Hyungmin Rho

Limitation Factors for Photosynthesis in 'Bluecrop' Highbush Blueberry (Vaccinium corymbosum) Leaves in Response to Moderate Water Stress

The levels of stomatal, mesophyll and biochemical limitations in CO2 assimilation of ‘Bluecrop’ highbush blueberry leaves were compared at two different levels of leaf water potential. The leaf water potentials were −1.49 and −1.94 MPa in daily-irrigated (DI) and non-irrigated (NI) shrubs, respectively. The NI shrubs represented plants under moderate water stress. Mesophyll conductance (gm) and chloroplastic CO2 concentration (Cc) were estimated by combined measurements of gas exchange and chlorophyll fluorescence under various intercellular CO2 concentrations (Ci). Net CO2 assimilation rates (An) as a function of Cc were used for calculating maximum carboxylation efficiency (αcmax) at the real sites of CO2 assimilation. Maximum An (Anmax) from the light response curves at 400 μmol mol−1 air of ambient CO2 concentration (Ca) were lower in the leaves of NI shrubs than in those of DI ones. However, electron transport rates were higher in the leaves of NI shrubs than in those of DI ones. The decrease in CO2 assimilation following water stress may be caused by a decrease in gm rather than a decrease in stomatal conductance (gs) according to limitation analysis. Limitation rates by gs, calculated at 400 μmol mol−1 air of Ca in An-Ci curves, were not significantly different between the leaves of DI and NI shrubs. However, limitation rates by gm from An-Cc curves were significantly higher in the leaves of NI shrubs than in those of DI ones. Maximum carboxylation efficiency (αcmax) values calculated from the An-Cc curve, contrary to those calculated from the An-Ci curve, were higher in the leaves of NI shrubs than in those of DI ones. Consequently, mesophyll limitation than stomatal and biochemical limitations mainly down-regulated the photosynthesis in the leaves of ‘Bluecrop’ blueberry shrubs during moderate water stress.

Photosynthetic characteristics of highbush blueberry (Vaccinium corymbosum cv. Bluecrop) leaves in response to water stress and subsequent re-irrigation

SUMMARY Gas exchange and photosystem II (PSII) activities in the leaves of 2-year-old ‘Bluecrop’ highbush blueberry (Vaccinium corymbosum) were monitored during water stress and subsequent re-irrigation to investigate the effects of the intensity of water stress on changes in photosynthetic characteristics. The blueberry shrubs were not irrigated for 3 to 5 weeks, then re-irrigated daily up to 8 weeks. The decrease in soil water potential during water stress caused a progressive decrease in leaf water potential. Soil water potentials decreased to -0.26 MPa and -0.34 MPa at 3 and 5 weeks, respectively, following water stress, but recovered following subsequent re-irrigation, while the soil water potential in daily-irrigated shrubs was maintained at over -0.13 MPa throughout the experiment. Chlorophyll concentrations decreased with an increasing duration of water stress. Chlorophyll concentrations in leaves on shrubs subjected to water stress for 5 weeks did not recover following re-irrigation, unlike those subjected to water stress for 3 weeks. The leaves on shrubs subjected to water stress for 5 weeks maintained lower levels of chlorophyll during reirrigation. The net rate of CO2 assimilation (An) decreased significantly with an increasing duration of water stress. Reirrigation reversed the decrease in An in leaves on shrubs subjected to water stress for 3 weeks. Stomatal conductance (gs) exhibited a similar pattern to An. The actual quantum yield of photosystem II (ΦPSII) and the electron transport rate (ETR) also decreased significantly with an increasing duration of water stress, although the Fv/Fm ratio was not affected. ΦPPSII and ETR values in the leaves on shrubs subjected to water stress for 5 weeks did not recover after reirrigation, unlike those subjected to water stress for 3 weeks. Non-photochemical quenching increased with an increasing duration of water stress, but subsequent re-irrigation did not reverse the increase. These results indicate that the timing of re-irrigation of water-stressed ‘Bluecrop’ highbush blueberry is critical in order to maintain their photosynthetic capacity. Among the photosynthetic characteristics measured, ΦPSII and ETR could be used as sensitive indicators to assess the physiological status of leaves of ‘Bluecrop’ highbush blueberry growing under water stress conditions.

Estimation of carboxylation efficiency from net CO2 assimilation rate as a function of chloroplastic CO2 concentration in strawberry (Fragaria × ananassa cv. Maehyang) leaves

Carboxylation efficiency in fully expanded leaves of strawberry (Fragaria × ananassa cv. Maehyang) was estimated based on net CO2 assimilation rate (An) as a function of chloroplastic CO2 concentration (Cc). To estimate the mesophyll conductance (gm) and then construct An-Cc curves, An and electron transport rate as a function of intercellular CO2 concentration (Ci) were simultaneously determined at a range of 50 to 2,200 μmol CO2/mol air at a saturating photosynthetic photon flux (PPF), 1,200 μmol·m−2·s−1. Mitochondrial respiration rate (Rd) and CO2 compensation point in the absence of Rd required for calculating gm were found to be 0.15 μmol·m−2·s−1 and 44.1 μmol CO2/mol air, respectively, as determined from An-Ci curves below 200 μmol CO2/mol air at three different PPFs. Both stomatal conductance (gs) and gm decreased with increasing Ci. However, the gm responded more sensitively to various Ci than the gs. The gm was significantly lower than the gs at Ci beyond 600 μmol·mol−1 air examined. Maximum carboxylation efficiency (αcmax) derived from An-Cc curves was 0.28 μmol·m−2·s−1 and 2.2 times higher than that from An-Ci curve. Since the An-Ci curve was based on infinite gm, the αcmax derived from the An-Ci curve might be underestimated. Actually, gm was rather dynamic with changing Ci and thus a crucial component of the diffusional limitation of An. For estimating photosynthetic characteristics in strawberry leaves more accurately, An-Cc curve should be constructed in consideration of gm, especially for closed plant production systems.