Combining branch-scale COS/CO2 exchange and canopy-scale SIF measurements to disentangle the effects of high VPD and low soil moisture in mature pine trees under field conditions

<p>During the regular seasonal drought conditions in our semi-arid pine forest, soil water content decrease below the 16% threshold of no transpirable soil water availability, and VPD increase to values of >5 kPa. Soil drought in one forest plot was eliminated by using supplemental drip irrigation during summer. We used automatic branch chambers to measure CO<sub>2</sub>/H<sub>2</sub>O exchange together with laser-based COS exchange, and retrieving canopy sun induced fluorescence (SIF) using a high-resolution spectrometer above the canopy that was moved between the control and irrigated plots on a weekly basis. Using these research tools, we investigated the ecophysiological response (including rates of gas exchange, conductances, and photochemical response) of the mature pine trees to the differential effects of soil and atmospheric droughts. Leaf relative uptake (LRU) ratio of COS to CO<sub>2</sub> fluxes was used to constrain estimates of carbon assimilation (An) and changes in the An/gl ratio (where gl is leaf conductance), and leaf COS and H<sub>2</sub>O exchange fluxes were also used to partition leaf conductance. We will report on the first seasonal cycle of COS and CO<sub>2</sub> fluxes, LRU and SIF<sub>A </sub>of mature pine trees under field conditions in the semi-arid forest, which includes the summer dry stress period, the recovery during the transition to the winter wet season, and the spring peak activity period.</p>

Effects of high temperature on photosynthesis and yield in mungbean

Temperature (36°C) at pre-flowering stage of mungbean showed lower leaf conductance than others. Photosynthesis decreased but transpiration rate was not affected by the temperature treatments. Ambient temperature showed the highest yield. Yields at the temperature 36°C at pre-flowering, flowering and grain filling stages were identical.

Analysis of leaf wetting effects on gas exchanges of corn using a whole-plant chamber system

A whole-plant chamber system equipped with a transpiration sap flow meter was developed for measuring the transpiration rate even if leaves are wetted. A preliminary experiment in which dynamics of transpiration rate and/or evaporation rate of wetted and non-wetted plants were measured and compared with each other demonstrated the validity of the measurement system. The system was then used to analyse leaf wetting effects on gas exchange of corn under slight water stress conditions of soil (a volumetric soil water content of 9.7%). Leaf wetting decreased vapour pressure in leaves by decreasing leaf temperature but it increased vapour pressure in the air; therefore, vapour pressure difference between leaves and air, as a driving force of transpiration, was significantly lower in wetted plant. As a result, transpiration rate decreased by 44% and leaf conductance as an index of stomatal aperture was increased by leaf wetting. Such increasing leaf conductance due to leaf wetting increased the photosynthetic rate by 30% and therefore it improved water use efficiency (2.4 times). These results suggest that morning leaf wetting due to night time dew formation may have an advantage in crop production in semi-arid regions.

Impacts of water stress, environment and rootstock on the diurnal behaviour of stem water potential and leaf conductance in pistachio (Pistacia vera L.)

Little information is available on the diurnal behaviour of water potential and leaf conductance on pistachio trees despite their relevance to fine tune irrigation strategies. Mature pistachio trees were subject to simultaneous measurements of stem water potential (Ψx) and leaf conductance (gl) during the day, at three important periods of the irrigation season. Trees were grown on three different rootstocks and water regimes. An initial baseline relating Ψx to air vapor pressure deficit (VPD) is presented for irrigation scheduling in pistachio. Ψx was closely correlated with VPD but with a different fit according to the degree of water stress. No evidence of the variation of Ψx in relation to the phenology of the tree was observed. Furthermore, midday Ψx showed more accuracy to indicate a situation of water stress than predawn water potential. Under well irrigated conditions, gl was positively correlated with VPD during stage II of growth reaching its peak when VPD reached its maximum value (around 4 kPa). This behaviour changed during stage III of fruit growth suggesting a reliance of stomatal behaviour to the phenological stage independently to the tree water status. The levels of water stress reached were translated in a slow recovery of tree water status and leaf conductance (more than 40 days). Regarding rootstocks, P. integerrima showed little adaptation to water shortage compared to the two other rootstocks under the studied conditions.