Photosynthetic Diffusional Constraints Affect Yield in Drought Stressed Rice Cultivars during Flowering

<div><p>Global production of rice (<i>Oryza sativa</i>) grain is limited by water availability and the low ‘leaf-level’ photosynthetic capacity of many cultivars. <i>Oryza sativa</i> is extremely susceptible to water-deficits; therefore, predicted increases in the frequency and duration of drought events, combined with future rises in global temperatures and food demand, necessitate the development of more productive and drought tolerant cultivars. We investigated the underlying physiological, isotopic and morphological responses to water-deficit in seven common varieties of <i>O. sativa</i>, subjected to prolonged drought of varying intensities, for phenotyping purposes in open field conditions. Significant variation was observed in leaf-level photosynthesis rates (<i>A</i>) under both water treatments. Yield and <i>A</i> were influenced by the conductance of the mesophyll layer to CO₂ (<i>g</i>_m) and not by stomatal conductance (<i>g</i>_s). Mesophyll conductance declined during drought to differing extents among the cultivars; those varieties that maintained <i>g</i>_m during water-deficit sustained <i>A</i> and yield to a greater extent. However, the variety with the highest <i>g</i>_m and yield under well-watered conditions (IR55419-04) was distinct from the most effective cultivar under drought (Vandana). Mesophyll conductance most effectively characterises the photosynthetic capacity and yield of <i>O. sativa</i> cultivars under both well-watered and water-deficit conditions; however, the desired attributes of high <i>g</i>_m during optimal growth conditions and the capacity for <i>g</i>_m to remain constant during water-deficit may be mutually exclusive. Nonetheless, future genetic and physiological studies aimed at enhancing <i>O. sativa</i> yield and drought stress tolerance should investigate the biochemistry and morphology of the interface between the sub-stomatal pore and mesophyll layer.</p></div

Measurements of (a) the intercellular [CO₂] (<i>C</i>_i) to the ambient [CO₂] (<i>C</i>_a) ratio (<i>C</i>_i/<i>C</i>_a), and (b) the chloroplastic [CO₂] (<i>C</i>_c) to the ambient [CO₂] ratio (<i>C</i>_c/<i>C</i>_a) in control and water-stressed leaves of the seven <i>Oryza sativa</i> genotypes.

<p>The measurements were made on the flag leaf in saturating PPFD (1400 µmol m⁻²s⁻¹), with relative humidity ranging between 45–55%, and a leaf temperature of 30°C. Data are means of 4 to 7 plants per treatment. Error bars as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109054#pone-0109054-g001" target="_blank">Figure 1</a>. Different letters denote significant differences among means derived using a factorial ANOVA and Tukey <i>post-hoc</i> test.</p

Carbon isotope discrimination (‰) of the flag leaf pellet (bulk) and soluble sugars of seven rice cultivars grown under drought and well-watered conditions in ‰.

<p>Values indicate the mean of six plants; ± indicates the standard error of mean; different letters indicate significant differences (P≤0.05) among means according to an ANOVA.</p><p>Carbon isotope discrimination (‰) of the flag leaf pellet (bulk) and soluble sugars of seven rice cultivars grown under drought and well-watered conditions in ‰.</p

Changes in yield and photosynthesis in relation to modification of diffusive resistances to CO₂ uptake following water-stress.

<p>Those varieties that experienced smaller reductions in parameters were more tolerant of drought. a) relationship between Δyield and Δ<i>g</i>_s (linear regression: R² = 0.337; <i>F</i>_1,4 = 2.032; <i>P</i> = 0.227); b) relationship between Δyield and Δ<i>g</i>_m (linear regression: R² = 0.134; <i>F</i>_1,4 = 0.618; <i>P</i> = 0.476); c) relationship between Δyield and Δ<i>g</i>_tot (linear regression: R² = 0.0818; <i>F</i>_1,4 = 0.356; <i>P</i> = 0.583); d) relationship between Δ<i>A</i> and Δ<i>g</i>_s (linear regression: R² = 0.0003; <i>F</i>_1,4 = 0.00106; <i>P</i> = 0.976); e) relationship between Δ<i>A</i> and Δ<i>g</i>_m (linear regression: R² = 0.742; <i>F</i>_1,4 = 11.527; <i>P</i> = 0.0274); f) relationship between Δ<i>A</i> and Δ<i>g</i>_tot (linear regression: R² = 0.715; <i>F</i>_1,4 = 10.042; <i>P</i> = 0.0339), and; g) relationship between Δyield and Δ<i>A</i> (linear regression: R² = 0.427; <i>F</i>_1,4 = 2.979; <i>P</i> = 0.159). Error bars as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109054#pone-0109054-g001" target="_blank">Figure 1</a>. Numbers next to data points indicate <i>Oryza sativa</i> variety as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109054#pone-0109054-g001" target="_blank">Figure 1</a>.</p

Interaction of yield and <i>A</i> with transpiration efficiency (<i>A</i>/<i>g</i>_s) and the ratio of <i>g</i>_m to <i>g</i>_s in well-watered (open symbols) and drought conditions (closed symbols): a) relationship between yield and <i>A</i>/<i>g</i>_s under full (linear regression: R² = 0.0595; <i>F</i>_1,4 = 0.253; <i>P</i> = 0.641) and water-stressed (linear regression: R² = 0.434; <i>F</i>_1,4 = 3.072; <i>P</i> = 0.155) conditions; b) relationship between harvest index (<i>HI</i>) and <i>A</i>/<i>g</i>_s under full (linear regression: R² = 0.205; <i>F</i>_1,4 = 1.032; <i>P</i> = 0.367) and water-stressed (linear regression: R² = 0.185; <i>F</i>_1,4 = 0.909; <i>P</i> = 0.394) conditions; c) relationship between yield and <i>g</i>_m:<i>g</i>_s (linear regression: R² = 0.456; <i>F</i>_1,10 = 8.379; <i>P</i> = 0.

<p>Error bars as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109054#pone-0109054-g001" target="_blank">Figure 1</a>. Numbers next to data points indicate <i>Oryza sativa</i> variety as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109054#pone-0109054-g001" target="_blank">Figure 1</a>.</p

Measurements of (a) photosynthesis rate (<i>A</i>), (b) stomatal conductance (<i>g</i>_s), (c) mesophyll conductance (<i>g</i>_m), and (d) intrinsic transpiration efficiency (<i>A</i>/<i>g</i>_s) in control and water-stressed leaves of the seven <i>Oryza sativa</i> genotypes.

<p>The measurements were made on the flag leaf in saturating PPFD (1400 µmol m⁻²s⁻¹), with relative humidity ranging between 45–55%, and a leaf temperature of 30°C. Data are means of 4 to 7 plants per treatment. Error bars as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109054#pone-0109054-g001" target="_blank">Figure 1</a>. Different letters denote significant differences among means derived using a factorial ANOVA and Tukey <i>post-hoc</i> test.</p

On the Use of Leaf Spectral Indices to Assess Water Status and Photosynthetic Limitations in <i>Olea europaea</i> L. during Water-Stress and Recovery

<div><p>Diffusional limitations to photosynthesis, relative water content (RWC), pigment concentrations and their association with reflectance indices were studied in olive (<i>Olea europaea</i>) saplings subjected to water-stress and re-watering. RWC decreased sharply as drought progressed. Following rewatering, RWC gradually increased to pre-stress values. Photosynthesis (<i>A</i>), stomatal conductance (<i>g</i>_s), mesophyll conductance (<i>g</i>_m), total conductance (<i>g</i>_t), photochemical reflectance index (PRI), water index (WI) and relative depth index (RDI) closely followed RWC. In contrast, carotenoid concentration, the carotenoid to chlorophyll ratio, water content reflectance index (WCRI) and structural independent pigment index (SIPI) showed an opposite trend to that of RWC. Photosynthesis scaled linearly with leaf conductance to CO₂; however, <i>A</i> measured under non-photorespiratory conditions (<i>A</i>_1%O2) was approximately two times greater than <i>A</i> measured at 21% [O₂], indicating that photorespiration likely increased in response to drought. <i>A</i>_1%O2 also significantly correlated with leaf conductance parameters. These relationships were apparent in saturation type curves, indicating that under non-photorespiratory conditions, CO₂ conductance was not the major limitations to <i>A</i>. PRI was significant correlated with RWC. PRI was also very sensitive to pigment concentrations and photosynthesis, and significantly tracked all CO₂ conductance parameters. WI, RDI and WCRI were all significantly correlated with RWC, and most notably to leaf transpiration. Overall, PRI correlated more closely with carotenoid concentration than SIPI; whereas WI tracked leaf transpiration more effectively than RDI and WCRI. This study clearly demonstrates that PRI and WI can be used for the fast detection of physiological traits of olive trees subjected to water-stress.</p></div

Comparison between the estimates of mesophyll conductance of CO₂ (<i>g</i>_m) obtained by applying two independent methods: the variable <i>J</i> method and the ‘δ13C of recently synthesised sugars’ method (linear regression: R² = 0.932; <i>F</i>_1,9 = 122.750; <i>P</i> = 1.515×10⁻⁶).

<p>Each data point represents the average value of three observations based upon the Δ13C of recently synthesised sugars and six to fourteen gas-exchange measurements utilising the variable J method. Error bars as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109054#pone-0109054-g001" target="_blank">Figure 1</a>. The regression line excludes the two data points on the right of the graph (IR64 and PS80) with anomalously high <i>g</i>_m derived from the δ13C of recently synthesised sugars. Numbers next to data points indicate <i>Oryza sativa</i> variety as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109054#pone-0109054-g001" target="_blank">Figure 1</a>.</p

Time courses of leaf (a) relative water content (RWC), dashed horizontal lines indicate the range of values (mean ± one standard deviation) recorded in the well-watered plants over the duration of the experiment; (b) photochemical reflectance index (PRI) (dashed horizontal lines as in 1a); (c) photosynthesis (<i>A</i>) (measured in both ambient air and in air with 1% [O₂] (<i>A</i>1%O2), and; (d) stomatal conductance (<i>g</i>_s), mesophyll conductance (<i>g</i>_m) and total conductance (<i>g</i>_t) of olive saplings grown during and after the drought cycle (days 1–23), dashed horizontal lines indicate the range of <i>g</i>_s values recorded in the well-watered plants. ↓ = end of the drying cycle. Data points are means of five plants (five leaves per plant) ±1 SEM.

<p>Time courses of leaf (a) relative water content (RWC), dashed horizontal lines indicate the range of values (mean ± one standard deviation) recorded in the well-watered plants over the duration of the experiment; (b) photochemical reflectance index (PRI) (dashed horizontal lines as in 1a); (c) photosynthesis (<i>A</i>) (measured in both ambient air and in air with 1% [O₂] (<i>A</i>1%O2), and; (d) stomatal conductance (<i>g</i>_s), mesophyll conductance (<i>g</i>_m) and total conductance (<i>g</i>_t) of olive saplings grown during and after the drought cycle (days 1–23), dashed horizontal lines indicate the range of <i>g</i>_s values recorded in the well-watered plants. ↓  =  end of the drying cycle. Data points are means of five plants (five leaves per plant) ±1 SEM.</p

Interaction of diffusive conductance parameters to CO₂ uptake with yield and photosynthesis (<i>A</i>) under well-watered (open symbols) and drought conditions (closed symbols): a) relationship between yield and stomatal conductance (<i>g</i>_s) (linear regression: R² = 0.696; <i>F</i>_1,10 = 22.900; <i>P</i> = 0.000740); b) relationship between <i>A</i> and <i>g</i>_s (linear regression: R² = 0.873; <i>F</i>_1,11 = 75.721; <i>P</i> = 2.911×10^–6); c) relationship between yield and mesophyll conductance (<i>g</i>_m) (linear regression: R² = 0.850; <i>F</i>_1,10 = 56.611; <i>P</i> = 2.911×10^–5); d) relationship between <i>A</i> and <i>g</i>_m (linear regression: R² = 0.952; <i>F</i>_1,11 = 217.071; <i>P</i> = 1.376×10^–8); e) relationship between yield and total conductance (<i>g</i>_tot) (linear regression: R<sup

<p>Error bars as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109054#pone-0109054-g001" target="_blank">Figure 1</a>. Numbers next to data points indicate <i>Oryza sativa</i> variety as in <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0109054#pone-0109054-g001" target="_blank">Figure 1</a>.</p