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 CO(2) 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 CO(2) 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 CO(2) concentration in chloroplasts of stressed leaves to be considerably lower than the intercellular CO(2) concentration. Measurements based on the photosynthetic response to chloroplast CO(2) often confirm that the photosynthetic capacity is preserved but photosynthesis is limited by diffusive resistances in drought and salt-stressed leaves.

Photosynthesis is particularly sensitive to heat stress and recent results provide important new insights into the mechanisms by which moderate heat stress reduces photosynthetic capacity. Perhaps most surprising is that there is little or no damage to photosystem II as a result of moderate heat stress even though moderate heat stress can reduce the photosynthetic rate to near zero. Moderate heat stress can stimulate dark reduction of plastoquinone and cyclic electron flow in the light. In addition, moderate heat stress may increase thylakoid leakiness. At the same time, rubisco deactivates at moderately high temperature. Relationships between effects of moderate heat on rubisco activation and thylakoid reactions are not yet clear. Reactive oxygen species such as H 2 O 2 may also be important during moderate heat stress. Rubisco can make hydrogen peroxide as a result of oxygenase side reactions and H 2 O 2 production by rubisco was recently shown to increase substantially with temperature. The ability to withstand moderately high temperature can be improved by altering thylakoid lipid composition or by supplying isoprene. In my opinion this indicates that thylakoid reactions are important during moderate heat stress. The deactivation of rubisco at moderately high temperature could be a parallel deleterious effect or a regulatory response to limit damage to thylakoid reactions.

Transitory starch is stored during the day inside chloroplasts and broken down at night for export. Maltose is the primary form of carbon export from chloroplasts at night. We investigated the influence of daylength and circadian rhythms on starch degradation and maltose metabolism. Starch breakdown was faster in plants of Arabidopsis (Arabidopsis thaliana) ecotype Wassilewskija growing in long days. Transcript levels of genes encoding enzymes involved in starch degradation and maltose metabolism showed a strong diurnal rhythm. Under altered photoperiods, the transcript levels and the rate of starch degradation changed within one day/night cycle. However, the amount of proteins involved in starch degradation was maintained relatively constant throughout the day/night cycle. To investigate whether the diurnal cycling of the transcript levels is only a response to light or is also regulated by a circadian clock, we measured the amount of messenger RNAs in Arabidopsis leaves under continuous light and continuous darkness. The expression of genes encoding starch degradationrelated enzymes was under very strong circadian control in continuous light. Under continuous light, the amount of maltose also showed a strong endogenous rhythm close to 24 h, indicating that maltose metabolism is under circadian control. Light is necessary for the cycling of transcript levels and maltose levels. Under continuous darkness, these genes were barely expressed, and no cycling of maltose levels was observed.Starch is the most abundant carbohydrate reserve in plants. There are two types of starch: storage starch and transitory starch. Transitory starch is stored during the day inside chloroplasts and broken down at night for export. At night, starch is converted to maltodextrin by several enzymes, such as debranching enzyme, and appears to be influenced by a glucan, water dikinase (GWD) and phosphoglucan, water dikinase (PWD) (Ritte et al., 2000(Ritte et al., , 2002Trethewey and Smith, 2000;Yu et al., 2001;Smith et al., 2003;Zeeman et al., 2004;Kotting et al., 2005). a-Amylase (AtAMY3) was thought to be involved in the conversion of starch to maltodextrin (Trethewey and Smith, 2000), but recent data indicate that AtAMY is not required for transitory starch breakdown in Arabidopsis (Arabidopsis thaliana) leaves (Yu et al., 2005). Maltodextrin is then converted to maltose and Glc by b-amylase (CT-BMY) and disproportionating enzyme (DPE1) in the chloroplast (Lao et al., 1999;Critchley et al., 2001;Scheidig et al., 2002;Schneider et al., 2002). Recent evidence indicates that maltose and Glc are the two major forms of carbon exported from chloroplasts at night (Weber et al., 2000;Servaites and Geiger, 2002;Weise et al., 2004). Maltose is exported by the maltose transporter MEX1 (Niittylä et al., 2004) and is metabolized in the cytosol by several enzymes, including disproportionating enzyme (DPE2) and possibly glucan phosphorylase (AtPHS2) Lu and Sharkey, 2004;Schupp and Ziegler, 2004). Localization of DPE2 in the literature is conflicting. ...

Restrictions to photosynthesis can limit plant growth at high temperature in a variety of ways. In addition to increasing photorespiration, moderately high temperatures (35-42 ∞ ∞ ∞ ∞ C) can cause direct injury to the photosynthetic apparatus. Both carbon metabolism and thylakoid reactions have been suggested as the primary site of injury at these temperatures. In the present study this issue was addressed by first characterizing leaf temperature dynamics in Pima cotton ( Gossypium barbadense ) grown under irrigation in the US desert south-west. It was found that cotton leaves repeatedly reached temperatures above 40 ∞ ∞ ∞ ∞ C and could fluctuate as much as 8 or 10 ∞ ∞ ∞ ∞ C in a matter of seconds. Laboratory studies revealed a maximum photosynthetic rate at 30-33 ∞ ∞ ∞ ∞ C that declined by 22% at 45 ∞ ∞ ∞ ∞ C. The majority of the inhibition persisted upon return to 30 ∞ ∞ ∞ ∞ C. The mechanism of this limitation was assessed by measuring the response of photosynthesis to CO 2 in the laboratory. The first time a cotton leaf (grown at 30 ∞ ∞ ∞ ∞ C) was exposed to 45 ∞ ∞ ∞ ∞ C, photosynthetic electron transport was stimulated (at high CO 2 ) because of an increased flux through the photorespiratory pathway. However, upon cooling back to 30 ∞ ∞ ∞ ∞ C, photosynthetic electron transport was inhibited and fell substantially below the level measured before the heat treatment. In the field, the response of assimilation ( A ) to various internal levels of CO 2 ( C i ) revealed that photosynthesis was limited by ribulose-1,5-bisphosphate (RuBP) regeneration at normal levels of CO 2 (presumably because of limitations in thylakoid reactions needed to support RuBP regeneration). There was no evidence of a ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) limitation at air levels of CO 2 and at no point on any of 30 A -C i curves measured on leaves at temperatures from 28 to 39 ∞ ∞ ∞ ∞ C was RuBP regeneration capacity measured to be in substantial excess of the capacity of Rubisco to use RuBP. It is therefore concluded that photosynthesis in field-grown Pima cotton leaves is functionally limited by photosynthetic electron transport and RuBP regeneration capacity, not Rubisco activity.

Isoprene-emitting plants lose a large portion of their assimilated C as isoprene. Because isoprene synthesis can be regulated, it has been assumed that isoprene benefits the plant. Since the rate of isoprene emission from leaves is highly responsive to temperature, we hypothesized that isoprene benefits plants by increasing their thermotolerance. We used three methods to measure isopreneinduced thermotolerance in leaves. Each technique assayed thermotolerance under conditions that suppressed endogenous isoprene isoprerie. None of the experiments was designed to determine the mechanism of thermotolerance, but we theorize that isoprene functions by enhancing hydrophobic interactions in membranes.