Effect of water stress on yield and physiological traits among selected African tomato (Solanum lycopersicum) land races

Expansion of tomato farming in dryland regions of Kenya has the potential to improve livelihoods and food security of rural farmers. However, the crop is very sensitive to water deficit that has made its expansion in dryland regions of the country to nearly impossible. Crop landraces have been continuously used to develop varieties adapted to abiotic stresses such as drought. In Africa, tomato has a rich genetic resource base which is largely undocumented and whose knowledge can aid in the identification of genotypes with desirable traits for breeding. The objective of this study was to evaluate the variation in response to water stress on yield and physiological traits of twenty (20) African tomato accessions from the World Vegetable Centre and the National Genebank of Kenya. Planting was done in a greenhouse in a randomized complete block design with three replications and subjected to four soil moisture levels of 100% Pot capacity (PC), 80% PC 60% PC and 40% PC. The response to water stress was mainly dependent on the genotype and reduction in moisture significantly reduced the SPAD value, leaf relative water content, stomatal conductance, the number of fruits per plant and fruit weight per plant. However, canopy temperature increased with the decrease in moisture level. Variations among accessions for fruit weight per plant ranged from 521-2404.3 g (100% PC), 421.3-2020.7 g (80% PC), 359.3-1768.3 g (60% PC) and 127.3-1487.7 g (40% PC). This variability shows the potential among the African tomato accessions for breeding drought-tolerant tomato varieties. * Corresponding Author: Kenneth O. Tembe  tembeken@gmail.com International Journal of Agronomy and Agricultural Research (IJAAR) ISSN: 2223-7054 (Print) 2225-3610 (Online) http://www.innspub.net Vol. 10, No. 1, p. 78-85, 2017 Int. J. Agron. Agri. R. Tembe et al. Page 79 Introduction Tomato (Solanum lycopersicum) is a fruit vegetable that belongs to the family Solanaceae which consists of approximately 100 genera and 2500 species, including several plants of agronomic importance such as potato, eggplant, pepper, and tobacco (Olmstead et al., 2008). The crop which is the second most important vegetable crop cultivated in the world (Foolad, 2007) is native to South America (Blanca et al., 2012). Tomato fruits are cooked as vegetables or used as salad and sometimes processed to tomato paste, tomato sauce, tomato juice and ketchup. According to Mbaka et al., (2013), tomato is an economically important horticultural crop in Kenya that has the potential of improving the livelihood of the poor rural farmers. Consumption of tomato fruit has gained importance due to its rich antioxidant property known to reduce cancer incidences (Wamache, 2005). According to Wang et al., (2011), tomato fruit contains lycopene, β-carotene, ascorbic acid and phenolic compounds, which have nutritional benefits to consumers. One of the major constraints to tomato production in dryland areas of Kenya is the lack of adequate rainfall. This is because the crop is very sensitive to water deficit that reduces fruit yield and results in possible crop failure (Sibomana et al., 2013). The current global warming, which causes fluctuations in precipitation distribution, further increases the risk of this plant being repeatedly exposed to drought (Miyashita et al., 2005). However, provisions of appropriate amount of water and breeding for drought tolerance are few of the practices that can ameliorate these challenges in dry lands. According to Nuruddin, (2003), sensitivity to water deficit varies among different crops and genotypes. Such variations have been found in crop landraces which are often characterized by good stress tolerance and local adaptability (Newton et al., 2010). According to Torrecillas et al., (1995), tolerance to water stress can be found in wild species of crops. These genotypes have the potential of growing under conditions that present minimum water. This characteristic is important and can be introduced into commercial varieties with good agronomic characteristics (Ashraf, 2010). African tomatoes are landraces with dynamic populations, distinct identities and lack of formal crop improvement. To date, a large number of these landraces have been collected and stored in genebanks and research organizations. However, very few of them have been systematically evaluated for their adaptability to drought (Robertson and Labate, 2007). It is for this reason that this study aimed at estimating the variability among selected African tomato accessions in yield and physiological traits under varying moisture levels. Materials and methods Study site The study was conducted at the University of Nairobi's Upper Kabete Campus field station, Kenya, in the year 2015. It is geographically located at an altitude of 1940 meters above sea level and between latitude 10 14’ 20’ South and 10 15’ 15’North and longitude 360 44’ East. Treatment and treatment allocation Seeds of 20 African tomato accessions sourced from the World Vegetable Centre (AVRDC) and the National Genebank of Kenya were used (Table 1). Four (4) weeks old seedlings were transplanted into 10 liter pots and arranged in a randomized complete block design (RCBD). Ten kilograms of sterilized air dried soil (a mixture of sand, topsoil, and manure at the ratio of 2:4:1) was used to fill the pots. Four pots with one seedling each were randomly assigned to each of the four watering regimes throughout their growth cycle. The amount of water added was determined based on the percentage of pot water capacity (Sibomana et al., 2013). Treatments included: 100% of pot capacity (PC) or control (3000 ml), while stress was achieved by applying 80% (80% of PC), 60% (60% of PC) and 40% (40% of PC) of the amount of water applied to the control plant (Sibomana et al., 2013). The pots were covered with black plastic material to prevent evaporation and placed on top of a plastic paper to avoid direct contact with the soil surface. Int. J. Agron. Agri. R. Tembe et al. Page 80 Table 1. List of selected African tomato landraces evaluated in the study. SL. No. Accession name Species name Origin 1 GBK 050580 S. lycopersicon Kenya 2 GBK 050580 S. lycopersicon Kenya 3 RVI01896 S .lycopersicon Madagascar 4 RVI02100 S. lycopersicon Madagascar 5 RVI02107 S. lycopersicon Madagascar 6 VI005871 S. lycopersicon Morocco 7 VI005874 S. lycopersicon Morocco 8 VI005876 S. lycopersicon Morocco 9 VI005895 S. lycopersicon Egypt 10 VI006826 S. lycopersicon Ethiopia 11 VI006841 S. lycopersicon Ethiopia 12 VI006847 S. lycopersicon Ethiopia 13 VI006881-B S. lycopersicon Zimbabwe 14 VI006972 S. lycopersicon Tanzania 15 VI007539 S. lycopersicon South Africa 16 VI007540 S. lycopersicon South Africa 17 VI008234 S. lycopersicon Nigeria 18 VI030379 S. lycopersicon Mauritius 19 VI030852 S. lycopersicon South Africa 20 VI037948 S. lycopersicon Zambia Data collection Data were taken at 50% days to flowering from three randomly tagged tomato plant accessions for SPAD value, leaf relative water content (LRWC), stomatal conductance and canopy temperature. Numbers of fruits per plant and weight of fresh fruits per plant were recorded as candidate plants had their fruits attain physiological maturity. Chlorophyll measurements were done on two fully opened leaves in each plant using SPAD (Minolta SPAD 502 chlorophyll meter). Leaf relative water content (LRWC) was calculated according to Yamasaki and Dillenburg (1999) formula, LRWC (%) = [(FM – DM)/(TM – DM)] x 100. Stomatal conductance was determined using a leaf porometer (Model Sc-1, Decagon Devices, Pullman, USA) and expressed in millimoles per meter squared seconds (mmol/m2s) as suggested by Chakhchar et al., (2016). Canopy temperature was measured according to Turner et al. (1986) using an infra-red thermometer (Model THI-500, TASCO, Japan). Statistical analysis of data All the data collected were subjected to the analysis of variance (ANOVA) using GENSTAT Release 7.2 Discovery Edition 15. Treatment means were separated using Fisher's least significant difference (F-LSD) at 5 % level of significance. Results SPAD value SPAD value was significantly (P<0.05) reduced by moisture deficit (Table 2). Similarly, variation among accessions was significant and ranged from 48.3 to 58.1 (100% PC), 47.6 to 57.1(80% PC), 46.8 to 56.9 (60% PC) and 46.3 to 56.8 (40% PC). Table 2. Mean values for SPAD value and leaf relative water content among the 20 selected tomato accessions grown in the greenhouse under different water levels. SPAD value Percentage leaf relative water content Accession name 100%PC 80%PC 60%PC 40%PC Mean 100%PC 80%PC 60%PC 40%PC Mean GBK 050580 52.00 50.67 50.03 49.97 50.67 94.38 92.73 62.24 43.05 73.10 GBK 050589 52.27 51.77 50.60 50.27 51.23 92.94 92.86 58.93 42.86 71.90 RVI01896 55.80 54.07 51.73 47.57 52.29 77.17 68.24 58.34 54.82 64.64 RVI02100 53.97 51.57 51.20 50.27 51.75 85.12 77.55 65.78 47.31 68.94 RVI02107 51.73 50.37 50.07 49.30 50.37 80.10 75.34 52.41 47.14 63.75 VI005871 54.77 53.53 51.73 49.27 52.33 77.94 75.76 60.65 56.00 67.59 VI005874 51.30 51.00 49.77 48.57 50.16 86.15 81.11 74.13 49.86 72.81 VI005876 57.67 57.60 56.93 56.77 57.24 80.55 69.29 54.61 49.35 63.45 VI005895 56.67 54.23 53.70 52.63 54.31 86.79 83.80 67.87 47.88 71.58 VI006826 52.10 50.73 49.73 49.40 50.49 89.27 80.50 72.25 63.62 76.41 VI006841 49.83 48.47 47.63 47.60 48.38 88.40 87.43 53.06 47.21 69.02 VI006847 57.27 54.17 53.53 50.27 53.81 84.14 76.36 65.81 46.36 68.17 VI006881-B 51.40 49.10 47.97 47.30 48.94 85.40 82.16 57.40 55.40 70.09 VI006972 55.43 54.87 52.43 51.73 53.62 84.04 82.91 54.62 49.85 67.85 VI007539 50.70 49.50 48.20 46.87 48.82 85.63 76.32 61.61 48.98 68.14 VI007540 53.63 52.37 50.60 50.30 51.73 76.75 72.02 68.18 53.91 67.72 Int. J. Agron. Agri. R. Tembe et al. Page 81 SPAD value Percentage leaf relative water content Accession name 100%PC 80%PC 60%PC 40%PC Mean 100%PC 80%PC 60%PC 40%PC Mean VI008234 48.30 47.57 46.80 46.30 47.24 81.38