Peanut cultivars
Nine peanut cultivars, including 3 with high resistance (Yuhanghua 7, Huayu 9624, Huayu 9623), 3 with medium resistance (Anhua 6, Huayu 958 and Huayu 9510), and 3 susceptible genotypes (Zhengnonghua 23, Huayu 917 and Huayu 51), were used in this study (Table 1).
Table 1
Pod rot scores and leaf palisade anatomical and shell biochemical features of the 9 peanut cultivars
Cultivar | Pod rot score | Leaf palisade anatomical feature | Shell lignin staining area (µm2/µm) | Shell biochemical feature (%) |
No. of cells/mm | Cell width (µm) | Index | Lignin | Hemicellulose | Cellulose |
Yuhanghua 7 | 0 | 103.33a | 8.97AB | 927.44A | 214.68A | 15.23 | 13.60 | 5.51 |
Huayu 9624 | 0 | 93.33ab | 10.21A | 959.97A | 182.59B | 15.07 | 13.27 | 4.73 |
Huayu 9623 | 0 | 80.00bcd | 10.35A | 833.72AB | 151.32CD | 15.70 | 14.30 | 4.40 |
Anhua 6 | 0 | 83.33abc | 8.80AB | 731.98ABC | 152.91C | 14.73 | 13.33 | 5.32 |
Huayu 958 | 1 | 80.00bcd | 7.44BC | 601.26ABC | 136.60CDE | 15.77 | 13.73 | 4.99 |
Huayu 9510 | 1 | 73.33bc | 8.31BC | 608.90ABC | 149.13CD | 13.13 | 12.53 | 4.94 |
Zhengnonghua 23 | 2 | 70.00bc | 7.50BC | 522.60BC | 124.99DE | 13.27 | 14.07 | 4.77 |
Huayu 917 | 2 | 66.67c | 7.05C | 474.74BC | 110.32E | 14.77 | 15.00 | 5.71 |
Huayu 51 | 3 | 63.33c | 6.94C | 439.28C | 114.71E | 13.67 | 13.03 | 4.81 |
Within each column, the figures followed by the same uppercase letter were not significantly different at 0.01 probability level, and those followed by the same lowercase letter were not significantly different at 0.05 probability level. |
Peanut cultivation, management, investigation and harvesting arrangements
The study was conducted at Shandong Peanut Research Institute Laixi Experimental Station. Peanuts were sown on May 13, 2022, under film mulch. Routine agronomic practices were followed (Wan 2003). Chemicals were sprayed to control insect pests, whereas no fungicides were used for disease management. Investigations on plant height, lodging, peg strength, leaf spots, and pod rot were made on September 5, 2022. Peanuts were harvested two days later (on September 7, 2022).
Sampling for studies on leaf and shell anatomy and shell biochemistry
For each cultivar, three mature pods, identified by the hull scrape method (Williams and Drexler 1981), were sampled on August 12 (92 days after sowing, DAS) to investigate sclerenchymatous mesocarps with phloroglucinol staining method.
To study the anatomy of peanut leaflets, the basal leaflet of the first leaf on the lower branches of each cultivar were sampled on August 18, 2022 (98 DAS). A section of the leaflet about 3 mm wide was further cut off in the middle of the leaflet, perpendicular to the medrib. Three replicates were made for each cultivar. Shells from mature peanut pods (Williams and Drexler 1981) were sampled on the same day to determine their lignin, cellulose and semi-cellulose content, and for each cultivar, three replicates of 10 g each were made.
Paraffin sectioning
Fresh tissues were fixed in FAA (formalin-aceto-alcohol) fixative for more than 24h. Thereafter, the tissue was removed from the fixative and trimmed flat with a scalpel in a fume hood. Trimmed tissue and its corresponding label were placed in a dehydration box.
The dehydration box was put in a Donatello Dehydrator (DIAPATH, Italy) for dehydration in a gradient alcohol sequence, followed by clearing and subsequent paraffin infiltration. The tissue was subjected to a series of treatments: 75% alcohol for 4h, 85% alcohol for 2h, 90% alcohol for 2h, 95% alcohol for 1h, absolute ethanol I for 30min, absolute ethanol II for 30min, alcoholic benzene for 5-10min, xylene (Sinopharm chemical reagent Co., Ltd., China) I for 5-10min, xylene II for 5-10min, 65°molten paraffin I 1h, 65°molten paraffin II 1h, 65°molten paraffin III 1h.
The paraffin-infiltrated tissue was then embedded using a RM2016 embedding machine (Wuhan Junjie Electronics Co., Ltd., China). Melted paraffin was first put into the embedding frame, and before the paraffin solidified, the tissue was removed from the dehydration box and placed into the embedding frame according to the requirements of the embedding surface and affixed with the corresponding label. The paraffin was cooled at -20°on a JB-L5 Frozen Platform (Wuhan Junjie Electronics Co., Ltd., China) and after the paraffin got solidified, the paraffin block was removed from the embedding frame and trimmed.
The trimmed paraffin block was cooled on the − 20°C Frozen Platform, and afterwards the cooled paraffin block was sectioned on a Leica RM 2016 Pathology slicer (Shanghai Leica Instruments Co., Ltd., China) at a thickness of 3 µm. The sections were floated on 40°C warm water in a KD-P organizer (Zhejiang Jinhua Kedi Instruments Co., Ltd., China) to flatten the tissue, and were picked up with a glass slide and baked in a 60°C GFL-230 Oven (Tianjin Lai Bo Rui Instruments Co. Ltd., China). After the water got dried and the paraffin was melted, the sections were removed and stored at room temperature for further use.
Section staining and microscopic examination
Peanut leaflet tissue
In turn the paraffin slides were placed into dewaxing clearing solution I (Servicebio, China) for 20min, dewaxing clearing solution II for 20min, absolute ethanol (Sinopharm Chemical Reagent Co., Ltd., China) I for 5min, absolute ethanol II for 5min, and 75% alcohol for 5min, followed by washing with tap water.
The slides were transferred into a plant safranine staining solution (Servicebio, China) and stained for 2h, and slightly washed with tap water to remove excess dye. The slides were then put into 50%, 70% and 80% gradient alcohols for 3-8s each in turn.
The sections were placed into a plant fast green staining solution (Servicebio, China) and stained for 6-20s. Dehydration was performed with three absolute ethanol treatments.
The sections were cleared in xylene for 5min, and mounted in neutral gum (Sinopharm chemical reagent Co., Ltd., China).
Anatomical observations were made under a Nikon Eclipse E100 Upright optical microscope (Nikon, Japan). Image acquisition was performed using a Nikon DS-U3 Imaging system (Nikon, Japan). Number of palisade cells within a length of 200 µm was counted with 3 replicates. Palisade cell width with 3 replicates was recorded. Index = Number of palisade cells/µm × Palisade cell width (µm) (Godoy et al. 1985).
Peanut shell tissue
The procedure for section dewaxing was the same as described above. After dewaxing and tap water washing, the sections were stained in a phloroglucinol C (Servicebio, China) solution for 5s, washed with water and air dried.
The tissues were enclosed with a histochemical pen, covered with an appropriate amount of a phloroglucinol B (Servicebio, China) solution using a Pasteur pipet, stained for 30s, and an appropriate amount of phloroglucinol A (Servicebio, China) solution was added on the tissue with another clean pipette and blew to mix evenly, and the tissue was stained for 2min.
After removal of part of the staining solution with a pipette, the section was sealed with a coverslip, and photos were taken within 3 min to avoid color fade. Shell lignin staining area every 500µm long was measured 9 times for each cultivar.
Measurement of main agronomic characters and disease and lodging rating
Data including main stem height, 100-pod weight, 100-seed mass, and shelling percentage were collected following the procedure described by Yu (2008). For each cultivar, four mature pods from the first nodes of the cotyledonary branches were chosen for peg strength measurement using a tensiometer (HandPI HP-50, China).
Chlorophyll content in the two apical leaflets on the third branch of peanut plants was measured quantitatively using a SPAD-502 chlorophyll meter (Konica Minolta, Japan). Two plants of each cultivar were measured, and the average of 2 SPAD readings was recorded.
Near infrared spectroscopy (NIRS) was used to predict the content of major fatty acids (oleic, linoleic and palmitic acids, as a percentage of total), oil, protein and total soluble sugars (TSS) in sun-dried bulk kernel samples (Wang and Zhang 2013). Each sample was measured three times, and the averages were used in analysis.
Leaf spot disease and pod rot severity, and lodging status were scored separately on a 0–3 scale, where 0 = free of symptom, 1 = slight, 2 = medium, and 3 = severe. Specifically, for leaf spots, 0 = no disease lesions, 1 = 0 < leaf lesion area ≤ 25%, 2 = 25%< leaf lesion area ≤ 50%, 3 = leaf lesion area > 50%; for pod rot, 0 = no rotten pods, 1 = 0 < rotten pods ≤ 25%, 2 = 25%<rotten pods ≤ 50%, 3 = rotten pods > 50%; for lodging status, 0 = no lodging, 1 = 0 < lodging plants ≤ 25%, 2 = 25%<lodging plants ≤ 50%, 3 = lodging plants > 50%.
Shell biochemical assay
Lignin, hemicellulose and cellulose content in peanut shells was determined by wet chemistry methods .
Statistical analysis
Analysis of variance and multiple comparisons based on Duncan’s multiple range method (DMRT), broad-sense heritability estimation and correlation analysis were performed using the DPS 14.50 package.