Direct and indirect effects of light on stomata II. In Commelina communis L.

Abstract. Two experiments are described which test the normal correlations that arise between stomatal conductance, net CO₂ assimilation rate, and intercellular CO₂ concentration (C_i), using whole shoots of Commelina communis L. In the first, conductance increased with decreasing C_i, at four different quantum flux densities, such that there was no unique relationship between conductance and quantum flux density or C_i, In the second, conductance increased hyperbolically with increasing quantum flux density while C_i was held constant at 466, 302, and 46 μmiolmol⁻¹, and the response differed at each C_i. In neither experiment was conductance consistently related to net CO₂ assimilation rate in the mesophyll. In both experiments high C_i suppressed the response of conductance to light, while there was a large response of conductance to light at low C_i, indicating an interaction between the effects of light and CO₂ on stomata. The results show that the parallel responses of assimilation and conductance to light result in constant intercellular CO₂ concentrations, and not that stomata maintain a 'constant C_i'.     

Canopy development and tillering of field-grown crops of two contrasting cultivars of winter wheat (Triticum aestivum) in response to CO2 and temperature

Elevated CO₂ (691 cf. 371 /miol CO₂ mol^-1 air) and warmer temperatures (over the range 1.0^UC below to 1.6^oC above ambient) increased light interception by crops of two contrasting cultivars (Hereward and Soissons) of winter wheat (Triticum aestivum L.) during winter growth in the field. The fractional interception of light by the canopy increased more rapidly initially in Soissons than in Hereward, but Hereward showed a much greater response to CO₂ (35% increase in Hereward but only 7% in Soissons) at 500^oCd after sowing. By terminal spikelet formation, in contrast, fractional interception was greater in Hereward than in Soissons, while the effect of CO₂ was the same in both cultivars (9%). Thus, although differences in the relative response of canopy development to CO₂ were detected between cultivars initially, differences were negligible during later development. The greater interception of light by the canopy in elevated CO₂, at any one temperature, resulted from increased tillering. The number of tillers plant“‘ at terminal spikelet was a linear function of main stem dry mass at this developmental stage but with a greater response in elevated CO₂, viz 2.3 and 3.8 tillers g^-1 main stem dry mass at 371 and 691 /μmol CO₂ mol^-1 air, respectively; these relations were unaffected by cultivar.     

Acclimation of photosynthesis to elevated CO2 in onion (Allium cepa) grown at a range of temperatures

Onion (Allium cepa) was grown in the field within temperature gradient tunnels (providing about ‐2.5°C to +2.5°C from outside temperatures) maintained at either 374 or 532 μmol mol^?1 CO₂. Plant leaf area was determined non‐destructively at 7 day intervals until the time of bulbing in 12 combinations of temperature and CO₂ concentration. Gas exchange was measured in each plot at the time of bulbing, and the carbohydrate content of the leaf (source) and bulb (sink) was determined. Maximum rate of leaf area expansion increased with mean temperature. Leaf area duration and maximum rate of leaf area expansion were not significantly affected by CO₂. The light‐saturated rates of leaf photosynthesis (A_sat) were greater in plants grown at normal than at elevated CO₂ concentrations at the same measurement CO₂ concentration. Acclimation of photosynthesis decreased with an increase in growth temperature, and with an increase in leaf nitrogen content at elevated CO₂. The ratio of intercellular to atmospheric CO₂ (C₁/C₃ ratio) was 7.4% less for plants grown at elevated compared with normal CO₂. A_sat in plants grown at elevated CO₂ was less than in plants grown at normal CO₂ when compared at the same C₁. Hence, acclimation of photosynthesis was due both to stomatal acclimation and to limitations to biochemical CO₂ fixation. Carbohydrate content of the onion bulbs was greater at elevated than at normal CO₂. In contrast, carbohydrate content was less at elevated compared with normal CO₂ in the leaf sections in which CO₂ exchange was measured at the same developmental stage. Therefore, acclimation of photosynthesis in fully expanded onion leaves was detected despite the absence of localised carbohydrate accumulation in these field‐grown crops.     

Temperature gradient chambers for research on global environment change. II. A twin-wall tunnel system for low-stature, field-grown crops using a split heat pump

A temperature gradient chamber (TGC) is described which enables elevated CO₂ concentrations and a dynamic temperature gradient to be imposed on field crops throughout their life cycle under standard husbandry. Air is circulated through two double-walled polyethylene-covered tunnels connected to a split heat pump system to give a near-linear temperature gradient along each tunnel. Solar energy gain along each tunnel and exchange with outer tunnel air flow contribute to the temperature gradient and also produce diurnal and seasonal temperature fluctuations corresponding to ambient conditions. Mean temperature gradients of between 3 and 5°C have been recorded throughout the growing seasons of crops of lettuce, carrot, cauliflower and winter wheat. Elevated or present CO₂ concentrations are maintained in each of two pairs of tunnels throughout the cropping season using pure CO₂ injected through motorized needle valves. This system can realistically simulate aspects of the effects of projected future environmental change on crop growth, development and yield, and in particular tin-possible interaction of the effects of increased CO₂ and temperature.     

Direct and indirect effects of light on stomata. I. In Scots pine and Sitka spruce

Abstract. The response of stomatal conductance to broadband blue and red light was measured in whole shoots of Scots pine and Sitka spruce, two species which have low stomatal sensitivity to CO². In Scots pine, blue light was more than three times more effective than red light (on an incident quantum basis) in opening stomata, particularly at low quantum flux densities (<100μmiol m⁻² s⁻¹). However, the apparent quantum yield of net CO² assimilation rate in blue light was only half that in red light. The contrasting effects of red and blue light on conductance and assimilation led to higher intercellular CO² concentrations (C_i) in blue light (up to 100 μmol mol⁻¹ higher) than in red light. Similar results were obtained with Sitka spruce shoots, though differences in the effectiveness of red and blue light were less marked. In both species, both red and blue light increased conductance in normal and CO²-free air, indicating that neither red nor blue light exert effects through changes in C_i or mesophyll assimilation. However, decreases in C_i caused increases in conductance in both red and blue light, suggesting that these direct effects of light are not wholly independent of CO².