2.8.13
REDUCTION IN RADIATION INTERCEPTION, RADIATION USE EFFICIENCY AND PHOTOSYNTHETIC RATE IN FIELD PEA (PISUM SATIVUM L.) BY ASCOCHYTA DISEASES

NA LUCAS, BA MCKENZIE, RE GAUNT, DJ MOOT

Plant Science Department, P.O. Box 84, Lincoln University, Canterbury, New Zealand

Background and Objectives
The objective of this study was to investigate the physiological response to ascochyta diseases in field pea crops. Waggoner and Berger [1] analysed the published yield response of a number of crops to disease. In wheat, potato, and peanut crops, dry matter production was highly correlated with radiation interception (RI) by healthy leaves. There was no evidence that radiation use efficiency (RUE) was affected by disease. In contrast, Madiera et al. [2] found that early season infection of field beans by Ascochyta fabae reduced RUE as well as reducing RI through a reduction in leaf area duration.

Materials and Methods
In 1996-97 and 1997-98 two field pea cultivars, Beacon" (determinate) and Whero (indeterminate) were subjected to five disease intensities. Plants were inoculated 2-3 ;weeks before flowering with a combination of Ascochyta pisi and Mycosphaerella pinodes. The amount of radiation transmitted through the canopy was measured weekly from 45 ;days after sowing until physiological maturity using a LICOR LAI-2000 plant canopy analyser and expressed as total intercepted photosynthetically active radiation (PAR). Total dry matter was measured at a final bulk harvest at the dry seed stage and used for the calculation of RUE. In 1997-98 photosynthetic rate was also measured weekly from 77 ;days after sowing until physiological maturity using a LICOR LI-6400 portable photosynthesis system. Photosynthetic measurements were taken repeatedly throughout the day on randomly chosen leaves exposed to 60% of total incident radiation as measured by a tuber solarimeter.

Results and Conclusions
In 1996-97 yields of both cultivars were related to RI for all disease levels. The full disease treatment reduced (P<0.001) PAR, in relation to the healthy treatment, from 543 ;MJ/m2 to 478 ;MJ/m2 for ‘Beacon’ and 663 ;MJ/m2 to 566 ;MJ/m2 for ‘Whero’. Disease also reduced RUE from 1.79 ;g DM/MJ PAR to 1.59 ;g DM/MJ PAR for ‘Beacon’ and 2.39 ;g DM/MJ PAR to 1.90 ;g DM/MJ PAR for ‘Whero’. Similar reductions in RI, RUE and net photosynthetic rate were observed in 1997-98. In conclusion, the effects of disease were described by a simple regression model, and disease had substantial effects on both RI and RUE.

References
1. Waggoner PE, Berger RD, 1987. Phytopathology 77, 393-98.
2. Madeira AC, Clark JA, Rossall S, 1994. Journal of Agricultural Science 123, 225-32.