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DISCUSSION 



Bourliere: I would just like to ask Dr. Yemm a few questions. First ; 

 what happens in permanent leaves of evergreen plants ; do you observe 

 the same modification as in deciduous leaves but occurring at a slower 

 rate ? And second ; are some ecological factors, for instance temperature, 

 able to modify the growth and ageing rate of the leaves in any way ? 



Yemm: Now, the first point is the question of different kinds of leaf. 

 Of the two examples which I gave you, one was an annual plant (Tropae- 

 olum) and the other one was what is effectively a much longer-lived 

 evergreen leaf, the Cherry Laurel. There is a general relationship between 

 the length of time which the leaf normally lives on the plant and the 

 rate at which the senescence changes occur. I think the Cherry Laurel 

 leaf takes about twice as long to pass through the yellowing phase as 

 the Tropaeolum. 



The second one: ecological factors and leaf senescence really are 

 extremely important. One of the things which increases the rate of 

 senescence of leaves very markedly in cereal plants, which I have been 

 interested in particularly, is water supply. One gets much earlier 

 yellowing and, of course, earlier ripening of the leaves under conditions 

 of restricted water supply. And there are many other factors — the 

 supply of nitrogen to the plant is an important one ; under conditions 

 of limited nitrogen supply again the leaves tend to become senescent 

 much earlier. 



Bourliere: And what about temperature? 



Yemm: Well, temperature is very difficult to distinguish from 

 humidity and water supply because it simply increases the rate of water 

 loss from the plant so the two factors interact. High temperatures and 

 relatively deficient water supply are both factors which tend to increase 

 the rate of senescence. 



Huggett: Does it happen at high temperatures with humidity? 



Yemm: It certainly does in apples. Temperature is a very important 

 factor in the ripening and senescence of apples. I do not think this has 

 been examined very carefully with leaves but the yellowing and ripening 

 of apples is very temperature-dependent. 



Huggett: What about leaves on tropical plants in humid climates like 

 Nigeria ? 



