The Effect of Light and Water Supply on Growth, Net CO2 Assimilation Rate and Mineral Content of Salak (Salacca zalacca (Gaertn.) Voss) Seedlings

The study was carried out in a greenhouse of at the Institute for Horticultural Sciences of the Humboldt Universität zu Berlin. Seven-months-old seedlings of salak cultivars “pondoh” from Indonesia were used for the study. At this age, plants are usually transferred from the nursery to the field in the growing region. An experimental block design was arranged for the study. A total of 48 experimental plants was separated into two blocks, namely shading (S) and non-shading (N). Three water supply treatments were assigned to each block. At the beginning of the study, the seedlings for N treatment were moved from shading to non-shading tables in the greenhouse. Three water supply treatments were assigned to each block. At the beginning of the study, the seedlings for N treatment were moved from shading to non-shading tables in the greenhouse. Two additional 400 W lamps (HQI-TS/D, OSRAM, Germany) were placed 2 m above the seedlings in the N treatment. Three watering treatments, applied to each block, were W1 (100 ml distilled water per plant every 2 days), W2 (100 ml per plant every 4 days) and W3 (100 ml per plant every 6 days). Therefore, the combinations of the treatments were SW1, SW2, SW3, NW1, NW2 and NW3. Responses to be analysed were shoot and root dry weight, increment of shoot length and leaf area, net CO2 assimilation rate (PN) and plant mineral contents, i.e. N, P, K, Ca and Mg. In addition, water content of the growing media was recorded. The study results showed that shading, which reduced sunlight by 70 %, was not beneficial for 7-months-old salak seedlings, due to lower growth rate, PN and N content as compared with plants in non-shading conditions. On the other hand, light intensities above 800 μmol m s (PAR) possibly cause leaf bleaching and should be avoided. Salak plants did not tolerate drought conditions. However, different water supply did not affect PN and mineral uptake (N, P, K, Ca, Mg) of salak plants.


Introduction
Environmental conditions, such as water availability and solar irradiation may become stress factors for plants if the dosage is too high or too low (Lüttge, 1997).However, the major factor in terms of plant stress is the complex interaction between plant and environment (Lüttge, 1997;Biswal and Biswal, 1999).Many environmental factors individually may not provoke any stress but in combination they can cause harmful situations for plants.
Water is among the most limiting factors for plant productivity and growth rate worldwide (Pugnaire et al., 1999), because of its essential role in plant metabolism, both at cellular and whole-plant level.Any decrease in water availability has an immediate effect on plant growth and physiological processes such as photosynthesis or mineral transport and accumulation (Hsiao et al., 1976).Water loss by transpiration causes transient water deficit, therefore the most plants suffer at least regular and daily water shortages.When drying, soil causes water absorption to lag behind transpiration and permanent water deficit may result in wilting and plant death due to dehydration (Pugnaire et al., 1999).
Photosynthesis is the fundamental physiological process for plant growth and production.Besides plant-related factors, environmental conditions influence net CO 2 assimilation rate (P N ), including irradiance, water availability, CO 2 concentration and temperature (Baswal and Baswal, 1999).Excessively high solar irradiance can damage the photosynthetic system, particularly in shade-adapted leaves or in leaves where the photosynthetic CO 2 assimilation has been inhibited by other stressors such as extreme temperature or water deficit (Jones, 1992).The damage can be a result of photooxidation where bleaching of chlorophyll occurs.Leaf injury not accompanied by bleaching is usually termed as photoinhibition (Jones, 1992).Salak (Salacca zalacca (Gaertn.)Voss) is an understorey palm tree species growing at low light conditions in tropical rain forests of South-East Asia.Salak belongs to a group of palms, which are 1.5 -5 m high, extremely spiny and sprout their leaves from the ground level.There are 30 cultivars of salak, which distribute across the Indonesian islands (Sudaryono et al., 1993;Kusumo, 1995).The availability of superior quality fruit of selected cultivars will promote the marketing of salak, such as cultivars "pondoh" from Yogyakarta and "gula pasir" from Bali provinces."Pondoh" is cultivated in the sub district Sleman of the Yogyakarta province.In Indonesia, salak has been cultivated throughout the islands and the fruit is widely used as fresh fruit.Salak production in Indonesia has increased from 423.5 t in 2000 to 862.5 t in 2009 (Ministry of Agriculture, 2011).Fresh salak fruits have been exported to Singapore, Middle-East, Malaysia, Hongkong and China (Ministry of Agriculture, 2009).
In Indonesia, research on many aspects of salak fruit including pre-and postharvest have been carried out.Up to now, there is only limited knowledge about the physiological responses of salak to environmental factors.To promote production and fruit quality of salak, knowledge about ecophysiological aspects is very important.Thus, the purpose of this study was to investigate the effects of light and water supply on growth and physiological responses of salak seedlings.

Location and experimental design
The study was carried out for a period of 6 weeks (April to May 2003) in a greenhouse at the Institute for Horticultural Sciences of the Humboldt Universität zu Berlin.The temperature of the greenhouse were adjusted to 20 °C/25 °C (day/night) and the relative humidity was between 40 and 80 %.
Seven-months-old seedlings of salak cultivars "pondoh" from Indonesia were used for the study.At this age, plants are usually transferred from the nursery to the field in the growing region.The plant growing media was compost/sand (1/1, weight/weight).The compost media (pH 6) comprised of 320 mg/l N, 320 mg/l P 2 O 5 and 450 mg/l K 2 O and was mixed with quartz sand (0.6 -1.2 mm).Salt content was less than 3 g/l NaCl.Seedlings were planted in 14.5 cm x 11 cm pots.All plants tested were kept on greenhouse tables under shade nettings from germination until the beginning of the treatments.Shading was achieved using double shading net (Varia N2 9011, Wunder Lich, Germany), 2 m above the seedlings.Full sun light was reduced by 70 %.
An experimental block design was arranged for the study.A total of 48 experimental plants was separated into two blocks, namely shading (S) and non-shading (N).Three water supply treatments were assigned to each block.At the beginning of the study, the seedlings for N treatment were moved from shading to non-shading tables in the greenhouse.Two additional 400 W lamps (HQI-TS/D, OSRAM, Germany) were placed 2 m above the seedlings in the N treatment.Three watering treatments, applied to each block, were W1 (100 ml distilled water per plant every 2 days), W2 (100 ml per plant every 4 days) and W3 (100 ml per plant every 6 days).Therefore, the combinations of the treatments were SW1, SW2, SW3, NW1, NW2 and NW3.
Responses to be analysed were shoot and root dry weight, increment of shoot length and leaf area, net CO 2 assimilation rate (P N ) and plant mineral contents, i.e.N, P, K, Ca and Mg.In addition to that, water content of the growing media was recorded.

Water content of the media
Water content of the media was measured with Time Domain Reflectometry (TDR), Soil Moisture Meter (70M/m/92, Easy Test Ltd., Poland) before the plants were watered according to the treatments.Digital readings ranged from 0 % (dry) to 100 % (saturated).During the study, 5 sets of TDR data were recorded (replicates of three media per treatment) from week 1 until week 5 of the experiment.

Growth
Shoot and root dry weight were measured at the end of the study after drying the fresh samples at 103 °C to constant weight (Maier, 1990).Leaf samples were weighed after drying at 65 °C for 8 h and were later used for the analysis of mineral contents.Shoot length and leaf area were first measured immediately after the seedling had been transplanted into the growing media.The second measurement was 14 weeks later.Shoot length was measured with a ruler, leaf area was measured with a leaf area meter (CI -202, CID Inc., USA).

Net CO 2 assimilation rate
Net CO 2 assimilation rate (P N ) of the leaves were measured at week 1, 2, 4 and 6 after the onset of treatments with a portable photosynthesis measuring system (CI-301PS, CID Inc., USA).The measurements were conducted starting from 10 am until 12 am, except at week 6 when measurements were taken from 11 am until 1.30 pm.

Mineral content
Leaf mineral contents were analysed from dried and ground samples (Mikro-Feinmühle-Cullati DCFH 48, Janke & Kunkel, Germany).N content was determined following a modified Kjeldahl method as has been described by Okoye (1980).500 mg leaf powder were digested (Büchi 430 Digestor, Büchi Labortechnik AG, Switzerland) in boiling 98 % H 2 SO 4 (20 ml) and selenium catalyst (15348 Merck, Germany).The samples were thereafter distilled in a Büchi-Scrubber B-412 (Büchi Labortechnik AG, Switzerland) into 0.1 mol H 3 BO 4 and subsequently were titrated with 0.1 N HCl.N content was calculated and expressed as % of DM.P, Ca, Mg and K were analysed according to a modification method as described by Evenhuis and de Waard (1980).The samples were dry-ashed at 490 °C for 4 h.Ashes were dissolved in 25% HCl and were evaporated in a sand bath for 20 min until the complete disappearance of solid residues.P was determined photometrically (Eppendorf 1101 M, Germany).The other minerals were analysed using atomic absorption spectophotometry (AAS) (905 A, GBC, Australia).The data of the minerals were expressed as mg/g DM.

Statistical analyses
All sample data were subjected to compare means and further analysed with least significant difference (LSD) test (Steel et al., 1997), with significant difference between means determined (P < 0.05) using the Statistic Program MINITAB 14 (Minitab Inc., State College, USA, 2001).The analyses comprised of solely the effect of light conditions or water supply treatments on growth, P N and mineral content of salak seedlings.The effect of interactions between light conditions and water supply on growth, P N and mineral content of salak seedlings was also analysed.

Water content of the media
Water content of the growing medium of salak seedlings grown under different light conditions is summarised in Table 1.No differences in water content of salak seedlings media were found between shading and non-shading treatments up to week 3. From this time on, medium moisture of non-shading treatment (20.51 %) was always slightly lower than that of shading treatment (24.39 %).

Growth
The effect of light on growth parameters of salak seedlings is summarised in Figure 1 and Table 3. Root and shoot dry weight as well as the increment of leaf area of seedlings growing in non-shading conditions were higher as compared with those in the shading variant (46 %, 31 % and 14 % respectively).Similar results were reported by Stoneman and Dell (1993) and Stoneman et al. (1995) on Eucalyptus marginata seedlings, i.e. growth of all plant parts increased in response to higher light level.In grape (Vitis vinifera), leaf, shoot, trunk, root and total plant dry weights were reduced when growing under lower light level (Van den Heuvel et al., 2004); root growth decreased by 84 % in vines that had been shaded for a period of 16 weeks ( Van den Heuvel, 2002).Root dry weight of vines growing under 80 % shading were reduced by 51 % in the following year (McArtney and Ferree, 1999).The increment of shoot length of seedlings in the non-shading variant was smaller as compared with those under shading conditions (29 %).This can be explained as etiolation effect indicating that under the shading net, light intensity was too low for proper growth of salak seedling.
The effect of water supply on growth of salak seedlings is presented in Table 4.No differences of growth parameters were found among water supply treatments.However, there was a strong effect of water supply on leaf area.The reduction by 26 % in the increment of leaf area of both W2 (watered every 4 days) and W3 (watered every 6 days) treatments as compared with W1 (watered every 2 days) was found.The reduction of leaf area appears to be largely affected by soil water status and by root hydration (Termaat et al., 1985).This could be correlated with the result of water content of the media that more frequent watering resulted in higher water content of the media.On the other hand, Levitt (1980) stated that decreased cell turgor (cell enlargement) is the most sensitive plant response to water stress, since cell growth is quantitatively related to cell turgor, and cell turgor decreases with any dehydration-induced decrease in cell water potential.Factors which postpone dehydration by reducing water loss, such as a decrease in leaf area, are known to decrease productivity (Turner, 1979).Moreover, it was found, that salak plants are drought susceptible, since the plants began to wilt during a 2 to 3 weeks drought period (Lestari and Ebert, 2002) The growth parameters affected by light and water supply treatments are summarised in Table 5. Seedlings growing in full light with highest water supply had the highest root and shoot weight as well as leaf area increment.This result indicated non-shading and high water supply were the most suitable conditions for growth of salak seedlings.On the other hand, seedlings of the shading variant and high water supply developed the lowest root and shoot dry weight.Shading in the study obviously reduced too much light, which was required by seedlings for growing more vigorously.On the other hand, more frequent water supply and shading caused excessive high water content of the growing media which possibly lead to oxygen depletion.These conditions are regarded to cause the inhibition of leaf extension and cessation of root growth (Marschner, 1995).

Net CO 2 assimilation rate
Net CO 2 assimilation rate (P N ) of salak seedlings under different light conditions is presented in Figure 2.Under non-shading (N) condition during the measurements photosynthesis active radiation (PAR) was about 400 to 700 µmol m -2 s -1 , whereas shading (S) reduced PAR by 70 %.Until week 4, P N of seedlings in non-shading (varying in between 3.4 and 3.6 µmol CO 2 m -2 s -1 ) was higher as compared with in shading condition (varying from 2.2 to 3.0 µmol CO 2 m -2 s -1 ).This result gave evidence that shading limited P N in salak seedlings.Higher light intensity led to higher P N in salak seedlings of non-shading condition as compared with that of seedlings under a shading net, except the midday P N measurement at week 6.A great number of publications give evidence, that photosynthetic CO 2 -assimilation is limited by light intensity (Wong et al., 1978;von Caemmerer and Farquhar, 1981;Stoneman et al., 1995).Higher P N resulted in more chemical energy in the form of carbohydrates and other organic compounds, which are ultimately used for plant growth.Similar results have been reported for banana plants, where P N was lower under shading conditions as compared to plants in full sun light (Thomas and Turner, 2001).This was due to the limitation of P N by low photosynthetic photon flux density.The mesophyll resistance, which is usually regarded as the sum of biophysical and biochemical resistances to CO 2 movement between the mesophyll cell wall and the site of carboxylation in the chloroplast, was reported to be higher in plants grown at low light intensities (Holmgren, 1968, Crookston et al., 1975).
There was no net CO 2 gain in non-shaded plants at week 6, measured over midday hours (Figure 2).The phenomenon of low P N at intensive solar irradiation and high water vapour saturation deficit is well known for many plants species ("midday depression").In our experiment, this effect was induced by high light radiation (up to PAR of 1636 µmol m -2 s -1 ) during the P N measurement.The "midday depression" in plants subjected to water stress is associated with photoinhibition.Both light and water stress evidently initiate an activation of the primary photochemical pathway associated with photosystem II (Björkman and Powles, 1984).This could be proved by the occurrence of bleaching in some salak leaves subjected to full sun light.It is well established that photobleaching is a secondary phenomenon following photoinhibition (Powles, 1984).Similar photoinhibitory effects or midday reduction of P N at high photon flux density (more than 1000 µmol m -2 s -1 ) have been reported for water-stressed carambola (Ismail et al., 1994) and citrus plants (Brakke and Allen Jr., 1995).This was typically attributed to large leaf-to-air vapour pressure difference or high atmospheric vapour pressure deficit.Therefore, in the case of light-susceptible salak, shade is an important factor for the survival of plants especially with respect to water stress.
As known from a previous study, salak palms are very susceptible to high light intensities (Lestari and Ebert, 2003).P N of salak seedlings at different water supply is presented in Figure 3. Until week 4 of the treatment, there was no clear influence of water supply on P N (ranging from 2.5 to 3.4 µmol CO 2 m -2 s -1 ).However, P N decreased at week 6, in all water supply treatments.Moreover, in some P N measurements at week 6, there was no net CO 2 gain in W3 plants, but only leaf respiration.Some studies have suggested that the change in stomatal conductance was the main cause for the decrease in P N in water-deficient plants (Boyer, 1976;Cornic, 2000).In Nerium oleander plants, subjected to water stress, full sun light resulted in an inhibition of non-stomatal components of photosynthesis.This was manifested as a reduced photon yield and light-saturated capacity of photosynthetic CO 2 uptake, measured after restoration of high leaf water potential and partial reopening of the stomata (Björkman and Powles, 1984).Either stomatal closure or non-stomatal limitation might have induced midday depression of P N as observed in non-shaded salak plants.In case of stomatal closure, CO 2 supply from ambient air becomes the limiting step in the CO 2 assimilation process (Pugnaire et al., 1999).P N of salak seedlings affected by interaction of light and water supply are presented in Table 6.P N of plants in all water supply treatments with non-shading condition tended to be higher than that of the interactions with shaded plants.P N of NW2 plants at week 1 and 4 was found to be the highest.The midday measurement of week 6 indicated that plants under full sun light failed to assimilate CO 2 .The results indicate that light had a stronger effect on P N in salak seedlings than water supply.Some studies also reported that "midday-depression" of P N under high irradiance can be observed either in stressed or non-stressed young carambola plants (Ismail et al., 1994), in well-irrigated Protea acaulos (Herppich et al., 1994) and in young, well-watered Welwitschia mirabilis (Herppich et al., 1996).P N of salak is generally low, even at high PAR.This phenomenon is a characteristic of shade plants, which have very low light compensation points i.e. 0.5 -2 µmol m -2 s -1 in the extreme shade species Alocasia macrorrhiza (Jones, 1992).At high PAR, P N of some other tropical tree species, such as peach palm (Bactris gasipaes Kunth) is about 10 µmol CO 2 m -2 s -1 (Oliveira et al., 2002), oil palm 25 µmol CO 2 m -2 s -1 (Dufrene and Saugier, 1993), mango 8 µmol CO 2 m -2 s -1 (Fukamachi et al., 1998), banana is 19 µmol CO 2 m -2 s -1 (Thomas and Turner, 2001) and citrus is 11 CO 2 m -2 s -1 (Brakke and Allen Jr., 1995).

Mineral content
N content of leaves was strongly influenced by light conditions.There was no influence on the uptake of other ions (Table 7).Estimates of the quantity of nutrients supplied to plants by mass flow are not only based on the nutrient concentration in the soil solution but also on the transpiration (Marschner, 1995).Movement of nutrients to the root surface via mass flow and subsequently from the root into leaves are predominantly influenced by transpiration (Shaner and Boyer, 1976).Higher transpiration rates under full-light condition will result in higher nutrient uptake.These could possibly explain the increased N concentration in the leaves of non-shaded plants as compared to that of shaded plants.This result might also relate with reduced growth and lower P N of salak seedlings in shading as compared with in non-shading conditions.A strong relationship exists between total plant N concentration and the fraction of plant dry matter that is produced in leaf tissue (Agren and Ingestand, 1987).Furthermore, Agren (1985) and Hirose (1988) modelled the plant relative growth rate as a linear function over a wide range of whole-plant internal N concentrations.There is also a tight relation between leaf N and maximum photosynthetic activity (Le Bot et al., 1998).Mathematical functions have been applied for modelling the effect of N on P N (DeJong and Doyle, 1985;Hirose and Werger, 1987;Sage and Pearcy, 1987;Hirose at al., 1988;Lim et al., 1990).Since the growth of plants is a function of their photosynthesic capacity and allocation to leaf tissue, interrelationships between N productivity, leaf growth, net assimilation rate and relative growth rate can be modelled (Agren and Ingestand, 1987;Hirose, 1988) On the other hand, there were no differences in mineral contents of salak seedlings in all water supply treatments (Table 8).This indicated that water supply did not limit the nutrient uptake of the seedlings.
The contents of N, P, K, Ca and Mg in salak seedlings affected by interaction of light and water supply are summarised in Table 9. N content of SW1 plants was lower than that of NW2 and NW3 plants, but not different to that of the other interactions tested.On the other hand, no differences of N content in salak leaf were found between similar water supply treatment in non-shading and shading condition.These results indicated that light had a stronger effect on N content of salak seedlings as compared to water supply.Interactions of light and water supply did not affect mineral contents of salak seedlings in this experiment.
According to Marschner (1995), the optimal ranges of mineral content in plant shoot for growth (in mg/g) are as follows: N = 20 -50; P = 3 -5; K = <20 -50, Mg = 1.5 -3.5, and Ca = 1 -> 50, whereas optimal leaf element concentration for oil palm (Elaeis guinensis) according to Fairhorst and Härdter (2003) as follows (in mg/g): N = 14, P = 1.5 -1.9, K = 10 -13, Mg = 1.6 -2.2, Ca = 1.4 -2.5.In our experiments, salak seedlings showed nutrient contents being in the optimal range (Marschner, 1995), with the exception of Mg.This might be due to a limited availability of this element in the media, since there was no supply of nutrients to the seedlings during the experiment.Only N was found to be significantly lower in shaded plants.The results clearly indicate that under the experimental condition as explained above, nutrient uptake of salak seedlings was neither affected by shading nor by water supply.

Figure 1 .Figure 2 Figure 3
Figure 1.Root and shoot dry weight of salak seedlings under different light conditions (shading (S) and non-shading (N)).Different letters in the same column indicate significant differences by LSD test (P<0.05)

Table 1 .
. Leaf conductance in relation to assimilation in Eucalyptus pauciflora Sieb.Ex Spreng.Influence of irradiance and partial pressure of carbon dioxide.Plant Physiol, 62, 670-674.Water content of the growing medium of salak seedlings grown under different light conditions (shading

Table 7 .
Mineral content of salak seedlings under different light conditions (shading (S) and non-shading (N)) Values represent means + SE.No significant differences by LSD test (P<0.05)werefoundTable 8. Mineral content of salak seedlings under different water supply (every 2 days (W1), every 4 days (W2) Values represent means + SE.No significant differences by LSD test (P<0.05)were found Table 9. Interactive effects of different light conditions (shading (S) and non-shading (N)) and water supply (every 2 days (W1), every 4 days (W2) and every 6 days (W3)) leaves of salak seedlings Values represent means + SE.No significant differences by LSD test (P<0.05)were found