Complex sentence: Lowland cultivars appear more plastic in their morphology, produce larger plants if stands become thin or when planted in wide rows, and they seem to be more sensitive to moisture stress than upland cultivars.
Split into simpler sentences: Lowland cultivars appear more plastic in their morphology, and produce larger plants if stands become thin or when planted in wide rows. On the other hand, lowland types seem to be more sensitive to moisture stress than upland cultivars.