Low night temperature-induced changes in photosynthesis and rubber accumulation in guayule (Parthenium argentatum Gray)

Three-year-old plants of Parthenium argentatum Gray cv. 11591 grown under n atural photoperiod were exposed for 60 d to low night temperature (LNT) of 15 degreesC (daily from 18:00 to 06:00). Effects of the treatment on net ph otosynthetic rates (PN), rubber accumulation, and associated biochemical tr aits were examined. LNT initially reduced PN with a parallel decline in the activities of ribulose-1,5-bisphosphate carboxylase, fructose bisphosphata se, and sucrose phosphate synthase for 20-30 d. Later, LNT enhanced PN and the activities of photosynthetic enzymes. Associated with high PN in LNT-tr eated guayule plants was a two-fold increase in rubber content and rubber t ransferase activity per unit of protein. The initial decrease in PN in LNT- treated guayule was associated with low content of chlorophyll (a+b), large starch accumulation, and higher ratio of glucosed-6-phosphate/fructose-6 - phosphate. Photosystem 2 activity in isolated chloroplasts was initially de creased, but increased after 30 d. There was a significant increase in the leaf soluble protein content in LNT-treated plants. Hence the photosyntheti c performance of plants grown at 15 degreesC night temperature for 50 d was superior to those grown under natural photoperiod in all parameters studie d. The high photosynthetic capacity may contribute to superior rubber yield s under LNT.