Photosynthetic and chlorophyll fluorescence regulation of upland cotton ( Gossiypium hirsutum L.) under drought conditions

To investigate physiological characteristics of photosynthesis at different stages of growth, the transgenic cultivar Lumianyan28 was treated under two conditions with relative water content of < 60% (drought treatment) and 70-80% (well watered control), respectively. Results obtained from leaves of main stem showed that drought decreased the transpiration efficiency and inner transpiration efficiency of functional leaves which refers to the forth main stem leave from top before tip pruning, and thereafter, refers to the third one or the second one in the later period. It can be inferred that plants were most sensitive to soil drought at the initial bloom stage. Due possibly to the reduction of thylakoid stacking, the content of chlorophyll a/b and net photosynthetic rate of the water-stressed plants were significantly lower than those of the well-watered control plants at the initial bloom stage. The transpiration rate of the functional leaves under drought treatment was remarkably higher than those of control plants at the bud stage, showing the relative significance of transpiration. The photosynthetic electron transport rate (ETR) of the main stem leaves correlated with the net photosynthetic rate under drought treatment (correlation coefficient=0.907*). However, such a correlation was not detected in the well-watered leaves. These results suggested that regulating photosynthetic system at crucial stages was the defense response of cotton plant to drought. However, this ability was very limited and progressively reduced along with prolonged drought.