South Africa close with a lead of 205
England collapse from 143-3 to 205ao
Root 78, Bairstow 45, Maharaj 3-21, Morris 3-38
SA 335: Amla 78, De Kock 68, Anderson 5-72
Root c De Kock b Morkel 78 (Eng 143-4)
Just when it looked like Morne Morkel might be tiring and could be withdrawn he gets the man South Africa really wanted. Joe Root drives at a full ball from the tall South African paceman, but instead he edges it and Quinton de Kock dives, almost full length in front of first slip, to take the catch. How big a moment could that be in the context of this Test match?
Root 78 Bairstow 21
Plenty of away swing for Duanne Olivier but the South Africa seamer just isn't landing the ball in the right area to really test England's batsmen. Olivier's final delivery is deemed as a wide - remarkably the first wide of the series to date.
Here's one of those crunching fours from Joe Root...
Michael Vaughan
Ex-England captain on BBC Test Match Special
South Africa just can't keep Root quiet. They are almost offering him a single to get him off the strike. Instead he's hitting fours.
Jonathan Agnew
BBC cricket correspondent on TMS
That was a poor ball from Morkel. A tired ball.
The slightest hint of anything short and Root is on to it in a flash. This time he plays he pulls the first ball of Morne Morkel's over round the corner for four. A loose delivery from the tall South Africa bowler.
Morkel has head in his hands with the final of the over as a ball outside off stump swings after passing Jonny Bairstow and runs away past South Africa wicketkeeper Quinton de Kock.
Duanne Olivier into the attack and he gives Jonny Bairstow a wide one which the England wicketkeeper crashes through the cover for four. That should settle a few nerves.
Interesting thoughts Jenny, thanks for that. I guess there comes added responsibility with what is perceived to be the 'correct' way to behave when you are skipper. There's also that extra scrutiny on everything you do.
tms@bbc.co.uk
Graeme Swann made a mild criticism of Joe Root for not removing his helmet to acknowledge the crowd's applause for his 50.
Root's innings is vital if England are going to have a chance in this match, so it's essential that he maintains his concentration. He had another ball to face before lunch and he had to keep his focus. He was seeing the ball well and his shots were almost all well timed and well placed. If he didn't want to take his helmet off and wave to all corners of the ground, that's absoluely fine by me. He is there to play cricket and win matches for England. I wouldn't have cared, even if I was there applauding, if he'd kept facing the bowler, raised his bat and waved it once, then got back down for the next ball.
Joe Root isn't necessarily a natural at some of the social aspects of captaincy, and he sounded a little uncomfortable in the post match interview after the victory in the first test. But he captained well and batted brilliantly, again to rescue England, in the first innings. I'll take that!
Jenny Billingsley
Michael Vaughan
Ex-England captain on BBC Test Match Special
I like Joe Root playing the pull shot to Morris and Philander, but it's very high risk to Morne Morkel because you can never be 100% certain on how high the ball will bounce.
Joe Root is back in the groove now. He guides a ball from Morne Morkel through third man for a boundary. A ball later he pulls it and runs a couple. Jonny Bairstow finishes the over with a play and a miss. He just needs to hang around and support Root here.
Jonathan Agnew
BBC cricket correspondent on TMS
Root has played some extraordinary shots today. The bowlers are pitching it only fractionally short and he's pulling it through mid-wicket. This is a remarkable innings.
Root 64 Bairstow 11
Vernon Philander drops a short-ish ball in but the pace is not quick enough and Joe Root picks it up early. The England skipper rocks on to the back foot and pulls the ball off the meat of the bat for four through midwicket. He then pinches the strike of the last ball of the over. Clever cricket.
Joe Root classically drives Morne Morkel's third ball of the over straight. The lanky South African stretches one of those long arms of his - Inspector Gadget-esque - and manages to prevent it going past him. A delivery later Root places one through the covers and picks up a single.
Image source, Getty Images#bbccricket
Jack Ranft: Could almost do with a nightwatchman in at this point. Just need to block, step back and watch the captain score the runs.
Stephan Shemilt
BBC Sport at Trent Bridge
The Trent Bridge honours board. The last two Test centuries here belong to Joe Root. Three in a row?
Root 56 Bairstow 11
Joe Root keeps the scoreboard ticking over as he gets in himself in line to face Vernon Philander and pick up three runs off the over. Jonny Bairstow a bit more watchful.
Ed Smith
Ex-England batsman on BBC Test Match Special
This is going to be the hardest batting Jonny Bairstow has had to do for a long time.
A few gremlins in the pitch. A full ball from Morne Morkel outside off stump to Jonny Bairstow skids on and stays very low. The England wicketkeeper puts that out of his mind a couple of deliveries later to guide the ball through the vacant third man area for a boundary.
Morkel ends the over with a ball short of good length which rears up and cuts Bairstow in half. England just need to tough it out here.
Here's how Gary Ballance was dismissed by Vernon Philander's first delivery after lunch.