Effect of Sink Removal and Magnesium Spraying on Photosynthetic Pigments and Sugar Yield of Two Sweet Sorghum Cultivars

The ever-increasing problem of water scarcity in arid and semi-arid regions of Iran has necessitated the demand to replacement of sugar beet plantation by more water-use efficient plants. Sweet sorghum is one of the sugars plants with low water requirements that potentiates cost-effective sugar production in different regions. However, physiological aspects of sugar yield in this plant is not completely understood. The present experiment studied the effect of sink (spike) removal (as a limiting factor for sugar production) and magnesium spraying (as an improving factor for plant growth) on photosynthetic pigments concentration and yield of two sweet sorghum cultivars. This research was carried out as a factorial experiment based on a randomized complete block design with three replications in two years (2014, 2015) in Shahroud Agricultural Research Center, Shahroud, Iran. Experimental factors included sweet sorghum cultivars (KFS2 and KFS3), sink removal treatments (control, mechanical removal and chemical removal of spike by ethephon) and spraying different concentrations of magnesium (0, 4 and 8 mM). Interaction of cultivars, sink removal and magnesium spraying levels was significant on photosynthetic pigments concentration, leading to increase in total sugar and sugar yield. The highest sugar yield in both cultivars was obtained from chemical removal of sink and 4 and 8 mM of magnesium application. In both of the studied cultivars, assimilate surface index was significantly affected by sink removal and the highest amount of this trait (2.65 g sugar per square meters of leaf area) was achieved from chemical removal of sink in KFS2. The latter results indicated that photosynthetic capacity of the sweet sorghum plants was enhanced by sink removal, especially when removed chemically, i.e. by ethephon. As no significant difference was observed between 4 and 8 mM in most examined traits, the 4 mM magnesium spraying was proven more suitable.