Abstract:

Drought stress has and continues to be a limiting factor to banana productivity. The effects are significant on crop growth and its yield. However significant increases in yield depend on moisture availability and expression of drought stress avoiding mechanisms. Physiological processes important in determining cultivar sensitivity to drought stress were not critically quantified in earlier studies hence this study. This study aimed at quantifying the threshold responses of physiological parameters such as plant biomass development, leaf orientation behaviour, chlorophyll breakdown and transpiration for their sensitivity to drought stress. The EAHB triploid cultivars of Mpologoma and Kisansa (AAA) were used in the study in comparison to considered drought tolerant cultivars of Kayinja (ABB), Sukali Ndiizi (AAB) and a low land cultivar Yangatnbi Km5 (AAA). All cultivars were subjected to three contrasting regimes of soil moisture. Moisture availability (M.A.) application levels were determined based on soil moisture retention factor (pF) ranging from 2.0-2.1 (0.1 - 0.13 bar) for free drainage soil level; 2.5-2.7 (0.32 - 0.51 bars) for semi moist level; 2.8-2.9 (0.64 - 0.8 bars) and drier soil. The progressive increase in moisture deficit from free drainage (90%) indicated a proportional loss in fresh weight ranging from 12.7% to 19.4% at semi moist soil level with 46.5-54.4% to cooking cultivars; 40-43.0% to dessert and plantain cultivars at drier soil level hence the sensitivity of expressive traits adaptable to drought. Most root sizes were less than 1cm in diameter with dense mass and length developed higher at semi moist soil level especially for cooking cultivars than at free drainage. The lowest specific root length (SRL) were significantly exhibited by cv Mpologoma (0.130m g'1), cv. Kisansa (0.198m g’1) and cv. Sukali Ndiizi (0.143 m g'1) at moist soil level correlating with larger leaf surface area for efficient utilization of available moisture. tiesuits for responsive biomass were significantly different with plant height across moisture regimes. Plant height ranged from 67 - 86cm and 64 - 93cm high at moist soil level (2008b & 2009a season) and (0 -83cm high for semi moist soil level and 46-64cm for drier soil level during 2009a season indicating the effect of moisture deficit stress on the plant’s growth potential. A decrease in soil moisture deficit from moist to semi moist soil level caused a significant reduction in height for cvs. Sukali Ndiizi and Kayinja than moisture reduction from semi moist to drier soil level. Leaf area expansion was large at moist soil level and diminished with increase in soil water deficit. The fresh tissue weight quantification at harvest showed the highest total weight with cv. Sukali Ndiizi at 2240g, 1181g, and 515.0g lor moist, semi moist and drier soil levels respectively and decreased at 58.0% and 48.0% upon soil moisture reduction from moist to semi moist and to drier soil level. The total fresh tissue weight of cooking cultivars were determined at 1887g, 1396g and 476g (cv. Kisansa) and 1700g, 998g and 466g (cv. Mpologoma) for moist, semi moist and drier soil levels respectively with proportional decrease of 69% and 52% when available soil moisture was reduced from semi moist to drier soil level. Cultivars exhibited regulatory survival mechanisms to drought stress through leaf folding. The cultivar potential for orienting leaves open showed significant differences across moisture regimes with cv. Yangambi Km5 (126°), cv. Kayinja (125°) and cv. Kisansa (124°) spreading leaves widely open at moist soil level. At drier soil level, cv. Sukali Ndiizi (74.2°), Mpologoma {109°) and cv. Kisansa (103.7U) exhibited more open leaves from 180° surface orientation increasing the leaf exposure for high transpiration. The decrease in soil water content from the field capacity led to plant dehydration and deterred leaf lamina expansion due to reduced leaf water potential (LWP). Leaves for all genotypes were turgid (93.2 - 95.4% RWC) at moist soil level enhancing cell expansion for co2 assimilation, high transpiration rate and biomass development. Moisture deficit increment semi moist to drier soil level indicated higher water saturation deficit especially for cultivars Mpologoma (20.5%) and Kisansa (21.4%) exhibiting severe sensitivity to drought stress. Full turgid leaves enhanced cells to remain extended for high transpiration causing the rate of lost water to be far higher than the replaceable in soil hence sensitivity and difficult survival in adverse dry conditions. Transpiration rates were significantly different across water regimes, varieties and time (weeks). The results from stressful treatment structure indicated the actual expression of traits for spending and saving water for plant growth. Generally average rates of transpiration were were quantified at 1.868 L wk’1 (cv. Sukali Ndiizi), 1.749 L wk’1 {Kisansa), 1.730 L wk’1 {Mpologoma), 1.693 L wk'1 {Yangambi Kni5) and 1.702 L wk'1 {Kayinja) at moist soil level. However, at drier soil level, cv Kayinja (0.421 L wk’1) transpired the least mean volume of water, validating its potential to save water. Cultivar Mpologoma (0.659 L wk’1) and Kisansa (0.696 L wk’1) significantly transpired more water and were therefore regarded as water spenders.The stressful condition put cv. Kisansa and Mpologoma to being waler spenders hence their sensitivity and therefore, could only gel acclimatized to the environment than being adaptive whereas cv. Sukalindiizi, Yangambi Km5 and Kayinja exhibited relative trails of waler saving. The potential of cultivars to save water was considered a good trait for plant adaptability to survive in stressful conditions but not productive. The identification of variables predicting transpiration rates were determined with a stepwise regression analysis and correlation coefficient. The regression coefficient variables identified were used to determine the rate of transpiration per unit change in variable development. The developed models for transpiration prediction across moisture deficit regimes were Cultivar Kayinja (ABB): Y= [a (0.517) + P, * bi (0.024)] and Y= [a (-3.68) + p, * b, (0.020) + p2 *b2 (0.338)] during 2008b&2009a seasons respectively for each developed and unfolded leaf. Cultivar Sukali Ndiizi (AAB): Y= [a (0.21 7) + pi * b3 (6.76)] and Y= [a (-2.408) + p,* bi (0.012) + p2* b2 (0.207) + p3* b4 (0.013)] during 2008A&B seasons respectively for increase in leaf area, leaf unfolding, developed healthy leaf and plant height accumulation. Cultivar Mpologoma (AAA): Y= [a (0.418) + Pi* b3 (0.001) and Y= [a (-0.890) + Pi* b| (0.016) + p2* be (0.001) during 2008b&2009a seasons respectively for developed leaf area, leaf unfolding and fresh leaf biomass. Cultivar Kisansa (AAA): Y= [ a (-0.160) + p, *b3 (9.00) + p2*b| (0.010) and Y= [ a (-0.139) + Pi*bi (0.009) + p2*b6 (0.001) during 2008b&2009a seasons respectively for developed leaf area, leaf unfolding and fresh leaf biomass. Cultivar Yangambi Km5 (AAA- Lowland):Y= [a (-0.134) + p,* b3 (1 1.54) and Y= [a (3.97) + Pi* b2 (0.232) + p2* b6 (0.002) + p3*b4 (- 0.04) during 2008b&2009a seasons respectively for developed leaf area, healthy leaf, fresh leaf biomass and gain in height All plants expressed high nitrogen levels across moisture deficit regimes with cv. Kayinja exhibiting the immobilization potential. Based on the results of this study important to carry out a molecular characterization to find which genes trigger the expression of drought avoidance mechanisms among cultivars with B-genome composition. During the study it was found that highly stressed plants were vulnerable to pest attack causing multiple stresses to the plant hence warrant to ascertain the cause.