Stay-green Sorghum Research Articles

Evaluation of Diverse Sorghum for Leaf Dhurrin Content and Post-Anthesis (Stay-Green) Drought Tolerance

Post-flowering drought tolerance (stay-green) in grain sorghum (Sorghum bicolor (L.) Moench) is an important agronomic trait in many arid and semiarid environments throughout the world. Stay-green has been associated with increased grain yields, as well as resistance to lodging and charcoal rot disease. Nonetheless, the relative effects of genotype, environment, and genotype × environment interactions are not well understood for this trait; similarly, the relationship between various leaf sugars and stay-green has not been sufficiently evaluated in diverse germplasm. Thus, the goals of this study were to determine the genotype, environment, and genotype by environment (GxE) effects for leaf dhurrin, sugars, and stay-green in ten diverse grain sorghum breeding lines, to evaluate the Pearson’s correlation coefficients (r) between these traits, and to determine entry-mean repeatability (R) for each of these traits. Of the compositional traits studied, we determined that leaf dhurrin had the highest correlation with the stay-green phenotypes (r = −0.62). We found that stay-green sorghum lines contained approximately 2–3 times as much dhurrin as non-stay-green lines, with B1778 containing the highest concentration of dhurrin (84.8 µg/cm2) and Tx7000 containing the least (20.9 µg/cm2). The differences between the environments for several of the traits were high, and all the traits examined had high repeatability (R = 0.89–0.92). These data demonstrate a relationship between leaf dhurrin and the stay-green phenotypes in sorghum, and further study will allow researchers to determine the causal effect that dhurrin has on post-flowering drought tolerance in sorghum.

Root system architecture analysis of sorghum genotypes and its effect on drought adaptation

Sorghum is a crucial crop in semi-arid regions because of its capacity to resume photosynthesis and physiological growth after being subjected to drought stress. Although it is one of the most resilient crops to drought stress, recurrent drought is affecting its productivity. It is thus of paramount importance to explore genes contributing to drought stress adaptation, thereby increasing the productivity of sorghum. A study was initiated to evaluate and determine the effect of root systems, particularly root angle traits, on drought stress adaptation and grain yield performance. A total of 428 sorghum genotypes from the Ethiopian breeding program were evaluated for their performance in three drought-stress environments. The experimental materials include stay-green, non-stay-green genotypes and released sorghum varieties. A row-column design with three replications was used for the field trials. For root system screenings a high throughput phenotyping platform were used and a row-column design with two replications applied for root trait analysis. The mean grain yield for non-stay green genotypes ranged from 1.63 to 3.1 tons/ha. However, for stay-green genotypes, it ranged from 2.4 to 2.9 tons per hectare. The analysis of the root system architecture showed highly significant variations among the genotypes. The root angle of non-stay-green genotypes ranged from 8.0 to 30.5°, while for stay-green sorghum genotypes it varied from 12.0 to 29.0°. At the same time, for improved varieties, it exhibited between 14.04 and 19.50°. The result of the principal component for stay-green genotypes was computed, and the largest variations were 52.7% and the least were 10.4%. The most contributing traits in dimension one were shoot dry weight and shoot fresh weight, followed by leaf width and shoot length. Positive and significant correlations were observed between leaf areas and shoot dry weight and leaf width and shoot dry weight at phenotypic and genotypic levels. Negative correlations were observed between root angle and leaf area. Root angle and root length traits had a negative phenotypic correlation (r = −0.018). In conclusion, in drought-stressed conditions, narrow root angle genotypes produced the highest grain production. Therefore, narrow root angle genotypes should be taken into account in sorghum breeding to boost sorghum gain yield in drought-stressed areas. Secondly, the association of the narrow root angle trait with grain yield revealed a connection between the two traits to maximize the productivity of sorghum, both for stay-green and non-stay-green sorghum genotypes. However, the productivity of narrow root angle genotypes was higher for stay-green gene introgressed sorghum genotypes. Finally, the negative correlation obtained between the root angle and grain yield traits for stay-green genotypes has justified the possibility of using the stay-green trait to select sorghum genotypes with narrow root angles.

Genetic modification of PIN genes induces causal mechanisms of stay-green drought adaptation phenotype.

The stay-green trait is recognized as a key drought adaptation mechanism in cereals worldwide. Stay-green sorghum plants exhibit delayed senescence of leaves and stems, leading to prolonged growth, a reduced risk of lodging, and higher grain yield under end-of-season drought stress. More than 45 quantitative trait loci (QTL) associated with stay-green have been identified, including two major QTL (Stg1 and Stg2). However, the contributing genes that regulate functional stay-green are not known. Here we show that the PIN FORMED family of auxin efflux carrier genes induce some of the causal mechanisms driving the stay-green phenotype in sorghum, with SbPIN4 and SbPIN2 located in Stg1 and Stg2, respectively. We found that nine of 11 sorghum PIN genes aligned with known stay-green QTL. In transgenic studies, we demonstrated that PIN genes located within the Stg1 (SbPIN4), Stg2 (SbPIN2), and Stg3b (SbPIN1) QTL regions acted pleiotropically to modulate canopy development, root architecture, and panicle growth in sorghum, with SbPIN1, SbPIN2, and SbPIN4 differentially expressed in various organs relative to the non-stay-green control. The emergent consequence of such modifications in canopy and root architecture is a stay-green phenotype. Crop simulation modelling shows that the SbPIN2 phenotype can increase grain yield under drought.

Effect of nitrogen supply on stay-green sorghum in differing post-flowering water regimes.

The expression of stay-green (SG) characteristic in sorghum under water stress was related to N supply. SG genotype performed better than a non-stay-green (NSG) genotype at medium and high N levels. The differences in physiological parameters between SG and NSG genotypes were not significant at low N level and severe water stress. Grain sorghum [Sorghum bicolor (L.) Moench] with stay-green (SG) trait has the potential to produce more biomass and use soil water and nitrogen (N) more efficiently under post-flowering water stress. Previous studies were mostly conducted without N deficiency and more information is needed for interactions among soil N availability, SG genotype, and post-flowering water stress. In this study, the differences in leaf growth and senescence, shoot and root biomass, evapotranspiration (ET), water use efficiency (WUE), leaf photosynthetic responses, and nitrogen use efficiency (NUE) between a SG genotype (BTx642) and a non-stay-green (NSG) genotype (Tx7000) were examined. The two genotypes were grown at three N levels (Low, LN; Medium, MN; High, HN) and under three post-flowering water regimes (No water deficit, ND; Moderate water deficit, MD; Severe water deficit, SD). The genotypic difference was generally significant while it frequently interacted with N levels and water regimes. At medium and high N levels, SG genotype consistently had greater green leaf area, slower senescence rate, more shoot biomass and root biomass, and greater WUE and NUE than the NSG genotype under post-flowering drought. However, differences in several variables (e.g., leaf senescence, ET, WUE and NUE) between genotypes were not significant under SD at LN. At HN and MN, photosynthetic function of SG genotype was better maintained under drought. At LN, SG genotype maintained greater green leaf area but had lower photosynthetic activity than the NSG genotype. Nonetheless, adequate N supply is important for SG genotype under drought and greater root biomass may contribute to greater NUE in SG genotype.

Physiological and biochemical basis for stay-green trait in sorghum

Drought is a major cause of sorghum [Sorghum bicolor (L.) Moench] yield losses in rain-fed agriculture, especially in the semi-arid and arid agro-ecological zones of Africa and Asia. Stay-green sorghum genotypes are able to maintain grain filling under drought conditions. The trait has been employed in the selection and breeding for post-flowering drought resistance, even though the genes regulating the trait are still being identified. The objective of this study was to assess how leaf area and chlorophyll are maintained in various sources of stay-green; and to determine whether the integrity of the photosynthetic apparatus and enzymes involved in the maintenance of photosynthesis during post-flowering drought stress are regulated differently. A glasshouse experiment was conducted using three stay-green sorghum lines (B35, KS19 and E36-1) and a senescent control, R16, under well-watered (WW) and water-limited (WL) conditions. The size of the canopy at anthesis varied significantly between genotypes, and this profoundly impacted leaf senescence patterns. For example, green leaf area (GLA) at anthesis was highly correlated with the decline in GLA during the first 21 days of grain filling, under both WW (r = 0.92) and WL (r = 0.86) conditions. These differences in senescence patterns were further exacerbated by the small pot size in this study (10 L). E36-1 is normally designated as a stay-green genotype, but the growth of this ‘high leaf area’, genotype in a small pot resulted in a senescent phenotype. Green leaf area retention was higher in B35 and KS19, and the loss of GLA started 14 days earlier in the WL E36-1 and R16 plants, compared to B35, with little change in KS19. Chlorophyll levels were higher in B35 and KS19 compared with R16 and E36-1 under WL conditions. FPSII, CO2 assimilation rate, leaf conductance, transpiration rate and leaf water use efficiency were higher in the stay-green genotypes under WL conditions compared to R16. Enzymes involved in leaf nitrogen metabolism and chlorophyll biosynthesis, and photosynthesis were retained at higher levels in the stay-green lines than in R16. Therefore, the stay-green mechanism resulted in reduced destruction of the photosynthetic apparatus, better nitrogen metabolism and chlorophyll turnover, and maintenance of active enzymes involved in photosynthesis.Key words: Green leaf area, photosynthesis, Sorghum bicolor

Case Study: Yield and quality of traditional senescent and stay-green sorghum and in situ ruminal disappearance of respective crop residues

ABSTRACT Nutritional composition of sorghum before grain harvest and ruminal disappearance of crop residues were evaluated in traditional senescent and stay-green sorghum hybrids grown under restricted water conditions. Hybrids were seeded in a randomized complete block design (experimental unit = 2.63-ha plots; n = 12, 6 plots per treatment) in limited water conditions (330 mm/season). Plants were sampled 129 d after seeding and botanically fractioned for yield and nutrient composition (Exp. 1) and in vitro true digestibility (Exp. 2). Crop residues were baled and in situ ruminal disappearance was evaluated using a crossover design (Exp. 3). Ruminally cannulated steers (n = 6; BW = 722 ± 65 kg) were randomly assigned to treatments: hybrid (traditional senescent vs. stay-green) and supplement (0 or 0.68 kg/animal daily; cottonseed meal). Experimental periods (n = 4) included a 10-d adaptation phase before incubations of 0, 3, 6, 12, 24, 3, 48, and 72 h. Data were analyzed using GLIMMIX procedures of SAS. Greater (P ≤ 0.05) whole-plant, grain, and stalk DM yields were observed with stay-green hybrid. Stay-green stalks contained less ash (P = 0.04) and greater fiber (P ≤ 0.03) than the traditional senescent cultivar. Projected whole-plant, grain, and stalk digestible OM yield was greater (P ≤ 0.05) with stay-green than the traditional senescent cultivar. Ruminal residue OM disappearance of both hybrids was increased (P

Bigger is not always better: Reducing leaf area helps stay-green sorghum use soil water more slowly

Stay-green, a trait that confers delayed leaf senescence and improved grain yield under post-anthesis drought, has been associated with smaller canopies at flowering and increased water uptake during the post-flowering period. It has been shown that the main stay-green quantitative trait loci reduce leaf area via reduced tiller number and smaller leaves. To show that these canopy characteristics are directly linked to water savings, we grew near-isogenic lines with and without stay-green introgressions in large lysimeter pots and measured their weekly pre-anthesis water use and main-stem and tiller leaf area. Paradoxically, age-related senescence of lower leaves in stay-green lines was accelerated before flowering, contributing to their smaller leaf area at flowering. This process of reducing leaf area by shedding old leaves lower in the canopy, has not previously been described for the stay-green introgressions. We found that tiller leaf area rather than transpiration efficiency, or transpiration per leaf area, was the main driver of weekly transpiration and the reduced pre-flowering water use in stay-green lines. In soils with good water-holding capacity, any water savings during the pre-anthesis period increases water availability during the post-anthesis period, therefore allowing plants to retain photosynthetic capacity for longer by “staying green” during grain filling.

Transcriptomic analysis comparing stay-green and senescent Sorghum bicolor lines identifies a role for proline biosynthesis in the stay-green trait.

Sorghum bicolor is an important cereal crop grown on the arid and semi-arid regions of >98 different countries. These regions are such that this crop is often subjected to low water conditions, which can compromise yields. Stay-green sorghum plants are able to retain green leaf area for longer under drought conditions and as such have higher yields than their senescent counterparts. However, the molecular and physiological basis of this drought tolerance is yet to be fully understood. Here, a transcriptomic approach was used to compare gene expression between stay-green (B35) and senescent (R16) sorghum varieties. Ontological analysis of the differentially expressed transcripts identified an enrichment of genes involved with the 'response to osmotic stress' Gene Ontology (GO) category. In particular, delta1-pyrroline-5-carboxylate synthase 2 (P5CS2) was highly expressed in the stay-green line compared with the senescent line, and this high expression was correlated with higher proline levels. Comparisons of the differentially expressed genes with those that lie in known stay-green qualitative trait loci (QTLs) revealed that P5CS2 lies within the Stg1 QTL. Polymorphisms in known cis-elements were identified in the putative promoter region of P5CS2 and these could be responsible for the differences in the expression of this gene. This study provides greater insight into the stay-green trait in sorghum. This will be greatly beneficial not only to improve our understanding of drought tolerance mechanisms in sorghum, but also to facilitate the improvement of future sorghum cultivars by marker-assisted selection (MAS).

Drought adaptation of stay-green sorghum is associated with canopy development, leaf anatomy, root growth, and water uptake.

Stay-green sorghum plants exhibit greener leaves and stems during the grain-filling period under water-limited conditions compared with their senescent counterparts, resulting in increased grain yield, grain mass, and lodging resistance. Stay-green has been mapped to a number of key chromosomal regions, including Stg1, Stg2, Stg3, and Stg4, but the functions of these individual quantitative trait loci (QTLs) remain unclear. The objective of this study was to show how positive effects of Stg QTLs on grain yield under drought can be explained as emergent consequences of their effects on temporal and spatial water-use patterns that result from changes in leaf-area dynamics. A set of four Stg near-isogenic lines (NILs) and their recurrent parent were grown in a range of field and semicontrolled experiments in southeast Queensland, Australia. These studies showed that the four Stg QTLs regulate canopy size by: (1) reducing tillering via increased size of lower leaves, (2) constraining the size of the upper leaves; and (3) in some cases, decreasing the number of leaves per culm. In addition, they variously affect leaf anatomy and root growth. The multiple pathways by which Stg QTLs modulate canopy development can result in considerable developmental plasticity. The reduction in canopy size associated with Stg QTLs reduced pre-flowering water demand, thereby increasing water availability during grain filling and, ultimately, grain yield. The generic physiological mechanisms underlying the stay-green trait suggest that similar Stg QTLs could enhance post-anthesis drought adaptation in other major cereals such as maize, wheat, and rice.

Effect of PEG induced osmotic stress on osmolytes and antioxidative enzymes in sorghum seedlings

The present investigations were undertaken to evaluate the levels of proline and glycine betaine osmolytes and activities of antioxidative enzymes under PEG-6000-induced osmotic stress in drought susceptible, tolerant and stay-green sorghum genotypes. A total of forty-eight drought susceptible, tolerant and stay-green rabi sorghum genotypes were screened to evaluate proline and glycine betaine accumulation under osmotic stress of PEG-6000. Among the tolerant genotypes, RSLG-262 and RSV-1366 accumulated the highest proline content, whereas, RSV-458 and Hadgaon local accumulated the highest glycine betaine content than susceptible genotypes. The high proline accumulating tolerant genotypes, RSLG-262 and RSV-1366 also exhibited correspondingly the highest ∆1_pyrroline-5-carboxylate synthetase activity. The mean activities of antioxidative enzymes such as superoxide dismutase, catalase, peroxidases and relative leaf water content were higher in the leaves of stay-green genotypes, followed by tolerant and susceptible types. Within stay-green genotypes, B-35 and E-36-1 and a drought tolerant genotype RSLG-262 recorded higher antioxidative enzyme activities as compared to other genotypes. The stay-green genotypes, B-35 and E-36-1 and a tolerant genotype RSLG-262 appeared to be the most promising ones.

Effect of PEG Induced Osmotic Stress on Peroxidase and Superoxide Dismutase Isozymes in Sorghum Seedlings

The present investigation was undertaken to understand the effect of PEG-6000– induced osmotic stress on isozymes of superoxide dismutase and peroxidase as well as their activities in four each of drought susceptible, tolerant and stay-green sorghum genotypes at seedling stage. Total number of isozymes of superoxide dismutase was more under stressed than unstressed condition, whereas the band intensity was also higher under stressed condition. Two to four new peroxidase isozymes with very high colour intensity bands were observed in most of the genotypes under stressed condition. The mean activity of superoxide dismutase and peroxidase were higher in the stay-green genotypes, followed by tolerant and susceptible genotypes.

Physiological and Biochemical Responses to Drought during Filling Stage in Stay Green Sorghum B35

Effects of drought on a typical stay green sorghum line B35 were investigated through analyzing the physiological and biochemical characteristics of sorghum plants with the treatments of weekly drought stress starting on the 10th day and the 30th day after anthesis respectively in pot experiment.Compared with CK,the stay green sorghum B35 showed higher chlorophyll content(SPAD value),photosynthetic rate and nitrogen content in leaf,stem and grain.Under the condition of drought stress,the increments of electrical conductivity rate,malondialdehyde content in B35 were less than those in CK,the increments of proline contents and decrements of leaf water potential in B35 were more than those in CK.The activities of superoxide dismutase(SOD) and peroxydase(POD) of B35 were higher than those of CK.We suggested that the stay green sorghum line could perform the physiological autoregulation effectively at the aspects of photosynthesis,nitrogen accumulation,cell membrane permeability of leaf,osmoregulation ability and activities of protective enzymes to adapt the drought.

Nitrogen Management Affects Sorghum Grown for Grain and Forage

Producers have to maximize crop production to stay economically viable. After grain harvest, sorghum [Sorghum bicolor (L.) Moench] stalks may be utilized for animal feed. The objective of this study was to determine the effect of fertilizer N rate and timing on yield, N concentration, and N removal in grain and the remaining stover of a stay-green sorghum hybrid harvested immediately after grain or left until a killing frost. Sorghum grain yield responded to N rates up to at least 120 lb/acre, but was unaffected by fertilizer N timing. Nitrogen removed in the grain increased linearly with N rate and was about 25% of the applied fertilizer N. Stover yield and N removal was reduced by applying two-thirds of the fertilizer N as a sidedress at the eight-leaf stage compared with applying two-thirds or more N preplant. Stover yield was greater when cut immediately after grain harvest than after a killing frost partly because of poor post-harvest growth. Small amounts of post-harvest plant growth resulted in greater stover N concentration, especially at higher N rates. Total N removal by stover and grain responded linearly to increasing N rates and was greater if stover was harvested after grain harvest and if post-harvest growth did not occur. Nitrogen management is important for sorghum grain, as well as for stover used for feed.