Studying heat stress adaptation of yam in the Mississippi Delta

How yam (Dioscorea spp.) copes with temperatures above 35 degrees Celsius during the critical tuber bulking phase is a practical dilemma for growers in tropical lowlands where heatwaves are becoming more frequent. This research examined heat stress adaptation mechanisms in 10 yam genotypes (6 D. rotundata, 4 D. alata) subjected to three temperature regimes (30/22, 35/27, and 40/32 degrees Celsius day/night) in controlled-environment chambers at the Indore Regional Center for Sustainable Agriculture, Madhya Pradesh, India, from April 2022 to March 2024. Tuber yield, heat shock protein expression, membrane thermostability, antioxidant enzyme activities, carotenoid content, and leaf gas exchange parameters were measured. Three genotypes (TDm-1, TDm-3, both D. rotundata, and TDa-8, D. alata) maintained tuber yields above 1.5 kg per plant at 40/32 degrees, compared to below 0.8 kg in sensitive genotypes. Heat shock protein expression was 3.1-fold higher in tolerant genotypes. Membrane stability index remained above 68% in tolerant versus below 47% in sensitive types at the highest temperature. These findings identify heat shock protein accumulation and membrane stability as the primary adaptive mechanisms and provide candidate genotypes for breeding heat-resilient yam cultivars.

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Drivers of Yam Heat ToleranceClose ×

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Yield (kg): TDm-1 vs TDm-3 vs TDa-8 vs TDm-5 vs TDa-2Close ×

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Introduction

Here is a practical dilemma facing yam farmers across the tropics: should they plant early to avoid late-season heat, risking poor establishment if rains are delayed, or plant at the normal time and gamble that temperatures during tuber bulking won't exceed the crop's tolerance threshold? As heatwave frequency increases under climate change, this decision becomes more consequential each year [1] . Yam (Dioscorea spp.) is a staple food for over 300 million people in West Africa, the Caribbean, and parts of South and Southeast Asia, with annual global production exceeding 72 million tonnes [2] . The two most widely cultivated species, D. rotundata (white yam) and D. alata (water yam), differ in their thermal optima: D. rotundata prefers 25-30 degrees Celsius, while D. alata tolerates slightly higher temperatures [3] . Both species show yield declines when daytime temperatures consistently exceed 35 degrees during the 2-4 month tuber bulking window. Heat stress triggers a cascade of cellular responses in plants: protein denaturation, membrane fluidity changes, reactive oxygen species accumulation, and disruption of photosynthetic electron transport [4] . Heat Shock Proteins (HSPs) act as molecular chaperones that refold denatured proteins and protect cellular machinery [5] . Antioxidant enzymes (SOD, CAT, APX) scavenge reactive oxygen species. Membrane thermostability reflects the integrity of lipid bilayers at high temperatures. Which of these mechanisms contributes most to heat tolerance in yam hasn't been determined.

Material and Methods

Material and Methods

Material

Ten yam genotypes were obtained from the ICAR-Central Tuber Crops Research Institute, Thiruvananthapuram, and the Indore center's own collection. Seed tubers (250-300 g minisetts) were sprouted and planted in 100-litre pots filled with a 2:1 mix of red soil and compost at the Indore Regional Center for Sustainable Agriculture, Indore, Madhya Pradesh, India (22.7 degrees N, 75.8 degrees E, elevation 553 m). Plants were grown outdoors for 60 days to establish vine systems before transfer to controlled-environment chambers. The three temperature regimes were: optimal (30/22 degrees C day/night), moderate heat (35/27), and severe heat (40/32), maintained under a 12-hour photoperiod at 600 micromoles PAR [6] .

Methods

The research ran from April 2022 to March 2024 at the Indore Regional Center for Sustainable Agriculture, covering two growth cycles. After 60 days of outdoor establishment, plants were transferred to growth chambers and maintained under treatment temperatures for 90 days (the tuber bulking phase). A factorial completely randomised design with 10 genotypes x 3 temperatures x 4 replications (120 pots per cycle) was used. HSP expression was assessed by SDS-PAGE and densitometry at 7 and 30 days after heat onset. Membrane stability index was measured by the electrolyte leakage method on leaf discs. Antioxidant enzyme activities (SOD, CAT, APX) were determined spectrophotometrically from leaf extracts. Chlorophyll fluorescence (Fv/Fm) and net photosynthesis were measured with a LI-6400 portable gas exchange system. Tuber carotenoid content was quantified by HPLC. At harvest, tuber yield per plant and tuber dry matter content were recorded. Data were analysed by two-way ANOVA in SAS 9.4 [7] .

Sensitivity analysis

A sensitivity analysis was conducted to assess which physiological parameter best predicted tuber yield under heat stress. Stepwise multiple regression with tuber yield as the dependent variable and all measured physiological parameters as independent variables identified three significant predictors: HSP expression (partial R-squared = 0.31), membrane stability index (partial R-squared = 0.24), and net photosynthesis (partial R-squared = 0.14). Together these three variables explained 69% of yield variation under heat, confirming that cellular protection mechanisms are more important than photosynthetic capacity per se for maintaining yam tuber production under high temperatures [8] . Genotype (F = 28.4, p<0.001) and temperature (F = 52.7, p<0.001) both significantly affected tuber yield. At 40/32 degrees, TDm-3 (D. rotundata) maintained the highest yield at 2.18 kg per plant, while TDa-2 (D. alata) produced only 0.61 kg (Table 1). HSP expression at 30 days was 3.1-fold higher in the three tolerant genotypes than in sensitive ones. Membrane stability index at 40/32 degrees ranged from 38.2% (TDa-2) to 74.1% (TDm-3). SOD activity was 2.1fold higher in tolerant genotypes, while CAT and APX showed 1.7 and 1.9-fold increases respectively. Carotenoid content was reduced by 18.3% under severe heat across all genotypes but the decline was smaller in tolerant types (12.1%) than in sensitive ones (26.8%).

Genotype	Species	Yield (kg)	MSI (%)	HSP fold
TDm-1	D. rotundata	1.84	72.4	3.4
TDm-3	D. rotundata	2.18	74.1	3.7
TDa-8	D. alata	1.62	68.3	2.9
TDm-5	D. rotundata	0.74	43.8	1.4
TDa-2	D. alata	0.61	38.2	1.1

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Mean tuber yield (kg/plant), membrane stability index

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Results

Results

Discussion

The dominance of HSP expression as a yield predictor (partial R-squared = 0.31) aligns with the established role of heat shock proteins as frontline defenders against thermal protein denaturation [4,5] . In yam tubers, where starch synthesis depends on temperature-sensitive enzymes like ADP-glucose pyrophosphorylase, maintaining enzyme function through chaperone activity may be the most direct route to sustaining yield under heat [9] . Membrane stability's contribution (partial R-squared = 0.24) reflects the dependence of cellular compartmentalisation on intact lipid bilayers. When membrane integrity collapses at high temperatures, ion leakage disrupts metabolic processes throughout the cell [10] . The MSI threshold of approximately 68% appears to separate tolerant from sensitive genotypes in this dataset and could serve as a rapid screening tool. The finding that D. rotundata genotypes (TDm-1, TDm-3) were among the most tolerant despite D. rotundata's generally lower temperature optimum compared to D. alata [3] suggests that heat tolerance is genotype-specific rather than species-wide. Breeding programmes should screen within both species rather than assuming D. alata will always outperform D. rotundata under heat [11] .

Conclusion

This research identified heat shock protein expression and membrane thermostability as the primary mechanisms underpinning yam heat tolerance, collectively explaining 55% of yield variation at 40/32 degrees Celsius. Three genotypes (TDm-1, TDm-3, TDa-8) demonstrated strong adaptation, maintaining tuber yields above 1.5 kg per plant under severe heat stress. These genotypes are recommended as parents in breeding programmes targeting heat-resilient yam varieties for tropical lowlands including the Mississippi Delta and analogous warm-climate zones. HSP expression fold-change above 2.5 and MSI above 68% are proposed as screening thresholds for early-generation heat tolerance selection.

Acknowledgement

Acknowledgements

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Proportional contribution of heat adaptation mechanisms to yield maintenance as determined by sensitivity analysis

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Distribution of tuber yield (kg/plant) under severe heat stress (40/32 degrees) by genotype

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