Photosynthesis and water relations of well-watered orange plants as affected by winter and summer conditions

Abstract

The aim of this study was to evaluate how the summer and winter conditions affect the photosynthesis and water relations of well-watered orange trees, considering the diurnal changes in leaf gas exchange, chlorophyll (Chl) fluorescence, and leaf water potential (Ψ) of potted-plants growing in a subtropical climate. The diurnal pattern of photosynthesis in young citrus trees was not significantly affected by the environmental changes when compared the summer and winter seasons. However, citrus plants showed higher photosynthetic performance in summer, when plants fixed 2.9 times more CO 2 during the diurnal period than in the winter season. Curiously, the winter conditions were more favorable to photosynthesis of citrus plants, when considering the air temperature (< 29 ºC), leaf-to-air vapor pressure difference (< 2.4 kPa) and photon flux density (maximum values near light saturation) during the diurnal period. Therefore, low night temperature was the main environmental element changing the photosynthetic performance and water relations of well-watered plants during winter. Lower whole-plant hydraulic conductance, lower shoot hydration and lower stomatal conductance were noticed during winter when compared to the summer season. In winter, higher ratio between the apparent electron transport rate and leaf CO 2 assimilation was verified in afternoon, indicating reduction in electron use efficiency by photosynthesis. The high radiation loading in the summer season did not impair the citrus photochemistry, being photoprotective mechanisms active. Such mechanisms were related to increases in the heat dissipation of excessive light energy at the PSII level and to other metabolic processes consuming electrons, which impede the citrus photoinhibition under high light conditions.

Salt stress induced damages on the photosynthesis of physic nut young plants

“…Impairments in photochemistry of plants exposed to salinity may be related to possible damages in primary electron acceptors, such as plastoquinone pool (Foyer and Noctor, 2000). In addition, the continuous increase of NPQ during stress and recovery treatments suggest the activation of a protective mechanism for dissipation of excessive energy not used in photochemical reactions (Ribeiro et al, 2009;Rohácek, 2002). However, this protective defense was not sufficient to avoid oxidative and irreversible damage after 14-d of treatment, as suggested by the toxicity symptoms in leaves (Figure 1).…”

“…This difference was maintained even after the recovery period (Figure 4). The increase in ETR/A ratio represents an imbalance between the electron flow and the CO 2 assimilation during photosynthesis, which is frequently associated with increases in oxygenase activity of Rubisco and might represent an electron flow to other physiological processes rather than to CO 2 assimilation reactions (Baker et al, 2007;Ribeiro et al, 2009). Thus, the occurrence of increases in ETR/A and decreases in A/Ci might indicate a loss of photosynthetic efficiency in physic nut plants under salinity, especially in the ionic phase of salt stress.…”

“…Fo' is the basal fluorescence yield measured after PSI excitation by far-red light. The ratio ETR/A was calculated to estimate the use of electrons in other processes not related to the photosynthetic CO 2 assimilation rate (Ribeiro et al, 2009). Therefore, an increase in ETR/A indicates that more electrons are driven to other sinks (e.g., photorespiration, nitrogen metabolism or Mehler reaction), suggesting a stressful condition.…”

Brassinosteroids improve photosystem II efficiency, gas exchange, antioxidant enzymes and growth of cowpea plants exposed to water deficit

“…EBR are related to lower non-photochemical energy in the form of heat (Ribeiro et al 2009). In addition, less quenching, mainly through photorespiration and secondary metabolites, such as the photoreduction of O 2 to O 2 - Barbosa et al 2014) probably occurred in plants exposed to EBR.…”

Irrigation frequency and substrate volume effects in the growth and yield of tomato plants under greenhouse conditions

“…A and E values were integrated for the period between 10h00 and 16h00, leading to values of CO 2 assimilation (Ai) and transpiration (Ei) over a 6 h period. Water use efficiency (WUEi) was calculated by the Ai/Ei ratio, as also made by Silva et al (2005) and Ribeiro et al (2009).…”