Photosynthetic thermotolerance of woody savanna species in China is correlated with leaf life span

Abstract

Background and aims: Photosynthetic thermotolerance (PT) is important for plant survival in tropical and sub-tropical savannas. However, little is known about thermotolerance of tropical and sub-tropical wild plants and its association with leaf phenology and persistence. Longer-lived leaves of savanna plants may experience a higher risk of heat stress. Foliar Ca is related to cell integrity of leaves under stresses. In this study it is hypothesized that (1) species with leaf flushing in the hot-dry season have greater PT than those with leaf flushing in the rainy season; and (2) PT correlates positively with leaf life span, leaf mass per unit area (LMA) and foliar Ca concentration ([Ca]) across woody savanna species.

Methods: The temperature-dependent increase in minimum fluorescence was measured to assess PT, together with leaf dynamics, LMA and [Ca] for a total of 24 woody species differing in leaf flushing time in a valley-type savanna in south-west China.

Key results: The PT of the woody savanna species with leaf flushing in the hot-dry season was greater than that of those with leaf flushing in the rainy season. Thermotolerance was positively associated with leaf life span and [Ca] for all species irrespective of the time of flushing. The associations of PT with leaf life span and [Ca] were evolutionarily correlated. Thermotolerance was, however, independent of LMA.

Conclusions: Chinese savanna woody species are adapted to hot-dry habitats. However, the current maximum leaf temperature during extreme heat stress (44·3 °C) is close to the critical temperature of photosystem II (45·2 °C); future global warming may increase the risk of heat damage to the photosynthetic apparatus of Chinese savanna species.

Fig. 1.

Leaf dynamics of two representative species, (A) Buchanania latifolia that flushes in the dry season (LF-D) and (B) Woodfordia fruticosa that flushes from the beginning of the rainy season onwards (LF-R). Cumulative leaf number and the number of fallen leaves are shown, as indicated in the key; values are means ± s.e. At the top of (A), the climatic conditions during the study period are indicated: cool-dry season (white), hot-dry season (grey) and hot-rainy season (black).

Fig. 2.

Temperature-dependent increase in minimum fluorescence (T – F_o) for Terminthia paniculata, a common species, as an example. Four parameters were calculated from the T – F_o curve: T_S20, the temperature at which the slopes of the response curve reach 20 % of its maximum; T_c, critical temperature, the intersection of the lines extrapolated from the slow and fast rise portion of the T – F_o curve; T₅₀, the temperature at which F_o reaches 50 % of its maximum; and T_max, the temperature at which F_o reaches its maximum.

Fig. 3.

Differences in T_S20, T_c, T₅₀ and T_max between species with leaf flushing in the late dry season (LF-D, n = 5) and species with leaf flushing in the rainy season (LF-R, n = 19). See Fig. 2 for definitions of the four photosynthetic thermotolerance parameters. Data are means ± s.e. *P < 0·05; **P < 0·05; ***P < 0·001.

Fig. 4.

Relationships between photosynthetic thermotolerance, leaf life span and Ca concentration across species. See Fig. 2 for definitions of T_c and T_max. **P <0·01; ***P <0·001.