Physiological basis of stay-green trait expression in post-rainy season sorghum genotypes

Introduction

Can a crop that stays greener yield more grain under terminal drought? Sorghum (Sorghum bicolor L. Moench) ranks fifth among cereals and feeds over 500 million people in the semi-arid tropics [1,2] . In East Africa, post-rainy sorghum faces progressive water deficit during grain filling [3] . Genotypes maintaining functional green leaf area beyond flowering keep photosynthesising when senescent types shut down [4,5] . Researchers distinguish cosmetic staygreen from functional stay-green, where carbon fixation persists [7] . Proposed mechanisms include higher pre-flowering chlorophyll [8] , better antioxidant defence [9] , and altered nitrogen remobilization [10] . Despite QTL mapping [11] , field confirmation in East African germplasm remains limited.

Material and Methods

Research Area and Material

The experiment was conducted at the University of Nairobi Upper Kabete Campus research farm (1.2465°S, 36.7339°E, 1,737 m asl) during October 2022 to February 2023. Soils are humic nitisols (clay loam, pH 5.8). Total rainfall during the crop period was 387 mm; the postflowering phase coincided with an 18-day dry spell in January 2023 [12] . Five stay-green sorghum lines (SG-14, SG-27, SG-33, SG-41, SG-56) from the National Genebank of Kenya and one non-stay-green check (Seredo) were evaluated [3] . Basal fertilizer was 60 kg N/ha and 40 kg P 2 O 5 /ha at planting, with 30 kg N/ha topdressed at knee-height.

Methods

The trial used a randomized complete block design with three replications in 4.5 m x 3.0 m plots at 75 cm row spacing. Chlorophyll was recorded using a SPAD-502Plus meter at 0, 7, 14, 21, 28, 35, and 42 Days After Flowering (DAF). Chlorophyll-a and -b were extracted in 80% acetone [13] . Net photosynthetic rate and stomatal conductance were measured with a LI-6400XT system (LI-COR, USA). Green Leaf Area at Maturity (GLAM) was estimated on five Tagged plants per plot. Stay-green score was assessed on a 1-5 visual scale at maturity. Stem reserve contribution was computed from dry weight differences between flowering and maturity. Grain yield was adjusted to 12.5 % moisture. Data were analysed by ANOVA in R 4.3.1 with LSD at P = 0.05 [14] . Comprehensive interpretation SG-33 had the highest total chlorophyll at 4.63 mg/g, 83.7 % more than the check (Table 1). Its Pn of 19.4 µmol/m²/s was 71.7% above the check. SG-33 also topped grain yield at 3.87 t/ha with 72.1 % GLAM retained, while the check managed 2.18 t/ha and 28.4 % GLAM. Harvest index in stay-green lines ranged from 33.6-41.4 % versus 26.3 % in the check. All five stay-green lines scored above 3.5 while the check scored 1.3 (Figure 1). SG-33 and SG-14 formed a distinct upper group. Correlation analysis showed all pairwise associations between stay-green traits and grain yield were positive and highly significant (p<0.01). The strongest were between stay-green score and GLAM (r = 0.968) and between stay-green score and grain yield (r = 0.951), indicating visual scoring at maturity can reliably predict yield potential under terminal drought. Total chlorophyll was also strongly linked to grain yield (r = 0.934).

Results

Discussion

The physiological separation between stay-green and senescent genotypes confirms that functional stay-green in sorghum is driven by sustained photosynthetic capacity, not mere pigment retention [7] . SG-33 maintained both high chlorophyll and high Pn through 42 DAF, ruling out cosmetic greenness. The biphasic chlorophyll decline in the check matches patterns described by Thomas and Howarth [7] , likely reflecting loss of antioxidant protection once a water-stress threshold is crossed. The strong yield-stay-green correlation (r = 0.951) aligns with reports where QTLs Stg1-Stg4 explained 10-30 % of yield variance [11] , and shows that simple visual scoring captures this relationship practical for breeding programmes with limited laboratory access. Stem reserves were 47-103 % greater in stay-green lines, suggesting the phenotype complements reserve translocation, giving plants a dual grain-filling source that explains elevated harvest index in SG-33 and SG-14.

Conclusion

Stay-green trait expression in post-rainy sorghum was grounded in sustained photosynthetic function. Stay-green genotypes maintained 53.8-72.1 % green leaf area at maturity, showed 40-72 % higher photosynthetic rates, and produced 35-78 % more grain than the check. SG-33, with the highest chlorophyll, stem reserves, and yield (3.87 t/ha), is the most promising donor parent for drought tolerance breeding. Chlorophyll retention and glam correlated strongly with yield, confirming their value as selection criteria for semi-arid East African conditions.

References

1. Borrell AK, Mullet JE, George-Jaeggli B, van Oosterom EJ, Hammer GL, Klein PE, Jordan DR. Drought adaptation of stay-green sorghum is associated with canopy development, leaf anatomy, root growth, and water uptake. J Exp Bot. 2014;65(21):6251-6263.
2. FAOSTAT. Food and Agriculture Organization statistical database. Rome: FAO; 2023.
3. Ngugi K, Kimani W, Kiambi D. Sorghum germplasm characterization and evaluation for drought tolerance in semi-arid Kenya. East Afr Agric For J. 2013;79(2):87-94.
4. Kassahun B, Bidinger FR, Hash CT, Kuruvinashetti MS. Stay-green expression in early generation sorghum backcross lines. Euphytica. 2010;172(3):351-362.
5. Thomas H, Smart CM. Crops that stay green. Ann Appl Biol. 1993;123(1):193-219.
6. Rosenow DT, Clark LE. Drought and lodging resistance for a quality sorghum crop. In: Proceedings of the 50th Corn and Sorghum Research Conference. Chicago: ASTA; 1995. p. 82-97.
7. Thomas H, Howarth CJ. Five ways to stay green. J Exp Bot. 2000;51(Suppl 1):329-337.
8. Borrell AK, Hammer GL, Henzell RG. Does maintaining green leaf area in sorghum improve yield under drought? II. Dry matter production and yield. Crop Sci. 2000;40(4):1037-1048.
9. Tao YZ, Henzell RG, Jordan DR, Butler DG, Kelly AM, McIntyre CL. Identification of genomic regions associated with stay green in sorghum. Theor Appl Genet. 2000;100(8):1225-1232.
10. van Oosterom EJ, Chapman SC, Borrell AK, Hammer GL, Henzell RG. Functional dynamics of the nitrogen balance of sorghum. II. Grain filling period. Field Crops Res. 2010;115(1):29-38.
11. Harris K, Subudhi PK, Borrell A, Jordan D, Rosenow D, Nguyen H, Klein P, Klein R, Mullet J. Sorghum stay-green QTL individually reduce post-flowering drought-induced leaf senescence. J Exp Bot. 2007;58(2):327-338.
12. Jaetzold R, Schmidt H, Hornetz B, Shisanya C. Farm management handbook of Kenya. Vol. II. 2nd ed. Nairobi: Ministry of Agriculture; 2006.
13. Arnon DI. Copper enzymes in isolated chloroplasts: Polyphenoloxidase in Beta vulgaris. Plant Physiol. 1949;24(1):1-15.
14. R Core Team. R: A language and environment for statistical computing. Vienna: R Foundation for Statistical Computing; 2023.