Philippine Journal of Crop Science 2004, 29(1) 29-39 Copyright 2005, Crop Science Society of the Philippines Released 15 March 2005 SCREENING CASSAVA GENOTYPES FOR RESISTANCE To WATER DEFICIT DURING CROP ESTABLISHMENT alREYNALDO V BERGANTIN', ANTRA YAMAUCHI2, JOSE R PARDALES JR' & DIOSCORO M BOLATETE 'Plant Environmental Response Laboratory, PhilRootcrops, Leyte State University, Baybay, Leyte 6521-A, Philippines, and 2Graduate School of Bioagricultural Sciences, Nagoya University, Chikusa, Nagoya 464-8601, Japan Although known to be drought-resistant, cassava is usually constrained in its development and productivity by lack of soil moisture especially during the earlier stages of growth, hence the need for screening for resistance. The performance of 28 cassava genotypes under deficit soil moisture content (smc) of 10% was studied during their establishment period. The plants were grown in plastic pots (24.5 cm wide, 26.5 cm deep) that were filled with sandy loam soil and kept under a rain shelter covered with transparent acetate 0.08 mm thick. This study was undertaken to determine the vegetative and physiological traits in juvenile cassava plants that may confer drought resistance to the crop. The results showed that with reference to the well- watered plants (25% SMC), the plant height, number of leaves, and total shoot dry weight were significantly (P<0.01) reduced by lack of water supply. In terms of variety, these traits varied significantly (P<0.01) in their degree from one variety to another. In droughted plants, the stomatal resistance was significantly higher (P<0.01) and the rate of transcription significantly lower (P<0.05) than in the well-watered controls. The effect of water regime on leaf water potential OW and water use efficiency (WUE) was not at all significant, however. The values of these physiological traits generally varied among the varieties, although significant effects (P<0.01) were noted only for 1111, but not for transpiration rate. Overall, the results made it possible for some genotypes to be identified as resistant to limited soil water supply. With the well-watered plants as basis, genotypes that performed well under limited soil water conditions were observed to manifest any of the following traits: lesser inhibition in shoot vegetative development, higher stomatal resistance, lower rate of transpiration, higher dry biomass, and higher WUE. cassava, drought, leaf water potential, limited water supply, stomata]. resistance, transpiration rate, water use efficiency INTRODUCTION (www.iwmui.org). Withthe growing world population and increasing water scarcity, major Increasingly, water scarcityis becoming a efforts and strategies in agriculture have to be universal problem at the present time. Report geared towards sustaining crop production and shows that water insufficiency threatens the food increasingproductivityunderlimitedwater supply of nearly 3 billion people, as well as the resources. Among the crops in cultivation today, productivity of many ecosystems around the world root crops deserve particular attention not only (www.worldwaterforum.org). Currently, some 450 because many of the world's poorest and most food- million people in 29 countries face water shortage insecure households are highly dependent on these problems. By 2025, about 2.7 billion people in 29 crops as a contributing, if not the principal, source of countries will live in regions facing severe water food,nutrition, and cash income (Alexandratos scarcity. Asia and Sub-Saharan Africa, containing 1995), but also that the crops' individual ability to the most heavily populated and poorest regions in grow even in areas with limited water supply can be theworld,willbemostseverelyaffected exploited to help ensure food security, especially that about 2 billion people in the tropics and drought resistance on the crop can also be improved subtropicsdepend onthesecropsfortheir by breeding & selection in order to come up with sustenance and livelihood (Scott et al 2002). varieties that will perform well under limited soil Cassava (Manihot esculenta) is one of the most moisture supply. widely grown rootcrops in the world. Millions of However, varietal response of cassava to soil farmers,processorsandtradersearntheir water deficiency is not yet well understood. It is livelihood from this crop. As food, it is the basic postulated that different varieties show different staple of about 500 million people in the tropical and reactions to drought and that the mechanism of sub-tropical parts of the world (www.fao.org), being drought tolerance varies from one variety to another a very reliable and cheap source of body energy. It among the resistant ones. A basic understanding of follows that cassava can then be an important the drought response of different cassava varieties engine for growth in many countries if production, is therefore important so that the traits that give diversification, and the commercial uses of the crop the crop the ability to withstand drought stress and are improved or developed. thus help stabilize the productivity of the crop, can Cassava is commonly known to be resistant to be identified. This information will be beneficial to drought and withstands prolonged periods of water plant breeders for them to have a good idea of the shortage (Cock 1985, El-Sharkawy & Cock 1987) traits that can be directly worked on in designing especially after it has fully established itself in the new drought-resistant varieties of the crop. field. This can be the reason why the crop is commonly planted in marginal areas including MATERIALS & METHODS those that are frequented by water shortage or drought. Generally, however, the economic yield of The study was conducted in an open-air non- the crop depends on the growing conditions to which shaded space at the Philippine Rootcrops Research it is exposed, and water appears to be the most and Training Center (PhilRootcrops) located in the limiting growth factor (Pardales et al 2001). The campus of Leyte State University, Baybay, Leyte studies of Pardales & Esquibel (1996) and Agili & (10°44' N: 124°47' E: 12 m ask). A rigid rain shelter Pardales(1997) have shown that cassavais (9.5 meters wide, 16 meters long, 2.5 meters high) sensitivetosoilwaterdeficiencyduring the with a transparent acetate (0.08 mm thick) roof was establishment period,ie, the first 3 months ofconstructed to protect the experimental pots (of growth from planting. Drought stress during this polyethylene, 24.5 cm wide, 26.5 cm deep) against periodsignificantlyreducesthegrowthand the rain, thereby allowing the regulation of the development of both the roots and shoots, which in moisture content of the soil in the pots. turn affects the succeeding development of the plant including tuber development, even if the drought Test Varieties And Plant Establishment stress is alleviated later. This generally results in Twenty-eight cassava genotypes obtained from either low tuber yield at the normal maturity time the cassava germplasm collection of PhilRootcrops of the crop, or calls for a delay in the timing of the were used inthisstudy.These included10 harvest to allow an increase in storage root yield to traditional varieties (Golden Yellow, Indang 2, a desirable level. Kadabao, Kaplutan, Namaya, Nito-nito, Pintuyan 3, In places where the soil moisture supply is Siasi,Tandang 2,and Zapote),15Philippine highly dependent on rainfall, the production of Seedboard- (PSB) released varieties (Datu 1, Lakan cassava is often unstable particularly when the rain 1, Sultan 1, Vassourinha, Sultan 2, VC-1, VC-2, VC- pattern is fluctuating or when the genotype planted 4, PSB Cv-11, PSB Cv-12, PSB Cv-13, PSB Cv-14, PSB is sensitive to water stress. The potential for genetic Cv-15, PSB Cv-16, and PSB Cv-19), and 3 introduced improvementincassavahasbeenclearly varieties from Thailand (KU-5, Rayong 5, and demonstrated in increasing the yield and improving Rayong 60). Six stem cuttings (20 cm long) of each the traits of the crop that has something to do with variety were planted upright at one cutting to a pot the yield like harvest index and root dry matter filled with 9 kg of air dried sandy loam soil mixed content (Mariscal & Bergantin 1998, Kawano et al with 1.6 grams of powdered complete fertilizer (14- 1998). It is also assumed that traits conferring 14-14 NPK). Two days prior to planting, the soil in each pot was saturated with 1,125 mL tap water to Experimental Design & Statistical Analysis bring the soil to field capacity (ie, 25% SMC). This The treatments were laid out insplit-plot amount of water in the soil was maintained in all arranged in a randomized complete block design the pots from planting to 30 days after planting (RCBD) with three replications. Water regimes (well- (DAP) by regularly determining the SMC with a watered and droughted) served as the main plots, digital readout volumetric water content measuring the varieties as the subplots. device (Hydro Sense: Decagon, USA). The amount of The analysis of variance was applied on the water lost through evaporation and transpiration data obtained following the General Linear Model from each pot every other day was re-supplied (SAS 1989). When necessary, differences among accordingly. A total of 180 pots were prepared. variety means were separated using Duncan's Multiple Range Test (DMRT) at (P<0.05) level. Least Imposition Of Drought Significant Difference (LSD) was used to determine Beginning at 30 DAP (referred to as Day 0 of the level of significance of the difference in water stress), half of the total number of pots for between the well-watered and the droughted plants each genotype were not applied with water until in every variety. their SMC leveled off at around 10%, to simulate a severedroughtcondition.Thisamount was RESULTS & DISCUSSION maintained throughout the whole duration of the drought stress (ie, until 85 DAP). The SMC of the Plant Development remaining half of the pots was kept at its original The stem elongation (expressed as plant height) level (25%) to serve as well-watered controls.