Effects of salt stress on photosynthetic characteristics and canopy structure of edible sunflower leaves at budding stage

To explore the effects of the degree of salt-stress on the photosynthesis of edible sunflowers (hereafter referred to as “edible sunflower”) at the budding stage, the photosynthetic characteristics of the leaves and their canopy structures were obtained after three degrees of salt stress classed as lower (LS), medium (MS), and higher (HS) total dissolved solids values (1.00, 2.68, and 4.93 g∙kg⁻¹, respectively). The results showed that the three salt stress treatments dramatically decreased the net photosynthetic rate (P_n) of the leaves and reduced the number of leaves with high P_n. When compared with that in the LS treatment, the maximum P_n in the MS and HS treatments decreased by 5.09% and 38.69%, respectively. In addition, salt stress reduced the P_n of the top leaves. The maximum P_n values of the top leaves in the MS and HS treatments were 8.08% and 14.66% lower than those in the LS treatment, respectively. The P_n of all leaves was mainly affected by the synergistic effects of stomatal and non-stomatal factors in the LS treatment, whereas it was mainly affected by non-stomatal factors in the MS and HS treatments. Salt stress also changed the plant type of the edible sunflowers. The canopy structure of LS treatment had a pagoda shape; however, the canopy structures of MS and HS treatments were flat and cylindrical, respectively. Salt stress reduced the total and main functional leaf areas of edible sunflower. Compared with the LS and MS treatments, the total leaf area of the HS treatment decreased by 56.03% and 47.74%, respectively; the maximum single leaf area of the HS treatment also decreased by 38.71% and 49.46%, respectively. Contrarily, the maximum leaf inclination angle of the HS treatment increased by 30.92% and 14.59% compared to those of the LS and MS treatments, respectively. Concludingly, salt stress significantly reduced the photosynthetic performance and leaf area of the main functional leaves of edible sunflowers, causing the canopy structure to change from extended to contracted, thereby inhibiting normal plant growth.