WATER-STRESS IN LEAVES OF PHASEOLUS-VULGARIS INFECTED WITH XANTHOMONAS-CAMPESTRIS PV PHASEOLI

Infection of bean leaves (Phaseolus vulgaris) by Xanthomonas campestris pv. phaseoli in the field frequently resulted in the appearance of isolated flaccid areas in green leaf tissue adjacent, to necrotic and chlorotic lesions. The flaccid leaf areas had significantly higher stomatal resistances compared to nearby turgid areas on the same leaf, and the turgid areas had stomatal resistances that were the same or only moderately elevated compared to those of healthy leaves. The flaccid tissues also had significantly lower relative water contents than turgid tissues on the same leaf demonstrating that pathogen-induced water stress was localized. The levels of free proline, another indicator of water stress, were directly correlated (r(2)=0.556) with disease severity. The change in free proline content implied that water stress increased in direct proportion with the amount of tissue infected. Water stress may be due to the disruption of xylem elements by the invasion of X. c. phaseoli from nearby lesions. One result of xylem invasion could have been severe water deficits which were sufficient to cause stomatal closure and leaf flaccidity; however, this effect was highly localized and the remainder of the diseased leaf was either significantly less water stressed or not affected.