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In mature and young leaves of sunflower (Helianthus annuus L. cv. Catissol-01) plants grown in the greenhouse, photosynthetic rate, stomatal conductance, and transpiration rate declined during water stress independently of leaf age and recovered after 24-h rehydration. The intercellular CO 2 concentration, chlorophyll (Chl) content, and photochemical activity were not affected by water stress. However, non-photochemical quenching increased in mature stressed leaves. Rehydration recovered the levels of non-photochemical quenching and increased the F v /F m in young leaves. Drought did not alter the total Chl content. However, the accumulation of proline under drought was dependent on leaf age: higher content of proline was found in young leaves. After 24 h of rehydration the content of proline returned to the same contents as in control plants.Additional key words: chlorophyll; drought; intercellular CO 2 concentration; leaf water potential; non-photochemical quenching; photochemical activity; rehydration; stomatal conductance to water vapour; transpiration rate. ---The leaf water status and carbon uptake is under stomata control and stomata closure is one of the earliest responses to drought, resulting in protection of the plants against harmful dehydration but also inevitably results in reduction of CO 2 uptake for photosynthesis. The stomata control is important only under mild stress (Cornic 2000). However, metabolic inhibition of photosynthesis also takes place at mild water stress (Tezara et al. 1999) and it becomes more important as the water stress intensifies. Most of the studies in this area were done on mature leaves and the few available data comparing the effects of water stress on mature and young leaves indicate that the photosynthetic responses to water stress are strongly dependent on leaf age (David et al. 1998). In addition, there are indications that water stress accelerates leaf senescence (Olsson 1995).The accumulation of osmolyte compounds in the cells as a result of water stress is often associated with a possible mechanism to tolerate the harmful effect of water shortage. The contribution of sugars as an osmotic solute in expanded and partly expanded sunflower leaves was studied by Jones and Turner (1980). They found that contents of sugars did not change in fully expanded leaves. In opposition, the contents of soluble sugars in partly expanded leaves were reduced. In addition to sugars, some plants also accumulate other low molecular mass compounds, such as proline (Gzik 1996, Bajji et al. 2001. These osmolyte accumulations in plant cells might contribute, via lowering the cell osmotic potential, to maintaining several physiological processes, such as photosynthesis, stomatal conductance, and leaf expansion even under stressed conditions. The present experiment was designed to study the responses of photosynthesis to water stress in fully expanded and expanding leaves of sunflower plants. Additionally, the change in proline content and the ability of plants to recover from the water stres...

Leaf senescence is characterized by decreased chlorophyll content in leaves. The objectives of this study were to determine whether heat‐induced chlorophyll decline is due to inhibited chlorophyll synthesis or accelerated chlorophyll degradation and to determine whether genetic variations in heat tolerance of bentgrass (Agrostis spp.) species were associated with differential chlorophyll‐enzymatic responses to heat stress. Five turfgrass lines, including two transgenic creeping bentgrass (A. stolonifera L.) lines overexpressing isopentenyl transferase (ipt) gene ligated to a senescence‐activated promoter (SAG12) or heat shock promoter (HSP18.2) for controlling cytokinin synthesis, two thermal bentgrass (A. scabra Willd.) lines, and a wild‐type (WT) creeping bentgrass (‘Penncross’) were subjected to heat (38/33°C, day/night) or optimal temperature (22/18°C, day/night) (nonstress) for 42 d in growth chambers. The physiological parameters of turf quality, chlorophyll content, chlorophyll index, and dark green color index were measured. The data suggested significant genetic variations in the level of heat‐induced leaf senescence among the bentgrass lines. The enzyme activity of a key chlorophyll‐synthesizing enzyme, porphobilinogen deaminase, did not differ significantly across all the lines after 42 d of heat stress. The activities of chlorophyll‐degrading enzymes, including chlorophyllase and chlorophyll‐degrading peroxidase, increased significantly after heat stress, whereas pheophytinase activity was unchanged. Heat‐tolerant transgenic lines and thermal bentgrass maintained significantly lower activities of chlorophyll‐degrading enzymes than the WT under heat stress. Heat‐induced chlorophyll loss in bentgrass could be mainly due to accelerated chlorophyll degradation. Selecting for low chlorophyll degradation enzyme activity will help to facilitate the development of elite stay‐green bentgrass lines.

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