Field studies on factors controlling leaf photosynthesis in dryland rice [Philippines]
1986
Dingkuhn, M. | Cruz, R.T. (International Rice Research Inst., Los Banos, Laguna (Philippines))
Net photosynthesis, leaf abscisic acid (ABA) content, and leaf water status of four rice cultivars were studied in two dry seasons during line source sprinkler differential irrigation. Under well-watered conditions (120% Epan), photosynthetic rates were within the range occurring in C4 crops (up to 45 micron mol m-2S-1 or 71 mg dm-2 h-1). In the stress treatment, stomatal closure and decline in photosynthesis were detected at threshold leaf water potentials of -0.8 MPa in the dryland-adapted variety Azucena and -1.0 MPa in IR20 (lowland-adapted). No differences in maximum photosynthesis, however, could be detected among the four rice cultivars tested. The main factor controlling photosynthesis at light saturation was stomatal closure, leading to a characteristic reduction in assimilation during early afternoon. This midday or afternoon depression occurred under well-watered upland conditions and increased after stress onset. Another significant factor apart from stomatal closure was a non-stomatal inhibition of CO2-assimilation detectable in the afternoon and during severe water stress. This should be shown by a comparison of intercellular CO2 concentration (Ci) and assimilation rate. This non-stomatal inhibition might be induced by high endogenous levels of ABA. Exogenous application of ABA via transpiration stream caused both stomatal and non-stomatal inhibition of photosynthesis. Furthermore, elevated endogenous concentrations of ABA were detected in the afternoon and during water stress. In contrast to stomatal closure, non-stomatal inhibition of CO2-assimilation reduces water use deficiency (WUE).
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