Whole Plant and Leaf Steady State Gas Exchange during Ethylene Exposure in Xanthium strumarium L.

Abstract

The effects of ethylene evolved from ethephon on leaf and whole plant photosynthesis in Xanthium strumarium L. were examined. Ethylene-induced epinasty reduced light interception by the leaves of ethephon treated plants by up to 60%. Gas exchange values of individual, attached leaves under identical assay conditions were not inhibited even after 36 hours of ethylene exposure, although treated leaves required a longer induction period to achieve steady state photosynthesis. The speed of translocation of recently fixed "C-assimilate movement was not seriously impaired following ethephon treatment; however, a greater proportion of the assimilate was partitioned downward toward the roots. Within 24 hours of ethephon treatment, the whole plant net carbon exchange rate expressed on a per plant basis or a leaf area basis had dropped by 35%. The apparent inhibition of net carbon exchange rate was reversed by physically repositioning the leaves with respect to the light source. Ethylene exposure also inhibited expansion of young leaves which was partially reversed when the leaves were repositioned. The data indicated that ethylene indirectly affected net C gain and plant growth through modification of light interception and altered sink demand without directly inhibiting leaf photosynthesis.The extent to which ethylene can affect photosynthetic CO2 fixation and potentially alter biomass accumulation and yield has been examined in several papers; however, the results have been inconsistent (10,(12)(13)(14)(15)(16)22). Studies ofwhole plant or attached whole leaf gas exchange have indicated that net carbon uptake is reduced in some but not all of the 30 species and cultivars surveyed (10, 12-16, 21, 22 stimulated ethylene release (5). Steady state laminar gas exchange in Zea mays L. and Pisum sativum L. was not inhibited by continuous exposure to ethylene gas (12) and the stomatal aperture of intact leaves was not affected by exposure to 10 ,L L' ethylene (1) even after a 24 h exposure (4).We recently reported (22) that continuous exposure to ethylene derived from ethephon had no effect on steady state photosynthesis and transpiration rates of Lycopersicon esculentum Mill. leaves. The partitioning of recently assimilated '4CO2 into sugar, amino acid, organic acid, and starch pools in the leaf was not altered by ethylene exposure, but during the 24 h following ethephon application there was an alteration in the partitioning of current photosynthate between root and shoot sinks, (e.g. developing flower clusters). Ethephon also caused a rapid epinastic response extending through the petioles and the leaves (22) suggesting that whole plant CO2 exchange (i.e. measured in a whole plant cuvette or a whole leafcuvette enclosing the petiole) could be significantly altered due to the reduction in total light interception (21).We have selected Xanthium strumarium L. as the experimental material for this study in order to avoid potentially anomalous conclusions arising from studies (21) on a single ethylene sensitive species s...

Ethylene Directly Inhibits Foliar Gas Exchange in Glycine max

“…These data support the hypothesis that C2H4 exerts its effects on gas exchange directly. Although Glycine max does respond to exogenous C2H4 with epinastic changes in leaf angle, the hypothesis (17) that these changes in leaf angle account entirely for the reductions in PN is not supported by these studies. In fact, the magnitude of the reduction in gas exchange reported here is comparable to that reported previously for G. max in nonsteady-state single leaf cuvette studies (PN reduced to 69-75% of control, g, reduced to 32-52% of control; 3, 1 1,12).…”

“…They further suggest (16)(17)(18) that reports of direct effects on gas exchange such as we have reported in soybean (3,11,12) have been biased by nonsteady-state conditions and leaf angle changes. They have concluded that reduced light interception and lower photosynthetic induction states of epinastic leaves account for any reported reductions in gas exchange in response to C2H4.…”

“…Other researchers have reported no change in gas exchange and have concluded that for some or all species, gas exchange is unresponsive (6,(16)(17)(18). Interspecific differences in responsiveness to C2H4 seem to explain the discrepancies in many cases; species differences have been reported in several studies (4, 5, 7, 9, 1 1), but differences in technique could also be a factor.…”

“…Interspecific differences in responsiveness to C2H4 seem to explain the discrepancies in many cases; species differences have been reported in several studies (4, 5, 7, 9, 1 1), but differences in technique could also be a factor. For example, it has recently been proposed (16)(17)(18) …”

“…Interspecific differences in responsiveness to C2H4 seem to explain the discrepancies in many cases; species differences have been reported in several studies (4, 5, 7, 9, 1 1), but differences in technique could also be a factor. For example, it has recently been proposed (16)(17)(18)) that C2H4 has no direct effect on PN or gs, but rather that it exerts its effect on gas exchange by causing leaf epinasty and reduced light interception by the leaf surface. For example, Woodrow et al (17) found no changes in gs or PN per unit leaf area in Xanthium strumarium (17) or Lycopersicon esculentum (16) that could not be reversed by returning the epinastic leaves to ' …”