- Email: [email protected]
Soil water stress and the growth and yield of potato plants grown from microtubers and conventional seed tubers
Field Crops Research 95 (2006) 89–96 www.elsevier.com/locate/fcr
Soil water stress and the growth and yield of potato plants grown from microtubers and conventional seed tubers Jackson Kawakami, Kazuto Iwama *, Yutaka Jitsuyama Department of Botany and Agronomy, Graduate School of Agriculture, Hokkaido University, N9, W9, Kita-ku, Sapporo 060-8589, Japan Received 10 December 2003; received in revised form 4 February 2005; accepted 4 February 2005
Abstract Tuber yields of potato plants grown from microtubers in fields are more variable than yields from conventional seed tubers (CT). One reason could be their higher susceptibility to water stress. This study clarified the effect of soil water stress from 1 month after emergence on the growth and yield of plants grown from conventional seed tubers and microtubers in fields. Microtubers (0.5–3 g) and conventional seed tubers (50 g) were grown in Hokkaido, Japan, over three field seasons. One month after emergence, polyshelters were placed over the plots to prevent rainfall, and either irrigated (wet plot) or non-irrigated (dry plot) treatments were formed. At mid-flowering (about 50 days after emergence) leaf area index (LAI) in microtuber plants was decreased relatively more due to soil water stress than LAI in conventional seed tuber plants. However, at maximum shoot growth (about 80 days after emergence) both microtuber and conventional seed tuber plants had a similar relative decrease in LAI due to soil water stress. At midflowering and maximum shoot growth microtuber and conventional seed tuber plants had reduced stomatal conductance due to soil water stress, but the reduction in stomatal conductance was greater in conventional seed tuber plants than in microtuber plants. Microtuber and conventional seed tuber plants had similar root development at maximum shoot growth. Tuber production from midflowering until plant maturity was similarly affected by soil water stress in microtuber and conventional seed tuber plants. At harvest, plants affected by soil water stress had about 87% of the tuber dry weight of irrigated plants. We conclude, that the greater variation on tuber yield of microtuber plants cannot be attributed to soil water stress from 1 month after emergence. # 2005 Elsevier B.V. All rights reserved. Keywords: Drought tolerance; Field cultivation; Leaf area index; Root growth; Solanum tuberosum L.; Stomatal conductance
1. Introduction Potato (Solanum tuberosum L.) microtubers (MT) are small tubers (about 1 cm diameter) and are * Corresponding author. Tel.: +81 11 706 3877; fax: +81 11 706 3878. E-mail address: [email protected] (K. Iwama).
produced in vitro. The field performance of plants grown from MT has been studied (Wattimena et al., 1983; Leclerc and Donnelly, 1990; Haverkort et al., 1991b; Ranalli et al., 1994; Kawakami et al., 2003); the MT tuber yield is comparable or lower than plants grown from conventional seed tubers (CT). The use of MT as seed tubers in potato production is advantageous in areas where the production of healthy seed
0378-4290/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2005.02.004
90
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
tubers is difficult (Iwama et al., 2000; Kawakami et al., 2003). However, MT plants have greater year-to-year variation in tuber yield compared with CT plants (Leclerc and Donnelly, 1990; Kawakami et al., 2003). Some studies suggested that soil water stress causes greater variation in tuber yield in MT plants: Lommen (1994) working with small tubers produced in pots (minitubers) reported that plants grown from minitubers of lighter weight had a smaller root system and a higher shoot to root ratio at emergence than plants from heavier minitubers and therefore a higher susceptibility to shortage of soil water at this stage. Leclerc and Donnelly (1990) working with MT transplanted to fields suggested that MT plants were more susceptible to low precipitation in the field due to an earlier production of stolons. Kawakami et al. (2003) working with directly planted MT in fields found a smaller root system in MT than CT plants until 1 month after emergence. Although from these results MT plants appear to be more susceptible to soil water stress than CT plants, up to our knowledge there are no studies on the effects of soil water stress on the growth and yield of directly planted MT in fields. This study compared the effect of soil water stress on the growth and yields of potato plants grown from MT and CT and investigated if soil water stress affects tuber yield differently when plants are grown from these two different propagules.
2. Materials and methods The experiments were done from 2000 to 2002 on the Experimental Farm of the Field Science Center for Northern Biosphere, Hokkaido University, Sapporo, Japan (438040 N, a brown lowland soil, Typic Udifluvent). Conventional seed tubers weighing about 50 g were produced by the National Center for Seeds and Seedlings (Hokkaido, Japan), and MT of 0.5–1 g (2000) and 1–3 g (2001, 2002) were produced by Kirin Brewery Co. Ltd. (Japan). Two potato cultivars of late maturity, Norin 1 and Konafubuki, were used as propagules. Konafubuki has a smaller root mass and is more sensitive to soil water stress than Norin 1 (Iwama et al., 1993). Tubers were planted by hand 7 cm deep for CT and 3 cm deep for MT. The planting distance was 75 cm (2000, 2001) or 70 cm (2002) between
rows and 25 cm between hills. The plots consisted of four rows of 13 plants each (9.75 m2) in 2000 and 2001, and nine rows of 12 plants each (18.9 m2) in 2002. In each irrigation treatment, the propagules were planted in a randomized complete block design with three replications on May 10, 2000, and May 13, 2001, and with two replications on May 3, 2002. Fertilizer was applied just before planting: 70, 48, 75, and 18 kg ha1 of N, P, K, and Mg, respectively. The plots (soil and tubers) were covered with an un-woven fabric polyester soil cover (Passlight, Unitika Ltd., Japan) for 2 weeks after planting to ensure a uniform initial plant stand. Plots were hand-weeded during the early growth stage and were rainfed until the start of the irrigation treatments. Weeds, insects, and diseases were controlled according to the standard practice of the Experimental Farm of the Field Science Center for Northern Biosphere, Hokkaido University. To protect the crops from rainfall, two poly-shelters of 6.3 m 25 m were placed over the plots from approximately 1 month after emergence and then the irrigation was started. In 2000 and 2001, one shelter (wet plot) was irrigated twice a week (20 mm application) until around physiological maturity by using furrow irrigation tubes, while the other (dry plot) was not irrigated. In 2002, each shelter had both wet and dry plots. The soil water potential (Cs) was measured once a week at four depths (20, 50, 100 and 150 cm from the hill surface) by using porous-cup soil moisture tensiometers placed in the inner row of the first and second replications in both MT and CT of cultivar Norin 1, totaling four places in each irrigation treatment. Readings were done using a handy manometer (PG-100-102 G, Copal Electronics Co. Ltd., Japan) on each tensiometer. At 55 (mid-flowering) and 83 (maximum shoot growth) days after emergence (70%, DAE) in 2000, at 51 (mid-flowering) and 83 (maximum shoot growth) DAE in 2001, and at 75 (maximum shoot growth) and 104 (late bulking) DAE in 2002, three (2000, 2001) or four (2002) plants of each cultivar and tuber type were harvested from each replication. All leaves of each plant were stripped and the leaf area of a randomly taken sub-sample of about 1000 cm2 for each plant was measured by an automatic area meter (AMM-9, Hayashi Denko Co. Ltd., Japan). The dry weights (DW) of tuber and leaf were measured after ovendrying at 70 8C for more than 72 h, and the leaf area
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
index (LAI) was calculated as a product of the measured leaf area/leaf DW ratio and the total leaf DW. At maximum shoot growth (2000), 6–10 soil cores (each 5 cm diameter, 100 cm3 volume) were sampled from each soil layer at 10–20 cm depth intervals until 150 cm deep for one plant of each cultivar and tuber type in each irrigation treatment to find root length density. The roots in the soil core sample were first separated from the soil by using a root washing device (hydropneumatic elutriation system, Gillison’s Variety Fabrication Inc., USA) and then they were separated manually from the soil and plant residue. The length of collected roots was measured by using the line intersect method (Tennant, 1975). In 2000 and 2001, the stomatal conductance (gs) of both the adaxial (upper) and abaxial (lower) leaf surface in three (2000) or two (2001) of the youngest fully grown leaves of each cultivar and tuber type in each replication was measured by using a Steady State Porometer (LI-1600, LI-COR Inc., USA). These measurements were made between 8:00 a.m. and 13:00 p.m. at 51, 60, 90, and 98 DAE (2000) and at 57, 67, 86, and 96 DAE (2001). In all 3 years, the DW of marketable tubers (heavier than 20 g) was recorded after physiological maturity (70% of the leaves of plants turned yellow) for 10 plants of each cultivar and seed tuber type in each replication. To find differences between CT and MT plants on LAI, tuber DW and growing period in the response to the irrigation treatment, data from all replicates of the two cultivars for 2 or 3 years were combined to produce a regression line for each tuber type (treated as a dummy variable) showing the association between the dry and wet plots data. The lines were then compared to find differences between tuber type in the response to the irrigation treatment. In this model, as the irrigation was started after the initial plant development (approximately 30 DAE), intercepts were assigned to pass through the origin and only the slope was compared between MT and CT plants. To analyze the effect of the water treatment on LAI, tuber DW and growing period of MT and CT plants, t-test {t = (line slope 1)/standard error} was performed for each tuber type. Analysis of variance using the mean value of the two cultivars in each water treatment was used to test for the significance of stomatal conductance between seed tuber type, year, and their interaction by the method of Petersen (1994).
91
All calculations were made by using the SPSS Base 7.5.1 J for Windows (SPSS Inc., USA).
3. Results Analysis of variance was performed and it was found that the cultivar Konafubuki had a greater decrease in the variables analyzed due to soil water stress, but the cultivar response to soil water stress was not affected by seed tuber type. Also, in our previous study (unpublished), the range of the MT size used in this study, i.e., 0.5–3 g, did not affect the final tuber yield. Therefore, the cultivar and MT size effect was not included in the analysis. 3.1. Soil water conditions As the trends for the difference between the two plots were similar at the different depths, only data from the 50 and 150 cm depths are shown (Fig. 1). Until the start of irrigation (about 30 DAE), Cs was less in 2001 and 2002 compared with 2000. After the start of the water treatment, Cs in the wet plots was always greater than 20 kPa at both depths throughout the growing period in all 3 years. In the dry plots, Cs at 50 cm started to decrease gradually until about 60 DAE when it reached the peak at about 60 kPa in 2000 and 2001, and at about 40 kPa in 2002. Similar trends were observed for Cs at 150 cm in the dry plots, but the decrease started later and had a smaller peak than at 50 cm, particularly in 2002. 3.2. LAI At mid-flowering, the LAI of MTand CT plants were differently affected bysoil water stress (Fig. 2).Although both CTand MT plants showed a smaller LAI in dry plots compared with the wet plots, the difference was relatively greater for MT plants (i.e., 65%) than for CT plants (i.e., 85%). At maximum shoot growth, MT and CT plants were similarly affected (reduced on average 35%) in LAI due to soil water stress. 3.3. Stomatal conductance The patterns in gs in both years and at both growth stages were similar (Table 1). In the wet plots, MT and
92
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
Fig. 1. Time trends in soil water potential at 50 and 150 cm depths of plots irrigated (wet plot) and non-irrigated (dry plot) from about one month after emergence from 2000 to 2002. Vertical bars are S.E. (n = 4).
CT plants had similar gs. In addition, gs was smaller in the dry plots compared with the wet plots. However, the difference in gs between wet and dry plots was greater for CT than for MT plants. Microtuber had a greater gs than CT plants in the dry plots, but the greater gs of MT plants in the dry plots at maximum shoot growth was not consistent between 2000 and 2001 (Y T significant).
3.4. Root growth Analysis of variance showed that the irrigation treatment and cultivar effect were significant, but the interaction of them with tuber type was not significant. Therefore, the average of the data from irrigation treatment and cultivar is presented. The root length
Fig. 2. Comparison of leaf area index (LAI) between MT and CT plants in wet and dry plots at mid-flowering (2000, 2001) and after maximum shoot growth (2000, 2001, 2002). (a) Data of all replicates and two cultivars are shown. Regression lines were fitted with tuber type treated as a dummy variable, and the slopes (b1) were compared between MT and CT plants. The overall R2 of the regression at mid-flowering was 0.965 (P < 0.001), and after maximum shoot growth was 0.906 (P < 0.001). ns, not significant (P 0.05), (**) significant at P < 0.01. (b) t statistics, (b1 1)/S.E., was performed for each tuber type to find significant difference in LAI between wet plots and dry plots.
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
93
Table 1 Stomatal conductance (cm s1), as affected by water treatment of CT and MT plants at two growth stages in 2000 and 2001 Tuber type (T)a
Stomatal conductance (cm s1) Mid-floweringb
After maximum shoot growthc
d
Wet plot
CT MT Year (Y) T YT
Dry plot
Wet plot
Dry plot
2000
2001
2000
2001
2000
2001
2000
2001
1.14 1.14
1.12 1.20
0.82 0.95
0.45 0.56
0.50 0.60
1.14 1.25
0.21 0.34
0.36 0.36
nse ns ns
**
**
*
*
ns ns
*
ns
*
a
CT: conventional seed tubers, MT: microtubers. 2000: average of measurements made at 51 and 60 days after emergence, 2001: average of measurements made at 57 and 67 days after emergence. c 2000: average of measurements made at 90 and 98 days after emergence, 2001: average of measurements made at 86 and 96 days after emergence. d Wet plot: 20 mm irrigation twice a week from about 30 days after emergence to physiological maturity. Dry plot: without irrigation and protected from rainfall from about 30 days after emergence to physiological maturity. e ns: not significant (P 0.05). * Significant at P < 0.05. ** Significant at P < 0.01. b
density in both plant types was highest from the soil surface to a depth of 50 cm, below which it decreased sharply (Fig. 3). At this growth stage the root length density, root density profile and rooting depth showed no significant difference between MT and CT plants. 3.5. Tuber production and length of growing period The tuber production until mid-flowering was not different between the dry and wet plots (Fig. 4). Plants
from dry plots had produced on average 95% of the tuber DW in the wet plots. Also, at maximum shoot growth, the tuber DW of both plant types was similarly affected by soil water stress. Tuber DW was on average 18% lower in the dry plots compared with the wet plots. There was no significant effect of soil water stress on the growing period of MT and CT plants (Fig. 5). At harvest, tuber DW in the dry plots was lower than in the wet plots. The relative decrease in tuber DW was similar for MT and CT plants.
4. Discussion
Fig. 3. Root length density profile to 150 cm soil depth at 83 days after emergence (2000) of MT and CT plants. Data are the average of the irrigation treatments and cultivars. Horizontal bars are S.E. (n = 4). (a) ns, not significant (P 0.05).
Potato plants are susceptible to Cs less than 25 kPa (Epstein and Grant, 1973; MacKerron and Jefferies, 1986). Therefore, the plants at the dry plots in this study were under a stressed soil water condition after the start of the irrigation treatment (Fig. 1). The initial response of potato plants to soil water stress is a reduced leaf area (Gregory and Simmonds, 1992). The initially lower LAI of plants exposed to soil water stress in this study reflects the effect of water shortage in the soil. Also, the greater decrease in LAI in MT plants than in CT plants in the dry plots at midflowering (Fig. 2) indicates a greater susceptibility to
94
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
Fig. 4. Comparison of tuber dry weight (DW) between MT and CT plants in wet and dry plots at mid-flowering and after maximum shoot growth. (a) Data of all replicates and two cultivars are shown. Regression lines were fitted with tuber type treated as a dummy variable, and the slopes (b1) were compared between MT and CT plants. The overall R2 of the regression at mid-flowering was 0.954 (P < 0.001), and after maximum shoot growth was 0.960 (P < 0.001). ns, not significant (P 0.05). (b) t statistics, (b1 1)/S.E., was performed for each tuber type to find significant differences in tuber DW between wet plots and dry plots.
soil water stress in MT plants than in CT plants from the start of irrigation treatment (about 30 DAE) to the flowering stage (about 53 DAE). Possible explanations for the greater initial susceptibility to soil water stress in MT plants are the smaller root system of young MT plants (Kawakami et al., 2003) or the greater shoot to root ratio at emergence (Lommen, 1994; Struik and Lommen, 1999), or both. However, the similar lower LAI in MT and CT plants in the dry plots than in wet plots at maximum shoot growth (Fig. 2) suggests that plants of both tuber types are similarly affected by low Cs at the later growth stage. At around the maximum shoot growth, MT plants have a similar or higher LAI compared with CT plants (Kawakami et al., 2003). In this study, the LAI at maximum shoot growth of MT and CT plants
was similar in both dry (2.40 and 2.18) and wet (3.77 and 3.24) plots, suggesting that at soil water stress condition of this study, MT plants can develop a good LAI. In potato plants grown under conditions of plentiful soil water, gs is primarily affected by photosynthetic photon flux density (Gordon et al., 1997), but when plants are subject to soil water stress, gs is more affected by Cs (Vos and Groenwold, 1989; Gordon et al., 1997). The reduced gs of plants in the dry plots compared with the wet plots (Table 1) agrees with these findings. Also, the higher gs of MT plants than CT plants in the dry plots suggests that MT plants suffered less from the effect of soil water stress than CT plants. One possible explanation is that MT plants had more soil water available because they had a lower dry matter production than CT plants.
Fig. 5. Comparison of days from emergence to physiological maturity (growing period) and tuber dry weight (DW) at harvest between MT and CT plants in wet and dry plots from 2000 to 2002. (a) Data of all replicates and two cultivars are shown. Regression lines were fitted with tuber type treated as a dummy variable, and the slopes (b1) were compared between MT and CT plants. The overall R2 of the regression for growing period was 0.998 (P < 0.001), and for tuber DW was 0.967 (P < 0.001). ns, not significant (P 0.05). (b) t statistics, (b1 1)/S.E., was performed for each tuber type to find significant differences in growing period and tuber DW between wet plots and dry plots.
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
Root growth affects dry matter production (Iwama, 1988), and root density is important for water uptake from the soil (van Loon, 1981; Gregory and Simmonds, 1992; Iwama et al., 1999). Therefore, the similar root length density and rooting depth between MT and CT plants supports the speculation that both have similar potential to take up water from the soil and that they respond similarly to soil water stress at maximum shoot growth. The relatively lesser effect of soil water stress on tuber production than on LAI at mid-flowering may be caused by the timing of the start of the irrigation treatment in this study. MacKerron and Jefferies (1986) reported that the critical stage for soil water stress was at tuber initiation. In our previous study (Kawakami et al., 2003) tuber initiation of MT plants was at about 26 DAE and was 2–10 days later than CT plants. In this study, we started the irrigation treatment at about 1 month after emergence, so CT and MT plants did not suffer from soil water stress at the time of tuber initiation. Another factor that may be related to the lesser effect of soil water stress on tuber production than on LAI is that most of the incoming radiation is intercepted by a LAI of around 3 (Haverkort et al., 1991a, and in this study MT and CT plants have already developed a LAI of this size at mid-flowering in the dry plots, resulting in a small difference in the intercepted radiation between plants from the wet and dry plots. Soil water stress affects tuber DW by reducing the interception of radiation as a result of a reduced rate and duration of leaf growth (Jefferies and MacKerron, 1989; Jefferies, 1995). In this study, plants in the dry plots had a reduced LAI that contributed to a reduced tuber DW at harvest. However, soil water stress had no effect on the growing period of CT and MT plants. The similar lower tuber DW in the dry than in wet plots between CT and MT plants at harvest is probably a result of a similar LAI, a similar root mass at maximum shoot growth and a similar growing period in the dry plots.
5. Conclusion The soil water stress affecting potato plants in this study was probably greater than the soil water stress that may occur under conventional cultivation,
95
because no year is without rainfall during the long period of potato growth. Despite the severe soil water stress imposed in this study the relative reduction in growth from mid-flowering and in final tuber yield caused by this stress, was similar for MT and CT plants. Therefore the higher year-to-year variability in tuber yields of MT plants reported in previous studies (Leclerc and Donnelly, 1990; Kawakami et al., 2003) may not be attributed to soil water stress after tuber initiation. More studies on the effect of early soil water stress and other climatic factors (i.e., temperature, day length) and the possible interaction between them on the growth and yield of potato plants grown from MT are necessary for a better understanding of the yield variation of MT plants and to assure a more stable tuber yield of MT plants under field cultivation.
Acknowledgements We thank N. Moki and S. Ichikawa, Field Science Center for Northern Biosphere, Hokkaido University, for their technical assistance in field management. The collaboration of the students of Crop Science Laboratory, Hokkaido University, and the measurements of stomatal conductance by A. Kubota and K. Sugihara are gratefully acknowledged. We are thankful to Dr. T. Hasegawa for his guidance during this study and to Dr. J. Gopal for comments on the manuscript.
References Epstein, E., Grant, W.J., 1973. Water stress relations of the potato plant under field conditions. Agron. J. 65, 400–404. Gordon, R.J., Brown, D.M., Dixon, M.A., 1997. Stomatal resistance of three potato cultivars as influenced by soil water status, humidity and irradiance. Potato Res. 40, 47–57. Gregory, P.J., Simmonds, L.P., 1992. Water relations and growth of potatoes. In: Harris, P. (Ed.), The Potato Crop. The Scientific Basis for Improvement. Chapman & Hall, London, pp. 214– 246. Haverkort, A.J., Uenk, D., Veroude, H., van de Waart, M., 1991a. Relationships between ground cover, intercepted solar radiation, leaf area index and infrared reflectance of potato crops. Potato Res. 34, 113–121. Haverkort, A.J., van de Waart, M., Marinus, J., 1991b. Field performance of potato microtubers as propagation material. Potato Res. 34, 353–364.
96
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
Iwama, K., 1988. Difference in root growth of potato plants among years and cropping seasons. Jpn. J. Crop Sci. 57, 346–354. Iwama, K., Hasegawa, T., Nakaseko, K., 1999. New potato lines with high productivity and drought tolerance. In: Horie, T., Geng, S., Amano, T., Inamura, T., Shiraiwa, T. (Eds.), Proceedings of the International Symposium on World Food Security and Crop Production Technologies for Tomorrow, Kyoto University, Kyoto, pp. 189–193. Iwama, K., Hasegawa, T., Zheng, X., 2000. Potato growth and yield using micro-tuber as seeds in Qibainong, the southwest China. In: Nakatani, M., Komaki, K. (Eds.), Proceedings of the 12th ISTRC Symposium on Potential of Root Crops for Food and Industrial Resources, Tsukuba, pp. 443–445. Iwama, K., Hukushima, T., Yoshimura, T., 1993. Influence of planting density on root growth and yield in potato. Jpn. J. Crop Sci. 64, 628–635. Jefferies, R.A., 1995. Physiology of crop response to drought. In: Haverkort, A.J., MacKerron, D.K.L. (Eds.), Potato Ecology and Modelling of Crops under Conditions Limiting Growth. Kluwer Academic Publishers, Dordrecht, pp. 61–74. Jefferies, R.A., MacKerron, D.K.L., 1989. Radiation interception and growth of irrigated and droughted potato (Solanum tuberosum). Field Crops Res. 22, 101–112. Kawakami, J., Iwama, K., Hasegawa, T., Jitsuyama, Y., 2003. Growth and yield of potato plants grown from microtubers in fields. Am. J. Potato Res. 80, 371–378.
Leclerc, Y., Donnelly, D.J., 1990. Seasonal differences in the field performance of micropropagated potato under a short growing season in Quebec. Am. Potato J. 67, 507–516. Lommen, W.J.M., 1994. Effect of weight of potato minitubers on sprout growth, emergence and plant characteristics at emergence. Potato Res. 37, 315–322. MacKerron, D.K.L., Jefferies, R.A., 1986. The influence of early soil moisture stress on tuber number in potato. Potato Res. 29, 299–312. Petersen, R.G., 1994. Agricultural Field Experiments: Design and Analysis. Marcel Dekker Inc., New York. Ranalli, P., Bassi, F., Ruaro, G., del Re, P., di Candilo, M., 1994. Microtuber and minituber production and field performance compared with normal tubers. Potato Res. 37, 383–391. Struik, P.C., Lommen, W.J.M., 1999. Improving the field performance of micro- and minitubers. Potato Res. 42, 559–568. Tennant, D., 1975. A test of a modified line intersect method of estimating root length. J. Ecol. 63, 995–1001. van Loon, C.D., 1981. The effect of water stress on potato growth, development, and yield. Am. Potato J. 58, 51–69. Vos, J., Groenwold, J., 1989. Characteristics of photosynthesis and conductance of potato canopies and the effect of cultivar and transient drought. Field Crops Res. 20, 237–250. Wattimena, G., McCown, B., Weis, G., 1983. Comparative field performance of potatoes from microculture. Am. Potato J. 60, 27–33.
Soil water stress and the growth and yield of potato plants grown from microtubers and conventional seed tubers Jackson Kawakami, Kazuto Iwama *, Yutaka Jitsuyama Department of Botany and Agronomy, Graduate School of Agriculture, Hokkaido University, N9, W9, Kita-ku, Sapporo 060-8589, Japan Received 10 December 2003; received in revised form 4 February 2005; accepted 4 February 2005
Abstract Tuber yields of potato plants grown from microtubers in fields are more variable than yields from conventional seed tubers (CT). One reason could be their higher susceptibility to water stress. This study clarified the effect of soil water stress from 1 month after emergence on the growth and yield of plants grown from conventional seed tubers and microtubers in fields. Microtubers (0.5–3 g) and conventional seed tubers (50 g) were grown in Hokkaido, Japan, over three field seasons. One month after emergence, polyshelters were placed over the plots to prevent rainfall, and either irrigated (wet plot) or non-irrigated (dry plot) treatments were formed. At mid-flowering (about 50 days after emergence) leaf area index (LAI) in microtuber plants was decreased relatively more due to soil water stress than LAI in conventional seed tuber plants. However, at maximum shoot growth (about 80 days after emergence) both microtuber and conventional seed tuber plants had a similar relative decrease in LAI due to soil water stress. At midflowering and maximum shoot growth microtuber and conventional seed tuber plants had reduced stomatal conductance due to soil water stress, but the reduction in stomatal conductance was greater in conventional seed tuber plants than in microtuber plants. Microtuber and conventional seed tuber plants had similar root development at maximum shoot growth. Tuber production from midflowering until plant maturity was similarly affected by soil water stress in microtuber and conventional seed tuber plants. At harvest, plants affected by soil water stress had about 87% of the tuber dry weight of irrigated plants. We conclude, that the greater variation on tuber yield of microtuber plants cannot be attributed to soil water stress from 1 month after emergence. # 2005 Elsevier B.V. All rights reserved. Keywords: Drought tolerance; Field cultivation; Leaf area index; Root growth; Solanum tuberosum L.; Stomatal conductance
1. Introduction Potato (Solanum tuberosum L.) microtubers (MT) are small tubers (about 1 cm diameter) and are * Corresponding author. Tel.: +81 11 706 3877; fax: +81 11 706 3878. E-mail address: [email protected] (K. Iwama).
produced in vitro. The field performance of plants grown from MT has been studied (Wattimena et al., 1983; Leclerc and Donnelly, 1990; Haverkort et al., 1991b; Ranalli et al., 1994; Kawakami et al., 2003); the MT tuber yield is comparable or lower than plants grown from conventional seed tubers (CT). The use of MT as seed tubers in potato production is advantageous in areas where the production of healthy seed
0378-4290/$ – see front matter # 2005 Elsevier B.V. All rights reserved. doi:10.1016/j.fcr.2005.02.004
90
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
tubers is difficult (Iwama et al., 2000; Kawakami et al., 2003). However, MT plants have greater year-to-year variation in tuber yield compared with CT plants (Leclerc and Donnelly, 1990; Kawakami et al., 2003). Some studies suggested that soil water stress causes greater variation in tuber yield in MT plants: Lommen (1994) working with small tubers produced in pots (minitubers) reported that plants grown from minitubers of lighter weight had a smaller root system and a higher shoot to root ratio at emergence than plants from heavier minitubers and therefore a higher susceptibility to shortage of soil water at this stage. Leclerc and Donnelly (1990) working with MT transplanted to fields suggested that MT plants were more susceptible to low precipitation in the field due to an earlier production of stolons. Kawakami et al. (2003) working with directly planted MT in fields found a smaller root system in MT than CT plants until 1 month after emergence. Although from these results MT plants appear to be more susceptible to soil water stress than CT plants, up to our knowledge there are no studies on the effects of soil water stress on the growth and yield of directly planted MT in fields. This study compared the effect of soil water stress on the growth and yields of potato plants grown from MT and CT and investigated if soil water stress affects tuber yield differently when plants are grown from these two different propagules.
2. Materials and methods The experiments were done from 2000 to 2002 on the Experimental Farm of the Field Science Center for Northern Biosphere, Hokkaido University, Sapporo, Japan (438040 N, a brown lowland soil, Typic Udifluvent). Conventional seed tubers weighing about 50 g were produced by the National Center for Seeds and Seedlings (Hokkaido, Japan), and MT of 0.5–1 g (2000) and 1–3 g (2001, 2002) were produced by Kirin Brewery Co. Ltd. (Japan). Two potato cultivars of late maturity, Norin 1 and Konafubuki, were used as propagules. Konafubuki has a smaller root mass and is more sensitive to soil water stress than Norin 1 (Iwama et al., 1993). Tubers were planted by hand 7 cm deep for CT and 3 cm deep for MT. The planting distance was 75 cm (2000, 2001) or 70 cm (2002) between
rows and 25 cm between hills. The plots consisted of four rows of 13 plants each (9.75 m2) in 2000 and 2001, and nine rows of 12 plants each (18.9 m2) in 2002. In each irrigation treatment, the propagules were planted in a randomized complete block design with three replications on May 10, 2000, and May 13, 2001, and with two replications on May 3, 2002. Fertilizer was applied just before planting: 70, 48, 75, and 18 kg ha1 of N, P, K, and Mg, respectively. The plots (soil and tubers) were covered with an un-woven fabric polyester soil cover (Passlight, Unitika Ltd., Japan) for 2 weeks after planting to ensure a uniform initial plant stand. Plots were hand-weeded during the early growth stage and were rainfed until the start of the irrigation treatments. Weeds, insects, and diseases were controlled according to the standard practice of the Experimental Farm of the Field Science Center for Northern Biosphere, Hokkaido University. To protect the crops from rainfall, two poly-shelters of 6.3 m 25 m were placed over the plots from approximately 1 month after emergence and then the irrigation was started. In 2000 and 2001, one shelter (wet plot) was irrigated twice a week (20 mm application) until around physiological maturity by using furrow irrigation tubes, while the other (dry plot) was not irrigated. In 2002, each shelter had both wet and dry plots. The soil water potential (Cs) was measured once a week at four depths (20, 50, 100 and 150 cm from the hill surface) by using porous-cup soil moisture tensiometers placed in the inner row of the first and second replications in both MT and CT of cultivar Norin 1, totaling four places in each irrigation treatment. Readings were done using a handy manometer (PG-100-102 G, Copal Electronics Co. Ltd., Japan) on each tensiometer. At 55 (mid-flowering) and 83 (maximum shoot growth) days after emergence (70%, DAE) in 2000, at 51 (mid-flowering) and 83 (maximum shoot growth) DAE in 2001, and at 75 (maximum shoot growth) and 104 (late bulking) DAE in 2002, three (2000, 2001) or four (2002) plants of each cultivar and tuber type were harvested from each replication. All leaves of each plant were stripped and the leaf area of a randomly taken sub-sample of about 1000 cm2 for each plant was measured by an automatic area meter (AMM-9, Hayashi Denko Co. Ltd., Japan). The dry weights (DW) of tuber and leaf were measured after ovendrying at 70 8C for more than 72 h, and the leaf area
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
index (LAI) was calculated as a product of the measured leaf area/leaf DW ratio and the total leaf DW. At maximum shoot growth (2000), 6–10 soil cores (each 5 cm diameter, 100 cm3 volume) were sampled from each soil layer at 10–20 cm depth intervals until 150 cm deep for one plant of each cultivar and tuber type in each irrigation treatment to find root length density. The roots in the soil core sample were first separated from the soil by using a root washing device (hydropneumatic elutriation system, Gillison’s Variety Fabrication Inc., USA) and then they were separated manually from the soil and plant residue. The length of collected roots was measured by using the line intersect method (Tennant, 1975). In 2000 and 2001, the stomatal conductance (gs) of both the adaxial (upper) and abaxial (lower) leaf surface in three (2000) or two (2001) of the youngest fully grown leaves of each cultivar and tuber type in each replication was measured by using a Steady State Porometer (LI-1600, LI-COR Inc., USA). These measurements were made between 8:00 a.m. and 13:00 p.m. at 51, 60, 90, and 98 DAE (2000) and at 57, 67, 86, and 96 DAE (2001). In all 3 years, the DW of marketable tubers (heavier than 20 g) was recorded after physiological maturity (70% of the leaves of plants turned yellow) for 10 plants of each cultivar and seed tuber type in each replication. To find differences between CT and MT plants on LAI, tuber DW and growing period in the response to the irrigation treatment, data from all replicates of the two cultivars for 2 or 3 years were combined to produce a regression line for each tuber type (treated as a dummy variable) showing the association between the dry and wet plots data. The lines were then compared to find differences between tuber type in the response to the irrigation treatment. In this model, as the irrigation was started after the initial plant development (approximately 30 DAE), intercepts were assigned to pass through the origin and only the slope was compared between MT and CT plants. To analyze the effect of the water treatment on LAI, tuber DW and growing period of MT and CT plants, t-test {t = (line slope 1)/standard error} was performed for each tuber type. Analysis of variance using the mean value of the two cultivars in each water treatment was used to test for the significance of stomatal conductance between seed tuber type, year, and their interaction by the method of Petersen (1994).
91
All calculations were made by using the SPSS Base 7.5.1 J for Windows (SPSS Inc., USA).
3. Results Analysis of variance was performed and it was found that the cultivar Konafubuki had a greater decrease in the variables analyzed due to soil water stress, but the cultivar response to soil water stress was not affected by seed tuber type. Also, in our previous study (unpublished), the range of the MT size used in this study, i.e., 0.5–3 g, did not affect the final tuber yield. Therefore, the cultivar and MT size effect was not included in the analysis. 3.1. Soil water conditions As the trends for the difference between the two plots were similar at the different depths, only data from the 50 and 150 cm depths are shown (Fig. 1). Until the start of irrigation (about 30 DAE), Cs was less in 2001 and 2002 compared with 2000. After the start of the water treatment, Cs in the wet plots was always greater than 20 kPa at both depths throughout the growing period in all 3 years. In the dry plots, Cs at 50 cm started to decrease gradually until about 60 DAE when it reached the peak at about 60 kPa in 2000 and 2001, and at about 40 kPa in 2002. Similar trends were observed for Cs at 150 cm in the dry plots, but the decrease started later and had a smaller peak than at 50 cm, particularly in 2002. 3.2. LAI At mid-flowering, the LAI of MTand CT plants were differently affected bysoil water stress (Fig. 2).Although both CTand MT plants showed a smaller LAI in dry plots compared with the wet plots, the difference was relatively greater for MT plants (i.e., 65%) than for CT plants (i.e., 85%). At maximum shoot growth, MT and CT plants were similarly affected (reduced on average 35%) in LAI due to soil water stress. 3.3. Stomatal conductance The patterns in gs in both years and at both growth stages were similar (Table 1). In the wet plots, MT and
92
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
Fig. 1. Time trends in soil water potential at 50 and 150 cm depths of plots irrigated (wet plot) and non-irrigated (dry plot) from about one month after emergence from 2000 to 2002. Vertical bars are S.E. (n = 4).
CT plants had similar gs. In addition, gs was smaller in the dry plots compared with the wet plots. However, the difference in gs between wet and dry plots was greater for CT than for MT plants. Microtuber had a greater gs than CT plants in the dry plots, but the greater gs of MT plants in the dry plots at maximum shoot growth was not consistent between 2000 and 2001 (Y T significant).
3.4. Root growth Analysis of variance showed that the irrigation treatment and cultivar effect were significant, but the interaction of them with tuber type was not significant. Therefore, the average of the data from irrigation treatment and cultivar is presented. The root length
Fig. 2. Comparison of leaf area index (LAI) between MT and CT plants in wet and dry plots at mid-flowering (2000, 2001) and after maximum shoot growth (2000, 2001, 2002). (a) Data of all replicates and two cultivars are shown. Regression lines were fitted with tuber type treated as a dummy variable, and the slopes (b1) were compared between MT and CT plants. The overall R2 of the regression at mid-flowering was 0.965 (P < 0.001), and after maximum shoot growth was 0.906 (P < 0.001). ns, not significant (P 0.05), (**) significant at P < 0.01. (b) t statistics, (b1 1)/S.E., was performed for each tuber type to find significant difference in LAI between wet plots and dry plots.
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
93
Table 1 Stomatal conductance (cm s1), as affected by water treatment of CT and MT plants at two growth stages in 2000 and 2001 Tuber type (T)a
Stomatal conductance (cm s1) Mid-floweringb
After maximum shoot growthc
d
Wet plot
CT MT Year (Y) T YT
Dry plot
Wet plot
Dry plot
2000
2001
2000
2001
2000
2001
2000
2001
1.14 1.14
1.12 1.20
0.82 0.95
0.45 0.56
0.50 0.60
1.14 1.25
0.21 0.34
0.36 0.36
nse ns ns
**
**
*
*
ns ns
*
ns
*
a
CT: conventional seed tubers, MT: microtubers. 2000: average of measurements made at 51 and 60 days after emergence, 2001: average of measurements made at 57 and 67 days after emergence. c 2000: average of measurements made at 90 and 98 days after emergence, 2001: average of measurements made at 86 and 96 days after emergence. d Wet plot: 20 mm irrigation twice a week from about 30 days after emergence to physiological maturity. Dry plot: without irrigation and protected from rainfall from about 30 days after emergence to physiological maturity. e ns: not significant (P 0.05). * Significant at P < 0.05. ** Significant at P < 0.01. b
density in both plant types was highest from the soil surface to a depth of 50 cm, below which it decreased sharply (Fig. 3). At this growth stage the root length density, root density profile and rooting depth showed no significant difference between MT and CT plants. 3.5. Tuber production and length of growing period The tuber production until mid-flowering was not different between the dry and wet plots (Fig. 4). Plants
from dry plots had produced on average 95% of the tuber DW in the wet plots. Also, at maximum shoot growth, the tuber DW of both plant types was similarly affected by soil water stress. Tuber DW was on average 18% lower in the dry plots compared with the wet plots. There was no significant effect of soil water stress on the growing period of MT and CT plants (Fig. 5). At harvest, tuber DW in the dry plots was lower than in the wet plots. The relative decrease in tuber DW was similar for MT and CT plants.
4. Discussion
Fig. 3. Root length density profile to 150 cm soil depth at 83 days after emergence (2000) of MT and CT plants. Data are the average of the irrigation treatments and cultivars. Horizontal bars are S.E. (n = 4). (a) ns, not significant (P 0.05).
Potato plants are susceptible to Cs less than 25 kPa (Epstein and Grant, 1973; MacKerron and Jefferies, 1986). Therefore, the plants at the dry plots in this study were under a stressed soil water condition after the start of the irrigation treatment (Fig. 1). The initial response of potato plants to soil water stress is a reduced leaf area (Gregory and Simmonds, 1992). The initially lower LAI of plants exposed to soil water stress in this study reflects the effect of water shortage in the soil. Also, the greater decrease in LAI in MT plants than in CT plants in the dry plots at midflowering (Fig. 2) indicates a greater susceptibility to
94
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
Fig. 4. Comparison of tuber dry weight (DW) between MT and CT plants in wet and dry plots at mid-flowering and after maximum shoot growth. (a) Data of all replicates and two cultivars are shown. Regression lines were fitted with tuber type treated as a dummy variable, and the slopes (b1) were compared between MT and CT plants. The overall R2 of the regression at mid-flowering was 0.954 (P < 0.001), and after maximum shoot growth was 0.960 (P < 0.001). ns, not significant (P 0.05). (b) t statistics, (b1 1)/S.E., was performed for each tuber type to find significant differences in tuber DW between wet plots and dry plots.
soil water stress in MT plants than in CT plants from the start of irrigation treatment (about 30 DAE) to the flowering stage (about 53 DAE). Possible explanations for the greater initial susceptibility to soil water stress in MT plants are the smaller root system of young MT plants (Kawakami et al., 2003) or the greater shoot to root ratio at emergence (Lommen, 1994; Struik and Lommen, 1999), or both. However, the similar lower LAI in MT and CT plants in the dry plots than in wet plots at maximum shoot growth (Fig. 2) suggests that plants of both tuber types are similarly affected by low Cs at the later growth stage. At around the maximum shoot growth, MT plants have a similar or higher LAI compared with CT plants (Kawakami et al., 2003). In this study, the LAI at maximum shoot growth of MT and CT plants
was similar in both dry (2.40 and 2.18) and wet (3.77 and 3.24) plots, suggesting that at soil water stress condition of this study, MT plants can develop a good LAI. In potato plants grown under conditions of plentiful soil water, gs is primarily affected by photosynthetic photon flux density (Gordon et al., 1997), but when plants are subject to soil water stress, gs is more affected by Cs (Vos and Groenwold, 1989; Gordon et al., 1997). The reduced gs of plants in the dry plots compared with the wet plots (Table 1) agrees with these findings. Also, the higher gs of MT plants than CT plants in the dry plots suggests that MT plants suffered less from the effect of soil water stress than CT plants. One possible explanation is that MT plants had more soil water available because they had a lower dry matter production than CT plants.
Fig. 5. Comparison of days from emergence to physiological maturity (growing period) and tuber dry weight (DW) at harvest between MT and CT plants in wet and dry plots from 2000 to 2002. (a) Data of all replicates and two cultivars are shown. Regression lines were fitted with tuber type treated as a dummy variable, and the slopes (b1) were compared between MT and CT plants. The overall R2 of the regression for growing period was 0.998 (P < 0.001), and for tuber DW was 0.967 (P < 0.001). ns, not significant (P 0.05). (b) t statistics, (b1 1)/S.E., was performed for each tuber type to find significant differences in growing period and tuber DW between wet plots and dry plots.
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
Root growth affects dry matter production (Iwama, 1988), and root density is important for water uptake from the soil (van Loon, 1981; Gregory and Simmonds, 1992; Iwama et al., 1999). Therefore, the similar root length density and rooting depth between MT and CT plants supports the speculation that both have similar potential to take up water from the soil and that they respond similarly to soil water stress at maximum shoot growth. The relatively lesser effect of soil water stress on tuber production than on LAI at mid-flowering may be caused by the timing of the start of the irrigation treatment in this study. MacKerron and Jefferies (1986) reported that the critical stage for soil water stress was at tuber initiation. In our previous study (Kawakami et al., 2003) tuber initiation of MT plants was at about 26 DAE and was 2–10 days later than CT plants. In this study, we started the irrigation treatment at about 1 month after emergence, so CT and MT plants did not suffer from soil water stress at the time of tuber initiation. Another factor that may be related to the lesser effect of soil water stress on tuber production than on LAI is that most of the incoming radiation is intercepted by a LAI of around 3 (Haverkort et al., 1991a, and in this study MT and CT plants have already developed a LAI of this size at mid-flowering in the dry plots, resulting in a small difference in the intercepted radiation between plants from the wet and dry plots. Soil water stress affects tuber DW by reducing the interception of radiation as a result of a reduced rate and duration of leaf growth (Jefferies and MacKerron, 1989; Jefferies, 1995). In this study, plants in the dry plots had a reduced LAI that contributed to a reduced tuber DW at harvest. However, soil water stress had no effect on the growing period of CT and MT plants. The similar lower tuber DW in the dry than in wet plots between CT and MT plants at harvest is probably a result of a similar LAI, a similar root mass at maximum shoot growth and a similar growing period in the dry plots.
5. Conclusion The soil water stress affecting potato plants in this study was probably greater than the soil water stress that may occur under conventional cultivation,
95
because no year is without rainfall during the long period of potato growth. Despite the severe soil water stress imposed in this study the relative reduction in growth from mid-flowering and in final tuber yield caused by this stress, was similar for MT and CT plants. Therefore the higher year-to-year variability in tuber yields of MT plants reported in previous studies (Leclerc and Donnelly, 1990; Kawakami et al., 2003) may not be attributed to soil water stress after tuber initiation. More studies on the effect of early soil water stress and other climatic factors (i.e., temperature, day length) and the possible interaction between them on the growth and yield of potato plants grown from MT are necessary for a better understanding of the yield variation of MT plants and to assure a more stable tuber yield of MT plants under field cultivation.
Acknowledgements We thank N. Moki and S. Ichikawa, Field Science Center for Northern Biosphere, Hokkaido University, for their technical assistance in field management. The collaboration of the students of Crop Science Laboratory, Hokkaido University, and the measurements of stomatal conductance by A. Kubota and K. Sugihara are gratefully acknowledged. We are thankful to Dr. T. Hasegawa for his guidance during this study and to Dr. J. Gopal for comments on the manuscript.
References Epstein, E., Grant, W.J., 1973. Water stress relations of the potato plant under field conditions. Agron. J. 65, 400–404. Gordon, R.J., Brown, D.M., Dixon, M.A., 1997. Stomatal resistance of three potato cultivars as influenced by soil water status, humidity and irradiance. Potato Res. 40, 47–57. Gregory, P.J., Simmonds, L.P., 1992. Water relations and growth of potatoes. In: Harris, P. (Ed.), The Potato Crop. The Scientific Basis for Improvement. Chapman & Hall, London, pp. 214– 246. Haverkort, A.J., Uenk, D., Veroude, H., van de Waart, M., 1991a. Relationships between ground cover, intercepted solar radiation, leaf area index and infrared reflectance of potato crops. Potato Res. 34, 113–121. Haverkort, A.J., van de Waart, M., Marinus, J., 1991b. Field performance of potato microtubers as propagation material. Potato Res. 34, 353–364.
96
J. Kawakami et al. / Field Crops Research 95 (2006) 89–96
Iwama, K., 1988. Difference in root growth of potato plants among years and cropping seasons. Jpn. J. Crop Sci. 57, 346–354. Iwama, K., Hasegawa, T., Nakaseko, K., 1999. New potato lines with high productivity and drought tolerance. In: Horie, T., Geng, S., Amano, T., Inamura, T., Shiraiwa, T. (Eds.), Proceedings of the International Symposium on World Food Security and Crop Production Technologies for Tomorrow, Kyoto University, Kyoto, pp. 189–193. Iwama, K., Hasegawa, T., Zheng, X., 2000. Potato growth and yield using micro-tuber as seeds in Qibainong, the southwest China. In: Nakatani, M., Komaki, K. (Eds.), Proceedings of the 12th ISTRC Symposium on Potential of Root Crops for Food and Industrial Resources, Tsukuba, pp. 443–445. Iwama, K., Hukushima, T., Yoshimura, T., 1993. Influence of planting density on root growth and yield in potato. Jpn. J. Crop Sci. 64, 628–635. Jefferies, R.A., 1995. Physiology of crop response to drought. In: Haverkort, A.J., MacKerron, D.K.L. (Eds.), Potato Ecology and Modelling of Crops under Conditions Limiting Growth. Kluwer Academic Publishers, Dordrecht, pp. 61–74. Jefferies, R.A., MacKerron, D.K.L., 1989. Radiation interception and growth of irrigated and droughted potato (Solanum tuberosum). Field Crops Res. 22, 101–112. Kawakami, J., Iwama, K., Hasegawa, T., Jitsuyama, Y., 2003. Growth and yield of potato plants grown from microtubers in fields. Am. J. Potato Res. 80, 371–378.
Leclerc, Y., Donnelly, D.J., 1990. Seasonal differences in the field performance of micropropagated potato under a short growing season in Quebec. Am. Potato J. 67, 507–516. Lommen, W.J.M., 1994. Effect of weight of potato minitubers on sprout growth, emergence and plant characteristics at emergence. Potato Res. 37, 315–322. MacKerron, D.K.L., Jefferies, R.A., 1986. The influence of early soil moisture stress on tuber number in potato. Potato Res. 29, 299–312. Petersen, R.G., 1994. Agricultural Field Experiments: Design and Analysis. Marcel Dekker Inc., New York. Ranalli, P., Bassi, F., Ruaro, G., del Re, P., di Candilo, M., 1994. Microtuber and minituber production and field performance compared with normal tubers. Potato Res. 37, 383–391. Struik, P.C., Lommen, W.J.M., 1999. Improving the field performance of micro- and minitubers. Potato Res. 42, 559–568. Tennant, D., 1975. A test of a modified line intersect method of estimating root length. J. Ecol. 63, 995–1001. van Loon, C.D., 1981. The effect of water stress on potato growth, development, and yield. Am. Potato J. 58, 51–69. Vos, J., Groenwold, J., 1989. Characteristics of photosynthesis and conductance of potato canopies and the effect of cultivar and transient drought. Field Crops Res. 20, 237–250. Wattimena, G., McCown, B., Weis, G., 1983. Comparative field performance of potatoes from microculture. Am. Potato J. 60, 27–33.