EFFECT OF SOWING DATES AND MINERAL FERTILIZER RATES ON THE NET PHOTOSYNTHETIC PRODUCTIVITY (NET ASSIMILATION RATE) OF SUNFLOWER HYBRIDS

Abstract

The net assimilation rate (NAR) — the net photosynthetic productivity of the crop — integrates the efficiency with which leaf area is converted into dry matter and is a key physiological indicator for evaluating cultivars and agronomic treatments. A three-year field experiment (2024–2026) on meadow-alluvial soils of the Experimental Farm of Urgench State University (Khorezm region, Uzbekistan) examined the NAR of three sunflower hybrids — the local ‘KK-52’ F₁ (Uzbekistan) and the introduced ‘Dushko’ F₁ (Serbia) and ‘Svetlana’ F₁ (Russia) — under two sowing dates (spring primary crop, second decade of April; summer repeat crop, third decade of June) and four fertilizer rates (N₁₂₀P₈₅K₁₂₀ to N₂₁₀P₁₄₅K₂₁₀ kg ha⁻¹). NAR was determined for five inter-phase periods. In the spring crop NAR rose to a distinct maximum during the head-formation–flowering period (11.7 g m⁻² day⁻¹ on average, up to 13.3 g m⁻² day⁻¹), whereas in the summer crop the peak shifted to the flowering–seed-setting period. NAR increased with fertilizer up to the N₁₈₀P₁₂₅K₁₈₀ rate and declined at the highest rate for every hybrid, identifying N₁₈₀ as the physiological optimum. NAR was positively correlated with the fertilizer rate (r = 0.76) and strongly correlated with the final biological dry matter (r = 0.84, p < 0.001), confirming NAR as a principal driver of biomass accumulation. Analysis of variance showed highly significant hybrid and fertilizer effects (p < 0.001). KK-52 exhibited the highest NAR, followed by Dushko and Svetlana, and the spring crop exceeded the summer crop.