Ecophysiological and Molecular Analysis of Contrasting Genotypes for Leaf Senescence in Sunflower (Helianthus annuus L.) Under Differential Doses of N in Soil

Abstract

Leaf senescence in plants is the last stage of leaf development and is characterized by a decline in photosynthetic activity, an active degeneration of cellular structures, and the recycling of accumulated nutrients to areas of active growth, such as buds, young leaves, flowers, fruits, and seeds. This process holds economic significance as it can impact yield, influencing the plant's ability to maintain an active photosynthetic system during prolonged periods, especially during the grain filling stage, which affects plant weight and oil content. It can be associated with different stresses or environmental conditions, manifesting itself widely in the context of climate change and limiting yield, especially in crops of agronomic relevance. In this work, we study the stability of two widely described sunflower (Helianthus annuus L.) genotypes belonging to the INTA Breeding Program against differential N conditions, to verify their yield stability in control conditions and under N supply. Two inbred lines were utilized, namely R453 (early senescence) and B481-6 (late senescence), with contrasting nitrogen availability in the soil but sharing the same ontogeny cycle length. It was observed that, starting from R5.5, the B481-6 genotype not only delayed senescence but also exhibited a positive response to increased nitrogen availability in the soil. This response included an increase in intercepted radiation, resulting in a statistically significant enhancement in grain yield. Conversely, the R453 genotype did not show significant differences under varying nitrogen availability and exhibited a tendency to decrease grain yield when nitrogen availability was increased. The response to nitrogen can vary depending on the specific genotype.

Keywords: nitrogen; senescence; sunflower; yield.

Figure 1

Meteorological conditions during the crop cycle. Values are daily means of medium (Tm), maximum (Tmax), and minimum (Tmin) temperature and daily photosynthetic incident radiation (Rad) and accumulated rainfall (Rainfall). Bars indicate the phenology of the B481-6 and R453 genotypes. R1 corresponds to the “star visible” stage, while R5.5 marks mid-anthesis, R7 indicates when the rear of the head initiates a transformation to a pale-yellow hue, and, at R9, the bracts become yellow and brown, following the scale proposed by Schneiter and Miller (1981) [23].

Figure 2

The net photosynthesis (Pn) and stomatal conductance (gs) data for the 5th leaf (c,f), 10th leaf (b,e), and last leaf (a,d) in genotypes B481-6 and R453 with different soil nitrogen levels as a function of thermal time to emergence (°Cd). R1 corresponds to observations taken at 727 °Cd, R5.5 corresponds to observations taken at 1112 °Cd, and R8 corresponds to observations taken at 1540 °Cd, following the scale proposed by Schneiter and Miller (1981) [23]. Vertical bars indicate ± standard error. Different letters at each phenological stage indicate significant differences for Tukey’s test at p = 0.05.

Figure 3

Intercellular CO₂ concentration (Ci), quantum yield of PSII photochemistry (ΦPSII), and photochemical fluorescence quenching coefficient (qP) data for the 5th leaf (d–f), 10th leaf (g–i), and last leaf (a–c) in genotypes B481-6 and R453 with different soil nitrogen levels as a function of thermal time to emergence (°Cd). R1 corresponds to observations taken at 727 °Cd, R5.5 at 1112 °Cd and R8 corresponds to observations taken at 1540 °Cd, following the scale proposed by Schneiter and Miller (1981) [23]. Vertical bars indicate ± standard error. Different letters at each phenological stage indicate significant differences for Tukey’s test at p = 0.05.

Figure 4

The green leaf area (GLA) per plant (a) and fraction of intercepted PAR (iPAR %) (b) as a function of thermal time to emergence (°Cd). Insets (c): Yield (g m⁻²) as a function of intercepted PAR accumulated during the period from R5.5 to physiological maturity (R5.5-PM; MJ m⁻²) in genotypes B481-6 and R453 with different soil nitrogen levels. A dashed line indicates mid-anthesis (R5.5). Symbols indicate nitrogen availability and colors indicate line. Vertical bars indicate ± standard error. Different letters at each phenological stages indicate significant differences for Tukey’s test at p = 0.05.

Figure 5

Relative yield changes compared to the overall average yield (%) for the entire experiment under the unfertilized condition (N0) and the fertilized condition (N1) in lines R453 and B481-6. Insets: Yield (g m⁻²) as a function of fertilized conditions. Different letters indicate significant differences (p < 0.05) between treatments.

Figure 6

qPCR Assays: Relative expression profiles of transcription factors under unfertilized (N0) and fertilized (N1) conditions in genotypes R453 and B481-6. (a,b) HaCAB2 (senescence marker gene; SDG in sunflower) and (c,d) HaNAC01—SAGs in sunflower. Relative transcript levels are depicted as the ratio (log2 scale) between the gene expression of the premature senescence genotype and its contrasting delayed senescence genotype.