Effect of photoperiod and temperature on resistance againstPhytophthora infestans in susceptible and resistant potato cultivars: Effect on deposition of structural phenolics on the cell wall and resistance to penetration

Deposition of phenolic compounds before and after inoculation withPhytophthora infestans was evaluated in two Mexican cultivars (Malinche and Tollocan) with major unknown R genes for resistance to potato late blight, two cultivars (393295.236=CIP1 and 391046.22=CIP2) without R genes from the International Potato Center (CIP) and the susceptible cultivar Atlantic. Before inoculation, plants were grown in growth chambers at two temperatures (16 or 24 C) and two photoperiods (PPD 12 or 16 h day length). Forty-eight hours after inoculation, the number of penetrations was recorded and depositions of phenolic compounds were classified according to detection and location in (a) the anticlinal cell wall, (b) the whole cell, (c) the stomatal cells, and (d) without detectable depositions of phenolic compounds. The concentration of phenolic compounds in the epidermal cells was slightly increased at 16 C and 16 h PPD and penetration frequency was lower at 16 C (12 h PPD). Concentration of phenolic compounds was not correlated with penetration frequency, but was correlated with the resistance level of the different potato cultivars. Atlantic had the highest number of penetrations followed by Tollocan, CIP1, CIP2, and Malinche. The cytological observations indicated that four types of deposition of phenolic compounds occurred in all five potato cultivars irrespective of their type and level of resistance. These results suggest that deposition of phenolic compounds on epidermal cells is a general resistance mechanism in potato leaves that does not have a specific relation with resistance to the penetration ofP. infestans. Phenolic depositions were intrinsically similar in potato cultivars with and without R genes, which stresses the difficulty in differentiating between horizontal and vertical resistance.