Non-stomatal limitations of photosynthesis in grassland species under artificial drought in the field

Abstract

As drought stress is expected to occur more frequently in future climate in central Europe, survival and productivity of grassland species are an important issue. Non-stomatal limitation processes related to the drought-stress inhibition of photosynthesis of selected grassland species were analysed at three locations using leaf gas exchange and chlorophyll fluorescence. The effect of an artificial drought on the non-stomatal limitations differed considerably between species present in the same grassland plot. The maximum efficiency of photosystem II (F_v/F_m), indicator for the intactness of the photosynthetic electron transport, showed only small differences under drought. On the other hand, more pronounced effects were observed for the carboxylation velocity of Rubisco (V_c,max). V_c,max was in Phleum pratense about 20% lower under drought than in control plants, while other species in the same plot were far less affected. The carboxylation velocity of Rubisco is highly sensitive to water deficit and might represent a tool to evaluate the drought response of various species in order to address the performance of grasslands.

Introduction

From recent investigations it can be concluded that extreme events (e.g. heat waves, extreme drought periods) will become more relevant in Central Europe during the next decades (Schär et al., 2004). It was previously demonstrated that the feedbacks between the land surface and the atmosphere were predominantly the cause of the increase in the variability of summer temperature (Seneviratne et al., 2006). Thus, a better understanding of the mechanisms involved in the interactions between land and atmosphere seems to be crucial in the future. Drought stress is an important environmental factor inhibiting plant growth and reducing yield worldwide (Li et al., 2000, Li and Wang, 2003, Zhang et al., 2005). Since less precipitation is expected to be available for plants in Switzerland during the next decades, drought must be considered as one of the main environmental factors limiting plant productivity in future (Boyer, 1982). Several studies have shown previously that photosynthesis is one of the first physiological processes affected under drought (Lawlor, 1995, Munns, 2002). Nevertheless, it is still unclear and under debate whether water deficit affects plant photosynthesis over stomatal limitation (Sharkey, 1990, Chaves, 1991, Loreto et al., 2003, Reynolds-Henne et al., 2010) or over metabolic alterations (Boyer, 1976, Lawlor, 1995, Cornic and Massacci, 1996). But, at different levels of water stress, the part of stomatal or non-stomatal limitation to photosynthesis seems to vary regarding the species analysed (Chaves et al., 2002). However, recent studies have been conducted to extend the knowledge and the understanding of the plant response to water stress and its effect on photosynthetic parameters (Flexas and Medrano, 2002, Lawlor and Cornic, 2002, Chaves et al., 2003). It seems now that stomatal limitations could largely reduce CO₂ assimilation under mild to moderate drought stress, whereas non-stomatal limitation could account for a larger part under more severe drought (Flexas et al., 2004, Grassi and Magnani, 2005). Thus, studies allowing more detailed insights into the mechanisms responsible for the capacity of plants to maintain growth and productivity under drought stress are needed. It has been shown in some studies that overcoming stomatal limitation with high CO₂ concentration was not sufficient to fully restore maximum capacity of photosynthesis under drought, showing that non-stomatal processes were still involved (Tezara et al., 1999, Lawlor, 2002). Non-stomatal limitations include diffusive resistance at the leaf level, with mesophyll and chloroplast conductance for CO₂ (Roupsard et al., 1996, Flexas and Medrano, 2002) and also metabolic impairments. One of the most frequently used techniques to highlight non-stomatal processes is to identify the key biochemical limitation to photosynthesis. Such limitations can be determined from the response of the leaf net assimilation rate (A_n) to sub-stomatal CO₂ concentration (C_i) (A_n–C_i curve). Thus, stomatal and metabolic limitations can be derived from these A–C_i curves (Tezara et al., 2002). But it is still unclear whether these determinations of metabolic limitation to photosynthesis are acceptable under drought, because two problems remain for the C_i calculation: the irregular stomatal closure over the whole surface of the leaf (Laisk, 1983, Buckley et al., 1997) and the variation in the cuticular conductance at different ranges of vapor pressure (Boyer et al., 1997). Furthermore, changes in the mesophyll conductance to CO₂ have been also described under drought leading to a wrong interpretation of the A–C_i curves in this context (Flexas and Medrano, 2002, Centritto et al., 2003, Flexas et al., 2004). However, a novel “curve-fitting” method was introduced recently to allow the calculation of non-stomatal limitation parameters using A_n–C_i curves (Sharkey et al., 2007). In this context, only a few studies focused on grasslands (Galmes et al., 2007).

Although stomatal conductance is a major factor influencing photosynthetic performance under drought stress, non-stomatal limitations may considerably reduce CO₂ assimilation. The work reported here was based on the hypothesis that non-stomatal limitations may be relevant for the overall performance of grassland species in the field.

To identify non-stomatal restrictions, changes in the maximum efficiency of photosystem II (F_v/F_m), a “sensitive indicator of plant photosynthetic performance” (Maxwell and Johnson, 2000), as well as changes in the maximum velocity of Rubisco for carboxylation (V_c,max) and maximum carboxylation rate at high CO₂ (A_max) were analysed in grassland species subjected to an artificial drought.