Drought constraints on C4 photosynthesis: stomatal and metabolic limitations in C3 and C4 subspecies of Alloteropsis semialata

Abstract

The C4 photosynthetic pathway uses water more efficiently than the C3 type, yet biogeographical analyses show a decline in C4 species relative to C3 species with decreasing rainfall. To investigate this paradox, the hypothesis that the C4 advantage over C3 photosynthesis is diminished by drought was tested, and the underlying stomatal and metabolic mechanisms of this response determined. The effects of drought and high evaporative demand on leaf gas exchange and photosynthetic electron sinks in C3 and C4 subspecies of the grass Alloteropsis semialata were examined. Plant responses to climatic variation and soil drought were investigated using a common garden experiment with well-watered and natural rainfall treatments, and underlying mechanisms analysed using controlled drying pot experiments. Photosynthetic rates were significantly higher in the C4 than the C3 subspecies in the garden experiment under well-watered conditions, but this advantage was completely lost during a rainless period when unwatered plants experienced severe drought. Controlled drying experiments showed that this loss was caused by a greater increase in metabolic, rather than stomatal, limitations in C4 than in the C3 leaves. Decreases in CO2 assimilation resulted in lower electron transport rates and decreased photochemical efficiency under drought conditions, rather than increased electron transport to alternative sinks. These findings suggest that the high metabolic sensitivity of photosynthesis to severe drought seen previously in several C4 grass species may be an inherent characteristic of the C4 pathway. The mechanism may explain the paradox of why C4 species decline in arid environments despite high water-use efficiency.

The Responses of Plant Leaf CO2/H2O Exchange and Water Use Efficiency to Drought: A Meta-Analysis

“…This suggested that C 4 plants have higher carbon sequestration under drought conditions. These results are consistent with previous research that C 4 plants have an advantages in Pn in drought conditions compared with C 3 plants [30,44,45] because the C 4 CO 2 -concentrating mechanism offers C 4 photosynthesis a greater buffering capacity against CO 2 shortages due to partial stomatal closure under water stress [46]. However, drought increased C 3 WUE overall, but tended to decrease C 4 WUE (Figure 1).…”

Baixa temperatura noturna e deficiência hídrica na fotossíntese de cana‑de‑açúcar

“…Este mecanismo concentra CO 2 próximo à enzima Rubisco, o que permite sua maior atividade como carboxilase e resulta em maior eficiência do uso de água e de nitrogênio e na ausência de fotorrespiração (Sage & Kubien, 2007). Dessa forma, a L M da fotossíntese pode estar relacionada a alterações na atividade enzimática da PEPcase e da Rubisco, com regeneração dos substratos das enzimas e transporte de elétrons (Ripley et al, 2007;Carmo-Silva et al, 2008;Ghannoum, 2009;Soares-Cordeiro et al, 2010). A eficiência aparente de carboxilação da PEPcase (V pmáx ) foi menor do que a da testemunha, no primeiro dia, em todos os tratamentos.…”

Reduced plant water status under sub-ambientpCO2limits plant productivity in the wild progenitors of C3and C4cereals

“…Our results could therefore be a reflection of direct effects of leaf water status on A mediated via non-stomatal (metabolic) limitations that are independent of P i (Lawlor & Tezara, 2009). Furthermore, it has been demonstrated that C 4 plants are sensitive to non-stomatal limitations mediated via plant water status, and the effect was greater than in C 3 leaves (Ripley et al, 2007;Taylor et al, 2011). Alternatively the variance introduced into the calculation of P i from both H 2 O and CO 2 flux measurements could have diminished the power of statistical analyses for P i .…”