deficiency. K deficiency did not influence stem expansion during the first 3 d of treatment, but the stem did not expand in diameter thereafter (Fig. 2). K deprivation treatment initially increased daytime shrinkage. The effect on night-time expansion was also depressed by K deficiency at 15 DAT and subsequently (Fig. 3E).
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Fruit diameter:
The diameter of fruit increased with the passage of time in both control and K-deficient plants during the treatment period (Fig. 4). K deficiency reduced fruit expansion, and the rate of expansion slowed down with passage of time, leading to divergence of the curves denoting fruit expansions in control and K-deficient situations. On day 1 (Fig. 5A), daytime shrinkage of fruit was minimal, but night-time expansion was enormous in both control and K-deficient plants. At 6 DAT, K deficiency increased daytime shrinkage of fruit diameter as much as its effect on inhibiting night-time expansion compared with the control (Fig. 5B). Such a tendency due to K deficiency intensified with passage of time up to day 18 (Fig. 5C).
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Apparent photosynthetic rate, stomatal conductance, and leaf internal CO2 concentration:
The apparent photosynthetic rate (P0) increased with time in both control and K-deficient plants, with no significant difference up to 11 DAT (Fig. 6A). However, after 12 d of treatment, P0 decreased in the K-deficient plants compared with the control, and the difference increased with passage of time. The response of stomatal conductance to K deprivation was similar to that of P0 except for the first few days of the treatment (Fig. 6B). Leaf internal CO2 concentration remained similar in both control and K-deficient plants, except on some occasions after day 15.
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K uptake and partitioning:
Among the plant organs, the K concentration relative to dry matter was highest in the fruits and lowest in leaves, irrespective of the K available in the culture medium (Fig. 7). K deprivation decreased the concentration of K in all the parts during the 21 d period of treatment. The reduction was remarkable in the vegetative parts, while it was marginal in fruits. The percentage of K partitioning in fruit increased with passage of time in both control and K-deficient conditions, and peaked on days 14 and 21, respectively (Fig. 8). Conversely, the K partitioning percentage in leaves, stem, and roots decreased with time as the element was re-distributed to the growing fruits. K deprivation enhanced partitioning in favour of the fruits significantly; on day 21, the concentration was 44% in control and 68% in K-deficient conditions.
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13C partitioning:
13C atom% excess was the highest in the fed leaf, followed by the fruits, and least in the matured leaves (Table 1). It increased in the fed leaf but decreased in other parts, particularly in fruits. K deficiency decreased the export rate of 13C from the fed leaf to other parts, marginally on day 7 and significantly on day 14 after treatment (Fig. 9). Most of the 13C exported to various plant parts was partitioned into the fruits, followed by the stem (Fig. 10). K deficiency slightly increased the partitioning of 13C to fruits on day 7 after treatment. However on day 14, it had no significantly effect.
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Sugar concentration:
K-deficient treatment increased leaf sugar concentration in comparison with the control at 7 DAT (Fig. 11). Similarly, fruit sugar concentration in K-deficient plants was higher than that of the control throughout the experimental period. However, root sugar concentration decreased due to K deficiency treatment, and stem sugar concentration showed a similar trend only on day 21 after treatment.
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Experiment 2
Plant biomass production:
The increase in whole plant dry mass accumulation was slower in the K-deficient plants compared with the control during the 7 d period of treatment (Fig. 12). K deficiency led to suppressed stem growth more than growth of the other organs during the first 3 d of treatment. At 7 DAT, dry weight in the shoot apex, leaves, stem, and roots decreased to 96, 81, 74, and 80% relative to the control, respectively.
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Changes in stem diameter:
Stem diameter increased in both control and K-deficient plants during the treatment period. It exhibited a similar pattern of daytime shrinkage and night-time expansion in both control and K-deficient treatment at the end of 2 DAT (Fig. 13). However, K suppressed the increase in stem diameter at 3 DAT, and the depression was accelerated with the elapse of time up to 8 DAT.
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14C partitioning:
Among the plant parts studied, 14C specific activity was the highest in the fed leaf, followed by the stem and shoot apex (data not shown). The 14C specific activity was marginal in the other leaves and roots. These two tendencies of the 14C specific activity were observed over all measurements and were not affected by K deficiency. K deficiency reduced the amount of activity of 14C in the stem and increased the amount of activity of the fed leaf, but no major fluctuation occurred in the shoot apex, other leaves, and root on 3 DAT and 7 DAT (data not shown). The export rate of 14C into other plant parts from the fed leaf was higher on 3 DAT than on 7 DAT (Fig. 14). K deficiency decreased the export rate of 14C from the fed leaf into other parts on 3 DAT (control and K deficiency; 54% and 48%, respectively). A 7 DAT, this phenomenon was enhanced (control and K deficiency 48% and 39%, respectively). Most of the 14C partitioning was received by the stem. This tendency did not change throughout the treatment period in either control or K-deficient plants (data not shown). The 14C partitioning rates of the stem in the control and K deficiency treatments were 88% and 86%, respectively at 7 DAT.
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Discussion |
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Effect of K deficiency on growth and water relations
Increased K supply to plants increases translocation of assimilates (Huber, 1984), but the mechanism of this type of effect does not include any improvement of net carbon exchange rate (Geiger and Conti, 1983). In the present study, withdrawal of K from the culture media impaired tomato plant growth (Fig. 1). Similar responses to suboptimal K supply were observed in sugarcane (Hart, 1969, 1970) and sweet potato (Tsuno and Fujise, 1965). However, other researchers could not record such a response to K deficiency in Ricinus communis (Peuke et al., 2002) and tomato (Pujos and Morard, 1997). It is possible that K supply to plants improves phloem loading, unloading, and conversion of assimilates in sink tissues, and a deficiency retards such activities (Zhao et al., 2001). In this context, the present study has helped to resolve some of the controversies existing in the literature. Withdrawal of K from the culture medium did not alter the carbon exchange rate of the plant for some time (Fig. 6), but the effect was more constant on the sink organs such as stem and fruit (Figs 2, 3). K deficiency altered the water relations of the plant as evidenced by an enhanced degree of stem shrinkage during daytime, and the night-time recovery was only lower than the control at 15 d (Fig. 3). This pattern of response of the tomato plant to K deficiency is identical to the response of Japanese persimmon to salinity stress (Fujita et al., 2003a) and of tomato plants grown under P deficiency (Fujita et al., 2003b). Changes of stem diameter of Scots pine (Pinus sylvestris L.) are known to consist of two components: an irreversible component due to growth and a variable component due to moisture content that shows a diurnal cycle of expansion and shrinkage (Sevanto et al., 2002). Our previous studies showed that the stem diameter changes were similar in both woody plants, such as Japanese pear (Ito et al., 2002) and Japanese persimmon (Fujita et al., 2003a), and herbaceous plants such as tobacco (Moghaieb et al., 2006; Suwa et al., 2006) and tomato (Fujita et al., 2003b). The current results (Figs 2, 3) confirmed that K deficiency tended to reduce water supply to the plant by affecting the variable component, as daytime shrinkage of the stem was larger. According to the Münch hypothesis, phloem sap flow is driven by a turgor pressure gradient. The present study suggests that low stem water potential reduces phloem turgor, thereby decreasing assimilates moving into the sieve tube for growth of the sink organs from the source. These tissues regained water and expanded by night because of high turgor owing to the closure of stomata. On the other hand, the irreversible component of stem growth (Sevanto et al., 2002) reflects changes in wooden parts composed of cell wall materials such as lignin, cellulose, hemicellulose, etc. Synthesis of these materials is susceptible to deficiency of assimilates, but not to any deficiency of water at night.
Effect of K deficiency on source–sink relationship
Any change in stem diameter from one night to the succeeding night reveals the capacity for stem growth during one day. A similar estimation can be done for fruit growth. In the present experiment, positive correlations were observed between changes of fruit diameter build-up (Fig. 4) and fruit dry weight accumulation (Fig. 1) (control R2=0.99, K deficiency R2=0.83). Withdrawal of K from the culture media inhibited the stem and fruit diameter expansion on the fourth (Fig. 2) and sixth (Fig. 5B) days after treatment, respectively, suggesting that K deficiency suppressed the growth of stem and fruit on the corresponding days after treatment. This observation provides a precise assessment of the effect of K deficiency on tomato plant growth at an early production stage and facilitates quick replenishment of stock solution. In contrast, correct assessment of the stress on fruit harvest is not possible because of remobilization of the element from older organs (Pujos and Morard, 1997), and farmers are disadvantaged by not being able to take any corrective measures to achieve good production.
In addition, the study showed that K deficiency impairs photosynthesis activities of the tomato plant (Fig. 6). This result is consistent with reports in sugarcane (Hart, 1969), sugar beet (Terry and Ulrich, 1973), and cotton (Bednarz et al., 1998; Zhao et al., 2001). However, K deficiency did not influence leaf area of the plant as no difference could be observed between the control and K-deficient plants at 14 DAT (data not shown). The study also revealed that in K-deficient tomato plants, growth inhibition in stem and fruit occurred prior to depression of photosynthetic activity in the source leaves in Experiment 1 (Figs 1, 6). The phenomenon is supported by decreases in biomass and 14C partitioning in the stem by K deficiency prior to depression of the photosynthetic rate, as shown in Experiment 2 (Figs 12, 15). The adverse effect of K deficiency on the growth of stem and fruits might be consequential to reduction in assimilate translocation to these organs. There is evidence that K may exert a direct effect on the translocation of photosynthates in potato (Haeder et al., 1973), sugarcane (Hart, 1969, 1970), and tomato (Haeder and Mengel, 1972). In the present study, K deficiency marginally depressed the export rate of 13C from the fed leaf to other plant parts at 7 DAT (Fig. 9), although growth of the stem and fruit diminished at 4 d and 6 d, respectively. Consequently, more 13C-labelled photosynthates accumulated in the leaf (Table 1). These phenomena led to the increase of the leaf sugar concentration at 7 DAT in the K-deficient plants (Fig. 11). Similar effects of K deficiency, leading to elevation of leaf sugar concentration, were noticed in cotton (Pettigrew, 1999) and soybean (Huber, 1984). In addition, a suboptimal K level led to poor growth of the fruit (Fig. 1), which resulted in the accumulation of sugars (Fig. 11). It is assumed that increased sugar concentration in fruit may simply be related to strongly inhibited expansion of fruit (Fig. 4). It might be possible that the sugar concentration of the fruit increased because of enhanced activity of acid invertase (Huber, 1984). Feedback inhibition of photosynthesis as a result of decreased sink demand is a long known phenomenon (Roitsch, 1999). Different experimental approaches have shown that sugars play a key role in this regulation by repression of the expression of photosynthetic genes (Kock, 1996). The specific inhibitory effect of sugars on photosynthesis or on the expression of photosynthetic genes is further supported by some other reports (Morcuende et al., 1997; Felitti and Gonzalez, 1998; Winder et al., 1998). The present study identified that a suboptimal K level impairs growth of tomato plants due to depression of photosynthetic activity, but the depression of leaf photosynthesis is a consequence of the feedback inhibition induced by accumulation of assimilates not used in growth of the sink organs. The study also identified fruits as the strongest sink for K among the plant organs (Figs 7, 8). The high concentration of sugar and preferential allocation of K in the fruits under a suboptimal K level of the growing medium can ensure fruit quality, although not quantity. In addition, the micro-morphometry technique can be used as a reliable tool for the detection of K deficiency at the fruiting stage of tomato.
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Comparison of K and P deficiencies in tomato
Monteith's (1977) approach envisages that the effect of any limiting factor on crop growth can be analysed on the basis of dry matter accumulation, and the process is a function of the interception of incident light and its conversion to chemical energy. According to observations, both P (Fujita et al., 2003b) and K (present study) limitations in the growing medium affected the primary production of the tomato plant and partitioning of assimilates to the sink, and a concurrent change in water status was observed. P deficiency lowered the water status of the plant, possibly by affecting the activity of water channel proteins (Clarkson et al., 2000; Steudle et al., 2000) and maintenance of root hydraulic conductivity (Radin and Eidenbock, 1984). In contrast, the present study suggests that water status may decline under the present conditions, because K nutrition is necessary for root hair elongation (Rigas et al., 2001) and maintenance of cell turgor (Taiz and Zeiger, 2002). P deficiency did not affect leaf photosynthesis and diameters of stem and fruit compared with the control until a lag period of 12 d (Fujita et al., 2003b). However, under K deficiency, stem and fruit diameter increase was reduced within the first few days of treatment. Conversely, source activity (photosynthesis) declined at 12 DAT, possibly because of end-product inhibition. In the Münch pressure flow hypothesis, adequate phloem turgor is required for partitioning of carbon solutes from the source to the sink (Patrick, 1997). Both P and K deficiencies affected phloem turgor, leading to shrinkage of stem diameter and reduction in partitioning of solutes. However, the effect of the former was less imminent compared with the latter. Unlike P, K is a major solute for cell sap, and it can modulate cell water potential directly and instantaneously. For the first time, the studies reported here have identified that K nutrition is more important for tomato plants than P nutrition.
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