Safflower gas exchange, leaf water potential, and water-saving irrigation in surface and subsurface drip irrigation systems at different lateral spacings

Over two growing seasons, we evaluated how irrigation strategies and lateral spacings under drip irrigation methods shape safflower physiology. A split-split plot design tested full irrigation (FI), partial root-zone drying (PRD), and deficit irrigation (DI) across three lateral spacings (L1 = 20, L2 = 40, and L3 = 60 cm) with surface (SD) and subsurface (SSD) drip irrigation systems. We monitored leaf area index (LAI), photosynthesis rate (A_n), stomatal conductance (g_s), leaf transpiration rate (T_r), leaf water potential (LWP), and leaf to air vapor pressure deficit (VPD) and related these to seed yield. Relative to FI, water-saving irrigation strategies reduced seasonal irrigation by ~ 25-26%, and SSD applied ~ 7% less water than SD. Peak LAI was 3.95 (2018) and 4.88 (2019), and it significantly reduced by an average of 14% and 18% with water-saving irrigation and wider later spacing, respectively, in comparison to FI and 20 cm lateral spacing. Gas exchange parameters (A_n, g_s, and T_r) decreased with wider lateral spacing and peaked under FI and 20 cm lateral spacing, though PRD and 40 cm spacing resulted in an acceptable physiological performance under water scarcity. The LWP was less negative under FI by roughly 18% compared to water-saving irrigation and by 24% under L1 compared to L3. The extinction coefficient (k) of spring safflower for SD and SSD was 0.62 and 0.70 on average during the two growing seasons. Subsurface drip irrigation showed slightly higher light extinction coefficient, suggesting altered canopy light absorption. Collectively, treatments optimized for gas exchange properties and seed yield were FI-L1-SSD. When water is scarce, the most effective saving strategy was PRD-L2-SSD, which maintained A_n, g_s, LWP, and A_n/T_r better than DI while saving ~ 25% seasonal water compared to FI.