Photosynthetic Induction State of Leaves in a Soybean Canopy in Relation to Light Regulation of Ribulose-1-5-Bisphosphate Carboxylase and Stomatal Conductance

Abstract

Photosynthetic induction state, stomatal conductance and light regulation of ribulose-1,5-bisphosphate carboxylase (rubisco) were examined for leaves in a mature, closed soybean (Glycine max) canopy (leaf area index approximately 5) with the objective to determine the extent to which these factors may be limiting the capacity to respond to light transients during sunflecks (17,19). After long periods in low light a photosynthetic induction requirement can limit the capacity of leaves to respond to sunflecks (3). However, during short sunflecks, postillumination CO2 fixation can contribute substantially to assimilation (4). The objective of this study was to examine the extent to which the induction requirement of CO2 assimilation limited the capacity of leaves within a soybean canopy to respond to transient light increases. Since the induction response is primarily determined by light modulation of rubisco activity and the light-mediated increase in stomatal conductance (10, 23), variations in these parameters within the canopy were examined. MATERIALS AND METHODS Plant MaterialPlots of soybeans (Glycine max var Williams) were planted in rows spaced 75 cm apart and oriented east-west. Three 5 by 20 m plots were planted on May 15, June 15, and July 2, 1988. The plots were irrigated two to three times weekly and were fertilized with a commercial N:P:K (20:10:10) fertilizer at the time of planting and 3 weeks later for a total amount of 200 kg ha-'. Measurements were started after the canopies were fully developed to a height of 1 to 1.2 m and plants were in the flowering stage. Photosynthesis MeasurementsA and g, were measured on leaves at about midpoint in the canopy with a transportable, open-system gas exchange apparatus that has been described previously (16,21). A single attached leaflet was enclosed in a temperature controlled, nickel-plated aluminum chamber with a gas window in the top. Fans within the chamber kept the air well mixed and gave a high boundary layer conductance for water vapor of 2 mol m-2 s-'.

The role of enzyme activation state in limiting carbon assimilation under variable light conditions

“…4), but the total lightfleck assimilation was enhanced by PICF, with a resultant net increase in LUE (Fig. 3) (Pearcy 1990). The PICF became increasingly significant at lower lightfleck frequencies as shown by the calculated individual lightfleck assimilation (At.)…”

“…Periodic shading limits net carbon uptake of Leaves often experience fluctuating PFD regimes leaves within canopies upon subsequent reillumination because of the pattern of sun and shade cast by othat high PFD due to decay of the photosynthetic inducer leaves. A significant proportion of the total light tion state (Pearcy and Seemann 1990). While this loss received by leaves within canopies is in the form of induction reduces carbon assimilation, part of it can of sunflecks (brief high light periods) (Pearcy et al be regained by post-illumination CO2 fixation (PICF) 1990).…”

“…PICF increases the efficiency of utilization of short duration lightflecks ( 1-10 s), during which it may constitute a majority of the carbon gain (Pearcy 1990). Sharkey et al (1986) showed that utilization of carbon reduction cycle intermediate pools that are built up during the lightfleck itself are responsible for PICE The PICF arises due to the continued activity of the first step of carbon reduction, rubisco.…”

A interação ecofisiológica planta-ambiente

“…As previously reported, the time required to reach full photosynthetic induction depends in part on the length of the period and PFD of low irradiance (PONS et al, 1992;WHITEHEAD & TESKEY, 1995). The IS state of a leaf is determined by the immediate past irradiance (PEARCY & SEEMANN, 1990) and declines when an induced leaf is shaded longer than a minute or so, and can be increased by exposure of a shaded leaf to a series of sunflecks (PEARCY, 1989).…”

“…As previously reported, the time required to reach full photosynthetic induction depends in part on the length of the period and PFD of low irradiance (PONS et al, 1992; WHITEHEAD & TESKEY, 1995). The IS state of a leaf is determined by the immediate past irradiance (PEARCY & SEEMANN, 1990) and declines when an induced leaf is shaded longer than a minute or so, and can be increased by exposure of a shaded leaf to a series of sunflecks (PEARCY, 1989).Differences in T 50 and T 90 among plants exposed to 10 and 30 min of darkness were higher in the gap than in the understorey (Table 1) After 10 and 30 min of darkness the differences between ecological groupswere not clearly detected in the gap for T 50 and T 90 , indicating that eco-physiological characteristics of each ecological group did not influence the IS time of the species evaluated herein. On the other hand, after both darkness times, the LS-species showed lower T 50 in the understorey indicating a faster IS.…”