Plants under Climatic Stress

Abstract

Photosynthetic rates of both C₄- and C₃-pathway plants grown at 25 C were measured before and during a period of chilling stress at 10 C, and then again at 25 C following various periods at 10 C. When temperatures are first lowered photosynthetic rates drop immediately, then undergo a further reduction which is quite rapid in species such as Sorghum, maize, and Pennisetum; slower in soybean; and very slow in Paspalum and ryegrass. Visible light causes progressive permanent damage to the photosynthetic capacity of leaves during this period of lowered photosynthesis. The extent of damage increases with light intensity and the length of time leaves are held at 10 C but varies greatly between species, being roughly correlated with the extent to which chilling initially and subsequently lowers photosynthesis. Three days of chilling (10 C) at 170 w·m⁻² reduces the photosynthetic capacity of youngest-mature Paspalum leaves only 30 to 40% while Sorghum leaves are essentially inoperative when returned to 25 C after the same stress. Root temperature has a substantial rapid effect on photosynthesis of soybean and little immediate effect on Sorghum. Photosynthesis of stress-intolerant species (Sorghum) is reduced only slightly more than that of semitolerant species (Paspalum) when temperatures are lowered at mid-photo-period, but to a far greater extent if temperatures are reduced at the commencement of a photoperiod.