OVERCOMING ABIOTIC SHADES FOR SWEETPOTATO (IPOMOEA BATATAS L.) IN THE HIGHLAND OF PAPUA, INDONESIA: THE ADAPTATION OF VARIETIES AND STICKS INCLINATION ANGLES

Abiotic shades due to the high level of cloud are the major problem causing a low level of sweetpotato (Ipomoea batatas L.) photosynthesis in the higland of Papua, Indonesia. In order to optimize the growth and production of sweetpotato, it needs to engineer the growing environment for the crop. This study aims to learn the growth, production and photosynthesis efficiency (Eμ) by combining varieties and sticks inclination angles on (1.560 m asl) from April to September 2016. It was conducted using a Split-Plot Design with three replications. The main plot was three sweet potato varietes: Siate, Papua Salosa and Cangkuang, while the subplot was the inclination of sticks with three angles: 90o, 60o, 45o and without using sticks. The result shows that a combination of Cangkuang and 90o stick inclination produces the highest total dry matter and tubers (326.9 g/plant and 248.7 g/plant respectively). The highest net assimilation rate was obtained in combination of Cangkuang with stick inclination angles treatments 90o and 60o for 125.2 mg.cm2.day-1 and 105.3 mg.cm2.day-1, the leaf area index ranged from 0.9-5.5. The highest tubers yield was achieved by the Cangkuang with 90o and 60o sticks inclination angles treatments, which are 31.53 t.ha-1 and 28.86 t.ha-1 respectively, with the harvest indexes around 36-62 %, in which the tubers yield and the harvest indexes have significant correlations. The highest photosynthesis efficiency was achieved by the Cangkuang with 90o (3.83 %) and 600 (3.34 %) sticks inclination angles treatments, while the lowest one was done by the Siate and without sticks (0.92 %) treatment. As a conclusion, the higher sticks inclination angles applied on the sweetpotato varieties, particularly whose wider leafs, under abiotic shade conditions may increase crop growth rates, total and tuber dry matters, tubers yield, harvest indexes, and photosynthesis efficiency rates.


Introduction
The Baliem valley has an area of 960 Km 2 , located in the Jayawijaya Regency of Papua, with an elevation of 1560 m asl. This region is unique as it has a vast variey of bio-diversity which has strongly attached to the socio-culture of indigenous people since hundred years ago. According to de Fretes et al. (1999) and Kogoya et al. (2014), Papua has the highest endemic species in the world. Meanwhile, Arobaya and Pattiselanno (2007) indicate there are 35 uselful crops used by the Dani people who live in the Baliem valley for many purposes, in which Pandanus and Sweetpotato are the most well-known crops. Sweetpotato is staple food for people, feed for livestock, and widely used for traditional ceremonies (Wydiastuti, 1994). The Dani call it as hipere, regarded as a mother or woman crop which has 200 different cultivars cultivated in the Baliem valley and the other 400 ones grown around the valley (La Achmady et al., 1993;Rauf et al., 2009). Soenarto (1997); Saraswati et al. (2013) mention that sun intensity radiation in the Baliem valley is only 1.38 kJ.cm -2 .d -1 with an average duration of sunlight around 3.98 hours/day and mostly cloudly. As a result, sunlight reached to soil surface and crops for photosynthesis becomes limited. To overcome the limitation of sun radiation intensity, the Dani adopts local knowledge in which they grow long-vine cultivars which are creeped to sticks whose 120-150 cm in height aiming to prevent leaves from shading one to the others, while short-vine ones are grew in the edge of gardens (Widyastuti, 1994). This practice aims to enable crop leafes get enough sunlight for photosynthesis and the crops produce big tubers. Johnston and Onwueme (1998); Onwueme and Johnston (2000) report that sweetpotato is the most intolerant tropical root crop to shade in which the higher shade rates the lower dry weight produce. Osvald, Alkaumper and Midmore (1994); Osvald, Alkaumper and Midmore (1995) also report that shade is responsible for the decrease of forming initiation, numbers, and sizes of tubers.
The Indigenous knowledge of Dani people as sweetpotato farmers in the highland of Papua has widely been studied by a number of scientists, e.g. Widyastuti, 1994;Soenarto, 1997 andSaraswati et al (2013). However, most of the studies only looked at the socio-cultural aspects, some did the agronomic ones, and none study the effect of abiotic shade due to cloudly high rates on the physiologi of sweetpotato crops. For the crops to be efficient in utilizing sun radiation and more photosynthesis results be stored in tubers, it needs environmental engineering by choosing appropriate varieties and the inclination of sticks to increase the interception and absortion of sun radiation by leafs.
According to Wargiono (1980), sweetpotato varieties whose wide leafs can have more effective photosynthesis processes and produce higher tubers compared to those whose narrow and fingering ones. Maryasa (1990) mentions that the use of sticks enables crops to catch sunlight efficently and reduces shade rates. Applying different inclination angles of sticks will change the position of leafs which can affect the absortion rate of sun energy by crops. This application follows the law of Lambert in Arifin (1987) which states that the absortion rate of sun energy by any surfaces is greatly affected by the edge of incoming sunlight.
This study aims to examine the capability of varieties with leafs morphology and different inclination angles of sticks to increase the efficiency of crops in utilizing sun radiation. The increase rate of efficiency in utilizing sun radiation by crops during their growing period can be

Experimental Designs, Treatments, and Data Collection
The experiment was conducted using a random spli-plot design with three replications. Sweetpotato variety is the main plot with three of type: Siate (V1), Papua Sollosa (V2) and Cangkuang (V3). Four different inclination angels of sticks are the sub plot: Without stick (A0), and sticks inclination of 45º (A1), 60º (A2), and 90º (A3). The land for the experiment was traditionaly prepared using local shovel, and plots were divided into three group which each has 3 sub-groups with a size of 5.25 m x 6.50 m. The distance between groups was 100 cm, while the distance between sub-groups was 50 cm. The planting space was 75 x 50 cm with only one cutting per planting hole in a single planting row. The weeding was done on 15, 45 and 80 Days After Planting (DAP). Both chemical ferilizer and pest-diseases control were not applied in this experiment following the local government prohibitation regulation. The observation was conducted using a destrucive method by taking 2 non-edge plants as samples on 40, 70, 100 and 130 DAP. These samples were taken using a local wooden stick called sege in the local languange. Sun radiation interception was measured on and under the leaf canopy using a lux meter LX 1330 B. The measurement was done three times on and under the leaf canopy, and also three times on the surface of soil at 11.00 AM when the wheater was bright. Global sun radiation data was also collected from a local meteorological station (Class III Wamena).

Crop Measurement
The observation variables included Leaf Area Index (LAI), Crop Growth Rate (CGR), Net Assimilation Rate (NAR), dry weight, tuber produce, Harvet Index (HI) and Photosynthesis Efficiency (Eµ). The leaf wide was measured using the Puch method (Etje and Osiru,1988), while the LAI, CGR and NAR were done using the Gardner et al. (1985) method, and the Eµ was measured using the Yoshida (1981) method. The LAI was calculated from the area of leafs per land area. The CGR was done by the sum of total dry weight of crops per land area and per the age of crops. The NAR was done by the sum of of total dry weight of crops per leaf area unit and per the age of crops. The crop was separated into roots, stakes, leafs, dan tubers which were dried until the weight is constant in the oven with a temperature of 80º C. The HI was defined as total tubers dry weight tubers divided by total crops dry weigth. Finaly, the Eµ was measured basing on total crops dry weight (gm -2 ) divided by cumulative sun radiation (calcm -2 day -1 ) after harvest.

Statistical Analysis
Statistical analysis was conducted using Analysis of Variance (ANOVA) with a different significancy level of 5%. The difference between treatments was tested by Duncan Multiple Range Test (DMRT 5%). The overall data compilation and analysis were processed using the GenStat 17 statistical program and Microsoft Office Excel 2010.

Leaf Are Index (LAI)
In the early stage of vegetative periods, there is no significant difference in Leaf Area Index. However, at 70 -130 DAP, the LAI begin to show significant differences (p<0.05) in between all of the treatment combinations. Changes in the LAI for the combination of varieties and sticks inclination angels according to crop growing periods are shown in Figure 1. After planting, the LAI increase fastly in line with crop growing periods reaching the peak at 100 DAP, then begin to fall approaching crop harvesting periods at 130 DAP. The highest LAI is achieved by the V3A2 (5.5) combination, while the optimum ones is done by the V3A3 (4.2) one, however is not significantly different from V3A2 (4.1). The difference in morphology causes the difference in physiological functions. Cangkuang whose has wide leafs compared to Siate and Papua Salosa whose have narrower leafs affects its LWIs. Physiologically, LAI and crop header architecture are important as they determine the capability of crops to absorb sunlight. The higher LAI the more sunlight can be caught by leafs. Brown (1992) estimates an LAI of 3 -4 is required to intercept 95% of PAR radiation in sweetpotato crops. Polynomial models to predict the highest and lowest LAI for the combination of varieties and sticks inclination angels are presented using the following equations: y = -0.

Crop Growth Rate (CGR) and Net Assimilation Rate (NAR)
The effect of the combination of varieties and sticks inclination angels on Crop Growth Rates is presented at Figure 2a. of 3 -4 is achieved at 56 -112 DAP, in which the CGRs does a maximum rate which produces dry weight of 106 -133 g.m 2 .
Net Assimilation Rates (NARs) are net assimilation results per leaf wide unit and time. The NARs are significantly differences (p<0.05) among all the combination of varieties and sticks inclination angels from the early stage of growing to harvesting periods. The CGRs tend to be higher at vegetative phases, then fall gradually following the age of crops until generative ones. It is similar to Gardner (1991) that the CGRs are not constant and tend to fall following the age of crops as the older crops the more photosynthesis results are utilized to form tubers. At the early stage of growing periods, the V3A3 dan V3A2 combinations produce the highest NARs (125.2 mg.cm 2 .day -1 dan 105.3 mg.cm 2 .day -1 respectively; Figure 2b). This finding is higher than

Tuberous Root Dry Weight (TW) and Total Dry Weight (TDW)
The results show that Tuberous Root Dry Weight (TWs) increase following the age of crops.

Tuber Yield (TY) and Harvest Index (HI)
The treatments combination of varieties and sticks inclination angels shows significant differences in TY (P<0.05) ranging from 12.60 t/ha to 31.53 t/ha (Figure 3b). The V3A3 and V3A2 treatments achieve the highest TY (31.53 t/ha and 28.86 t/ha respectively), while the V1A0, V1A3, V2A0 and V2A3 treatments do the lowest ones. The treatments combination of varieties, whose wider leaf in particular, and higher sticks inclination angles locates the position of leafs more vertical, in which in a low sunlight condition enables the leafs to catch more sunlight for photosynthesis. In contrast, the treatments combination of varieties and without sticks causes only leafs on the top catching more sunlight, while the below ones doing less as they are shaded which also mean less photosynthesis. In addition, the low TY on the Siate and Papua Sollosa with the sticks inclination angle of 90 0 treatments is also due to some leafs become dry and fall.
Translocation efficiency to the economic of TYs defines HI, which is a biological weight proportion unit transformed to yields. The ANOVA of HI at 150 DAP is presented at Figure 4a. The treatments combination of varieties and sticks inclination angles have significant differences (p<0.05) in HIs ranging from 36 % to 62 %. The highest HIs is achieved by the V1A1-V1A3 treatments, however they are not sigificantly different from the V2A1-V2A3 and V1A1-V1A2 treatments. This finding means the use of sticks for all the varieties may increase HIs, except Siate with the sticks inclination angle of 90 o due to many senescence leafs. The higher HIs the more efficient for the crop to utilize photosynthesis results for the formation of tubers. It explains that the higher position of leafs or the more vertical order of leafs facing incoming-sunlight, the more sunlight being distributed into all leafs in every canopy layer, which means the more efficient for the crop to utilize sunlight. Wigham and Wooley (1974) state that the orientation of leafs affects the radiation interception of sunlight by header crops, in other words, a vertical header crop is more efficient to intercept sunlight radiation than a horizontal one. Moreover, Wang et al. (2014) state that low interception efficiency due to shade reduces TY and dry matter accumulation significantly, however it increases shoot/root ratios. The TYs has strong correlations with the HIs as they show in the following linier equation: y = 69.663x -17.15, R 2 = 0.7164, coefficient correlation, r = 0.8464 (Figure 4b). This finding similar to Bhagsari (1990) who concludes that both fresh and dry sweetpotato TYs have significant correlations with HIs.

Photosynthesis Efficiency (Eµ)
Eµ is a comparison between energy needed for crops to produce chemical energy or organic materials and total energy received by the crops (Yoshida, 1981;Sitaniapessy, 1985). The ANOVA for the Eµs in this study is presented at Figure 5a. The average Eµs for the treatments combination of varieties and sticks inclination angels show significant differences (p<0.05) ranging from 0.92 to 3.83%. The V3A3 and V3A2 treatments show the highest Eµs (3.83 and 3.34 %), while the V1A0 treatment does the lowest one (0.92 %). This finding is considered to be high as Eµs for sweetpotato crops, according to Rana and Rana (2014) is generally only 1.6 -1.9 %. The correlations between the interception efficiency and the Eµs are shown at Figure 5b. The interception efficiency increases in all the treatments, except the V1A3 and V2A3 ones due to its leaf morphological characteristics (narrower and fingery) causing even at the sticks inclination angle of 90 o , still much radiation escape from leafs catching. This finding means that the varieties whose wider leafs and more vertical of orientation leafs order treatments can increase their Eµs due to higher PARs intercepted by each leaf in every header layers. Goudriaan (2016) states that the distribution of leafs plays an important role in determining sunlight interception. If water and nutrients supply is sufficient, the use efficiency of sunlight radiation by crop-headers will be determined by the interception of sunlight by crop-headers and the distribution pattern of sunlight in crop-headers (Monteith, 1977;Sitaniapessy, 1985). Govindjee and Thomas D. Sharkey (2016) conclude that if LAIs are high, photosynthesis canopy will be maximized by the vertical inclination of leafs.

Conclusions
Tubers and total dry weights increase on all the sweetpotato varieties following the higher inclination angles of sticks, except on the combination of Siate and Papua Salosa varieties with the inclination angle of sticks of 90º. The major factor which affects the dry weight produce was the leaf wide index. The crops growth rates increase from vegetative to early genarative periods in which the highest (11.1 g.plant -2 ) was achived by the combination of Cangkuang variety and the inclination angle of stick of 90º, on 100 DAP, following the increase pattern of leafs area index. The crops growth rates on the combination of Papua Salosa and Cangkuang varieties with sticks tend to be higher compared to the combination of Siate variety with sticks, except during the early growing period. The components related to the economic result of sweetpotato are tubers produce and harvest index. Cangkuang variety whose wide leafs morphology achives the highest produce when it combines with the inclination angles of sticks of 90 o and 60 o (31.53 t.ha -1 and 28.86 t.ha -1 respectively). The harvest indexes increase following the increase inclination angles of sticks on all the varieties, except Siate. Finally, the photosynthesis effciency rates also increase in line with the increase efficiency of interception in which the highest is achieved by the combination of Cangkuang variety and the inclination angles of sticks of 90 o and 60 o (3.83 % and 3.34 % respectively).