INTRODUCTION 492 ECOLOGICAL ASPECTS OF PHOTOSYNTHETIC TEMPERATURE ADAPTATION 493 Photosynthetic Temperature Dependence in Thermally Contrasting Climates ........ 493 Photosynthetic Temperature Acclimation 497 Seasonal acclimation in natural habitats ...... ....... 497 Studies in controlled environments 499 THE MECHANISTIC BASIS FOR PHOTOSYNTHETIC RESPONSE AND ADAPTATION TO TEMPERATURE 504 Reversible Temperature Respon.ses 505 Stomata! effec� o� the . temJH!.rature response 0/ photo.rynthesis ......... ....... 505 Interacttons with /lght mtenslty 507 C, photo.rynthesis (lM photorespiration 507 C, photo.rynthesis 515 Comparison 0/ plants from contrasting thermol regimes ...... ...... 517 ["eversible Temperature Respon.ses 519 Low temperature sensitMty ....... 519 High temperature sensitivity 524 Adoptive responses in the heat stability 0/ the photosynthetic apparatus 530 CONCLUDING REMARKS 532

Photosynthetic Response and Adaptation to Temperature in Higher Plants

Growth of autotrophic plants is directly and dramatically influenced by the intensity of light — the driving force of photosynthesis — which provides nearly all of the carbon and chemical energy needed for plant growth. Moreover, light intensity (quantum flux density) is perhaps the most conspicuous environmental variable with which plants must cope. Contrasting terrestrial habitats may differ by at least two orders of magnitude in the daily quantum flux available for photosynthesis. In any habitat the quantum flux density varies seasonally, diurnally, and spatially (such as within a canopy of a given plant stand).

Responses to Different Quantum Flux Densities

Photorespiration and photoinhibition: Some implications for the energetics of photosynthesis

Chilling injury to crop plants was first described 70 years ago and has been systematically investigated with electron microscopy since the late 1960s. Chloroplasts are the first and most severely impacted organelle. Thylakoids swell and distort, starch granules disappear, and a peripheral reticulum (vesicles arising from inner membrane of chloroplast envelope) appears. Chloroplast disintegration follows prolonged chilling. Mitochondria, nuclei and other organelles are less susceptible to chilling injury. Organellar development and ontogeny may also be disrupted. The inherent chilling sensitivity of a plant, as well as the ability of some species to acclimate to chilling, influence the timing and appearance of ultrastructural injury with the resulting outcome being mild, moderate, or severe. Other environmental factors that exacerbate injury are irradiance, chilling duration, and water status. The physiological basis for chloroplast swelling may be linked to chilling-stable starch-degrading enzymes that produce soluble sugars thus lowering stromal water potential at a time when chloroplast photosynthate export is reduced. Thylakoid dilation appears to be related to photo-oxidative conditions produced during chilling in the light. The peripheral reticulum is proposed to increase surface area of the transport-limiting membrane (chloroplast inner membrane) in response to the chilling-induced reduction in metabolite transport. Many of the ultrastructural symptoms appearing during moderate stress resemble those seen in programmed cell death. Future research directions are discussed.

https://www.onlinelibrary.wiley.com/doi/pdf/10.1046/j.1365-3040.2000.00560.x

The ultrastructure of chilling stress

The functional significance of different photosynthetic CO2 fixation pathways is a question which can be answered in many ways, each being appropriate to certain scales of enquiry. In physiological ecology our purpose should be to integrate these different scales of enquiry as comprehensively as possible, and to show how photosynthesis contributes, directly or indirectly, to performance and survival of plants in diverse habitats. Studies of photosynthetic CO2 fixation were afflicted with a post-Calvin cycle chauvinism in the 1950’s, which may have been responsible for the slow and tentative revelation of the C4 pathway of photosynthetic carbon assimilation in the USA and USSR (Burr et al. 1957; Karpilov 1960; Kortschak et al. 1965). Elucidation of this pathway undoubtedly stimulated new interest in the carbon metabolism of photosynthesis in the next decade (Hatch and Slack 1966, 1970; CC Black 1973) and led to an upsurge in comparative studies of higher plant photosynthesis (Black 1971; Bjorkman 1973). Largely as a result of this stimulus, the peculiar dark CO2 fixation processes of succulent plants, known as crassulacean acid metabolism (CAM), were also recognized as a distinctive photosynthetic process (Kluge and Ting 1978; Osmond 1978). It also led to the present revival of interest in the photosynthesis of aquatic plants.

Functional Significance of Different Pathways of CO 2 Fixation in Photosynthesis

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.

A. O. Taylor

Papers

Plants under Climatic Stress: I. Low Temperature, High Light Effects on Photosynthesis