Coordination between stem water transport capacity and photosynthesis in corn and sorghum cultivars during progressive soil drying

Despite the fact of a strong linkage has been noted between xylem water transport capacity and photosynthesis in several woody plant species, not much research has studied such linkage in herbaceous plants. Herbaceous plants could be more vulnerable to drought-induced embolism than woody plants due to their lower xylem structure. Hence in a pot experiment, this paper investigated the coordination between stem water transport capacity and photosynthesis in corn cultivars ("Shaandan 21" and "Zhengdan 958") and sorghum cultivars ("Jinzhong 405" and "Jinza 12") during progressive soil drying. The aim of the study was to demonstrate the differences in sensitivity of photosynthesis to drought-induced embolism in maize and sorghum cultivars and the related hydro-physiological mechanisms of drought resistance. The results indicated that: (1) Drought-resistant corn cultivar "Zhengdan 958" had higher leaf water potential, net photosynthetic rate (Pn), maximum photochemical quantarum efficiency (Fv/Fm), maximum actual quantarum efficiency (ΦPSⅡ) and lower stomatal conductance (Gs) and transpiration rate (Tr) than drought-sensitive corn cultivar "Shaandan 21". Also drought-resistant sorghum cultivar "Jinza 12" had higher Pn and ΦPSⅡ than drought-sensitive sorghum cultivar "Jinzhong 405". This suggested that "Zhengdan 958" corn cultivar and "Jinza 12" sorghum cultivar had relatively stronger drought-resistance. (2) Drought induced different degrees of decreases in leaf water potential, photosynthetic gas exchange (Pn, Gs and Tr) and in chlorophyll fluorescence parameters (Fv/Fm and ΦPSⅡ) in both corn and sorghum cultivars. Gas exchange decreased earlier than fluorescence parameters during progressive soil drying. This implied that stomata closed earlier to prevent drought-induced damage to PSⅡ activity. (3) Drought-resistant corn cultivar "Zhengdan 958" and sorghum cultivar "Jinza 12" had higher water transport capacities and stronger cavitation resistance than drought-sensitive "Shaandan 21" and "Jinzhong 405", respectively. Sorghum had a weaker cavitation resistance than corn, suggesting that sorghum cultivars reduced water loss through rapid embolism. (4) Pn and Gs in corn cultivars showed greater sensitivity to declining water transport capacity than those in sorghum cultivars. Also Gs in drought-resistant corn and sorghum cultivars showed greater sensitivity to declining water transport capacity than those in drought-sensitive cultivars. Sorghum maintained certain photosynthesis at low water potential compared with maize, reflecting its higher conservative water use strategy.