Zantedechia aethiopica is a south african native, so I don,t think the sun will be the cause, and as they can be grown as a pond marginal, I wouldn,t think the water would be the cause either, I would think some sort of physiological disorder, sorry I can,t be more help to you.

Perhaps they are water-logged, rather than just in moist soil? Too much water could result in a loss of nutrients, such as nitrogen? Too much direct sun can burn the leaves, but that is different to yellowing of the leaves.

It may have something to do with hot weather, too. Z. aethiopica goes dormant when the temps go above 32º C, especially if the nights are warmer, too.

thanks for the info