REGULATION OF COTTON PHOTOSYNTHESIS DURING MODERATE CHILLING

This study was initiated to determine the effect of long-term, moderat e chilling at moderate light intensity on cotton (Gossypium hirsutum c v. Coker 312) leaf net photosynthesis (A). Upon exposure of warm-grown plants to 15 degrees C and a photosynthetic photon flux density of si milar to 500 mu mol m(-2) s(-1), A declined 40% in 2 days and then an additional 20% from days 2 to 8, A parallel decline occurred in A meas ured at saturating CO2 and 28 degrees C. Maintaining the roots at 27 d egrees C had no effect on the decline in A. The initial, 40% reduction in A was associated with an 88% reduction in stomatal conductance (g( s)), a 33% reduction in internal CO2 concentration (C-i), and a 29% re duction in the ratio of the variable to maximum chlorophyll fluorescen ce (Fv/Fm), but the in vivo capacities of ribulose 1,5-bisphosphate ca rboxylase (Rubisco) and stromal fructose 1,6-bisphosphatase remained h igh, suggesting that the initial decline in A was primarily a photoinh ibitory response coupled with some restriction by g(s). From days 2 to 8, g(s) and Fv/Fm remained constant, C-i, rose gradually, and the inh ibition of A by ambient O-2 decreased from 25% to 4% implicating an in creasing phosphate limitation to A. We hypothesized that this putative increase in phosphate limitation was the result of a slowing of sucro se and starch synthesis and not increased phosphate sequestration sinc e the total pool of leaf hexose phosphate rose only 11% from day 1, st arch accumulation during the photoperiod declined, and the extractable activity of sucrose phosphate synthase on day 8 was 60% of pre-chilli ng values. After the plants were returned to warm conditions, a 1-day lag in the recovery of A occurred that was associated with a decrease of in vivo Rubisco capacity, some photoinhibition, and a low g(s).