Identification of Heat Tolerant Cotton Lines Showing Genetic Variation in Cell Membrane Thermostability, Stomata, and Trichome Size and Its Effect on Yield and Fiber Quality Traits

Heat stress in cotton reduces its productivity. The development of heat-tolerant cotton varieties having resilience against changing climate is feasible. The purpose of this study was to probe the genetic variability in upland cotton for heat tolerance, the association of cell membrane thermostability (CMT), stomata, and trichome size with cotton adaptation to high temperature and effective breeding strategy to advance the valued traits. Relative cell injury percentage (RCI%) in studied genotypes ranged from 39 to 86%. Seventeen genotypes were found heat tolerant on the basis of low RCI%, heat susceptibility index (HSI<1), higher number of boll/plant, and seed cotton yield (SCY). Scanning electron microscopy (SEM) of heat-tolerant genotypes revealed the presence of different size of stomata (21.57 to 105.04 μm2) and trichomes (177 to 782.6 μm) on leaves of selected genotypes. The regression analysis showed a strong and negative association of RCI% and stomata size with SCY. However, no association was observed between the trichome size, yield, and fiber traits. On the overall location basis, a significant genotype × environment interaction was observed. All selected genotypes produced a higher SCY as compared with check varieties. But the stability analysis showed that the high yielding genotypes NIA-M-30, NIA-80, NIA-83, and CRIS-342 were also wide adaptive with unit regression (bi∼1) and non-significant deviation from the regression line (S2d∼0). The ability for the combination of some heat-tolerant genotypes was estimated by using the line × tester method among nine hybrids along with their 3 testers (i.e., male) and 3 lines (i.e., females). Genotypes, CRIS-342 and NIA-Perkh, were observed as best general combiners for SCY with a negative general combining ability effects for RCI%. Five hybrids showed a positive specific combining ability and heterotic effects for studied traits and also found lowest for HSI. RCI% and SCY/plant displayed higher estimates of heritability and genetic advance, indicating the heritability due to additive gene effects and chances of effective selection. The identified heat-tolerant and wide adaptive germplasm can be further advanced and utilized in cotton breeding programs for developing heat-tolerant cultivars. Selection criteria involving CMT and stomata size concluded to be an effective strategy for the screening of heat-tolerant cotton.

Intra-Plant Variability for Heat Tolerance Related Attributes in Upland Cotton

Abiotic stress, particularly heat stress, affects various parts of the cotton plant and ultimately impacts the seed cotton yield. Different portions of a single cotton plant of a cultivar exhibit variable responses to stress during reproductive and vegetative phases. To test this hypothesis, physiological and morphological traits related to heat stress were observed for two flowering positions in 13 genotypes of upland cotton. These genotypes were sown in field conditions in triplicate following a randomized complete block design. Data were collected for pollen germination, pollen viability, cell membrane thermostability, chlorophyll content, boll weight, and boll retention for both the top and bottom branches of each genotype. The collected data were analyzed for the identification of variability within and between genotypes for these two flowering positions. Tukey’s test was applied to estimate the significance of differences between genotypes and positions within each genotype. Results showed that the two positions within the same plant statistically varied from each other. The bottom branches of the genotypes performed significantly better for all traits measured except boll weight. The genotype AA-933 performed best for pollen germination and boll retention, while CYTO-608 exhibited maximum pollen viability in both the bottom and top flower positions compared with other genotypes. Overall, MNH-1016 and CIM-602 showed better cell membrane thermostability and chlorophyll content, respectively. This intra-plant variability can be further exploited in breeding programs to enhance the stress tolerance capabilities of the resulting varieties.

Heat Tolerance in Sugarcane: Optimum Temperature and Phenological Stage to Determination of Thermotolerance as Selection Criteria

Heat stress is the major abiotic stressor in agriculture which reduces crop productivity and yield. Six sugarcane (Sacharum officinarum L.) genotypes were studied to investigate the impact of three temperature levels at four phenological stages on tissue electrolyte production and the feasibility of using the cell thermostability method (CTM) for the identification and selection of heat tolerant sugarcane genotypes. The cell membrane thermostability was quantified by measuring relative cell injury percentage with a modification in the temperature treatment on four phenological stages in a field experiment. Our results suggest that heat tolerance based on cell membrane thermostability can be improved using the existing genetic variability available within the commercial or experimental sugarcane germplasm. We conclude that the cell membrane thermostability test can be a useful screening procedure for selecting sugarcane genotypes that tolerate high temperature stress. The test can be used in conjunction with a temperature trait of 60 °C during the maturity stage. This procedure predicts the ability of sugarcane genotypes to maintain yield and juice quality under stressful field conditions.

Garden Chrysanthemum Cell Membrane Thermostability and Flowering Heat Delay Differences Among U.S. and South Korean Germplasm

Global warming has created increased nighttime temperatures both in field and greenhouse production of chrysanthemums during flower bud initiation (FBI) and development, causing heat delay or complete cessation of flowering. Integration of breeding and selection for heat delay insensitivity (HDI) has become imperative for greenhouse (cut, potted types) and must be accomplished on a genotypic basis, similar to winterhardiness. This is a breeding objective in the joint garden chrysanthemum breeding project between the Chungnam Provincial Agricultural Research and Extension Services and the University of Minnesota. The objectives of this research were to test 10 genotypes (cultivars, seedlings) from both breeding programs when grown in low-temperature (LT) and high-temperature (HT) short-day (SD) and long-day (LD) conditions (four environments: LTSD, LTLD, HTSD, and HTLD); determine the extent of heat delay and HDI for visible bud date (VBD), flowering, and other phenotypic traits; evaluate relative injury (RI) and cell membrane thermostability (CMT), and to select future parents with lowered RI values, higher CMT, shorter heat-induced flowering delay, and/or HDI. ‘Magic Ball’ and ‘Minnwhite’ had the shortest plant height in HTLD and HTSD, whereas ‘Geumbangul’ had stability for height in all treatments. Lowest long day leaf numbers (LDLN) occurred under LTSD in seven genotypes. However, both ‘Geumbangul’ and ‘Magic Ball’ had complete stability for LDLN across all environments. Sigmoid curves for RI% and temperature were found for all genotypes and environments with R2 = 0.79–0.89. Only ‘Mellow Moon’ had stability or equal VBDs in HTSD, LTSD, and LTLD conditions. This is the first-ever report of stability for VBD across inductive and noninductive HT/LT treatments. Only ‘Centerpiece’ flowered in all environments and also had 0 day of heat for VBD in LT and 1 day of heat delay in HT, as well as three others (Mn. Sel’n. 01-210-43, ‘Autumn Fire’, and ‘Geumbangul’). Few had linear regressions with positive slopes for heat-induced VBD or flowering delay regressed with RI%; most had no slope (R2 ≈ 0.0) for all treatments (‘Centerpiece’, Mn. Sel’n. 01-210-43), whereas others were negative (‘Mammoth™ Dark Bronze Daisy’, Flw LTLD–LTSD). Surprisingly, one linear regression had a slope of R2 = 1.0 (‘Geumbangul’, Flw LTLD–LTSD). These responses are all novel in chrysanthemums. Selecting the best parents in both breeding programs to maximize stability of all traits across these four environments with minimal crossing and selection across generations could be accomplished by stacking parental traits. A crossing scheme involving just three parents is proposed to incorporate stability for all traits in just a few generations.

Heat Tolerance and Flowering-heat-delay Sensitivity in Relation to Cell Membrane Thermostability in Chrysanthemum

Flowering of many chrysanthemum [Dendranthema ×grandiflora (Ramat.) Kitam.] cultivars is reduced or delayed under high temperatures. Identification and rapid selection of heat-tolerant and flowering-heat-delay-insensitive chrysanthemum genotypes for commercial production is desirable. An electrolyte leakage technique was used to measure cell membrane thermostability of chrysanthemum cultivars. The relationship between the relative injury (RI) value occurring in leaf tissue discs and the treatment temperature was sigmoidal. The RI values at the approximate midpoint of the sigmoid response curve occurred at 47 to 53 °C for summer- and fall-flowering cultivars and at 45 to 46 °C treatments for winter- and spring-flowering cultivars. Regressing the delay in days to flowering for the cultivars grown at day/night temperature of 30/25 °C compared with those grown at 20/15 °C versus their associated RI values at 50 °C treatment showed a linear relationship. Reduced RI was more apparent in the heat-tolerant ‘Kaa Luoh-Lii’ than the heat-intolerant ‘Repulse’ after 30/25 °C treatment for 24 to 27 days. When 30/25 and 20/15 °C treatments were compared, the former did not alter leaf malondialdehyde (MDA) content in ‘Kaa Luoh-Lii’ but increased MDA content in ‘Repulse’.