In the interest of genetic engineering and agricultural applications, the authors analyze the relative importance of genes that control both metabolic and light reactions as well as the structure, arrangement, and orientation of photosynthesis.
This pattern of metabolic changes supports the assertion by ) that stomatal closure is the primary cause of the reduction in photosynthetic rate under mild drought, but shows that metabolic damage or down‐regulation—this analysis cannot distinguish between them—is progressive and commences with small changes in g under mild drought. In particular, decreased ATP content, implying impaired synthesis [and thus supporting the observations of ) and ) of impaired photophosphorylation and loss of ATP synthase, respectively] is important. To our knowledge, only one reference () reported no inhibition of ATPase under mild to moderate drought. A major consequence of loss of ATP would be limited RuBP regeneration under mild drought, shown clearly as an early effect of drought by our analysis. Nevertheless, despite the decreased capacity of these metabolic processes, decreased Ci confirms the predominance of stomatal limitation in restricting photosynthetic rate in the early phase of water loss. However, the metabolic changes are responsible for loss of photosynthetic potential during this phase ().
Metabolism/Photosynthesis Flashcards | Quizlet
N2 - Drought and salinity are two widespread environmental conditions leading to low water availability for plants. Low water availability is considered the main environmental factor limiting photosynthesis and, consequently, plant growth and yield worldwide. There has been a long-standing controversy as to whether drought and salt stresses mainly limit photosynthesis through diffusive resistances or by metabolic impairment. Reviewing in vitro and in vivo measurements, it is concluded that salt and drought stress predominantly affect diffusion of CO2 in the leaves through a decrease of stomatal and mesophyll conductances, but not the biochemical capacity to assimilate CO 2, at mild to rather severe stress levels. The general failure of metabolism observed at more severe stress suggests the occurrence of secondary oxidative stresses, particularly under high-light conditions. Estimates of photosynthetic limitations based on the photosynthetic response to intercellular CO2 may lead to artefactual conclusions, even if patchy stomatal closure and the relative increase of cuticular conductance are taken into account, as decreasing mesophyll conductance can cause the CO2 concentration in chloroplasts of stressed leaves to be considerably lower than the intercellular CO2 concentration. Measurements based on the photosynthetic response to chloroplast CO2 often confirm that the photosynthetic capacity is preserved but photosynthesis is limited by diffusive resistances in drought and salt-stressed leaves.
Plant Energy Transformations-Photosynthesis - …
Comparing results from different authors is difficult due to interspecific differences in the response of photosynthesis to leaf water potential and/or relative water content (RWC), the parameters most commonly used to assess the degree of drought (). To overcome this, we have exploited the relationship between stomatal conductance (g) and photosynthetic CO2 assimilation (), since an early and progressive effect of drought is stomatal closure (; ; ; ; ; ). We have recently demonstrated (; ) that, by relating photosynthetic parameters to their corresponding light‐saturated g, a pattern of responses is revealed which is independent of acclimation processes and only slightly dependent on the cultivars and species. For instance, the relationships between different photosynthetic parameters and the absolute values of g in grapevines (Vitis vinifera) and several Mediterranean sclerophyll shrubs were very similar. This applied even when maximum g reached approx. 500 mmol H2O m–2 s–1 in grapevines, and only 200 mmol H2O m–2 s–1 in sclerophyll shrubs (). The relationship between different photosynthetic parameters and g was not observed with relative water content or leaf water potential, i.e. decreased photosynthesis caused by drought occurred at different leaf water status in different species, albeit at similar stomatal conductance. Based on these previous findings and using data from the literature, we have analysed at what values of g—and thus at different severity of drought—some photosynthetic metabolic processes are impaired.