Abstract:

Globally elevated temperatures and changed precipitation distributions may lead to deficits of fresh water that reduce crop yields and degrade natural ecosystems. Plant carbon (C) and nitrogen (N) metabolism and its abiotic environmental regulation are responsible for net primary productivity and plant nutrient status. We review the relationship between C and N and regulation by environmental factors such as temperature, water moisture and CO₂ enrichment at multiple levels of plant organization, including molecule, tissue, organ, whole plant and ecosystem. For cereal crops including wheat and rice, grain N mainly includes: 1) N reallocated in vegetative organs before anthesis, and 2) N absorbed from soil after anthesis. Their proportions depend on the activity and size of grains as an N sink and species and cultivars, affecting the grain yield and quality. Leaf N level can explain 45%-75% of leaf photosynthesis, and 71%-88% of leaf N can be allocated into protein, with Rubisco, the key enzyme for photosynthesis, accounting for 30%-50% of total leaf soluble protein, making it the protein using most N. Furthermore, the N proportions among the photosynthetic organs and the ratio between soluble sugar and starch may be associated with the Rubisco gene. Therefore, plant N level may be assessed by photosynthetic capacity.
Many studies have demonstrated that drought can promote C allocation to below-ground parts of plants, increasing root:shoot biomass ratio. There is, however, evidence that this enhancement of roots due to moderate drought can be negated by severe drought. On the other hand, drought also increases N concentration in sink organs, such as wheat grains, and decreases mature leaf N concentration, decreasing leaf net photosynthetic rate. However, high temperature does not significantly increase C allocation to roots, but may decrease leaf N concentration and affect Rubisco level. Thus, a decline of photosynthetic capacity induced by above optimal temperature, particularly at night, may be ascribed to an adverse effect on photosystem Ⅱ (PSⅡ). Generally, elevated CO₂ dilutes plant tissue N, leading to a lower C:N ratio that may come from the effect on Rubisco expression. Whole ecosystem N allocation and cycle can be affected by elevated CO₂, thereby changing ecosystem structure and function. The interaction between severe drought and high temperature can lead to a decrease in leaf N, reducing plant C-fixing ability, depending on the time and severity of stresses. Under high CO₂ concentration and drought, the C allocation into below-ground parts can be enhanced and the C:N ratio may increase. The interaction between elevated CO₂ and high temperature can alter plant tissue N allocation, with elevated CO₂ increasing CO₂ site activity of Rubisco and decreasing N investment in photosynthetic apparatus, but high temperature increasing O₂ site activity of Rubisco and increasing N investment. Nevertheless, under global change conditions, the combined effects from various stress factors are complex, and may include both positive and negative relationships. Research is urgently needed to 1) elucidate plant C and N allocation models from molecular to ecosystem levels; 2) address synergistic effects of multiple environmental stresses; 3) predict C and N allocation based on different global change scenarios; 4) quantify the threshold for change in C and N allocation in response to global change; and 5) strengthen knowledge of the key role of C and N allocation in agricultural and forest productivity and conservation of natural ecosystems.

Key words: global change, elevated carbon dioxide (CO₂), drought, high temperature, carbon and nitrogen metabolism