265 1. Introduction Potato (Solanum tuberosum L.) is one of the major crops contributing to the world’s food requirement because it is a rich source of starch, having protein of high biological value (Eppendorfer and Eggum, 1994). In Lebanon, po- tato occupies an area of 19,700 ha with a total production of 460,000 t, but average yield is only 23 t ha-1, which is much below the crop’s potential productivity. Such a low yield seems to be due to the imbalance in nutrients applied for the agricultural production of this crop. Owing to the current trend of intensive cropping in Lebanon, soils have developed multi-nutrient deficiencies. Farmers usually diagnose and correct the deficiencies of nitrogen (N) and phosphorus (P), but often neglect the ef- fect of deficiencies of other essential macronutrients such as potassium (K). In addition to N and P, potato is a heavy remover of soil potassium and its response to potassium varies with variety, source and method of potassium fer- tilizer application (Sharma and Sud, 2001; Kumar et al., 2007; Abd El-Latif et al., 2011). Since biomass and bulking rate of potato tubers are positively affected by synthesis and accumulation of starch, K plays a key role in this regard as it is the most efficient monovalant cation that stimulates the activity of the starch synthase enzyme, catalyzing the incorporation of simple glucose molecules into complex molecules of starch (Moinuddin et al., 2004). Starch accumulation is coupled with cell and tissue growth of the tubers as K en- hances the overall growth of the plants (Singh and Singh, 1996), and facilitates the translocation of assimilates to the sinks/tubers (Moinuddin et al., 2005), which could ulti- mately increase the tuber bulking capacity and, thereby, its biomass and yield. Thus, potato removes large quantities of K and other soil nutrients, particularly N and P, in a short period coupled with a high rate of dry matter pro- duction (Perrenoud, 1993; Singh and Trehan, 1998). An optimum K level, along with optimum levels of N and P would, therefore, be required to exploit the full genetic po- tential of the crop and achieve an improved level of tuber yield and quality. In spite the efforts aimed at optimizing potato response to K fertilization, little has been done on the time of K ap- plication for potato farming. This study, conducted in the Central Bekaa Valley of Lebanon, aims at assessing po- tato response to increasing in-season potassium rates (four progressive rates of K) applied at different times of tuber Potato response to potassium application rates and timing under semi-arid conditions F. Karam*, R. Massaad**, S. Skaf**, J. Breidy**, Y. Rouphael*** * International Center for Agricultural Research in Dry Areas, PO Box 5466, Aleppo, Syria. ** Lebanese Agricultural Research Institute, PO Box 287, Zahleh, Lebanon. *** Department of Crop Production, Faculty of Agricultural Engineering and Veterinary Medicine, Lebanese University, Dekwaneh-El Maten, Beirut, Lebanon. Key words: aggregate tuber yield, potassium application rates, potassium application timing, Solanum tuberosum L., specific gravity, tuber dry matter. Abstract: A two-year experiment (2004-2005) was conducted at Tal Amara Research Station in the Bekaa Valley of Leba- non to evaluate the influence of progressive application of K rates and application timing on yield, yield components and tuber quality of potato (Solanum tuberosum L. cv. Agria). Four levels of potassium (0 (K0), 75 (K75), 150 (K150), and 225 (K225) kg K2O ha -1) and two application timings (tuber initiation and tuber bulking stages) were used in a split-plot design. The progressive application of potassium fertilizer from 0 to 225 kg K2O ha -1 significantly affected the yield and yield components of potato. In both years, small grade tubers and aggregate tuber yield increased quadratically with increas- ing K application rates up to 150 kg K2O ha -1, reaching a plateau thereafter, showing luxury consumption of the nutrient at 225 kg K2O ha -1. In 2004 when averaged over K application rates, large and medium grade tubers and aggregated tuber yield were 120%, 22%, and 12% greater, respectively, with K application at tuber bulking than at tuber initiation. A similar trend was also observed in 2005, when the small grade tubers and aggregate tuber yield were 20% and 12% higher, respectively, with K application at tuber bulking than at tuber initiation stage. Finally, no significant difference among treatments was observed for tuber dry matter (avg. 19.8%) and specific gravity (1.08 g cm-3). Adv. Hort. Sci., 2011 25(4): 265-268 Received for publication 2 August 2011 Accepted for publication 20 October 2011 Short note 266 growth (tuber initiation and tuber bulking stages) and to depict the optimal rate to achieve target yield. 2. Materials and Methods Experimental site Field experiments were conducted from April to Au- gust during the 2004 and 2005 growing years at Tal Amara Research Station in the Central Bekaa Valley of Lebanon (33º 51’ 44’’ N lat., 35º 59’ 32’’ N long., 905 m a.s.l). The details of the experimental site have been described else- where (Karam et al., 2003, 2005, 2006, 2007, 2009 a,b, 2011). Tal Amara has a well-defined hot and dry season from May to October and very cold conditions for the re- mainder of the year. Average seasonal rainfall is 592 mm, with 95% of the rain occurring between November and March, and a maximum of 145 mm in January. Historical data indicate no rain occurrence at Tal Amara from June to September. Rainfall amounts during the growing period were 35 and 25 mm during 2004 and 2005, respectively. Soils of the experimental site were deep, non-calcareous, clay Eutric Cambisols with an average bulk density of 1.2 g cm-3. Soil chemical and physical properties were: avail- able N content 45.5 g kg-1, available P content 17.0 g kg-1, and available K content 11.5 g kg-1, organic matter content 1.2% and pH 7.9. Crop management, K-treatments and experimental design Potato (Solanum tuberosum, L.) seeds of cultivar Agria were sown under field conditions on 5 April 2004 and 11 April 2005. The soil was plowed and disked each year in anticipation for bed preparation. In both years, seeds were planted in a conventional “hill” system, where single soil beds were separated by relatively deep furrows spaced 70 cm apart, giving a theoretical plant density of 70000 plants ha-1. The experiments were conducted under opti- mum irrigation conditions in both years. At planting, the soil surface was thoroughly moistened using a sprinkler irrigation system at the application rate of 4.5 mm h-1. When plants reached 8 to 10 cm in height (two weeks after emergence), a drip irrigation system was installed along the furrows. The drip system consisted in polyethylene (PE) distribution lines, 16 mm in diameter, 40 cm spaced drippers, delivering each 4 L h-1 at 1 bar of head pressure. Experiments were set up in a split plot design (main plot: potassium application rate; sub-plot: application timing). The trial covered four levels of potassium (0 (K 0 ), 75 (K 75 ), 150 (K 150 ), and 225 (K 225 ) kg K 2 O ha-1) and two applica- tion times (tuber initiation and tuber bulking stages), with five replications. Each experimental unit consisted of six rows, 5 m in length. In both years, preplant fertilizer was broadcast (150 kg·ha-1; 17N - 17P -17K) and incorporated into the soil. Moreover, a fertilizer dose of 144 kg N and 96 kg P 2 O 5 ha-1 was applied in two splits after planting (35 days after planting, DAP) and at tuber initiation (60 DAP) uniformly to all the plots to assure rigorous shoot development. Potassium was applied as K 2 O (0-0-46) in one split at tuber initiation (60 DAP) and tuber bulking (80 DAP) stages in four application rates with irrigation water. The experiments were concluded on 3 August 2004 (120 days after planting) and on 8 August 2005 (119 days after planting). Data collection After harvest, the tuber yield was grouped into three grades, grade 1 (200-400 g), grade 2 (85-200 g) and grade 3 < 85 g. The grade-wise and aggregate tuber yields were recorded. Tubers were dried in a forced-air oven at 80°C for 72 h and weighed to determine the tuber dry matter (DM). Tuber specific gravity (tuber weight in air/tuber weight in water) (Dunn and Nylund, 1945) was deter- mined on subsamples of acceptable tubers. Statistical analysis All data were statistically analyzed by ANOVA using the SPSS software package (SPSS 10 for Windows, 2001). Duncan’s multiple range test was performed at p=0.05 on each of the significant variables measured. 3. Results and Discussion In experiment 1 (2004), small grade tubers and aggre- gate tuber yield were significantly affected by K applica- tion rates, whereas large and medium grade tubers and ag- gregate tuber yield were highly influenced by K application timing, with no ‘K rates x K timing interaction’ (Table 1). While in experiment 2 (2005), small grade tubers and ag- gregated tuber yield were significantly influenced by K application rates, K application timing, with no significant ‘K rates x K timing interaction’ (Table1). In both years, no significant difference among treatments was observed for tuber dry matter (avg. 19.8%) and specific gravity (1.08 g cm-3) (Table 1). These results on tuber quality (i.e. dry matter and specific gravity) are consistent with the find- ings of Davenport and Bentley (2001) who observed no response in tuber quality, mainly specific gravity, in re- sponse to increasing K rates. In contrast, others have re- ported that excess K fertilizer reduces dry matter content and specific gravity of tubers (Westermann et al., 1994 a, b). Explanations for this disagreement could be the dif- ferent environments in which the plants were grown, and variations between potato genotypes in response to potas- sium application rates. In 2004 when averaged over K application rates, large and medium grade tubers and aggregated tuber yield were 120%, 22%, and 12% greater, respectively, with K appli- cation at tuber bulking than at tuber initiation. A similar trend was also observed in 2005, when the small grade tu- bers and aggregate tuber yield were 20% and 12% higher, respectively, with K application at tuber bulking than at tuber initiation stage (Table 1). In both years, irrespective of K application timing, the highest small grade tubers yield was recorded with K application rates of 150 kg K 2 O ha-1 (avg. 30 and 33 t ha-1, in 2004 and 2005 respectively), 267 whereas the highest aggregate tuber yield was observed at both K 150 and K 225 with no significant difference observed between the two K application rates followed by K 75 and finally K 0 treatment. In both experiments, aggregate tuber yield increased quadratically with increasing K application rates up to 150 kg K 2 O ha-1, reaching a plateau thereafter, indicating the luxury consumption of the nutrient at 225 kg K 2 O ha-1 (Table 1). Significant increase in tuber yield of potato as a result of K application is well documented (Cordova and Valverede, 2001; Singh et al., 2001; Tawfik, 2001; Umar and Moinuddin, 2001; Moinuddin et al., 2004, 2005). In fact, potato has a higher potassium requirement for op- timum production compared to cereals, pulses, oilseeds, and other commercial crops and produces much more dry matter in short growth duration. It produces large amounts of starch due to K-mediated carbohydrate metabolism (Perrenoud, 1993; Singh and Trehan, 1998). In addition, it helps in efficient translocation of photoassimilates to the developing sinks/tubers (Beringer, 1978) and enabling the plants to fully utilize applied N and P fertilizers (Mengel and Kirkby, 1987). Thus, K helps the potato tubers to at- taine large size and heavier weight. This was evident in the current study, as we observed a progressive increase in aggregate tuber yield. These results are consistent with the findings of Moinuddin et al. (2004, 2005) and Abd El- Latif et al. (2011) who showed an increase in tuber yield with a progressive application of K fertilizer from 0 to 225 kg K 2 O ha-1 (Moinuddin et al., 2004, 2005) and from 72 to 120 kg K 2 O fed.-1 (Abd El-Latif et al. 2011). Moreover, in line with our results, Singh et al. (1997) reported that an increase in K application rates resulted in an increase in the yield of small-grade tubers. To summarize, we can conclude that the progressive ap- plication of potassium fertilizer from 0 to 225 kg K 2 O ha-1 significantly affected the yield and yield components of potato. In both experimentation years, small grade tubers and aggregate tuber yield increased quadratically with in- Table 1 - Effects of potassium application rates and K application timing on grade-wise and aggregate tuber yield, tuber dry matter and specific gravity of potato plants grown in 2004 and 2005 Year K timing K rate Grade-wise tuber yield (t ha-1) Aggregate yield Tuber dry matter Specific gravity Kg K2O ha -1 Grade 1 Grade 2 Grade 3 t ha-1 % g cm-3 2004 Tuber initiation Stage 0 0.6 30.7 19.9 51.2 20.0 1.078 75 0.3 23.4 28.1 51.8 19.8 1.079 150 0.9 24.2 28.9 54.0 19.5 1.077 225 0.2 26.0 29.4 55.6 20.2 1.081 Tuber bulking Stage 0 1.0 27.4 25.5 53.9 19.5 1.077 75 1.7 30.8 24.3 56.8 19.2 1.075 150 0.4 33.0 31.1 a 64.5 20.0 1.079 225 1.3 36.5 25.2 63.0 20.0 1.080 Significance (z) K rate ns ns ** ** ns ns K timing * * ns * ns ns K rate x K timing ns ns ns ns ns ns 2005 Tuber initiation Stage 0 0.8 29.1 22.7 52.6 19.8 1.078 75 1 27.1 26.2 54.3 19.5 1.077 150 0.6 28.0 30 58.6 19.8 1.078 225 0.8 31.2 27.3 59.3 20.1 1.081 Tuber bulking Stage 0 0.9 28.2 27.3 56.4 19.7 1.077 75 1.3 29 31.4 61.7 19.3 1.076 150 0.5 31.0 36 66.5 19.9 1.079 225 1 33.9 32.8 67.7 20.0 1.080 Significance (z) K rate ns ns * ** ns ns K timing ns ns * * ns ns K rate x K timing ns ns ns ns ns ns (z) ns * **. ***Non significant or significant at P < 0.05, or 0.01 respectively. creasing K application rates up to 150 kg K 2 O ha-1, reach- ing a plateau thereafter, showing luxury consumption of the nutrient at 225 kg K 2 O ha-1, indicating the detrimental effect of over fertilization. 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