ST1A-1-3: Physiological Characteristics of Paper Mulberry ...posaa.kapi.ku.ac.th/Document/PDF/FinalRep2001_V1/... · ST1A-1-3: Physiological Characteristics of Paper Mulberry in Experimental

ST1A-1-3: Physiological Characteristics of Paper Mulberry in Experimental Plot

Suntaree Yingjajaval1, Bhuripong Damrongwut1, Jintana Bangjan1, Tada Chaisiha1,Cuttaliya Chutteang1 and Sutin Hiran-on1

ABSTRACTPaper mulberry, Si Satchanalai and Kozo cultivars were grown in the experimental field of

Kamphaeng Saen campus. The light response and CO2 compensation were measured for mature leaves of 24 days old. The photochemical efficiency, α, is in the range of 0.060 – 0.063 mol mol-1, and the Pm

values are 25.6 – 31.6 µmol m-2 s-1, with light saturation of about 880 µmol m-2 s-1. The CO2

compensation is 61, and the carboxylation conductance is in the range of 107 – 111 µmol m-2 s-1. The parameters indicate no distinct difference in the photosynthesis apparatus between the two cultivars. The similar light reaction rate is further confirmed by the measurement of cholrophyll fluorescence. The average quantum yield of dark-adapted leaf (Fv/Fm) is 0.822 for SS and 0.808 for KZ. The reaction centers of paper mulberry employed a photoprotection mechanism when the PPF intensified during the course of the day.

One measurement of diurnal change in gas exchange and water potential was made on leaves of 30 days old of 8 month-old plants in September, 2000. The matric potential of active root zone of 30 cm, was low at –53 kPa at the start of the day. So the root was in mild water stress. The sky was overcast, with only 2 hours of strong radiation during 10 – 12 hr. Kozo showed consistently higher rates of net photosynthesis (A), transpiration (E) and greater stomatal conductance (gs). The peak A was only half of Pm, which was the result of both low PPF and lower gs under stress. The gs of KZ was more responsive to change in total potential (ψt) of the leaf and was higher than SS’s at the same level of ψt. The solute concentration of leaf sap changed following the rate of A, which was higher for KZ. Averaged over day, KZ leaves were at lower total potential, but the higher solute concentrations enabled the leaves to gain slightly higher turgor than the SS’s. The biomass determination of each

1Faculty of Liberal Arts and Science, Kasetsart University Kamphaeng Saen, Nakhonprathom 73140, Thailand

Final ReportThe Research Project for Higher Utilization of Forestry and Agricultural Plant Materials in Thailand (HUFA)

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plant at 5 months old showed that KZ plant had more dry mass. We propose from this study that the faster growing rate of KZ does not come from the higher performance of the potential photosynthesis, but from the more dynamic opening of stomata, thus the higher rates of gas exchange under the field condition.

Paper mulberry grows better under a forest canopy. Cultivating the plant in farmer’s field results in low yield. The plant is prone to damage caused by unfavorable soil water regime. We set up the study of paper rmulbery in the experimental plot to measure primarily the basic process of its gas exchange rate. The data would shed more light to the understanding of the relations of environmental factors and the gas exchange process of the plant.

MATERIALS AND METHODSTwo cultivars of paper mulberry are under study to obtain basic data on their physiological

behavior. They are Si Satchanalai (SS), a local cultivar, and Kozo (KZ), a cultivar from Japan. The seedlings, provided by KAPI are transplanted into the experimental polt of the Tropical Vegetable Research Center in Kamphaeng Saen campus starting from January of 2000. The planting spacing was 2x2 m2, and there were 20 trees of each cultivar. The soil in the field is Kamphaeng Saen soil series (Typic Haplustalfs), which is neutral with pH in the range of 6.5 – 7.5, and high in phosphorus and potassium content. The field has high water table in the rainy season, which can be as high as 30 cm from the soil surface. Several basic data are obtained with details as follow.

Growth rate of the leafThe development of leaf is based on 3 leaves from 3 trees of each cultivar. Leaf age is the

number of days since budbreak. Length was measured everyday when leaf length reached 1 cm until the leaf reached 45 days.

Result is shown in Figure 1. Leaves of both cultivars have similar rate of development, with a rapid length extension from days 4 – 20 since budbreak. Growth rate for SS is 0.775 cm d-1, and 0.755 cm d-1 for KZ. During the period of 21 – 25 days, length growth started to slow down and stopped after 25 days. Leaf growth was complete after 25 days. Averaged maximum length of SS is 12.7 cm, while of KZ is 13.0 cm.


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One hundred mature and healthy leaves of each variety were selected to determine the leaf area, length and width with a leaf area meter (Cl-203CA by CID, U.S.A.), then they were weighed to obtain the fresh mass. The leaf statistics and the relations between parameters are shown in Table 1.

Figure 1. The extension of leaf length of two cultivars of paper mulberry

Table 1. Basic statistics of paper mulberry leaf, derived from 100 mature leavesa. Si Satchanalai

Area (A),cm2

Length (L), cm Width (W),cm

Fresh mass(Mf), g

Mf/Ag,-2

Average 185.48 20.00 19.94 3.49 186.58Standard deviation 62.41 3.73 1.92 1.35 26.55Maximum 332.52 28.14 16.68 6.75 386.76Minimum 23.53 8.40 8.15 0.91 163.01


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b. KozoArea (A),

cm2Length (L), cm Width (W),

cmFresh mass

(Mf), gMf/A

g,-2

Average 136.05 18.24 13.17 2.50 180.38Standard deviation 54.72 3.98 2.58 1.12 17.71Maximum 235.47 24.85 15.88 4.44 213.09Minimum 43.91 9.37 7.34 0.75 127.49

Table 2. Relations of leaf area to other parameters (see abbreviations and units in Table 1)a. Si SatchanalaiRelationships R2

A = 26.301 + 45.596 *Mf 0.966A = -218.44 + 28.967 *W 0.791A = -131.74 = 15.860 *L 0.897A = - 19.297 + 0.718 *W*L 0.924W = 4.995 + 0.447 *L 0.757b. KozoRelationships R2

A = 15.591 + 48.134 *Mf 0.972A = -131.23 + 20.301 *W 0.919A = -109.08 + 13.438 *L 0.956A = - 6.845 + 0.572 *W*L 0.972W = 1.776 + 0.624 *L 0.926

Most of Si Satchanalai leaves have a lobe on each side of the margin, while the leaves of Kozo are, in all large proportion, simple leaves. This gives Kozo a better correlation of leaf area to its legth and width. For both cultivars, the fresh mass of the leaf gives a good estimate of the area.


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hight response functionMeasurement of net photosynthesis is by portable photosynthesis open system (Ll 6400 by

Licor, Nebraska, U.S.A). Conditions inside the leaf chamber were as follow: the carbon dioxide concentration going into the leaf chamber was set constant at 400 µmolCO2 mol-1 air (= 40 pa), RH at 60%, leaf temperature at around 320, with leaf-air vapor pressure deficit averaged at 1.33 kPa. The net photosynthesis rate is measured under photosynthetic photon flux (PPF) ranging from 0 – 2,000 µmol mol-2 s-1. Two mature leaves of each cultivar were selected at the age of 24 days. The 4 leaves were measured within the same day, from morning to early afternoon, starting with Si Satchanalai’s leaves.

Figure 2. Light response function of paper mulberry, (a) net photosynthesis as a function of photosynthetic photon flux; (b) the corresponding stomatal conductance for water vapor. SS is Si Satchanalai and KZ is Kozo, 1 and 2 are leaf sample number


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Result is shown in Figure 2a. Net photosyntesis reaches the maximum of 24.2 – 25.1 µmol mol-2s-1 for SS and 21.9 25.6 µmol mol-2s-1for KZ. The data is fitted to a non-rectangular hyperbola function (Thornley and Johnson, 1990) of:

(1)

Where A is net photosynthesis; I is photosynthetic photon flux; Pm is maximum net photosynthesis rate; Rd is dark respiration rate; α is quantum or photochemical efficiency; and θ is the curvature factor. The result gives the following parameters:

Table 3. Parameters for light response function for 4 mature leaves, 24 days old, of Si Satchanalai and Kozo cultivars of paper mulberry

Si Satchanalai KozoParameters

Leaf 1 Leaf 2 Leaf 1 Leaf 2α, mol mol-1 0.0632 0.0601 0.0601 0.0635θ 0.6101 0.5827 0.6268 0.4624Pm, µmol mol-2s-1 29.3113 29.1204 25.6481 31.6458Rd, µmol mol-2s-1 1.4683 1.7456 1.6375 1.6931gd/gc 0.6391 0.7160 0.5954 1.1627lc, µmol mol-2s-1 24 30 28 27ls, µmol mol-2s-1 853 898 799 967

The two leaves of Si Satchanalai gave more consistent values than the Kozo’s. In all, the 4 leaves gave similar average values of the paprameters. The light compensation, lc, had average value of 27 and light at saturation, ls, was 880 µmol mol-2s-1. The curvature factor, θ, has the physical meaning of θ = rd/(rd+rc), where rd is total diffusive resistance of CO2 from the atmosphere to chloroplast, and rc is the carboxylation resistance. With the conductance being the inverse of


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resistance, the ratio of diffusive to carboxylation conductance (gd/gc) can be derived from the θ value. The higher ratio of gd/gc indicates that the leaf has a higher diffusive conductance. This can be expressed by the difference in stomateal conductance for vapor as a function of PPF as shown in Figure 2b. The higher conductance gives higher Pm. Later with the measurement of CO2 compensation, we can show that the two cultivars have similar carboxylation conductance. Therefore, the two cultivars show clearly the difference in the stomatal conductance of the leaf.

The light response functions of the above leaves show that there can be variation is the sample and the condition of the environment, which has stronger influence on the stomatal behavior of the leaf than locally imposed by the conditions set inside the leaf chamber. In conclusion, the measurement shows that the difference between the two cultivars, if it exists, is not distinct. The light apparatus seems to function with equal efficiency. The clearer difference is in the magnitude of the stomatal opening.

Carbon dioxide compensation pointThe same leaves that light responses were obtained, i.c., SS, leaf 2 and KZ, leaf 1, were used

for the study. The measurement was made in the late afternoon of the same day, after finishing the light response. With Ll 6400, the net photosynthesis was obtained by setting the PPF at 2,000 µmol mol-2s-1, leaf temperature at 33C and RH at about 60%, the resulting leaf-air VPD values were 1.3 – 1.8 kPa. The CO2 concentration varied ranging from 50 – 400 µmolCO2 mol-2 air. The relation of net photosynthesis (A) to the concentration of CO2 in the intercellular space (Cl) of the leaves is linear, which is

A = gc (Cl – Γ) , (2)

Where gc is the carboxylation or mesophyll conductance and Γ is the CO2 concentration is the chloroplast or the compensation point. CO2 compensation point is Cl when the net photosynthesis is zero, and the linear plot of A as a function of Cl yields the slope being the carboxylation conductance. Typically, a C3 plant has Γ value in the range of 50 – 100 µmol mol-1, while the C4 plant has the


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value approaches zero. Environmental factors that cause injury of the cell will increase the value of Γ(Farquhar and von Caemmerer, 1982).

The linear functions for the two cultivars are shown in Figure 3. Both cultivars yielded the parameter of similar magnitudes. CO2 compensation for SS was 61.1 µmolCO2 mol-1, and 61.8 µmolCO2 mol-1 for KZ. The carboxylation conductances were 111 and 107 mmol m-2 s-1, respectively. It is interesting to see that the linear function of Cl vs. net photosynthesis extends up to the maximum net photosynthesis rate, indicating that A is strongly coupled to CO2 content.

The conclusion derived from the measurement is that the photosynthesis and respiration of the two cultivars proceed at the same rates. The CO2 compensation is within the range of C3 plant, but the carboxylation conductance is higher than the values obtained for mango and tangerine, indicating that the net photosynthesis rate should be higher. Together with the light response curve, we can conclude tha both the light and dark reactions in the leaves of the two cultivars are of the same rates. The same efficiency of light reaction is further substantiated by the subsequent measurement of chlorophyll fluorescence. The difference in the net phogosynthesis rate between the two cultivars is a function of the magnitude of stomatal conductance.

Figure 3. Relation of the intercellular CO2 concentration and the net photosynthesis of paper mulberry. The intercept of the linear function is the CO2 compensation point, and the inverse of the slope is the carboxylation conductance


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Diurnal changes in the chlorophyll fluorescence, gas exchange and water potentialOn 28 September 2000, when the trees were 8 months old, we carried out an hourly

measurement of leaf chlorophyll fluorescence, gas exchange and water potential. The day was overcast with bright sunshine only during the hour of 10 –12. altogether about 20 – 23 mature, healthy leaves, of 30 days old, from about 10 trees of each cultivar were selected. The round started with the measurement of dark-adapted followed by light chlorophyll fluorescence (Mini-PAM by Heinz Walz, Germany). We then measured the transpiration rate of both sides of the epidermis with the steady state porometer (Ll-1600M by Licor, Nebraska, U.S.A.). After the leaf was exposed to light for no less than 15 minutes, we made the measurement of transpiration and photosynthesis with portable photosynthesis close system (Ll-6200 by Licor, Nebraska, U.S.A.). The last step was to excise the leaf for the measurement of total potential using the pressure chamber (SoilMoisture Equipment, Santa Barbara, U.S.A.). The leaf tissue was submerged in the liquid nitrogen, which was later used in the measurement of sap solute potential with the dewpoint micorvoltment (HR-33T by Wescor, Logan, U.S.A.).

Chlorophyll fluorescenceThe part in the tip region of the leaf was covered with black paper for about half an hour,

then a dark leaf clip (DLC-8) was inserted for another 5 minutes. Dark-adapted measurement (mode 13) was made setting the intensity = 5 and gain = 4. After that the clip was removed to expose the part to light for about 2 minutes. Measurement under light was made with mode 1, maintaining the same intensity and gain setting, and using leaf-clip holder (2030-B).

The result is shown in Figure 4. The dark-adapted quantum yield (φ =Fv/Fm), or the photosynthetic efficiency was relatively the same for the two cultivars. The average for the day was 0.822 for SS and 0.808 for KZ. The values are not much deviated from the typical value of 0.832 +0.004 at 77K for vascular plants. The maximum, fm, and minimum, Fo, fluorescence under dark adaptation was also very similar between the two cultivars. The result means that the light use efficiency at PSll is of the same magnitude for SS and KZ. The data support the finding of light response study, the the light reaction rate is the same for both cultivars.


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The light quantum yield varies in response to the intensity of radiation. The resulting changes in light quantum yield in Figure 4a clearly shows that light use efficiency was lowest when the PPF was strongest during the 10 – 12 hr. of the day. The relation can be shown more explicitly in Figure 5a. Leaf of paper mulberry could use most of PPF at the efficiency level of 50 – 80% when PPF was under 100 µmol mol-2s-1. The quantum efficiency dropped quidkly as the radiation intensity increased, with the level decreased to 10 – 20% when PPF was over 600 µmol mol-2s-1. This is close to the result of light response study that the saturation light for maximum net photosynthesis rate is about 880 µmol mol-2s-1.

The Fm value also decreased as the PPF intensified. The decrease in Fm indicates that the PSll has engaged a photoprotection mechanism, which is mostly associated with the xanthophyll zeaxanthin cycle at LHCll (Epron et al., 1992; Angelopoulos et al., 1996). When the PPF becomes strong, the excess energy is dissipated by the leaf as photochemical quenching, qP, and non-photochemical, NPQ, mostly the thermal quenching. During the day when the PPF exceeded 600 µmol m-2s-1, the value of qP dropped to zero, while that of NPQ increased. For Walz Mini-PAM fluorometer, the values of qP are valid in first approximation only, while NPQ values which reflect heat-dissipation of excitation energy in the antenna system, are a good indicator for excessive light energy (Heinz Walz GmbH, 1996).

The chlorophyll fluorescence study indicates that the PSll reaction center of paper mulberry uses PPF at high efficiency, with light saturation level around 600 µmol m-2s-1. The reaction center also employs a photoprotection mechanism when the PPF intensifies. Although both cultivars have similar magnitude of light use efficiency, the SS plant seems to adapt a more pronounced degree of photoprotection. This means that at high PPF, SS leaf can use the light more efficiently, and can dissipate more of the excess light energy via the non-photochemical quenching process.


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Figure 4. Chlorophyll fluorescence yield (a) of paper mulberry leaves under daylight and dark adaptation. Quantum yield or the photosynthesis efficiency, φ = Fv/Fm, where the variable fluorescence, Fv = Fm-Fo, with Fm and Fo are the maximum and minimum fluorescence of dark-adapted sample. Maximum and minimum fluorescence under (b) dark adaptation, and (c) daylight


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Figure 5. Quantum yield measured under daylight shows a rapid decrease as the PPF increases. When the radiation is strong, photochemical quenching, qP = (M-F)/(M-Fo) diminishes and the non-photochemical quenching, NPQ=(Fm-M)/M plays an increasing role. Fm and Fo are the maximum and minimum fluorescence of dark-adapted sample; M and F are the maximum and minimum fluorescence for sample under daylight


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Gas exchange of the two sides of the epidermisMeasurement of transpiration with the steady state proromenter (Ll-1600M) is useful for

differentiating the transpiration rates between the two sides of the leaf epidermis, the abaxial or the lower side and the adaxial or the upper side.

From the same side of the leaves, KZ lost more water than SS, espcially around noon (Figure 6a, b), and the lower epidermis transpired more than the upper.

The corresponding changes in the total (boundary and stomatal) conductance are shown in Figure 6c and d. The leaf of KZ had a much higher total conductance, especially during noon period than the SS’s. the ratio of the total conductance, gad:gab, for SS is 3.62 and 2.93 for KZ. It remains to be verified by anatomical study of the two sides of the leaf of both cultivars.

Transpiration (E) and net photosynthesis (A) of the leavesMore details on the gas exchange are obtained with the measurement of both transpiration (E)

and net photosynthesis (A) with a closed system. Figure 7 shows the PPF and the leaf-air vapor pressure deficit of the leaf throughout the day. The changes in E and A are shown in Figure 8a and b. The transpiration rate was within the range obtained in the light response study. The obvious difference was is the net photosynthesis rate. The maximum values from the light response measurement was about 24 – 25 µmol m-2s-1, but the present values reached the peak of only about 12 µmol m-2s-1, namely half of the potential. This may due partially to the low PPF and the small stomatal conductance as shown in Figure 8c. The stomatal conductance of SS was very high before 9 hr., then stomates seemed to close abruptly and remained partially close afterwards. The KZ leaves had higher conductance, mostly in the morning session. The partial closure of stomates in the afternoon was caused by the low radiation of the day, which was in the range of below 300 µmol m-2s-1. Strong sun lasted only 2 hr. from 10 – 12 hr. During this period, the leaf-air VPD increased to the level of 2 – 3.5 kPa. That the stomates did not open fully, even though the atmospheric condition was mild, was because the water potential of the active root zone was low. The top 30 cm of root zone had the matric potential in the range of –52 to –80 kPa during the daytime. So the water stress in the root zone could contribute to the partial closure of the stomaters.


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Si Satchanalai seemed to be effected by the soil water stress more than the Kozo. With the closing down of the stomates, the ratio of water loss to net CO2 gain, namely the ratio of E to A was relatively low for paper mulberry for the day. The interesting point is that KZ leaves carried out higher rates of both E and A making its E to A ratio comparable to that of SS (Figure 8d).

Figure 6. Two sides of leaf epidermis of paper mulberry have different rates of transpiration (a) and (b) and total leaf conductance (c) and (d). Kozo lost more water than Si Satchanalai and the total conductance was higher during the high vapor pressure deficit period of the day


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Figure 7. The atmospheric conditions when the diurnal gas exchang was measured. The sky was overcast with low radiation in the afternoon. The leaf-air VPD was mild througout the day and was high only for two hours in the middle of the day. (a) the photosynthetic photon flux, (b) the leaf-air vapor pressure deficit and (c) the difference of temperature between the leaf and the air


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Figure 8. The change in gas exchange rates during the day of paper mulberry leaves. (a) the net photosynthesis, A; (b) the transpiration, E; (c) the stomatal conductance to vapor and (d) the ratio or transpiration to photosynthesis. Kozo showed consistently higher values in all parameters


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Net photosynthesis as a function of PPF druing the day is shown in Figure 9a. The feature is similar to the previously obtained light response function, with light saturation at about 700 µmol m-2s-1. The transpiration rate as a function of its driving force, i.e., the leaf-air VPD, as shown in Figure 9b differentiates clearly between the two cultivars. At the same level of leaf-air VPD, the KZ’s leaves transpire more than the SS’s. The transpiration rate increases linearly according to the driving force, up to leaf-air VPD of 3.5 kPa, after which athe transpiration rate starts to drop off. The difference in transpiration at the same level of the driving force means that the leaf conductance is not equal.

Figure 9. The gas exchange rates of paper mulberry leaves during the day show that the rate is controlled by its determining factor. (a) photosynthetic photon flux was a controlling factor at the level below 700 µmol m-2s-1, (b) transpiration increased linearly following the leaf-air vapor pressure deficit up to 3 kPa


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Water potentialSoil matric potentials (ψm) at 4 depths in the root zone were recorded on an hourly basis

with tensiometers. The result is shown in Figure 10. Matric potentials for the depth of down to 40 cm show the soil water to be relatively dry with ψm in the range of –40 to –55 kPa. Normally, we will irrigate the root zone of 30-cm layer so that potential remains not less than –30 kPa. The deeper soil at 50 cm is still wet. So the condition was that the plants were in a mild water stress, especially for the active root zone of 30-cm layer. During the day we can see the loss of water in the top 30 cm, which is obvious during the period of 10-14 hr of the day. This period corresponds to the time of high transpiration rates when PPF became strong. There is a brief recovery at 15 hr, after which the potential fall gradually. So during the day, we can see the loss of water from the soil due to root uptake. The increase in potential in the 40-cm layer means that there is an upward movement of water from the 50 cm soil depth. There is water movement from the lower depth upwards towards the active zone of the root.

Total potential (ψm) of leaf is obtained from the pressure chamber. The leaf is then frozen and thawed to press out the sap of the tissue to determine the solute potential (ψπ). Turgor pressure is the difference between the two ((ψp = (ψr - ψπ) . The diurnal changes of all the 3 potentials are shown in Figure 11.

Figure 10. Changes in soil matric potential during the day. The active root zone of 30 cm was in a mild stress condition. There was higher water potential in the deeper zone, which would flow up to replenish the upper layer


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Figure 11. Diurnal changes in leaf water potential of paper mulberry ; (a) total potential, (b) solute potential and (c) turgor potential. With a big drop in total potential around noon, the leaf lost its turgor to a large extent. Negative turgor may be due to the fact that the apoplast fraction of the leaf tissues was not accounted for in the values of solues of solute potential


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The response of total potential ot atmospheric condition is apparent during the day. The initial values in the early morning were at –200 for SS and –500 kPa for KZ, which are there was a corresponding sharp drop in the total potential for both cultivars. The change in KZ potential was greater than SS. Averaged throughout the day, the total potential for SS was – 789 kPa and –820 kPa for KZ.

The solute potential for KZ went through a more pronounced cyclic change than SS. The average for the day was –1,437 for SS and –1,495 kPa for KZ. In other words, the solute concentration (Cl) of KZ sap is more concentrated, with the corresponding values being 580 and 604 mol m-3. The interesting feature is the coupling of ψπ with net photosynthesis rate (A). This is shown in Figure 12. During the first half of the day, Cl follows closely the change in A. As noon passed, PPF dropped sharply, so did the photosynthesis. The Cl remained high in the leaf, still followed the change in A. Towards the end of the day, Cl dropped gradually. The change in solute concentration of leaf sap seems to follow closely the photosynthesis process.


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Figure 12. The change in solute concentration of leaf sap (ci) seemed to correspond to the rate of net photosynthesis (A) in the first half of the day. In the afternoom, A decrease was due to low radiation, together with the low stomatal conductance. The translocation rate may be affected, resulting in solute concentration remaining high in the leaf

The turgor potential shows that KZ leaf was in a better water status than SS during the morning session, but then it lost its turgor more severely than SS in the afternoon. It is doubtful about the 2 points of negative turgor. This is an intrinsic error in the methodology of not correcting for the apoplast fraction of the leaf tissue. The negative values should be taken simply that the leaf must have lost its water potential substantially when the leaf-air VPD becomes high while the matric potential is low. There is a temporarily lapse of water supply to meet the demand. KZ regained its turgor quite late in the day, at 16 hr.


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Stomatal conductanceThe movement of stomata is governed by 5 major factors, i.e. PPF, leaf temperature, leaf-air

VPD, leaf intercellular CO2 concentration and leaf water potential. These relations are shown in Figures 13 and 14. Higher radiation intensity is not favorable for the leaves. High PPF is the cause of high leaf temperature. For KZ, the threshold of 35 C seems to lead to partial closure of stomata. At leaf-air VPD of less than 2.0, the stomata opening is not effected, but will be when the value is higher than 3 – 3.5 kPa. Under all factors, the stomatal conductance of KZ leaf was consistently higher than SS’s. The factor that has the most impact seems to be the leaf water potential.

When we consider the conductance as a function of total potential as in Figure 14a, the KZ’s conductance showed the response at low potential, but the response of SS was not apparent. The stomates of SS were mostly closed for the day. However, when we show the stomatal conductance together with the total potential as a function of the hour of the day (Fig 14 b and c), the stomatal behavior of SS leaves showed a pattern. As day progressed, the drop in potential led to the closing of the stomates. After the stomata opening was narrow down, the water loss decreased, giving the leaf time for water replenishment. But from 11-15 hr., even though the water potential recovered to a higher value, the stomates did not open accordingly. The reopening of the stomates occurred around 15 hr, which caused the potential to drop again. After that the stomates seemed to close down in another cycle, with partial recovery towards the end of the day. Water optential then recovered and was back to the morning level. The closure of SS stomata was severe in response to water stress of the leaves, which was caused by the dryness of the soil in the active root zone.

The same plot foe KZ revealed another pattern. The opening of stomates brought about the higher rate of transpiration in the morning. The water loss then caused the leaf total potential to decrease and the closing down of the stomates around 11 hr. After the transpiration rate was reduced, the replenishment of water in the leaf was faster, resulting in the recovery of optential and the reopening of the stomates at 13 hour. In the afternoon, the closure of stomates after 15 hr. was in response to the low PPF, leaf water potential returned consequently to the same lavel as in the morning.


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The changes of stomatal conductance indicate that the KZ stomates are more dynamic than the SS’s. The conductance of SS remains at constantly low level once the stress

Figure 13. Changes in stomatal conductance to vapor of paper mulberry leaves as functions of environmental factors. Kozo had an higher conductance and showed the stomatal opening to be effected when a certain limit was reached. Si Satchanalai had a much lower conductance and the values remained unchanged throughout the day


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Figure 14. Stomatal conductance to vapor as a function of total potential. (a) Kozo had a higher conductiance than Si Satchanalai at the same level of leaf total potential; (b) as total potential dropped to –1,400 kPa in Si satchanalai leaves, conductance decreased further and remained unchanged almost the whole day; (c) Kozo’s stomatal conductance also decreased when total potential reached – 1,400 kPa, but recovered and even increased when the potential became higher


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Progresses and does not recover fully. KZ, on the other hand, shows a greater degree of stomatal closure and reopening during the day. The response of stomates to leaf water stress needs to be confirmed in later study.

BiomassOnly 1 tree of each cultivar was cut at the age of 5 months. The fresh (Mf) and the dry (Md)

masses of all sections of the tree are shown in Table 4. The KZ cultivar had a better growth rate than SS. The shoot part of KZ comprised more mass than SS, resulting in higher shoot to root ratio. However, in terms of partition of the total dry mass, SS and significantly higher mass and higher proportion of the lateral roots. The partition of dry mass shown in Figure 15.

Table 4. Partition of fresh and dry mass in various parts of paper mulberry tree; SS = Si Satchanalai and KZ = Kozo cultivar at the age of 5 months

a. Whole plantDetail UnitCultivar SS KZStem girth Cm 13.3 12.5Stem diameter Cm 4.4 4.0Plant height M 1.35 1.65Total fresh mass G 4,095 4,970Total dry mass G 668 933Dry mass percentage % 16.3 18.8Leaf area m2 5.95 7.79Leaf area index 1.5 1.9Number of leaf 423 741


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b. Fresh (Mf) and dry (Md) mass of various partsMf,g Md,g

PartsSS KZ SS KZ

Stem+Branch 1,446 2,107 402 476Leaf 1,305 1,618 149 349Petiole 240 164 38 35Lateral roots 880 748 272 152Stump 224 333 78 76Shoot total 2,991 3,889 589 857Root total 1,104 1,081 351 228Plant total 4,095 4,970 668 933Shootroot 2.71 3.60 7.51 11.24Leteral root:Shoot 0.294 0.192 0.462 0.177

Within 5 months, KZ plant had accumulated considerable more dry mass than SS. For the stem and branch part, it had 17% higher dry mass. This should mean that KZ must have higher rate of assimilation and growth metabolism. When we determined the light response and CO2 compensation, we see no clear evidence of a higher capability of the photosynthesis apparatus of KZ. But the data from the diurnal gas exchange and water potential lead to the proposal that the mechanism of stomatal opening is responsible for the higher rate of net assimilation rate under the mild water stress condition. KZ leaf showed higher stomatal conductance under the same level of PPF, leaf temperature, leaf-air vapor pressure deficit and total potential than SS’s. The higher conductance resulted in higher gas exchange. However, KZ managed to use water efficiently, by maintaining fairly constant E to A ratio throughout the day, even when the leaf-air VPD increased. The stomatal conductance of KZ also seemed to be more dynamic, being able to change more quickly in response to the dropping and regaining of total potential during the day. The stomata frequency and its structure remains to be verified by the study of leaf anatomy.


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Figure 15. The partition of dry mass into various parts of the 5 month-old paper mulberryplant

REFERENCESAngelopoulos, K., B. Dichio and C. Xiloyannis. 1996. Inhibition of photosynthesis in olive trees (Olea

europaea L.) during water stress and rewatering. J. Exp. Bot. 47: 1093 – 1100.Epron, D., E. Dreyer and N. Breda. 1992. Photosynthesis of oak trees [Quercus petraea (Matt.) Liebl.]

diromg drought under field conditions: diumal course of net CO2 assimilation and photochemical efficiency of photosystem II. Plant, cell and Environ. 15: 809-820.

Farquhar, G.D. and S. von Caemmerer. 1982. Modelling of photosynthetic response to environmental conditions. Pp. 549-587. In: Lange, O.L., et al. (eds.). Physiologica l Plant Ecology II: Water Relations and Carbon Assimilation. Encyclopedia of Plant Physiology, Vol: 12B. Springer-Verlag, Berlin-Heidelberg.

Heinz Walz GmbH. 1996. Photosynthesis Yield Analyzer Mini-PAM, Portable Chlorophyll Fluorometer, handbook of Operation. First edition. Eichenring, Germany.

Johnson, I.R. and J.H.M. Thornley. 1984. A model of instantaneous and daily canopy photosynthesis. J. theor Biol. 107: 531-545.

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