Study on physiology and genetic tendency of watermelon spotted leaf

Abstract:【Objective】Leaf color variation represents a common plant alteration. It is notably caused by genetic mutations that result in an abnormal chlorophyll metabolism leading to changes in leaf color. Hence, these mutations are popularly identified as chlorophyll mutations. The leaf color variance can serve as a phenotypic marker in plant breeding and as a germplasm resource for ornamental plants. In the realm of plant physiology, leaf color variants are recognized as ideal materials to investigate a spectrum of physiological processes like photosynthesis and hormone metabolism. In the context of genetics, variant analysis can aid in recognizing the function of corresponding genes. TNY1201, a watermelon germplasm, displays speckled attributes on each leaf. Thus, the exploration of its leaf structure, photosynthesis, and genetic features can provide a benchmark for its practical usage and subsequent gene mapping and cloning. The present investigation undertook a comparative analysis on the leaf phenotype, anatomical structure, and photosynthetic physiological characteristics between the spotted leaf watermelon TNY1201 and ordinary watermelon 1182.【Methods】The healthy and unblemished leaves were harvested from different individuals of spotted leaf watermelon TNY1201 and typical watermelon 1182 to ascertain the pertinent parameters of the leaves. The leaf length and width were measured. The praffin sections were crafted to observe and assess the anatomical structure of the leaves. The average area and density of single stomata were measured using a micrometer and nail polish imprinting technique. The photosynthetic parameters including net photosynthetic rate, stomatal conductance, transpiration rate, and intercellular CO2 concentration were quantified via GFS-3000 photosynthetic apparatus at 09:00—10:00 on a clear day. The chlorophyll content of the leaves was estimated by alcohol extractionmethod. The content of dissolved sugar was measured by Anthrone colorimetry, and the content of soluble protein was assayed by the Coomassie brilliant blue method. The seeds of F1, F’1, F2, BC1P1 and BC1P2 progeny were obtained via conventional field management and artificial pollination. The P1 (1182), P2 (TNY1201), orthogonal F1, reciprocal F’1, BC1P1, BC1P2 progeny were sowed in module trays. As the seedlings matured to three leaves, the count of individual plants of spotless leaves and spotted leaves was surveyed and the data were analysed by Chi square test to determine the genetic tendency.【Results】From the first real leaf, all leaves of the TNY1201 have yellow spots. The average single stomatal area of the TNY1201 leaves equated to 467.97 μm2 , substantially larger than that of the 1182 leaves. Conversely, the stomatal density of the 1182 leaves was notably higher than those of the TNY1201. The anatomical parameters demonstrated notable disparities between the TNY1201 and 1182 leaves. Referencing the leaves of 1182, the morphology of the epidermal cells of the TNY1201 leaves was irregular, the palisade tissue and spongy tissue were loosely aligned within the mesophyll tissue, and the spongy tissue occupied a smaller proportion of volume. The leaf width, leaf area and leaf thickness of the TNY1201 are 18.38 cm, 206.59 cm2 and 124.13 μm, respectively, markedly greater than those of the 1182. Contrarily, there was no significant difference in the leaf length between the two materials. The content of chlorophyll in the TNY1201 leaves was significantly lower than 1182. The content of chlorophyll a, chlorophyll b and total chlorophyll in the TNY1201 leaves amounted to 82.51%, 70.97% and 75.38% of the 1182. There was no significant disparity in carotenoid content between the TNY1201 and 1182 leaves. The net photosynthetic rate of the 1182 leaves was 7.90 μmol·m-2 ·s -1 . The net photosynthetic rate of the TNY1201 leaves was 6.98 μmol·m-2 ·s -1 . The net photosynthetic rate of the 1182 was significantly higher than that of the TNY1201, which accounted for a 1.13 times increase over the TNY1201. The stomatal conductance and intercellular CO2 concentration of the TNY1201 leaves exhibited significantly higher values than those of the 1182. The transpiration rates demonstrated no significant variance between the TNY1201 and 1182 leaves. The content of soluble protein in the TNY1201 leaves was 22.70 μg · g-1 , noticeably higher than that of the 1182 leaves. The total soluble sugar content of the TNY1201 leaves was 0.77 mg·g-1 , which was markedly lower than that of the 1182 leaves. The F2 segregation population comprised 188 plants, including 146 individuals with spotted leaves and 42 individuals without spotted leaves. In contrast, the BC1P1 population of the 142 plants included 74 individuals with spotted leaves and 68 without spotted leaves. There were 145 strains present in the BC1P2 population, and all displayed spots on their leaves. Furthermore, the proportion of leaves exhibiting spots in the F2 plant population followed an approximate 3∶1 segregation ratio, while the proportion in the BC1P1 population followed a 1∶1 separation ratio.【Conclusion】The mesophyll tissue compactness in the TNY1201 leaves was lower than that of the 1182, and the proportion of palisade tissue was minimal. It could be postulated that there was fewer chloroplast in the TNY1201 leaves compared with 1182, leading to decreased chlorophyll content and photosynthetic rate, resulting in limited accumulation of photosynthetic products. The development of leaf spots was attributed to the reduction of chlorophyll content in the leaves. It would be noteworthy that the genetic control of these spots in the TNY1201 leaves was governed by a pair of dominant nuclear genes.