Environmental Influences on Photosynthesis and Carbon Export in Greenhouse Roses during Development of the Flowering Shoot

Abstract

Photosynthesis and concurrent export rates of expanded leaves on the flowering shoot of Rosa hybrida L. `Samantha' were measured at three stages of shoot and flower bud development. At 35 and 90 Pa CO2 photosynthesis and concurrent export rates of the upper expanded leaves were greater at Stage 3 (i.e., when petal color of the flower bud was visible) than at the two earlier stages of shoot and flower development. The optimum for leaf photosynthesis and concurrent export at ambient CO2 and saturating irradiance were ≈25 °C. Export was more sensitive to increased temperature than was carbon fixation. For example, at 40 °C photosynthesis was 40% lower while the export rate during photosynthesis was reduced by 80%. Increasing the photon fluence flux rate from 200 to 1000 μmol·m-2·s-1 PAR increased the photosynthetic rate and the concurrent export rate at 35 and 90 Pa CO2, but the increase in export was proportionally greater than that of photosynthesis. At 35 Pa CO2, the rate of C export during photosynthesis increased from 31 to 59% of the concurrent C fixation rate. At 90 Pa CO2, export during photosynthesis increased from 38 to 62% of the photosynthesis rate. The importance of irradiance on translocation processes was further demonstrated by comparing the disappearance of label during the feed period and during an extended night period. Plants grown at each CO2 level exported about three times as much of the 14C fixed during a 2-hour feed period in the light as during a subsequent 15-hour dark chase period. The nighttime export and respiration rates of leaves which had been exposed to elevated CO2 levels during the feed were higher than those rates observed at ambient CO2. However, at the end of the chase period, the leaves of plants which had been exposed to CO2 enrichment during the feed also retained more 14C than did the leaves of the plants which were at ambient CO2. Thus, although more 14C was fixed and exported under high CO2, the same proportion of labelled assimilates were exported, respired, and retained in the dark as at ambient CO2.