Changes of photosynthesis and chlorophyll fluorescence under high temperature stress in cowpea seedling leaves

Two cowpea varieties (Vigna unguiculata W., 'ZaoyouNo.1' and 'Youxuanzhijiang28-2') were used to study the changes of photosynthesis and chlorophyll fluorescence under high temperature stress (45/33 degrees C). The results showed that as time went by, all of the net photosynthetic rate (P-N), photochemical efficiency of photosystem (PS) 2 in the darkness (F-v/F-m) and quantum yield of PS 2 electron transport (Phi PS2) of the two varieties decreased. Moreover, the stomata conductance (Gs) increased during the first three days, and then decreased. However, the intercellular carbon dioxide (CO2) concentration (G) increased. These results indicated: 1. Under high temperature stress, non-stomatal limitation was the major factor leading to photosynthesis limitation. 2. The high temperature tolerance of 'ZaoyouNo.1' was demonstrably stronger than that of 'Youxuanzhijiang28-2'. 3. P-N, Phi PS II and F-v/F-m of the higher tolerance cowpea variety to high temperature were greater at 45 degrees C/33 degrees C (day/night) and 35 degrees C/27 degrees C (day/night). 4. When identifying high temperature tolerance of cowpea plants, P-N, Phi PS II and F-v/F-m can be used as physiological indexes.