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Breeding progress for drought tolerance in maize has been slow since drought tolerance is a complex trait controlled by many genes. Breeders improving maize for drought tolerance have therefore been using secondary traits and selection indices for selecting the best genotypes under drought stress. The objectives of this study were to determine the combining ability of the inbred lines in stress and non stress environments as well as compare the use and efficacy of secondary traits and selection indices in selecting for drought tolerant genotypes. Fifty hybrids formed through a North Carolina Design II and four checks were evaluated using the 0.1 alpha lattice planting design under optimum and drought environments. General combining ability (GCA), specific combining ability (SCA) and seven drought indices (SDI), stress susceptibility index (SSI), tolerance (TOL), yield index (YI), yield stability index (YSI), mean productivity index (MPI), stress tolerance index (STI) and geometric productivity index (GMP) were used in the computation of results. Results showed that under drought conditions, GCA was highly significant (P<0.001) for grain yield, anthesis-silking interval (ASI) and ears per plant (EPP) while SCA was significant for grain yield and EPP. Selection indices STI and GMP had positive and significant correlation (P<0.01) with grain yield under both drought stress (Ys) and optimum (Yp) conditions. EPP also had significant (P<0.01) and positive correlation with Ys and Yp. ASI had significant and negative correlation with Ys (P<0.01) and TOL (P <0.05). In addition EPP had a positive and significant correlation with STI (P<0.05) and GMP (P<0.01). The results indicated that ASI, EPP, STI and GMP are effective in identifying high yielding genotypes under different moisture regimes. Narrow sense heritability showed that phenotypic variation attributed to genetic effects increased under stress conditions for EPP and ASI and making them more reliable parameters for use in selecting for genotypes under stress conditions. The two secondary traits (ASI and EPP) together with two drought indices; (STI and GMP) can therefore be used in tandem for increased efficiency in selecting for genotypes under stress environments. Key words: Combining ability drought stress, drought index, heritability.

Secondary traits can improve precision with which drought or low soil nitrogen (low‐N) tolerant maize (Zea mays L.) genotypes are identified, compared to measuring only grain yield under each stress. The objective of this study was to identify secondary traits for indirect selection for yield improvement under drought and/or low‐N. Two sets of experiments were conducted at four locations in Nigeria for 2 yr. In the first experiment, cultivars were evaluated under low (30 kg ha−1) and high (90 kg ha−1) N and drought and well‐watered environments in the second experiment. Most reliable traits for selection for yield under drought were ear aspect (EASP), plant aspect (PASP), anthesis‐silking interval (ASI), and number of ears per plant (EPP). Days to anthesis (DYA) and silking (DYS) were the most reliable under well‐watered environments. Under low‐N, most reliable traits were DYA, DYS, stay green characteristic (LDTH), ASI, plant height (PLHT), EPP, EASP, and PASP whereas PLHT, EPP, PASP, and EASP were most reliable traits under high‐N environments. The ASI, EPP, EASP, and PASP were identified as most reliable traits for simultaneous selection of drought and low‐N tolerant genotypes. Under drought, cultivars TZE‐Y DT STR C4, TZE‐W DT STR C4, and TZE Comp 3 DT C1F2 were outstanding and closest to the ideal cultivar. Seven cultivars were closest to the ideal under low‐N, two of which were among the three selected under drought stress. Faster progress in selecting for improved grain yield is possible using EPP, EASP, PASP, and ASI under either drought or low‐N.

Two greenhouse studies were conducted to investigate the relationship between water stress and N2 fixation among drought-resistant and susceptible cowpea [Vigna unguiculata (L.) Walp.] genotypes. In both experiments, seeds were planted in 7.6-liter black polyethylene pots containing composted sawdust medium and were inoculated with Rhizobium. Throughout the experiments, flowers were removed to maintain vegetative growth. Water stress treatments were imposed by withholding water, while the control plants were watered as needed. The treatments were applied 58 and 56 days after planting (DAP) in the first and 2nd experiments, respectively. In both experiments, leaf water potential (LWP), shoot fresh weight (SFW), shoot dry weight (SDW), root fresh weight (RFW), nodule fresh weight (NW), nodule number (NN), and plant specific activity (PSA) by both in situ and destructive acetylene reduction methods were measured. Repeated observations of in situ acetylene reduction were made 58, 63, and 71 DAP in the first experiment. All other variables were measured 77 to 78 DAP in the first experiment. Single observations of all variables, including in situ and destructive acetylene reduction were made 56, 67, and 81 DAP in the 2nd experiment. Results suggested that resistant genotypes are capable of maintaining LWP and biomass production (as measured by SDW and SFW) during water stress. In addition, the effect of water stress on N2 fixation was far greater than the influence of genotype when genotypes were selected for relative drought resistance. Path analysis revealed that LWP is correlated to N2 fixation in water-stressed plants, and improvement of plant water status via drought resistance should increase N2 fixation potential under drought conditions. Therefore, breeding for drought resistance in conjunction with N2 fixation may be more beneficial than breeding strictly for N2 fixation potential without regard for environmental adaptation. The in situ method of acetylene reduction was found to be useful for detecting physiological changes due to water stress and estimating its genotypic N2 fixation potential.

A green house study was conducted between July and August, 2009 to test and determine the drought tolerance levels of ten maize (Zea mays L.) genotypes. The top soil used was retrieved from Bediease series (Chromic lixisol). Ten genotypes, (six inbred lines and four varieties) having different genetic backgrounds were used. Two sets of the genotypes were established, of which one set received water up to the end of the experiment, 55 days after planting (DAP) and the remaining set was water stressed for 34 days starting from 21 days after planting. Through pairwise comparison of means for plant height, leaf relative water content, dry matter yield, root dry mass and leaf rolling under water stressed and non stressed conditions, Entry 24 showed no significant difference between the two water regimes in all the five indicators used, followed by Entry 5 which recorded four out of the five indicators of drought. Mamaba recorded two out of five indicators of drought tolerance among the four varieties. The factors chosen for the ranking procedure proved to be effective indicators for the selection of drought tolerant maize. Key words: Zea mays, genotype, drought tolerance, water stress, non water stress.

Five International Institute of Tropical Agriculture (IITA) drought tolerant and open pollinated varieties (OPVs) (lines) and four local varieties with various level of susceptibility to drought (testers) were used. These maize varieties were crossed using a line x tester mating design during the 2007 cropping season to determine the general combining ability (GCA) and specific combining ability (SCA); also the level of heterosis was investigated. Parental lines and hybrids were evaluated in Biu in 2008 and 2009 cropping seasons. Results from the analysis of variance and combining ability shows that there was high and significant level (P&lt; 0.05) of genetic variability among the parental lines used and their hybrids in days to tasseling, days to silking, anthesis silking interval (ASI), plant height, ear height, weight of cobs, dehusked cobs and grain yield, thus suggesting the possibility for genetic improvement. The study established that, there were significant (P&lt;0.05) differences of GCA effects of parents and that of SCA effects of hybrids. Estimates of GCA were consistently lower than SCA effects in almost all the traits evaluated. This suggests that high performing hybrids such as EVDT-99WSTRQPMC0 x EX-DAMBOA WHITE, EVDT-99WSTRC0 x EX-DAMBOA YELLOW andTZE-COMP3DTC1 x EX-DAMBOA WHITE may be used to develop potential varieties. The parents: EVDT-99WSTRC0, EVDT-99WSTRQPMC0, TZE-COMP3DTC1 and EX-BIU WHITE were identified as the best general combiners in terms of GCA for days to tasseling, days to silking, ASI, plant height, ear height, dehusked cobs and grain yield. &nbsp; Key words: Maize, combining ability, additive, non-additive genetic effects, drought tolerance.

Gray leaf spot (GLS) caused by Cercospora zeae maydis is one of the major biotic constraints limiting maize (Zea mays L.) production and productivity in Kenya. The objective of this study was to evaluate inbred lines and select lines resistance to GLS from a diverse collection of genotypes and develop GLS resistant maize varieties. Forty-two diverse genotypes were evaluated for GLS resistance at two disease hotspot sites (KALRO-Kakamega and Egerton Njoro, Field 7) for two seasons in Kenya. Data on mean disease severity on leafs, area under disease progress curve (AUDPC), stalk lodging (SL), root lodging (RL), ear height (EH), ear aspect (EA), ears per plant (EPP), plant aspect (PA), and grain yield (GY) were collected. Data was subjected to analysis of variance. The genotype &times; environment interaction (G x E) had significant (P &lt; 0.001) effect on all agronomic traits except RL and EPP. This is evidence of the presence of significant variation among screened inbred lines in GLS resistance across the screened sites. Maize inbred lines including CKDHL 120312, KTLN 10123, CKL 05022 and CKDHL 153502 exhibited lower values of GLS severity and AUDPC which are useful genetic resources for future GLS resistance breeding programs. Inbred lines CKDHL 153502 provided a mean grain yield of 4.3 ton/ha followed by CKDHL 120312, also (4.3 ton/ha) and CKL 05022 (4.2 ton/ha). Strong and negative correlation was found between AUDPC and GY (r = -0.65) revealing the effect of GLS. Therefore, GLS resistant and high yielding lines are ideal candidates for selection and advancement in many breeding programs. Key words: Inbred lines, disease severity, gray leaf spot (GLS), resistance, area under disease progress curve (AUDPC).

The association of different plant characters, under and without water stress conditions, was studied in one local and five exotic open-pollinated maize varieties, in field experiments during the summers of 1988/89 and 1989/90. A split plot design with three replications for each site was used. Genotypes were evaluated for yield and eleven characters. Correlation coefficients under water stress indicated that number of leaves, ear height, black layer maturity, senescence of ear leaf, prolificacy, interval between pollen shed and silking, plant height and grains per row significantly influenced grain yield. Under non-stress conditions, ear length, senescence of ear leaf, grains per row and prolificacy significantly influenced grain yield. Through path coefficient analysis, it was concluded that the improvement of drought tolerance can be achieved by using characters like the interval between pollen shed and silking, prolificacy, number of grains per row and senescence of ear leaf as selection criteria.

Drought tolerance is a complex characteristic and screening based on multiple traits rather than direct selection indices would result in more efficient selection. The study aims to select hybrids under optimal and water deficit conditions to achieve genetic gains in yield traits without compromising the other secondary traits viz., anthesis-silking interval, leaf chlorophyll content, delayed leaf rolling and leaf senescence, relative water content, and ROS scavenging using the multi-trait genotype ideotype distance index. The 30 teosinte-maize hybrids along with four checks were evaluated for two classes of traits: 16 morphometric and 14 physiological and biochemical traits, under well-watered (WW) and water-stress (WS) environments. Significant variations among the genotypes were observed for most traits and the presence of moderate to high heritability for most traits suggests direct selection for improvement of those traits. Significant correlation of the traits with yield and intercorrelations between traits indicate the advantage of indirect selection based on secondary traits. By assuming a selection intensity of 15% and equal weightage to all the traits, five genotypes were selected based on the MGIDI under each class of traits and in each environment. The genotypes viz. G19, G20, and G29 were commonly selected across both environments. The selection led to desired positive and negative selection gains for most of the traits studied and resulted in high positive gains for single plant yield of 35.6% under WW and 69.3% under WS. The strength and weakness plots effectively present the advantages and limitations of the selected genotypes under each environment. The multi-trait-based selection approach is an effective tool for selecting genotypes and designing breeding strategies for stress breeding.

Identification of diverse inbred lines and selection of heat tolerant genotypes based on key secondary traits that influence the grain yield is of great importance. In the present study, correlation coefficient and path analysis were used to identify potential traits for heat tolerance. The results on correlation studies revealed that grain yield per plant had positive relationship with number of kernels per cob (0.833), shelling percentage (0.592), chlorophyll content (0.575), plant height, ear height, 100-seed weight, pollen shed duration and cob length. The traits like days to 50% tasseling (−0.383), days to 50% silking (−0.382), anthesis silk interval (−0.381), senescence and cell injury (%) were negatively associated with grain yield. Further, path analysis showed that the number of kernels per cob had highest positive direct (0.6689) and indirect effect via chlorophyll content and days to 50% tasseling on grain yield followed by shelling percentage through number of kernels per cob, days to 50% tasseling and chlorophyll content. Cluster analysis based on phenotypic data showed presence of three main groups. The first group (GI) consisted of four inbred lines (5.1%) which were superior to mean of 75 inbred lines in respect of all the traits. The selection of genotypes based on number of kernels per cob, anthesis silk interval, shelling percentage, chlorophyll content, plant height and ear height will indirectly increase the maize yield under heat stress.

Strong associations between agronomic and physiologic traits and drought tolerance, high heritability and high genetic advance for such traits would allow plant breeder to use such traits as selection criteria for selecting drought tolerant genotypes. The objectives of the present investigation were: (i) to explain the relationships between the drought tolerance index (DTI) and 14 agronomic and physiologic traits of 22 maize genotypes and (ii) to estimate the broad-sense heritability (h2b) and genetic advance (GA) from selection for such traits, in order to determine the selection criteria for DTI. A two-year experiment was carried out using a split plot experiment with three replications. The main plots were devoted to irrigation regimes, i.e. well watering (WW), water stress at flowering (WSF) and at grain filling (WSG), and sub plots to maize genotypes. It is evident from results that the best selection criteria for drought tolerance in our study were: 100-kernel weight (100KW) and chlorophyll concentration index (CCI) under WSF and WSG, anthesis-silking interval (ASI), upper stem diameter (SDU) and lower stem diameter (SDL) under WSF and kernels/row (KPR) and ear leaf area (ELA) under WSG, since they show high correlation (r) values with grain yield/plant (GYPP), high h2b and high GA estimates under the respective environments. Under well watering conditions, KPR, 100KW, CCI, and SDL traits showed high (r) values, high h2b and high GA estimates and therefore could be considered selection criteria for GYPP under non-stressed environment. The results concluded that predicted selection gain would be higher if selection was practiced under WW for lower values of days to silking and higher values of ears/plant, rows/ear, KPR, 100KW, SDU and SDL, under WSF for lower values of ASI and higher values of GYPP and under WSG for lower values of plant height, ear height, and barren stalks, and higher values of CCI and ELA.

Plant water regulation plays a critical role in land–atmosphere coupling and ecosystem responses to climate extremes. Isohydricity is widely used to characterize how plants regulate water loss under water stress, yet its behavior under interacting drought and salinity remains poorly understood. Here, we investigated maize (Zea mays L.) water-use strategies under combined water and salinity constraints using a controlled pot experiment. Maize plants were exposed to two water availability regimes (well-watered and drought conditions) and irrigated with either fresh or saline water. Isohydric behavior was assessed using three complementary hydraulic relationships: (i) transpiration rate (normalized by leaf area) versus soil water potential, (ii) leaf water potential versus soil water potential, and (iii) stomatal conductance versus leaf water potential. In addition, the vulnerability of soil–plant hydraulic conductance was also examined.Under drought or salinity applied separately, maize tended to exhibit more anisohydric behavior, characterized by relatively weak reductions in transpiration and stomatal conductance with declining water potential and a broader range of leaf water potential variation. In contrast, when drought and salinity occurred simultaneously, maize shifted toward a more isohydric mode of regulation, clearly differing from responses under single stress conditions. Moreover, under drought conditions, isohydricity inferred from the leaf–soil water potential relationship tended toward a more isohydric behavior under saline treatment, whereas isohydricity inferred from transpiration- and stomatal conductance–based relationships under salinity indicated a more anisohydric behavior. This discrepancy highlights the influence of evaluation methods on isohydricity characterization. Furthermore, we conclude that maize isohydricity is closely linked to the vulnerability of soil–plant hydraulic conductance. Under drought or salinity conditions, maize tends to exhibit more anisohydric behavior, which is associated with enhanced resistance of the soil–plant hydraulic system to the loss of hydraulic conductance. These findings advance our understanding of crop water relations under combined water and salinity stress and support integrated irrigation and salinity management strategies to improve water use efficiency and sustain yields in salt-affected regions.

Water availability is the primary constraint to sorghum production worldwide. The crop is grown in water-limited environments of varying extent and timing with water stress during grain filling being common. Varying extent and timing of water availability generates substantial genotype × environment interaction (GEI) for sorghum production, which restricts genetic improvement of grain yield. The objective of this thesis is to better understand sorghum performance under drought conditions and identify potential strategies to improve the rate of genetic gain for grain yield. To better understand the impacts and to deconstruct GEI by removing the confounding effects of flowering time (DTF) and tillering (FTN) on grain yield, linear mixed models were used to evaluate the impacts of DTF and FTN on grain yield of 1741 unique test hybrids. Hybrids were derived from crosses between 1078 elite male parents and 3 female testers and grown in 21 yield testing trials at 15 locations across the major sorghum production regions in Australia in three seasons. DTF made a significant contribution to genetic variation in grain yield of male lines in 14 of the 48 tester/trial combinations, explaining 0.2% to 61.0% variation, and FTN in 12 tester/trial combinations, explaining 1.4% to 56.9% variation. The relationship of DTF and FTN with grain yield of hybrids in the whole dataset was frequently positive, but varied depending on the genetic background of testers and growing environments. However, between trial genetic correlations for grain yield were not significantly improved after accounting for the effects of DTF and FTN using linear models. These results suggests that other genetic factors affecting canopy development dynamics and grain yield might contribute more of the genetic variation in grain yield. It is possible that linear mixed models did not capture the non-linear effects of flowering time and fertile tiller number on yield. One of the consequences of water stress during grain filling is lodging, with major consequences on grain yield and quality. Hence, resistance to lodging has been a major target of selection in sorghum breeding programs in Australia. A retrospective analysis was conducted on 37 commercial hybrids grown in 83 yield testing trials in major sorghum production regions in Australia in 14 seasons to quantify the geographical and seasonal variations in lodging occurrence and severity. Lodging occurred more frequently in Central Highlands and less frequently in Liverpool Plains, in comparison to the overall average across regions. The severity of lodging also varied across regions, with the most severe lodging (>20%) occurring in the Central Highlands and Western Downs. Lockyer Valley was the only region free from lodging. In addition, seasonal patterns of lodging frequency and severity were also observed. Over the 14 growing seasons, the frequency of lodging varied from 0 to 100%, with the most severe lodging (>20%) observed in 2005, 2016, and 2017. The Southern Oscillation Index explained 29% of the seasonal variation in lodging frequency. Results also showed that the levels of lodging resistance in commercial hybrids were not improved. This is possibly because sorghum breeders trade of improvements in lodging resistance to increase grain yield due to the complex nature of lodging. Currently, sorghum breeders use strategies of direct selection against lodging susceptibility and direct selection for stay-green (delayed leaf senescence) rather than selection for lodging resistance to improved lodging resistance. A better understanding of the genetic architecture of lodging and its association with other traits would enable faster and greater genetic gain in lodging resistance and grain yield. Large scale genome-wide association studies on lodging were conducted using data from 17 Australian sorghum trials over three seasons involving 2308 unique test hybrids. The identification of 213 QTL for lodging clearly demonstrate the complexity of lodging. The majority (92%) of the lodging QTL showed a significant association with leaf senescence and plant height, suggesting lodging in grain sorghum is primarily driven by carbohydrate remobilisation and plant height. Additionally, the thesis found that one third of the previously reported QTL for stalk rots co-located with lodging QTL identified in this study, 11 of which were associated with leaf senescence and/or plant height. These results indicate that genes providing classical resistance to stalk rots are likely to be of limited effectiveness in breeding for enhanced lodging resistance. However, the observation of significant enrichment of genes in lignin biosynthesis pathways within lodging QTL identified in this thesis indicates that lodging resistance is likely to be improved by selecting for stem strength and composition traits such as lignin content.

Growth analysis provides better insight into the adaptability of cassava genotypes grown under drought conditions during canopy establishment and full irrigation. This study is intended to determine the growth rate and starch yield of different cassava genotypes grown under irrigation and drought treatments during canopy establishment. The experiment was conducted in two growing seasons at Khon Kaen University, Thailand, from August 2021 to August 2022 (2021/2022) and from August 2022 to August 2023 (2022/2023) using six cassava genotypes. A 2 × 6 split-plot design with four replications was used. The main plots were full irrigation and drought conditions during canopy establishment (90 to 150 days after planting (DAP)), and the cassava genotypes were assigned as subplots. The results showed that drought treatment applied during 90 to 150 DAP reduced relative water content (RWC) in cassava leaves for both growing seasons, storage root growth rate (SRGR) in the 2021/2022 growing season, and stem growth rate (SGR) and crop growth rate (CGR) in the 2022/2023 growing season. Re-watering after a drought supported cassava’s growth rate, resulting in desirable yield and biomass at final harvest. The Rayong 72 and CMR38-125-77 produced significantly higher storage root dry weight, harvest index (HI), and starch yield than the other tested genotypes. Growing under drought treatment, the best performance in storage root dry weight with statistical significance for both years was recorded for CMR38-125-77 (11.2 and 11.4 t ha−1 for the 2021/2022 and 2022/2023 growing seasons, respectively), and this was associated with a high crop growth rate (CGR, 12.3 g m−2 day−1 for the 2021/2022 growing season) and relative growth rate (RGR, 1.11 × 10−2 g g−1 day−1 for the 2022/2023 growing season) during 180 to 360 DAP. These favorable cassava genotypes should be utilized for future plant breeding programs and cultivation to achieve the desired productivity in the growing areas with drought during canopy establishment.

Identification of specific physiological differences among maize genotypes (six inbred lines, four single crosses and four three-way crosses) to drought would be useful to plant breeders for improving selection criteria. Two field experiments were conducted in optimum and stress moisture conditions at Gemmeiza Agricultural Re-search Station in 2001 and 2003 growing seasons. The treatments were 14 geno-types, (six inbreds and eight hybrids), which were separately established under three irrigation treatments in split plot design with four replications. The genotypes were compared under water-deficit conditions on the basis of certain rapid techniques to determine leaf area index (LAI), photosynthetic efficiency (photo), canopy tempera-ture depression (CTD), stomatal conductance (SC) and chlorophyll (CHL) content which were deleteriously affected by water stress in the sensitive genotypes. Results revealed that values of LAI, photosynthetic efficiency, canopy temperature depres-sion, stomatal conductance and chlorophyll content of maize hybrids surpassed those of inbred lines. This was true at 30, 45 and 60 days after planting (DAP). Dif-ferences among maize hybrids and among inbred lines were significant at different growth stages for the same studied traits, except of the LAI for the inbreds, at 30 DAP and CTD for the hybrids at 30 as well as 60 DAP. Detecting certain physio-logical criteria, i.e. LAI, photosynthetic efficiency (photo), canopy temperature de-pression (CTD), stomatal conductance (SC) and chlorophyll content (Chl) could be used as reliable techniques to explore the physiological activity of maize genotypes at early stages of growth and may be recommended in screening different genotypes in maize breeding programs.

Seventeen photoperiodic sensitive cotton (Gossypium hirsutum L.) stocks were grown for 2 years (1980 and 1981) at Big Spring, Tex. to evaluate the relationship between turgor maintenance and shoot dry matter accumulation (growth). During each year, leaf‐cutter thermocouple psychrometers were used to measure predawn and midday leaf water and osmotic potentials and to estimate turgor pressure at 50 and 80 days after planting (DAP). Shoot dry weights were measured also at 50 and 80 DAP. The 1st year was hot and dry while the 2nd year had about average temperatures and rainfall. Linear regressions between predawn and midday water and turgor potentials were used to estimate turgor maintenance as i) the change in turgor pressure per unit change of water potential (slope) and, ii) the osmotic potential where zero turgor pressure occurred (X intercept). Significant differences did not occur among the stocks for changes in turgor pressure per unit change in water potential (slope), but they did occur among stocks for osmotic potential at zero turgor (πo) all four sampling dates. Analysis over‐all of the sampling dates, identified a consistently low (T178) and high (T292) stock for πo. Linear correlation and regression coefficients between shoot dry weights and πo were negative and statistically significant during the sampling period (80 DAP, 1980) when the plants were water‐stressed. The linear correlation and regression coefficients were negative, but not statistically significant during the other sampling periods (50 DAP, 1980, and 50 and 80 DAP, 1981). Linear correlation coefficients between the change in shoot dry weights between 50 and 80 DAP in 1980 and the change in πo were negative (r = ‐0.82) and statistically significant. These relationships suggesthat if selection pressure is directed towards enhancing osmotic adjustment under water stress, a reduced growth potential may result.