1. Increase in temperature and velocity of wind cause an increase in transpiration initially but later it slows down. Why???

2. Water  potential in leaf tissue is positive during

a. excess transpiration

b. low absorption

c. low transpiration

d. guttation

 Ans 2 is ---> a. excess transpiration. During absortion, DPD of mesophyll cells becomes lower than the surrounding atmosphere [ As water mover from lower DPD to higher DPD ] .Since Water potential = - DPD, so in case of , so during absorption water pot. of leaf tissue is positive. But during low absorption, DPD of leaf cells increases ; thus making water pot.  value lower or -ve. But during excess transpiration, water potential of mesophyll cells increases and its value is positive. 

  When velocity of wind increases, it increases the rate of transpiration. But when velocity increases much more then stomata closes and thereby lowers transpiration. Again, at high temperature activity of enzymes decreases. I think that is the answer.......

this is the second answer

when the rate of transpiration is low, the potential of water in the leaf increases as there is no loss of water and therefore water potential is positive. so the correct option is (c).

[DO RATE!!]

 @ Abhishek , u said that  that when transpiration is low the potential of water in the leaf increases as there is no loss of water and therefore water potential is positive..... but we know that water always moves from the region of higher water pot. to lower water pot. Therefore , when transpiration is low , the difference in water pot. of leaf cells and exterior is low ; which means lower water pot. of leaf cells. when transpiration takes place in a rapid manner , water pot. of mesophyll cells is quite higher [ i.e. its value increases and ultimately becomes zero ]. Ultimately a time comes when no transpiration takes place.....Hope it is clear now......plz nudge me for any doubt or correction........

 hey..water potential can be positive...it is true that water potential of pure water is taken arbitrarily as 0.. but just like potential energy, water potential also depends on the reference level.. also, even if we take water potential of pure water as 0 at a certain gravitational potential, if we somehow increase the gravitational potential( just by raising the pure water above to a height), its water potential becomes positive.. i also think that during low transpiraton, water potential gets positive as there is a build up of excess free water molecule which raise the potential energy of the water hence its water potential i.e answer is c

for the 1st one, i havent read it anywhere, but tried to apply some physics and chemistry, and a little logic... 

when wind is blowing, pressure decreases..i.e pressure potential gets negetive above the surface of leaf.. now.. water molecules would move from high pressure to low pressure... hence, for time being, transpiration increase.. now, as water starts to leave the leaf, the free water molecules in the leaf ( mesophyll cells) decrease, in other words, its water potential decreases as the free water molecules are leaving...now, absorbtion of water from soil takes place.. as this process goes on, the water potential of soil decreases... and due to it, the decrease water potential of the mesophyl cells cant be compensated fully by absorption of water....hence now, the difference between the water potential of mesophyll cell and the atmosphere is lesser, which results in lesser rate of transpiration...

plzz correct me if wrong, and plzz tell me if it violates any rule or something like that...

 For water to go out of the leaf , DPD of the mesophyll cells has to be be low than the outer atmosphere.We know that DPD = - Water pot., so water pot. of the mesophyll cells is higher.

    I dont know whether my concept is true or not.But i m quite sure about the relations stated above.So, totally confused...plz sumbdy explain the thing clearly. If i look 4m the relations mentioned above, then the ans comes out to be (a).....dont know !!!!!

temperature and wind velocity influences the rate of transpiration. with rise in temperature there is increase in rate initially but continue increase reduces the enzyme activity which directly influences the opening & closing of stomata so to compensate the rate start decreasing. approx. for every 10 degree celsius rise the rate get doubled.

 the rate of transpiration is maximum in mild wind initially with increase in wind velocity the rate increases as there is removal of water vapours from atmosphereand lowering the relative humidity.by wind.but wind with higher velocity the rate decreases probably because the guard cells become flaccid and stomats get closed.

AnS 2 is C when the rate of transpiration will be low bocz the loss of water will be less so potential willbe higher simply why u r going in all those complicated details of relation of psi and DPD all that